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Jupiter and Minerva Terraces, Hot Springs, Yellowstone Park
THE STORY OF THE UNIVERSE
Told by Great Scientists
and Popular Authors
COLLECTED AND EDITED
By ESTHER SINGLETON
Author of “Turrets, Towers and Temples,” “Wonders of Nature,”
“The World’s Great Events,” “Famous Paintings,” Translator
of Lavignac’s “Music Dramas of Richard Wagner”
I.—THE EARTH’S CRUST — FORMATION OF THE EARTH — Élisée Reclus
THE STORY OF THE UNIVERSE
According to Laplace’s ideas, the whole
planetary system formed, in long past ages, a
portion of the sun. This luminary, composed solely
of gaseous particles much lighter than hydrogen,
pervaded with its enormous rotundity the whole of
the space in which the planets, including Neptune,
are now describing their immense orbits. The diameter
of the solar spheroid must then have been 6,500
times greater than it now is, and its bulk must have
surpassed its present volume by more than 860,000
millions of times. In the same way, the earth, before
it began to get cool and solidify, would have embraced
the moon within its limits, and its diameter
would have been nearly six times greater than that
of the planet Jupiter. But, unsubstantial and aerial
as it was, our earth had then nothing but a cosmical
life which could hardly be called material; it was
not until it became more solid and its outer crust
was hardened that it actually commenced its real
existence.
This brilliant hypothesis accounts better than any
other for the uniform translatory motion of the planets
in the direction of west to east; it also apparently
agrees in a remarkable way with certain facts in the
subsequent history of the earth, as disclosed to us by
geology; finally, the marvelous rings which surround
the planet Saturn seem to proclaim the truth of the
theory devised by Laplace. There have been some
experiments on a small scale which appeared to reproduce
in miniature the magnificent spectacle presented
in the primitive ages by the origin of the planets.
M. Plateau, a Belgian savant, managed to make
a globe of oil revolve in a mixture of water and spirits
of wine which was of exactly the same specific
gravity as the oil. When the revolution of the little
globe was sufficiently rapid, it was noticed to flatten
at the poles and to swell at the equator; after a time
it threw off rings which suddenly assumed the shape
of globules actuated by a rotatory motion of their
own, and turning round the central globe.
Another hypothesis connected with Laplace’s brilliant
astronomical theory must be added, in order to
describe the formation of the planetary crust. When
the gaseous ring became condensed into a globe, it
would not cease to contract, owing to the continued
radiation of its caloric. The whole mass, having become
liquid through the gradual cooling of its molecules,
would be changed into a sea of lava whirling
round in space; but this state was only one of transition.
After an indefinite term of centuries, the loss
of heat was sufficient to cause the formation of a light
scoria, like a thin sheet of ice over the surface of the
fiery sea, perhaps just at one of the poles where nowadays
the extreme cold produces icebergs and a frost-bound
sea. This first scoria was succeeded by a second,
and then by others; next they would unite into
continents floating on the surface of the lava, and,
finally, would cover the whole circumference of the
planet with a continuous layer. A thin but solid
crust would then have imprisoned within it an immense
burning sea.
This crust was frequently broken through by
the lava boiling beneath it, and then, by means of
the solidification of the scoriæ, was again united; the
cooling process would tend also to slowly thicken
it. After a lapse of time, which must have been immensely
protracted—since the interval during which
the temperature of the terrestrial crust would be lowered
from 2,000° to 200° has been estimated, at the
very least, at three and a half millions of centuries—the
pellicle at last became firm, and the eruptions of
the liquid mass within ceased to be a general phenomenon,
localizing themselves at those points where
the firm crust was the thinnest. The surrounding atmosphere,
replete with vapors and various substances
maintained by the extreme heat in a gaseous state,
would gradually get rid of its burden; all kinds of
matter, one after the other, would become disengaged
from the luminous and burning aerial mass,
and precipitate themselves on the solid crust of the
planet. When the temperature was lowered sufficiently
to enable them to pass from a gaseous to a
liquid state, metals and other substances would fall
down in a fiery rain on the terrestrial lava. Next,
the steam, confined entirely to higher regions of the
gaseous mass, would be condensed into an immense
layer of clouds, incessantly furrowed by lightning.
Drops of water, the commencement of the atmospheric
ocean, would begin to fall down toward the
ground, but only to volatilize on their way and again
ascend. Finally these little drops reached the surface
of the terrestrial scoria, the temperature of the
water much exceeding 100°, owing to the enormous
pressure exercised by the heavy air of these ages; and
the first pool, the rudiment of a great sea, was collected
in some fissure of the lava. This pool was constantly
increased by fresh falls of water, and ultimately
surrounded nearly the whole of the terrestrial
crust with a liquid covering; but, at the same time, it
brought with it fresh elements for the constitution of
future continents. The numerous substances which
the water held in solution formed various combinations
with the metals and soils of its bed; the currents
and tempests which agitated it destroyed its shores
only to form new ones; the sediment deposited at the
bottom of the water commenced the series of rocks
and strata which follow one another above the primitive
crust.
Henceforward the igneous planet was externally
clothed with a triple covering, solid, liquid, and gaseous;
it might therefore become the theatre of life.
Vegetables and lowly forms of animals were called
into existence in the water, and on the land which
had emerged from it; and, finally, when the temperature
of the surface of the globe had become less than
50°, allowing albumen to liquefy and blood to flow
in the veins, the fauna and the flora would be developed,
the remains of which are found in the earliest
fossil strata. The era of chaos was succeeded by that
of vital harmony; but in the immense series of ages
we are dealing with, the life which appeared on the
refrigerated planet was little else than the “mouldiness
formed in a day.”
According to the theory generally propounded, the
solid crust was not very completely formed; it is, indeed,
much thinner than the layer of air surrounding
the globe; for, following the common estimate,
which, however, is purely hypothetical, at 22 to 25,
or, at most, 50 miles below the surface of the earth,
the terrestrial heat would be sufficient to melt granite.
Compared to the diameter of the earth, which
is about 250 times greater, this crust is nothing more
than a thin skin, a just idea of which may be given by
a sheet of thin cardboard surrounding a liquid sphere
a yard in diameter. In the case of the earth, this
liquid is a sea of lava and molten rocks, having, like
the ocean above it, its currents, its tides, and perhaps
its storms.
It is, in fact, very probable that a great part of the
rocks which form the outer portion of our planet,
especially the most ancient formations, existed in former
times in a state of fusion like that of volcanic
lava. As most geologists are of opinion, granite and
other similar rocks, forming the principal building-blocks
in the architecture of continents, existed once
in a soft or semi-soft state.
Neither must it be forgotten that, under the hypothesis
admitted by those who assume the existence
of a central fire, our planet is to be considered as
actually a liquid mass, as the external crust is in comparison
but a thin skin. Under these conditions, it
would be difficult to believe that this great ocean of
lava is not, like the watery ocean, agitated by the alternating
motion of tides, and that it does not move
twice every day the raft, as it were, which is floating
on its surface. It is difficult to understand how it
is that the earth is not much more depressed at the
poles than it now is, and has not been transformed
into a real disk. This flattening of the poles is not
more considerable than the mere superficial inequalities
in the equatorial zone between the summits of the
Himalayas and the abysses of the Indian Ocean. M.
Liais attributes the slight flattening of the two poles
to the erosion which the water and ice in those parts,
irresistibly drawn as they are toward the equator, incessantly
cause, year after year and century after century,
by the enormous quantity of débris torn away
from the surface of the soil, which they bear with
them.
The principal argument of those who look upon
the existence of a central fire as a demonstrated fact
is that, in the external strata of the earth, so far as
they have been explored by miners, the heat keeps on
increasing in proportion to the depth of the excavation.
In descending the shaft of a mine we invariably
pass through zones of increasing temperature;
only the rate of increase varies in different parts of
the earth, and according to the strata through which
the shaft is sunk. The heat increases more rapidly in
schist than in granite, and in metallic veins more even
than in schist; in lodes of copper more than in those
of tin, and in beds of coal more than in metallic
veins. M. Cordier, being struck by all the objections
which presented themselves to his mind as to the thinness
of the terrestrial crust, has admitted that this
covering could not be stable without having at least
from 75 to 175 miles of thickness.
CLASSES OF ROCKS — Sir Charles Lyell
Of what materials is the earth composed, and in
what manner are these materials arranged?
These are the first inquiries with which geology is
occupied, a science which derives its name from the
Greek ge, the earth, and logos, a discourse. Previously
to experience we might have imagined that
investigations of this kind would relate exclusively
to the mineral kingdom, and to the various rocks,
soils, and metals which occur upon the surface of
the earth, or at various depths beneath it. But, in
pursuing such researches, we soon find ourselves led
on to consider the successive changes which have
taken place in the former state of the earth’s surface
and interior, and the causes which have given rise
to these changes; and, what is still more singular and
unexpected, we soon become engaged in researches
into the history of the animate creation, or of the various
tribes of animals and plants which have, at
different periods of the past, inhabited the globe.
By the “earth’s crust” is meant that small portion
of the exterior of our planet which is accessible to
human observation. It comprises not merely all of
which the structure is laid open in mountain precipices,
or in cliffs overhanging a river or the sea, or
whatever the miner reveals in artificial excavation;
but the whole of that outer covering of the planet on
which we are enabled to reason by observations made
at or near the surface.
The materials of this crust are not thrown together
confusedly; but distinct mineral masses, called rocks,
are found to occupy definite spaces, and to exhibit a
certain order of arrangement. The term rock is applied
indifferently by geologists to all these substances,
whether they be soft or strong, for clay and
sand are included in the term, and some have even
brought peat under this denomination.
The most natural and convenient mode of classifying
the various rocks which compose the earth’s
crust is to refer, in the first place, to their origin, and
in the second to their relative age.
The first two divisions, which will at once be understood
as natural, are the aqueous and volcanic, or
the products of watery and those of igneous action
at or near the surface. The aqueous rocks, sometimes
called the sedimentary or fossiliferous, cover a larger
part of the earth’s surface than any others. They
consist chiefly of mechanical deposits (pebbles, sand,
and mud), but are partly of chemical and some of
them of organic origin, especially the limestones.
These rocks are stratified, or divided into distinct
layers or strata. The term stratum means simply a
bed, or anything spread out or strewed over a given
surface; and we infer that these strata have been generally
spread out by the action of water, from what
we daily see taking place near the mouths of rivers,
or on the land during temporary inundations. For,
whenever a running stream, charged with mud or
sand, has its velocity checked, as when it enters a
lake or sea, or overflows a plain, the sediment, previously
held in suspension by the motion of the
water, sinks, by its own gravity, to the bottom. In
this manner layers of mud and sand are thrown down
one upon another.
If we drain a lake which has been fed by a small
stream, we frequently find at the bottom a series of
deposits, disposed with considerable regularity, one
above the other; the uppermost, perhaps, may be a
stratum of peat, next below a more dense and solid
variety of the same material; still lower a bed of
shell-marl, alternating with peat or sand, and then
other beds of marl, divided by layers of clay. Now,
if a second pit be sunk through the same continuous
lacustrine formation at some distance from the first,
nearly the same series of beds is commonly met with,
yet with slight variations; some, for example, of the
layers of sand, clay, or marl may be wanting, one or
more of them having thinned out and given place to
others, or sometimes one of the masses first examined
is observed to increase in thickness to the exclusion of
other beds.
The term formation, which I have used in the
above explanation, expresses in geology any assemblage
of rocks which have some character in common,
whether of origin, age, or composition. Thus we
speak of stratified and unstratified, fresh-water and
marine, aqueous and volcanic, ancient and modern,
metalliferous and non-metalliferous formations.
In the estuaries of large rivers, such as the Ganges
and the Mississippi, we may observe, at low water,
phenomena analogous to those of the drained lakes
above mentioned, but on a grander scale, and extending
over areas several hundred miles in length and
breadth. When the periodical inundations subside,
the river hollows out a channel to the depth of many
yards through horizontal beds of clay and sand, the
ends of which are seen exposed in perpendicular
cliffs. These beds vary in their mineral composition,
or color, or in the fineness or coarseness of their
particles, and some of them are occasionally characterized
by containing driftwood. At the junction of
the river and the sea, especially in lagoons nearly separated
by sand bars from the ocean, deposits are often
formed in which brackish and salt-water shells are
included.
In Egypt, where the Nile is always adding to its
delta by filling up part of the Mediterranean with
mud, the newly deposited sediment is stratified, the
thin layer thrown down in one season differing slightly
in color from that of a previous year, and being
separable from it, as has been observed in Cairo and
other places.
When beds of sand, clay, and marl containing
shells and vegetable matter are found arranged in a
similar manner in the interior of the earth, we ascribe
to them a similar origin; and the more we examine
their characters in minute detail, the more exact do
we find the resemblance. Thus, for example, at various
heights and depths in the earth, and often far
from seas, lakes, and rivers, we meet with layers of
rounded pebbles composed of flint, limestone, granite,
or other rocks, resembling the shingles of a sea-beach
or the gravel in a torrent’s bed. Such layers of
pebbles frequently alternate with others formed of
sand or fine sediment, just as we may see in the channel
of a river descending from hills bordering a
coast, where the current sweeps down at one season
coarse sand and gravel, while at another, when the
waters are low and less rapid, fine mud and sand
alone are carried seaward.
If a stratified arrangement and the rounded form
of pebbles are alone sufficient to lead us to the conclusion
that certain rocks originated under water, this
opinion is further confirmed by the distinct and
independent evidences of fossils, so abundantly included
in the earth’s crust. By a fossil is meant any
body, or the traces of the existence of any body,
whether animal or vegetable, which has been buried
in the earth by natural causes. Now the remains of
animals, especially of aquatic species, are found almost
everywhere imbedded in stratified rocks, and
sometimes, in the case of limestone, they are in such
abundance as to constitute the entire mass of the rock
itself. Shells and corals are the most frequent, and
with them are often associated the bones and teeth
of fishes, fragments of wood, impressions of leaves,
and other organic substances. Fossil shells of forms
such as now abound in the sea are met with far inland,
both near the surface and at great depths below
it. They occur at all heights above the level of the
ocean, having been observed at elevations of more
than 8,000 feet in the Pyrenees, 10,000 in the Alps,
13,000 in the Andes, and above 18,000 feet in the
Himalayas.
These shells belong mostly to marine testacea, but
in some places exclusively to forms characteristic of
lakes and rivers. Hence it is concluded that some
ancient strata were deposited at the bottom of the
sea, and others in lakes and estuaries.
The division of rocks, which we may next consider,
are the volcanic, or those which have been produced
at or near the surface, whether in ancient or modern
times, not by water, but by the action of fire or subterranean
heat. These rocks are for the most part
unstratified, and are devoid of fossils. They are
more partially distributed than aqueous formations,
at least in respect to horizontal extension. Among
those parts of Europe where they exhibit characters
not to be mistaken, I may mention not only Sicily
and the country round Naples, but Auvergne, Velay,
and Vivarais, now the departments of Puy de Dôme,
Haute Loire, and Ardêche, toward the centre and
south of France, in which are several hundred conical
hills having the forms of modern volcanoes, with
craters more or less perfect on many of their summits.
These cones are composed, moreover, of lava,
sand, and ashes similar to those of active volcanoes.
Streams of lava may sometimes be traced from the
cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid
rock, in the same manner as some modern flows of
lava in Iceland have been known to do, the rivers
either flowing beneath or cutting out a narrow passage
on one side of the lava. Although none of these
French volcanoes has been in activity within the
period of history or tradition, their forms are often
very perfect. Some, however, have been compared to
the mere skeletons of volcanoes, the rains and torrents
having washed their sides, and removed all the loose
sand and scoriæ, leaving only the harder and more
solid materials. By this erosion and by earthquakes
their internal structure has occasionally been laid
open to view, in fissures and ravines; and we then
behold not only many successive beds and masses of
porous lava, sand, and scoriæ, but also perpendicular
walls, or dikes, as they are called, of volcanic rock,
which have burst through the other materials. Such
dikes are also observed in the structure of Vesuvius,
Etna, and other active volcanoes. They have been
formed by the pouring of melted matter, whether
from above or below, into open fissures, and they
commonly traverse deposits of volcanic tuff, a substance
produced by the showering down from the
air, or incumbent waters, of sand and cinders, first
shot up from the interior of the earth by the explosions
of volcanic gases.
Besides the parts of France above alluded to, there
are other countries, as the north of Spain, the south
of Sicily, the Tuscan territory of Italy, the lower
Rhenish provinces, and Hungary, where spent volcanoes
may be seen, still preserving in many cases a
conical form, and having craters and often lava
streams connected with them.
There are also other rocks in England, Scotland,
Ireland, and almost every country in Europe, which
we infer to be of igneous origin, although they do
not form hills with cones and craters. Thus, for example,
we feel assured that the rock of Staffa and
that of the Giant’s Causeway, called basalt, is volcanic,
because it agrees in its columnar structure and
mineral composition with streams of lava which we
know to have flowed from the craters of volcanoes.
The absence of cones and craters, and long narrow
streams of superficial lava in England and many
other countries, is principally to be attributed to the
eruptions having been submarine, just as a considerable
proportion of volcanoes in our own times burst
out beneath the sea. The igneous, as well as the
aqueous rocks may be classed as a chronological series
of monuments, throwing light on a succession of
events in the history of the earth.
We have now pointed out the existence of two distinct
orders of mineral masses, the aqueous and the
volcanic; but if we examine a large portion of a continent,
especially if it contain within it a lofty mountain
range, we rarely fail to discover two other classes
of rocks, very distinct from either of those above alluded
to, and which we can neither assimilate to deposits
such as are now accumulated in lakes or seas,
nor to those generated by ordinary volcanic action.
The members of both these divisions of rocks agree
in being highly crystalline and destitute of organic
remains. The rocks of one division have been called
plutonic, comprehending all the granites and certain
porphyries, which are nearly allied in some of their
characters to volcanic formations. The members of
the other class are stratified and often slaty, and have
been called by some the crystalline schists, in which
group are included gneiss, micaceous-schist (or
mica-slate), hornblende-schist, statuary marble, the
finer kinds of roofing-slate, and other rocks afterward
to be described.
All the various kinds of granites which constitute
the plutonic family are supposed to be of igneous or
aqueo-igneous origin, and to have been formed under
great pressure, at a considerable depth in the earth,
or sometimes perhaps under a certain weight of incumbent
ocean. Like the lava of volcanoes, they
have been melted, and afterward cooled and crystallized,
but with extreme slowness, and under conditions
very different from those of bodies cooling in
the open air. Hence they differ from the volcanic
rocks, not only by their more crystalline texture, but
also by the absence of tuffs and breccias, which are
the products of eruptions at the earth’s surface, or
beneath seas of inconsiderable depth. They differ
also by the absence of pores or cellular cavities, to
which the expansion of the entangled gases gives
rise in ordinary lava.
The fourth and last great division of rocks are the
crystalline strata and slates, or schists, called gneiss,
mica-schist, clay-slate, chlorite-schist, marble, and
the like, the origin of which is more doubtful than
that of the other three classes. They contain no
pebbles, or sand, or scoriæ, or angular pieces of imbedded
stone, and no traces of organic bodies, and
they are often as crystalline as granite, yet are divided
into beds, corresponding in form and arrangement
to those of sedimentary formations, and are
therefore said to be stratified. The beds sometimes
consist of an alternation of substances varying in
color, composition, and thickness, precisely as we
see in stratified fossiliferous deposits. According to
the Huttonian theory, which I adopt as the most
probable, the materials of these strata were originally
deposited from water in the usual form of sediment,
but they were subsequently so altered by subterranean
heat as to assume a new texture. It is
demonstrable, in some cases at least, that such a complete
conversion has actually taken place, fossiliferous
strata having exchanged an earthy for a highly
crystalline texture for a distance of a quarter of a
mile from their contact with granite. In some cases,
dark limestones, replete with shells and corals, have
been turned into white statuary marble, and hard
clays, containing vegetable or other remains, into
slates called mica-schist or hornblende-schist, every
vestige of the organic bodies having been obliterated.
Although we are in a great degree ignorant of the
precise nature of the influence exerted in these cases,
yet it evidently bears some analogy to that which volcanic
heat and gases are known to produce; and the
action may be conveniently called plutonic, because
it appears to have been developed in those regions
where plutonic rocks are generated, and under similar
circumstances of pressure and depth in the earth.
Intensely heated water or steam permeating stratified
masses under great pressure have no doubt played
their part in producing the crystalline texture and
other changes, and it is clear that the transforming
influence has often pervaded entire mountain masses
of strata.
In accordance with the hypothesis above alluded
to, I proposed in the first edition of the Principles
of Geology (1833), the term Metamorphic,
for the altered strata, a term derived from meta,
trans, and morphe, forma.
Hence there are four great classes of rocks considered
in reference to their origin—the aqueous, the
volcanic, the plutonic, and the metamorphic. Portions
of each of these four distinct classes have originated
at many successive periods. They have all
been produced contemporaneously, and may even
now be in the progress of formation on a large scale.
It is not true, as was formerly supposed, that all
granites, together with the crystalline or metamorphic
strata, were first formed, and therefore entitled
to be called “primitive,” and that the aqueous and
volcanic rocks were afterward superimposed, and
should, therefore, rank as secondary in the order of
time. This idea was adopted in the infancy of the
science, when all formations, whether stratified or
unstratified, earthy or crystalline, with or without
fossils, were alike regarded as of aqueous origin.
From what has now been said, the reader will understand
that each of the four great classes of rocks
may be studied under two distinct points of view;
first, they may be studied simply as mineral masses
deriving their origin from particular causes, and having
a certain composition, form, and position in the
earth’s crust, or other characters, both positive and
negative, such as the presence or absence of organic
remains. In the second place, the rocks of each class
may be viewed as a grand chronological series of
monuments, attesting a succession of events in the
former history of the globe and its living inhabitants.
GEOLOGICAL CHRONOLOGY — Sir J. William Dawson
The crust of the earth, as we somewhat modestly
term that portion of its outer shell which is
open to our observation, consists of many beds of rock
superimposed on each other, and which must have
been deposited successively, beginning with the lowest.
This is proved by the structure of the beds themselves,
by the markings on their surfaces, and by the
remains of animals and plants which they contain; all
these appearances indicating that each successive bed
must have been the surface before it was covered by
the next.
As these beds of rock were mostly formed under
water, and of material derived from the waste of
land, they are not universal, but occur in those places
where there were extensive areas of water receiving
detritus from the land. Further, as the distinction
of land and water arises primarily from the shrinkage
of the mass of the earth, and from the consequent
collapse of the crust in some places and ridging of it
up in others, it follows that there have, from the
earliest geological periods, been deep ocean-basins,
ridges of elevated land, and broad plateaus intervening
between the ridges, and which were at some times
under water and at other times land, with many intermediate
phases. The settlement and crumpling
of the crust were not continuous, but took place at
intervals; and each such settlement produced not
only a ridging up along certain lines, but also an
emergence of the plains or plateaus. Thus at all
times there have been ridges of folded rock constituting
mountain ranges, flat expansions of continental
plateau, sometimes dry and sometimes submerged,
and deep ocean-basins, never except in some of their
shallower portions elevated into land.
By the study of the successive beds, more especially
of those deposited in the times of continental submergence,
we obtain a table of geological chronology
which expresses the several stages of the formation of
the earth’s crust, from that early time when a solid
shell first formed on our nascent planet to the present
day. By collecting the fossil remains imbedded in
the several layers and placing these in chronological
order, we obtain in like manner histories of animal
and plant life parallel to the physical changes indicated
by the beds themselves. The facts as to the
sequence we obtain from the study of exposures in
cliffs, cuttings, quarries, and mines; and by correlating
these local sections in a great number of places,
we obtain our general table of succession; though it
is to be observed that in some single exposures or
series of exposures, like those in the great cañons of
Colorado, or on the coasts of Great Britain, we can
often in one locality see nearly the whole sequence
of beds.
The evidence is similar to that obtained by Schliemann
on the site of Troy, where, in digging through
successive layers of débris, he found the objects deposited
by successive occupants of the site, from the
time of the Roman Empire back to the earliest tribes,
whose flint weapons and the ashes of their fires rest
on the original surface of the ground.
Let us now tabulate the whole geological succession
with the history of animals and plants associated
with it:
ANIMALS
SYSTEMS OF FORMATIONS
PLANTS
Age of Man and Mammalia
Kainozoic
Modern
Angiosperms and Palms dominant
Pleistocene
Pliocene
Miocene
Eocene
Age of Reptiles
Mesozoic
Cretaceous
Cycads and Pines dominant
Jurassic
Triassic
Age of Amphibians and Fishes
Palæozoic
Permian
Acrogens and Gymnosperms dominant
Carboniferous
Erian
Silurian
Age of Invertebrates
Ordovician
Cambrian
Huronian (Upper)
Age of Protozoa
Eozoic
Huronian (Lower)
Protogens and Algæ
Upper Laurentian
Middle Laurentian
Lower Laurentian
It will be observed, since only the latest of the systems
of formations in this table belongs to the period
of human history, that the whole lapse of time embraced
in the table must be enormous. If we suppose
the modern period to have continued for say ten
thousand years, and each of the others to have been
equal to it, we shall require two hundred thousand
years for the whole. There is, however, reason to
believe, from the great thickness of the formations
and the slowness of the deposition of many of them
in the older systems, that they must have required
vastly greater time. Taking these criteria into account,
it has been estimated that the time-ratios for
the first three great ages may be as one for the Kainozoic
to three for the Mesozoic and twelve for the
Palæozoic, with as much for the Eozoic as for the
Palæozoic. This is Dana’s estimate. Another, by
Hull and Houghton, gives the following ratios:
Azoic, 34.3 per cent; Palæozoic, 42.5 per cent; Mesozoic
and Kainozoic, 23.3 per cent. It is further held
that the modern period is much shorter than the other
periods of the Kainozoic, so that our geological table
may have to be measured by millions of years instead
of thousands.
We can not, however, attach any certain and definite
value in years to geological time, but must content
ourselves with the general statement that it has
been vastly long in comparison to that covered by
human history.
Bearing in mind this great duration of geological
time, and the fact that it probably extends from a
period when the earth was intensely heated, its crust
thin, and its continents as yet unformed, it will be
evident that the conditions of life in the earlier geologic
periods may have been very different from
those which obtained later. When we further take
into account the vicissitudes of land and water which
have occurred, we shall see that such changes must
have produced very great differences of climate. The
warm equatorial waters have in all periods, as superficial
oceanic currents, been main agents in the diffusion
of heat over the surface of the earth, and their
distribution to north and south must have been determined
mainly by the extent and direction of land,
though it may also have been modified by the changes
in the astronomical relations and period of the earth,
and the form of its orbit. We know by the evidence
of fossil plants that changes of this kind have occurred
so great as, on the one hand, to permit the
plants of warm temperate regions to exist within the
Arctic Circle; and, on the other, to drive these plants
into the tropics and to replace them by Arctic forms.
It is evident also that in those periods when the continental
areas were largely submerged there might
be an excessive amount of moisture in the atmosphere,
greatly modifying the climate in so far as
plants are concerned.
Let us now consider the history of the vegetable
kingdom as indicated in the few notes in the right-hand
column of the table.
The most general subdivision of plants is into the
two great series of Cryptogams, or those which have
no manifest flowers, and produce minute spores instead
of seeds; and Phænogams, or those which possess
flowers and produce seeds containing an embryo
of the future plant.
The Cryptogams may be subdivided into the following
three groups:
1. Thallogens, cellular plants not distinctly distinguishable
into stem and leaf. These are the Fungi,
the Lichens, and the Algæ, or sea-weeds.
2. Anogens, having stem and foliage, but wholly
cellular. These are the Mosses and Liverworts.
3. Acrogens, which have long tubular fibres as
well as cells in their composition, and thus have the
capacity of attaining a more considerable magnitude.
These are the Ferns (Filices), the Mare’s-tails
(Equisetaceæ), and the Club-mosses (Lycopodiaceæ),
and a curious little group of aquatic plants
called Rhizocarps (Rhizocarpeæ).
The Phænogams are all vascular, but they differ
much in the simplicity or complexity of their flowers
or seeds. On this ground they admit of a twofold
division:
1. Gymnosperms, or those which bear naked seeds
not inclosed in fruits. They are the Pines and their
allies, and the Cycads.
2. Angiosperms, which produce true fruits inclosing
the seeds. In this group there are two well-marked
subdivisions differing in the structure of the
seed and stem. They are the Endogens, or inside
growers, with seeds having one seed-leaf only, as the
grasses and the palms; and the Exogens, having outside-growing
woody stems and seeds with two seed-leaves.
Most of the ordinary forest trees of temperate
climates belong to this group.
On referring to the geological table, it will be seen
that there is a certain rough correspondence between
the order of rank of plants and the order of their appearance
in time. The oldest plants that we certainly
know are Algæ, and with these there are plants apparently
with the structures of Thallophytes but the
habit of trees, and which, for want of a better name,
I may call Protogens. Plants akin to the Rhizocarps
also appear very early. Next in order we find forests
in which gigantic Ferns and Lycopods and Mare’s-tails
predominate, and are associated with pines.
Succeeding these we have a reign of Gymnosperms,
and in the later formations we find the higher Phænogams
dominant.
THE SILURIAN BEACH — Louis Agassiz
The crust of our earth is a great cemetery where
the rocks are tombstones on which the buried
dead have written their own epitaphs. They tell us
not only who they were and when and where they
have lived, but much also of the circumstances under
which they lived. We ascertain the prevalence of
certain physical conditions at special epochs by the
presence of animals and plants whose existence and
maintenance requires such a state of things, more
than by any positive knowledge respecting it. Where
we find the remains of quadrupeds corresponding
to our ruminating animals, we infer not only land,
but grassy meadows and an extensive vegetation;
where we find none but marine animals, we know
the ocean must have covered the earth; the remains
of large reptiles, representing, though in gigantic
size, the half aquatic, half terrestrial reptiles of our
own period, indicate to us the existence of spreading
marshes still soaked by retreating waters; while the
traces of such animals as live now in sand and shoal
waters, or in mud, speak to us of shelving sandy
beaches and mud flats. The eye of the Trilobite tells
us that the sun shone on the old beach where he lived;
for there is nothing in nature without a purpose, and
when so complicated an organ was made to receive
the light there must have been light to enter it. The
immense vegetable deposits in the Carboniferous
period announce the introduction of an extensive
terrestrial vegetation; and the impressions left by the
wood and leaves show that these first forests must
have grown in a damp soil and a moist atmosphere.
In short, all the remains of animals and plants
hidden in the rocks have something to tell of the
climatic conditions and the general circumstances
under which they lived, and the study of fossils is to
a naturalist a thermometer by which he reads the
variation of temperature in past times, a plummet
by which he sounds the depths of the ancient oceans—a
register, in fact, of all the important physical
changes the earth has undergone.
The Silurian beach was a shelving one, and covered,
of course, with shoal waters; but the parallel
ridges trending east to west across the State of New
York, considered by some geologists as the successive
shores of a receding ocean, are believed by others to
be the inequalities on the bottom of a shallow sea.
Not only, however, does the general character of
these successive terraces suggest the idea that they
must have been shores, but the ripple marks upon
them are as distinct as upon any modern beach. The
regular rise and fall of the water is registered there
in waving, undulating lines as clearly as on the sand
beaches of Newport or Nahant; and we can see on
any of those ancient shores the track left by the waves
as they rippled back at ebb of the tide thousands of
centuries ago. One can often see where some obstacle
interrupted the course of the water, causing
it to break around it; and such an indentation even
retains the soft, muddy, plastic look that we observe
on the present beaches, where the resistance made by
any pebble or shell to the retreating wave has given
it greater force at that point, so that the sand around
the spot is soaked and loosened. There is still another
sign familiar to those who have watched the action
of water on a beach. Where a shore is very shelving
and flat, so that the waves do not recede in ripples
from it, but in one unbroken sheet, the sand and
small pebbles are dragged and form lines which
diverge whenever the water meets an obstacle, thus
forming sharp angles on the sand. Such marks are
as distinct on the oldest Silurian rocks as if they had
been made yesterday. Nor are these the only indications
of the same fact. There are certain animals
living always on sandy or muddy shores which require
for their well-being that the beach should be
left dry for a part of the day. These animals, moving
about in the sand or mud from which the water
has retreated, leave their tracks there; and if, at such
a time, the wind is blowing dust over the beach and
the sun is hot enough to bake it upon the impressions
so formed, they are left in a kind of mold. Such
trails and furrows made by small shells and crustacea
are also found in plenty on the oldest deposits.
Admitting it, then, to be a beach, let us begin with
the lowest type of the Animal Kingdom and see
what Radiates are to be found there. There are
plenty of Corals, but they are not the same kind of
Corals as those that build up our reefs and islands
now. The modern Coral animals are chiefly Polyps,
but the prevailing Corals of the Silurian age were
Acalephian Hydroids, animals which indeed resemble
Polyps in certain external features, and have
been mistaken for them, but which are, nevertheless,
Acalephs by their internal structure.
Of the Echinoderms, the class of Radiates represented
now by our Star-Fishes and Sea-Urchins, we
may gather any quantity, though the old-fashioned
forms are very different from the living ones. The
Mollusks were also represented then, as now, by their
three classes, Acephala, Gasteropoda, and Cephalopoda.
The Acephala or Bivalves we find in great
numbers, but of a very different pattern from the
Oysters, Clams, and Mussels of recent times.
Of the Silurian Univalves or Gasteropods, there is
not much to tell, for their spiral shells were so brittle
that scarcely any perfect specimens are known,
though their broken remains are found in such quantities
as to show that this class also was very fully
represented in the earliest creation. But the highest
class of Mollusks, the Cephalopods or Chambered
Shells, or Cuttle-Fishes, as they are called when the
animal is unprotected by a shell, are, on the contrary,
very well preserved, and they are very numerous.
Of Articulates we find only two classes, Worms
and Crustacea. Insects there were none—for, as we
have seen, this early world was wholly marine.
There is little to be said of the Worms, for their soft
bodies, unprotected by any hard covering, could hardly
be preserved; but, like the marine Worms of our
own times, they were in the habit of constructing envelopes
for themselves, built of sand, or sometimes
from a secretion of their own bodies, and these cases
we find in the earliest deposits, giving us the assurance
that the Worms were represented there. I
should add, however, that many impressions described
as produced by Worms are more likely to
have been the tracks of Crustacea. But by far the
most characteristic class of Articulates in ancient
times were the Crustaceans. The Trilobites stand in
the same relation to the modern Crustacea as the
Crinoids do to the modern Echinoderms. They
were then the sole representatives of their class, and
the variety and richness of the type are most extraordinary.
They were of nearly equal breadth
for the whole length of the body, and rounded at
the two ends, so as to form an oval outline.
We have found Radiates, Mollusks, and Articulates
in plenty; and now what is to be said of Vertebrates
in these old times—of the highest and most
important division of the Animal Kingdom, that to
which we ourselves belong. They were represented
by Fishes alone; and the fish chapter in the history
of the early organic world is a curious and,
as it seems to me, a very significant one. We shall
find no perfect specimens; and he would be a daring,
not to say a presumptuous, thinker who would venture
to reconstruct a fish of the Silurian age from
any remains that are left to us. But still we find
enough to indicate clearly the style of those old fishes,
and to show, by comparison with the living types, to
what group of modern times they belong. We should
naturally expect to find the Vertebrates introduced
in their simplest form; but this is by no means the
case: the common fishes, as Cod, Herring, Mackerel,
and the like, were unknown in those days.
I have spoken of the Silurian beach as if there
were but one, not only because I wished to limit my
sketch and to attempt, at least, to give it the vividness
of a special locality, but also because a single
such shore will give us as good an idea of the characteristic
fauna of the time as if we drew our material
from a wider range. There are, however, a
great number of parallel ridges belonging to the
Silurian and Devonian periods running from east
to west, not only through the State of New York,
but far beyond, through the States of Michigan and
Wisconsin into Minnesota; one may follow nine or
ten such successive shores in unbroken lines from the
neighborhood of Lake Champlain to the Far West.
Although the early geological periods are more
legible in North America, because they are exposed
over such extensive tracts of land, yet they have been
studied in many parts of the globe. In Norway, in
Germany, in France, in Russia, in Siberia, in
Kamtchatka, in parts of South America, in short,
wherever the civilization of the white race has extended,
Silurian deposits have been observed, and
everywhere they bear the same testimony to a profuse
and varied creation. The earth was teeming
then with life as now, and in whatever corner of its
surface the geologist finds the old strata, they hold
a dead fauna as numerous as that which lives and
moves above it. Nor do we find that there was any
gradual increase or decrease of any organic forms
at the beginning or close of the successive periods.
I think the impression that the faunæ of the early
geological periods were more scanty than those of
later times arises partly from the fact that the present
creation is made a standard of comparison for all
preceding creations. Of course, the collection of
living types in any museum must be more numerous
than those of fossil forms, for the simple reason that
almost the whole of the present surface of the earth,
with the animals and plants inhabiting it, is known
to us, whereas the deposits of the Silurian and
Devonian periods are exposed to view only over comparatively
limited tracts and in disconnected regions.
But let us compare a given extent of Silurian or
Devonian seashore with an equal extent of seashore
belonging to our own time, and we shall soon be convinced
that the one is as populous as the other. On
the New England Coast there are about one hundred
and fifty different kinds of fishes; in the Gulf of
Mexico two hundred and fifty; in the Red Sea about
the same. We may allow in present times an average
of two hundred or two hundred and fifty different
kinds of fishes to an extent of ocean covering about
four hundred miles. Now, I have made a special
study of the Devonian rocks of Northern Europe,
in the Baltic, and along the shore of the German
Ocean. I have found in those deposits alone one
hundred and ten kinds of fossil fishes. To judge of
the total number of species belonging to those early
ages by the number known to exist now is about as
reasonable as to infer that because Aristotle, familiar
only with the waters of Greece, recorded less than
three hundred kinds of fishes in his limited fishing-ground,
therefore these were all the fishes then living.
The fishing-ground of the geologist in the
Silurian and Devonian periods is even more circumscribed
than his, and belongs, besides, not to a
living but to a dead world, far more difficult to decipher.
Extinct animals exist all over the world; heaped
together under the snows of Siberia, lying thick
beneath the Indian soil, found wherever English
settlers till the ground or work the mines in Australia,
figured in the old encyclopedias of China,
where the Chinese philosophers have drawn them
with the accuracy of their nation, built into the most
beautiful temples of classic lands—for even the
stones of the Parthenon are full of the fragments of
these old fossils, and if any chance had directed the
attention of Aristotle toward them, the science of
Paleontology would not have waited for its founder
till Cuvier was born—in short, in every corner of
the earth where the investigations of civilized men
have penetrated, from the Arctic to Patagonia and
the Cape of Good Hope, these relics tell us of successive
populations lying far behind our own, and
belonging to distinct periods of the world’s history.
CARBONIFEROUS PERIOD — Louis Figuier
In the history of our globe the Carboniferous
period succeeds to the Devonian. It is in the
formations of this latter epoch that we find the fossil
fuel which has done so much to enrich and civilize
the world in our own age. This period divides itself
into two great sub-periods: 1. The Coal-measures;
and 2. The Carboniferous Limestone. The first, a
period which gave rise to the great deposits of coal;
the second, to most important marine deposits, most
frequently underlying the coal-fields in England,
Belgium, France, and America.
The limestone mountains, which form the base of
the whole system, attain in places, according to Professor
Phillips, a thickness of 2,500 feet. They are
of marine origin, as is apparent by the multitude
of fossils they contain of Zoophytes, Radiata, Cephalopoda,
and Fishes. But the chief characteristic
of this epoch is its strictly terrestrial flora—remains
of plants now become as common as they were rare in
all previous formations, announcing a great increase
of dry land.
The monuments of this era of profuse vegetation
reveal themselves in the precious Coal-measures of
England and Scotland. These give us some idea of
the rich verdure which covered the surface of the
earth, newly risen from the bosom of its parent waves.
It was the paradise of terrestrial vegetation. The
grand Sigillaria, the Stigmaria, and other fern-like
plants, were especially typical of this age, and
formed the woods, which were left to grow undisturbed;
for as yet no living Mammals seem to have
appeared; everything indicates a uniformly warm,
humid temperature, the only climate in which the
gigantic ferns of the Coal-measures could have attained
their magnitude. Conifers have been found
of this period with concentric rings, but these rings
are more slightly marked than in existing trees of
the same family, from which it is reasonable to
assume that the seasonal changes were less marked
than they are with us.
Everything announces that the time occupied in
the deposition of the Carboniferous Limestone was
one of vast duration. Professor Phillips calculates
that, at the ordinary rate of progress, it would require
122,400 years to produce only sixty feet of coal.
Geologists believe, moreover, that the upper Coal-measures,
where bed has been deposited upon bed for
ages upon ages, were accumulated under conditions
of comparative tranquillity, but that the end of this
period was marked by violent convulsions—by ruptures
of the terrestrial crust, when the carboniferous
rocks were upturned, contorted, dislocated by faults,
and subsequently partially denuded, and thus appear
now in depressions or basin-shaped concavities;
and that upon this deranged and disturbed foundation
a fourth geological system, called Permian, was
constructed.
Coal, as we shall find, is composed of the mineralized
remains of the vegetation which flourished in remote
ages of the world. Buried under an enormous
thickness of rocks, it has been preserved to our days,
after being modified in its inward nature and external
aspect. Having lost a portion of its elementary
constituents, it has become transformed into a species
of carbon, impregnated with those bituminous substances
which are the ordinary products of the slow
decomposition of vegetable matter.
Thus, coal is the substance of the plants which
formed the forests, the vegetation, and the marshes
of the ancient world, at a period too distant for human
chronology to calculate with anything like precision.
It is a remarkable circumstance that conditions of
equable and warm climate, combined with humidity,
do not seem to have been limited to any one part of
the globe, but the temperature of the whole globe
seems to have been nearly the same in very different
latitudes. From the equatorial regions up to Melville
Island, in the Arctic Ocean, where in our days eternal
frost prevails—from Spitzbergen to the centre of
Africa, the carboniferous flora is identically the same.
When nearly the same plants are found in Greenland
and Guinea; when the same species, now extinct, are
met with of equal development at the equator as at
the pole, we can not but admit that at this epoch the
temperature of the globe was nearly alike everywhere.
What we now call climate was unknown in
these geological times. There seems to have been then
only one climate over the whole globe. It was at a
subsequent period, that is, in later Tertiary times, that
the cold began to make itself felt at the terrestrial
poles. Whence, then, proceeded this general superficial
warmth, which we now regard with so much
surprise? It was a consequence of the greater or
nearer influence of the interior heat of the globe.
The earth was still so hot in itself that the heat which
reached it from the sun may have been inappreciable.
Another hypothesis, which has been advanced with
much less certainty than the preceding, relates to the
chemical composition of the air during the Carboniferous
period. Seeing the enormous mass of vegetation
which then covered the globe, and extended
from one pole to the other; considering, also, the
great proportion of carbon and hydrogen which exists
in the bituminous matter of coal, it has been
thought, and not without reason, that the atmosphere
of the period might be richer in carbonic acid than
the atmosphere of the present day. It has even been
thought that the small number of (especially air-breathing)
animals, which then lived, might be accounted
for by the presence of a greater proportion
of carbonic acid gas in the atmosphere than is the
case in our own times. This, however, is pure assumption,
totally deficient in proof. What we can
remark, with certainty, as a striking characteristic of
the vegetation of the globe during this phase of its
history, was the prodigious development which it
assumed. The Ferns, which in our days and in our
climate are most commonly only small perennial
plants, in the Carboniferous age sometimes presented
themselves under lofty and even magnificent forms.
Every one knows those marsh-plants with hollow,
channeled, and articulated cylindrical stems; whose
joints are furnished with a membranous, denticulated
sheath, and which bear the vulgar name of “mare’s-tail”;
their fructification forming a sort of catkin
composed of many rings of scales, carrying on their
lower surface sacs full of spores or seeds. These humble
Equiseta were represented during the coal-period
by herbaceous trees from twenty to thirty
feet high and four to six inches in diameter. Their
trunks, channeled longitudinally, and divided transversely
by lines of articulation, have been preserved
to us: they bear the name of Calamites.
The Lycopods of our age are humble plants,
scarcely a yard in height, and most commonly
creepers; but the Lycopodiaceæ of the ancient world
were trees of eighty or ninety feet in height. It was
the Lepidodendrons which filled the forests. Their
leaves were sometimes twenty inches long, and their
trunks a yard in diameter. Such are the dimensions
of some specimens of Lepidodendron carinatum
which have been found. Another Lycopod of this
period, the Lomatophloyos crassicaule, attained dimensions
still more colossal. The Sigillarias sometimes
exceeded 100 feet in height. Herbaceous
Ferns were also exceedingly abundant, and grew beneath
the shade of these gigantic trees. It was the
combination of these lofty trees with such shrubs (if
we may so call them) which formed the forests of
the Carboniferous period.
How this vegetation, so imposing, both on account
of the dimensions of the individual trees and the immense
space which they occupied, so splendid in its
aspect, and yet so simple in its organization, must
have differed from that which now embellishes the
earth and charms our eyes! It certainly possessed the
advantage of size and rapid growth; but how poor it
was in species—how uniform in appearance! No
flowers yet adorned the foliage or varied the tints
of the forests. Eternal verdure clothed the branches
of the Ferns, the Lycopods, and Equiseta, which composed
to a great extent the vegetation of the age. The
forests presented an innumerable collection of individuals,
but very few species, and all belonging to
the lower types of vegetation. No fruit appeared fit
for nourishment; none would seem to have been on
the branches. Suffice it to say that few terrestrial
animals seem to have existed yet; animal life was apparently
almost wholly confined to the sea, while the
vegetable kingdom occupied the land, which at a
later period was more thickly inhabited by air-breathing
animals. Probably a few winged insects (some
coleoptera, orthoptera, and neuroptera) gave animation
to the air while exhibiting their variegated colors;
and it was not impossible but that many pulmoniferous
mollusca (such as land-snails) lived at the
same time.
The vegetation which covered the numerous islands
of the Carboniferous sea consisted, then, of
Ferns, of Equisetaceæ, of Lycopodiaceæ, and dicotyledonous
Gymnosperms. The Annularia and Sigillariæ
belong to families of the last-named class,
which are now completely extinct.
The Annulariæ were small plants which floated
on the surface of fresh-water lakes and ponds; their
leaves were verticillate, that is, arranged in a great
number of whorls, at each articulation of the stem
with the branches. The Sigillariæ were, on the contrary,
great trees, consisting of a simple trunk, surmounted
with a bunch or panicle of slender drooping
leaves, with the bark often channeled, and displaying
impressions or scars of the old leaves, which,
from their resemblance to a seal, sigillum, gave origin
to their name.
The Stigmariæ, according to palæontologists, were
roots of Sigillariæ, with a subterranean fructification;
all that is known of them is the long roots
which carry the reproductive organs, and in some
cases are as much as sixteen feet long.
Two other gigantic trees grew in the forests of
this period: these were Lepidodendron carinatum
and Lomatophloyos crassicaule, both belonging to the
family of Lycopodiaceæ, which now includes only
very small species. The trunk of the Lomatophloyos
threw out numerous branches, which terminated in
thick tufts of linear and fleshy leaves. The Ferns
composed a great part of the vegetation of the Coal-measure
period.
The seas of this epoch included an immense number
of Zoophytes, nearly 400 species of Mollusca, and
a few Crustaceans and Fishes. Among the Fishes,
Psammodus and Coccosteus, whose massive teeth inserted
in the palate were suitable for grinding; and
the Holoptychius and Megalichthys, are the most important.
The Mollusca are chiefly Brachiopods of
great size. The Bellerophon, whose convoluted shell
in some respects resembles the Nautilus of our present
seas, but without its chambered shell, were then
represented by many species.
Crustaceans are rare in the Carboniferous Limestone
strata; the genus Phillipsia is the last of the
Trilobites, all of which became extinct at the close of
this period. As to the Zoophytes, they consist chiefly
of Crinoids and Corals. We also have in these rocks
many Polyzoa.
Among the corals of the period we may include
the genera Lithostrotion and Lonsdalea. Among the
Polyzoa are the genera Fenestrella and Polypora.
Lastly, to these we may add a group of animals which
will play a very important part and become abundantly
represented in the beds of later geological periods,
but which already abounded in the seas of the
Carboniferous period. We speak of the Foraminifera,
microscopic animals, which clustered either in
one body or divided into segments, and covered
with a calcareous, many-chambered shell, as Fusulina
cylindrica. These little creatures, which, during
the Jurassic and Cretaceous periods, formed enormous
banks and entire masses of rock, began to make
their appearance in the period which now engages
our attention.
This terrestrial period is characterized, in a remarkable
manner, by the abundance and strangeness
of the vegetation which then covered the islands and
continents of the whole globe. Upon all points of the
earth, as we have said, this flora presented a striking
uniformity. In comparing it with the vegetation of
the present day, the learned French botanist, M.
Brongniart, who has given particular attention to
the flora of the Coal-measures, has arrived at the conclusion
that it presented considerable analogy with
that of the islands of the equatorial and torrid zone,
in which a maritime climate and elevated temperature
exist in the highest degree. It is believed that
islands were very numerous at this period; that, in
short, the dry land formed a sort of vast archipelago
upon the general ocean, of no great depth, the islands
being connected together and formed into continents
as they gradually emerged from the ocean.
This flora, then, consists of great trees, and also
of many smaller plants, which would form a close,
thick turf, or sod, when partially buried in marshes
of almost unlimited extent. M. Brongniart indicates,
as characterizing the period, 500 species of plants
which now attain a prodigious development. The
ordinary dicotyledons and monocotyledons—that is,
plants having seeds with two lobes in germinating
and plants having one seed-lobe—are almost entirely
absent; the cryptogamic, or flowerless plants, predominate;
especially Ferns, Lycopodiaceæ, and
Equisetaceæ—but of forms insulated and actually
extinct in these same families. A few dicotyledonous
gymnosperms, or naked-seed plants forming genera
of Conifers, have completely disappeared, not only
from the present flora, but since the close of the period
under consideration, there being no trace of them
in the succeeding Permian flora. Such is a general
view of the features most characteristic of the coal-period,
and of the Primary epoch in general. It
differs, altogether and absolutely, from that of the
present day; the climatic condition of these remote
ages of the globe, however, enables us to comprehend
the characteristics which distinguish its vegetation.
A damp atmosphere, of an equable rather
than an intense heat like that of the tropics, a soft
light veiled by permanent fogs, were favorable to the
growth of this peculiar vegetation, of which we
search in vain for anything strictly analogous in our
own days. The nearest approach to the climate and
vegetation proper to the geological period which
now occupies our attention would probably be found
in certain islands, or on the littoral of the Pacific
Ocean—the island of Chloë, for example, where it
rains during 300 days in the year, and where the light
of the sun is shut out by perpetual fogs; where arborescent
Ferns form forests, beneath whose shade
grow herbaceous Ferns, which rise three feet and
upward above a marshy soil; which gives shelter also
to a mass of cryptogamic plants, greatly resembling,
in its main features, the flora of the Coal-measures.
This flora was, as we have said, uniform and poor
in its botanic genera, compared to the abundance and
variety of the flora of the present time; but the few
families of plants which existed then included many
more species than are now produced in the same
countries. The fossil Ferns of the coal-series in
Europe, for instance, comprehend about 300 species,
while all Europe now only produces fifty. The
gymnosperms, which now muster only twenty-five
species in Europe, then numbered more than 120.
Calamites are among the most abundant fossil
plants of the Carboniferous period, and occur also
in the Devonian. They are preserved as striated,
jointed, cylindrical, or compressed stems, with fluted
channels or furrows at their sides, and sometimes
surrounded by a bituminous coating, the remains of
a cortical integument. They were originally hollow,
but the cavity is usually filled up with a substance
into which they themselves have been converted.
If, during the coal-period, the vegetable kingdom
had reached its maximum, the animal kingdom, on
the contrary, was poorly represented. Some remains
have been found, both in America and Germany, consisting
of portions of the skeleton and the impressions
of the footsteps of a Reptile, which has received
the name of Archegosaurus. Among the animals of
this period we find a few Fishes, analogous to those
of the Devonian formation. These are the Holoptychius
and Megalichthys, having jawbones armed
with enormous teeth. Scales of Pygopterus have been
found in the Northumberland Coal-shale at Newsham
Colliery, and also in the Staffordshire Coal-shale.
Some winged insects would probably join
this slender group of living beings. It may then be
said with truth that the immense forests and marshy
plains, crowded with trees, shrubs, and herbaceous
plants, which formed on the innumerable isles of the
period a thick and tufted sward, were almost destitute
of animals.
Coal, as we have said, is only the result of a partial
decomposition of the plants which covered the
earth during a geological period of immense duration.
No one, now, has any doubt that this is its
origin. In coal-mines it is not unusual to find fragments
of the very plants whose trunks and leaves
characterize the Coal-measures, or Carboniferous
era. Immense trunks of trees have also been met
with in the middle of a seam of coal. In order to
explain the presence of coal in the depths of the
earth, there are only two possible hypotheses. This
vegetable débris may either result from the burying
of plants brought from afar and transported
by river or maritime currents, forming immense
rafts, which may have grounded in different places
and been covered subsequently by sedimentary
deposits; or the trees may have grown on the
spot where they perished, and where they are now
found.
Can the coal-beds result from the transport by
water, and burial under ground, of immense rafts
formed of the trunks of trees? The hypothesis has
against it the enormous height which must be conceded
to the raft, in order to form coal-seams as
thick as some of those which are worked in our collieries.
If we take into consideration the specific
gravity of wood, and the amount of carbon it contains,
we find that the coal-deposits can only be about
seven-hundredths of the volume of the original wood
and other vegetable materials from which they are
formed. If we take into account, besides, the numerous
voids necessarily arising from the loose packing
of the materials forming the supposed raft, as
compared with the compactness of coal, this may
fairly be reduced to five-hundredths. A bed of coal,
for instance, sixteen feet thick, would have required
a raft 310 feet high for its formation. These accumulations
of wood could never have arranged themselves
with sufficient regularity to form those well-stratified
coal-beds, maintaining a uniform thickness
over many miles, and that are seen in most coal-fields
to lie one above another in succession, separated by
beds of sandstone or shale. And even admitting the
possibility of a slow and gradual accumulation of
vegetable débris, like that which reaches the mouth
of a river, would not the plants in that case be
buried in great quantities of mud and earth? Now,
in most of our coal-beds the proportion of earthy
matter does not exceed fifteen per cent of the entire
mass. If we bear in mind, finally, the remarkable
parallelism existing in the stratification of the coal-formation,
and the state of preservation in which the
impressions of the most delicate vegetable forms are
discovered, it will, we think, be proved to demonstration
that those coal-seams have been formed in perfect
tranquillity. We are, then, forced to the conclusion
that coal results from the mineralization of
plants which has taken place on the spot; that is to
say, in the very place where the plants lived and died.
It was suggested long ago by Bakewell, from the
occurrence of the same peculiar kind of fireclay under
each bed of coal, that it was the soil proper for
the production of those plants from which coal has
been formed.
Fingal’s Cave, Staffa, Coast of Scotland
The clay-beds, “which vary in thickness from a few
inches to more than ten feet, are penetrated in all directions
by a confused and tangled collection of the
roots and leaves, as they may be, of the Stigmaria
ficoides, these being frequently traceable to the main
stem (Sigillaria), which varies in diameter from
about two inches to half a foot. The main stems are
noticed as occurring nearer the top than the bottom
of the bed, as usually of considerable length, the
leaves or roots radiating from them in a tortuous irregular
course to considerable distances, and as so
mingled with the under-clay that it is not possible
to cut out a cubic foot of it which does not contain
portions of the plant.”
It is a natural inference to suppose that the present
indurated under-clay is only another condition of
that soft, silty soil, or of that finely levigated muddy
sediment—most likely of still and shallow water—in
which the vegetation grew, the remains of which
were afterward carbonized and converted into coal.
In order thoroughly to comprehend the phenomena
of the transformation into coal of the forests and
of the herbaceous plants which filled the marshes
and swamps of the ancient world, there is another
consideration to be presented. During the coal-period,
the terrestrial crust was subjected to alternate
movements of elevation and depression of the
internal liquid mass, under the impulse of the solar
and lunar attractions to which they would be subject,
as our seas are now, giving rise to a sort of subterranean
tide, operating at intervals, more or less
widely apart, upon the weaker parts of the crust, and
producing considerable subsidences of the ground.
It might, perhaps, happen that, in consequence of a
subsidence produced in such a manner, the vegetation
of the coal-period would be submerged, and the
shrubs and plants which covered the surface of the
earth would finally become buried under water.
After this submergence new forests sprung up in the
same place. Owing to another submergence, the second
forests were depressed in their turn, and again
covered by water. It is probably by a series of repetitions
of this double phenomenon—this submergence
of whole regions of forest, and the development
upon the same site of new growths of vegetation—that
the enormous accumulations of semi-decomposed
plants, which constitute the Coal-measures,
have been formed in a long series of ages.
But, has coal been produced from the larger plants
only—for example, from the great forest-trees of the
period, such as the Lepidodendra, Sigillariæ, Calamites,
and Sphenophylla? That is scarcely probable,
for many coal-deposits contain no vestiges of the
great trees of the period, but only of Ferns and other
herbaceous plants of small size. It is, therefore, presumable
that the larger vegetation has been almost
unconnected with the formation of coal, or, at least,
that it has played a minor part in its production. In
all probability there existed in the coal-period, as
at the present time, two distinct kinds of vegetation:
one formed of lofty forest-trees, growing on the
higher grounds; the other, herbaceous and aquatic
plants, growing on marshy plains. It is the latter
kind of vegetation, probably, which has mostly furnished
the material for the coal; in the same way that
marsh-plants have, during historic times and up to
the present day, supplied our existing peat, which
may be regarded as a sort of contemporaneous incipient
coal.
To what modification has the vegetation of the ancient
world been subjected to attain that carbonized
state which constitutes coal? The submerged plants
would, at first, be a light, spongy mass, in all respects
resembling the peat-moss of our moors and marshes.
While under water, and afterward, when covered
with sediment, these vegetable masses underwent a
partial decomposition—a moist, putrefactive fermentation,
accompanied by the production of much carbureted
hydrogen and carbonic acid gas. In this
way, the hydrogen escaping in the form of carbureted
hydrogen, and the oxygen in the form of carbonic
acid gas, the carbon became more concentrated,
and coal was ultimately formed. This emission of
carbureted hydrogen gas would, probably, continue
after the peat-beds were buried beneath the strata
which were deposited and accumulated upon them.
The mere weight and pressure of the superincumbent
mass, continued at an increasing ratio during a long
series of ages, have given to the coal its density and
compact state.
The heat emanating from the interior of the globe
would also exercise a great influence upon the final
result. It is to these two causes—that is to say, to
pressure and to the central heat—that we may attribute
the differences which exist in the mineral
characters of various kinds of coal. The inferior
beds are drier and more compact than the upper
ones; or less bituminous, because their mineralization
has been completed under the influence of a higher
temperature, and at the same time under a greater
pressure.
THE PALÆONTOLOGICAL HISTORY OF ANIMALS — Hugh Miller
However much the faunas of the various
geologic periods may have differed from
each other, or from the fauna which now exists, in
their general aspect and character, they were all,
if I may so speak, equally underlaid by the great
leading ideas which still constitute the master types
of animal life. And these leading ideas are four in
number. First, there is the star-like type of life—life
embodied in a form that, as in the corals, the
sea-anemones, the sea-urchins, and the star-fishes,
radiates outward from a centre; second, there is the
articulated type of life—life embodied in a form
composed, as in the worms, crustaceans, and insects,
of a series of rings united by their edges, but more or
less movable on each other; third, there is the
bilateral or molluscan type of life—life embodied
in a form in which there is a duality of corresponding
parts, ranged, as in the cuttle-fishes, the clams,
and the snails, on the sides of a central axis or plane;
and fourth, there is the vertebrate type of life—life
embodied in a form in which an internal skeleton is
built up into two cavities placed the one over the
other; the upper for the reception of the nervous centres,
cerebral and spinal—the lower for the lodgment
of the respiratory, circulatory, and digestive organs.
Such have been the four central ideas of the faunas
of every succeeding creation, except, perhaps, the
earliest of all, that of the Lower Silurian System,
in which, so far as is yet known, only three of the
number existed—the radiated, articulated, and molluscan
ideas or types.
The fauna of the Silurian System bears in all its
three great types the stamp of a fashion peculiarly
antique, and which, save in a few of the mollusca,
has long since become obsolete. Its radiate animals
are chiefly corals, simple or compound, whose inhabitants
may have somewhat resembled the sea-anemones;
with zoophytes, akin mayhap to the sea-pens,
though the relationship must have been a remote
one; and numerous crinoids, or stone lilies,
some of which consisted of but a sculptured calyx
without petals, while others threw off a series of
long flexible arms, that divided and subdivided like
the branches of a tree, and were thickly fringed by
hair-like fibres.
The articulata of the Silurian period bore a still
more peculiar character. They consisted mainly of
the Trilobites—a family in whose nicely jointed
shells the armorer of the Middle Ages might have
found almost all the contrivances of his craft anticipated,
with not a few besides which he had failed
to discover; and which, after receiving so immense
a development during the middle and later times of
the Silurian period that whole rocks were formed
almost exclusively of their remains, gradually died
out in the times of the Old Red Sandstone, and disappeared
forever from creation after the Carboniferous
Limestone had been deposited. The mollusca
of the Silurians ranged from the high cephalopoda,
represented in our existing seas by the nautili and the
cuttle-fishes, to the low brachiopods, some of whose
cogeners may still be detected in the terebratulæ of
the Highland lochs and bays, and some in the
lingulæ of the Southern Hemisphere. The cephalopods
of the system are all of an obsolete type, that
disappeared myriads of ages ago. At length, in an
upper bed of the system, immediately under the base
of the Old Red Sandstone, the remains of the earliest
known fishes appear, blended with what also appears
for the first time—the fragmentary remains of a
terrestrial vegetation. The rocks beneath this ancient
bone-bed have yielded no trace of any plant
higher than the Thallogens, or at least not higher
than the Zosteraceæ—plants whose proper habitat
is the sea; but, through an apparently simultaneous
advance of the two kingdoms, animal and vegetable—though,
of course, the simultaneousness may be but
merely apparent—the first land-plants and the first
vertebrates appear together in the same deposit.
The earliest fishes—first-born of their family—seem
to have been all placoids. The Silurian System
has not yet afforded trace of any other vertebral
animal. With the Old Red Sandstone the ganoids
were ushered upon the scene in amazing abundance;
and for untold ages, comprising mayhap millions of
years, the entire ichthyic class consisted, so far as is
yet known, of but these two orders. During the
times of the Old Red Sandstone, of the Carboniferous,
of the Permian, of the Triassic, and of the
Oolitic Systems, all fishes, though apparently as numerous
individually as they are now, were comprised
in the ganoidal and placoidal orders. The period
of these orders seems to have been nearly correspondent
with the reign, in the vegetable kingdom, of the
Acrogens and Gymnogens, with the intermediate
classes, their allies. At length, during the ages of the
Chalk, the Cycloids and Ctenoids were ushered in,
and were gradually developed in creation until the
human period, in which they seem to have reached
their culminating point, and now many times exceed
in number and importance all other fishes. The
delicate Salmonidæ and the Pleuronectidæ—families
to which the salmon and turbot belong—were
ushered into being as early as the times of the Chalk;
but the Gadidæ or cod family—that family to which
the cod proper, the haddock, the dorse, the whiting,
the coal-fish, the pollock, the hake, the torsk, and
the ling belong, with many other useful and wholesome
species—did not precede man by at least any
period of time appreciable to the geologist. No trace
of the family has yet been detected in even the Tertiary
rocks.
Of the ganoids of the second age of vertebrate existence—that
of the Old Red Sandstone—some were
remarkable for the strangeness of their forms, and
some for constituting links of connection, which no
longer exist in nature, between the ganoid and placoid
orders. The Acanth family, which ceased with
the Coal-measures, was characterized, especially in
its Old Red species, by a combination of traits common
to both orders; and among the extremer forms,
in which palæontologists for a time failed to detect
that of the fish at all, we reckon those of the genera
Coccosteus, Pterichthys, and Cephalaspis. The more
aberrant genera, however, even while they consisted
each of several species, were comparatively short-lived.
The Coccosteus and Cephalaspis were restricted
to but one formation apiece; while the
Pterichthys, which appears for the first time in the
lower deposits of the Old Red Sandstone, becomes
extinct at its close. On the other hand, some of the
genera that exemplified the general type of their
class were extremely long-lived. The Celacanths
were reproduced in many various species, from the
times of the Lower Old Red Sandstone to those of
the Chalk; and the Cestracions, which appear in the
Upper Ludlow Rocks as the oldest of fishes, continue
in at least one species to exist still.
The ancient fishes seem to have received their
fullest development during the Carboniferous period.
Their number was very great: some of them
attained to an enormous size, and, though the true
reptile had already appeared, they continued to retain
till the close of the System the high reptilian
character and organization. Nothing, however, so
impresses the observer as the formidable character
of the offensive weapons with which they were furnished,
and the amazing strength of their defensive
armature. I need scarce say that the palæontologist
finds no trace in nature of that golden age of the
world, of which the poets delighted to sing, when all
creatures lived together in unbroken peace, and war
and bloodshed were unknown. Ever since animal
life began upon our planet there existed, in all the
departments of being, carnivorous classes, who could
not live but by the death of their neighbors, and
who were armed, in consequence, for their destruction,
like the butcher with his axe and knife, and
the angler with his hook and spear. But there were
certain periods in the history of the past during
which these weapons assumed a more formidable
aspect than at others; and never were they more formidable
than in the times of the Coal-measures.
The teeth of the Rhizodus—a ganoidal fish of our
coal-fields—were more sharp and trenchant than
those of the crocodile of the Nile, and in the larger
specimens fully four times the bulk and size of the
teeth of the hugest reptile of this species that now
lives. The dorsal spine of its contemporary, the
Gyracanthus, a great placoid, much exceeded in
size that of any existing fish; it was a mighty spearhead,
ornately carved like that of a New Zealand
chief, but in a style that, when he first saw a specimen
in my collection, greatly excited the admiration
of Mr. Ruskin. But one of the most remarkable
weapons of the period was the sting of the Pleuracanthus,
another great placoid of the age of gigantic
fishes. It was sharp and polished as a stiletto, but,
from its rounded form and dense structure, of great
strength; and along two of its sides, from the taper
point to within a few inches of the base, there ran a
thickly set row of barbs, hooked downward, like the
thorns that bristle on the young shoots of the wild
rose, and which must have rendered it a weapon not
merely of destruction, but also of torture. The defensive
armor of the period, especially that of its
ganoids, seems to have been as remarkable for its
powers of resistance as the offensive must have been
for their potency in the assault; and it seems probable
that in the great strength of the bony and enameled
armature of this order of fishes we have the secret
of the extremely formidable character of the teeth,
spines, and stings that coexisted along with it.
The oldest known reptiles appear just a little before
the close of the Old Red Sandstone, just as the
oldest known fishes appeared just a little before the
close of the Silurian System. What seems to be the
Upper Old Red of Great Britain, though there still
hangs a shade of doubt on the subject, has furnished
the remains of a small reptile, equally akin, it would
appear, to the lizards and the batrachians; and what
seems to be the Upper Old Red of the United States
has exhibited the foot-tracks of a larger animal of
the same class, which not a little resemble those
which would be impressed on recent sand or clay
by the alligator of the Mississippi, did not the alligator
of the Mississippi efface its own footprints (a
consequence of the shortness of its legs) by the trail
of its abdomen. In the Coal-measures the reptiles
hitherto found are all allied, though not without a
cross of the higher crocodilian or lacertian nature,
to the batrachian order—that lowest order of the
reptiles to which the frogs, newts, and salamanders
belong. It was not, however, until the Permian and
Triassic Systems had come to a close, and even the
earlier ages of the Oolitic System had passed away,
that the class received its fullest development in
creation. And certainly very wonderful was the development
which it then did receive. Reptiles became
everywhere the lords and masters of this lower
world. When any class of the air-breathing vertebrates
is very largely developed, we find it taking
possession of all the three old terrestrial elements—earth,
air, and water. The human period, for instance,
like that which immediately preceded it, is
peculiarly a period of mammals; and we find the
class free, if I may so express myself, of the three
elements, disputing possession of the sea with the
fishes, in its Cetaceans, its seals and its sea-lions, and
of the air with the birds, in its numerous genera of
the bat family. Further, not until the great mammaliferous
period is fairly ushered in do either the
bats or the whales make their appearance in creation.
Remains of Oolitic reptiles have been mistaken in
more than one instance for those of Cetacea; but it is
now generally held that the earliest known specimens
of the family belong to the Tertiary ages, while
those of the oldest bats occur in the Eocene of the
Paris basin, associated with the bones of dolphins,
lamantines, and morses. Now, in the times
of the Oolite it was the reptilian class that possessed
itself of all the elements. Its gigantic enaliosaurs,
huge reptilian whales mounted on paddles, were the
tyrants of the ocean, and must have reigned supreme
over the already reduced class of fishes; its pterodactyles—dragons
as strange as were ever feigned
by romancer of the Middle Ages, and that to the
jaws and teeth of the crocodile added the wings
of a bat and the body and tail of an ordinary mammal—had
the “power of the air,” and, pursuing the
fleetest insects in their flight, captured and bore them
down; its lakes and rivers abounded in crocodiles
and fresh-water tortoises of ancient type and fashion;
and its woods and plains were the haunts of a strange
reptilian fauna of what has been well termed “fearfully
great lizards”—some of which, such as the iguanodon,
rivaled the largest elephant in height, and
greatly more than rivaled him in length and bulk.
Judging from what remains, it seems not improbable
that the reptiles of this Oolitic period were quite
as numerous individually, and consisted of wellnigh
as many genera and species as all the mammals of
the present time. In the cretaceous ages, the class,
though still the dominant one, is visibly reduced in
its standing: it had reached its culminating point in
the Oolite and then began to decline; and with the
first dawn of the Tertiary division we find it occupying,
as now, a very subordinate place in creation.
Curiously enough, it is not until its times of humiliation
and decay that one of the most remarkable of
its orders appears—an order itself illustrative of
extreme degradation, and which figures largely in
every scheme of mythology that borrowed through
traditional channels from Divine revelation, as a
meet representative of man’s great enemy, the Evil
One. I, of course, refer to the ophidian or serpent
family. The earliest ophidian remains known to
the palæontologist occur in that ancient deposit of
the Tertiary division known as the London Clay,
and must have belonged to serpents, some of them
allied to the Pythons, some to the sea-snakes, which,
judging from the corresponding parts of recent
species, must have been from fourteen to twenty feet
in length.
Birds make their first appearance in a Red Sandstone
deposit of the United States in the valley of
the Connecticut, which was at one time supposed to
belong to the Triassic System, but which is now held
to be at least not older than the times of the Lias.
No fragments of the skeletons of birds have yet been
discovered in formations older than the Chalk; the
Connecticut remains are those of footprints exclusively;
and yet they tell their extraordinary story,
so far as it extends, with remarkable precision and
distinctness. They were apparently all of the Grallæ
or stilt order of birds—an order to which the cranes,
herons, and bustards belong, with the ostriches and
cassowaries, and which is characterized by possessing
but three toes on each foot (one species of ostrich
has but two), or, if a fourth toe be present, so imperfectly
is it developed in most of the cases that it
fails to reach the ground. And in almost all the footprints
of the primeval birds of the Connecticut there
are only three toes exhibited. The immense size of
some of these footprints served to militate for a time
against belief in their ornithic origin. The impressions
that are but secondary in point of size greatly
exceed those of the hugest birds which now exist;
while those of the largest class equal the prints of the
bulkier quadrupeds. There are tridactyle footprints
in the Red Sandstones of Connecticut that measure
eighteen inches in length from the heel to the middle
claw, nearly thirteen inches in breadth from the
outer to the inner toe, and which indicate, from their
distance apart in the straight line, a stride of about
six feet in the creature that impressed them in these
ancient sands—measurements that might well startle
zoologists who had derived their experience of the
ornithic class from existing birds exclusively. In a
deposit of New Zealand that dates little if at all in
advance of the human period, there have been detected
the remains of birds scarce inferior in size
to those of America in the Liassic ages. The bones
of the Dinornus giganteus, exhibited by Dr. Mantell
in Edinburgh in 1850, greatly exceeded in bulk those
of the largest horse. The larger thigh-bone referred
to must have belonged, it was held, to a bird that
stood from eleven to twelve feet high—the extreme
height of the great African elephant. Such were
the monster birds of a comparatively recent period;
and their remains serve to render credible the evidence
furnished by the great footprints of their remote
predecessors of the Lias. The huge feet of the
greatest Dinornus would have left impressions
scarcely an inch shorter than those of the still huger
birds of the Connecticut.
With the Stonesfield slates—a deposit which lies
above what is known as the Inferior Oolite—the remains
of mammaliferous animals first appear.
The Eocene ages were peculiarly the ages of the
Palæotheres—strange animals of, that pachydermatous
or thick-skinned order to which the elephants,
the tapirs, the hogs, and the horses belong.
It had been remarked by naturalists that there are
fewer families of this order in living nature than of
almost any other, and that of the existing genera not
a few are widely separated in their analogies from
the others. But in the Palæotheres of the Eocene,
which ranged in size from a large horse to a hare,
not a few of the missing links have been found—links
connecting the tapirs to the hogs, and the hogs to
the Palæotheres proper; and there is at least one
species suggestive of a union of some of the more
peculiar traits of the tapirs and the horses. It was
among these extinct Pachydermata of the Paris
basin that Cuvier effected his wonderful restorations,
and produced those figures in outline which
are now as familiar to the geologist as any of the
forms of the existing animals. The London Clay
and the Eocene of the Isle of Wight have also yielded
numerous specimens of these pachyderms, whose
identity with the Continental ones has been established
by Owen; but they are more fragmentary, and
their state of keeping less perfect than those furnished
by the gypsum quarries of Velay and Montmartre.
In the Middle or Miocene Tertiary, pachyderms,
though of a wholly different type from their predecessors,
are still the prevailing forms. The Dinotherium,
one of the greatest quadrupedal mammals
that ever lived, seems to have formed a connecting
link in this middle age between the Pachydermata
and the Cetaceæ. Each ramus of the under jaw,
which in the larger specimens are fully four feet in
length, bore at the symphysis a great bent tusk
turned downward, which appears to have been employed
as a pick-axe in uprooting the aquatic plants
and liliaceous roots on which the creature seems to
have lived. The head, which measured about three
feet across—a breadth sufficient, surely, to satisfy the
demands of the most exacting phrenologist—was provided
with muscles of enormous strength, arranged
so as to give potent effect to the operations of this
strange tool. The hinder part of the skull not a
little resembled that of the Cetaceæ; while, from the
form of the nasal bones, the creature was evidently
furnished with a trunk like the elephant. It seems
not improbable, therefore, that this bulkiest of
mammaliferous quadrupeds constituted, as I have
said, a sort of uniting tie between creatures still associated
in the human mind, from the circumstances of
their massive proportions, as the greatest that swim
the sea or walk the land—the whale and the elephant
The Mastodon, an elephantoid animal, also furnished,
like the elephant, with tusks and trunk, but marked
by certain peculiarities which constitute it a different
genus, seems in Europe to have been contemporary
with the Dinotherium; but in North America (the
scene of its greatest numerical development) it appears
to belong to a later age. In height it did not
surpass the African elephant, but it considerably
exceeded it in length—a specimen which could not
have stood above twelve feet high indicating a length
of about twenty-five feet: it had what the elephants
want—tusks fixed in its lower jaw, which the males
retained through life, but the females lost when
young; its limbs were proportionally shorter, but
more massive, and its abdomen more elongated and
slim; its grinder teeth, too, some of which have been
known to weigh from seventeen to twenty pounds,
had their cusps elevated into great mammæ-like
protuberances, to which the creature owes its name,
and wholly differ in their proportions and outline
from the grinders of the elephant. The much greater
remoteness of the mastodontic period in Europe than
in America is a circumstance worthy of notice, as it
is one of many facts that seem to indicate a general
transposition of at least the later geologic ages on
the opposite sides of the Atlantic.
EUROPEAN AND ASIATIC DELUGES — Louis Figuier
The Tertiary formations, in many parts of
Europe of more or less extent, are covered by
an accumulation of heterogeneous deposits, filling up
the valleys, and composed of very various materials,
consisting mostly of fragments of the neighboring
rocks. The erosions which we remark at the bottoms
of the hills, and which have greatly enlarged already
existing valleys; the mounds of gravel accumulated
at one point, and which is formed of rolled materials,
that is to say, of fragments of rocks worn smooth
and round by continual friction during a long period,
in which they have been transported from one point
to another—all these signs indicate that these denudations
of the soil, these displacements and transports
of very heavy bodies to great distances, are due to
the violent and sudden action of large currents of
water. An immense wave has been thrown suddenly
on the surface of the earth, making great
ravages in its passage, furrowing the earth and
driving before it débris of all sorts in its disorderly
course.
To what cause are we to attribute these sudden and
apparently temporary invasions of the earth’s surface
by rapid currents of water? In all probability
to the upheaval of some vast extent of dry land, to
the formation of some mountain or mountain range
in the neighborhood of the sea, or even in the bed
of the sea itself. The land, suddenly elevated by an
upward movement of the terrestrial crust, or by the
formation of ridges and furrows at the surface, has,
by its reaction, violently agitated the waters, that is
to say, the more mobile portion of the globe. By
this new impulse the waters have been thrown with
great violence over the earth, inundating the plains
and valleys, and for the moment covering the soil
with their furious waves, mingled with the earth,
sand, and mud, of which the devastated districts
have been denuded by their abrupt invasion.
There have been, doubtless, during the epochs anterior
to the Quaternary period many deluges such
as we are considering. Mountains and chains of
mountains were formed by upheaval of the crust into
ridges, where it was too elastic or too thick to be
fractured. Each of these subterranean commotions
would be provocative of momentary irruptions of the
waves.
But the visible testimony to this phenomenon—the
living proofs of this denudation, of this tearing away
of the soil—is found nowhere so strikingly as in the
beds superimposed, far and near, upon the Tertiary
formations, and which bear the geological name of
diluvium. This term was long employed to designate
what is now better known as the “bowlder” formation,
a glacial deposit which is abundant in Europe
north of the 50th, and in America north of the 40th,
parallel, and reappearing again in the Southern
Hemisphere; but altogether absent in tropical regions.
It consists of sand and clay, sometimes stratified,
mixed with rounded and angular fragments of
rock, generally derived from the same district; and
their origin has generally been ascribed to a series of
diluvial waves raised by hurricanes, earthquakes, or
the sudden upheaval of land from the bed of the sea,
which had swept over continents, carrying with them
vast masses of mud and heavy stones, and forcing
these stones over rocky surfaces so as to polish and
impress them with furrows and striæ. Other circumstances
occurred, however, to establish a connection
between this formation and the glacial drift. The
size and number of the erratic blocks increase as we
travel toward the Arctic regions; some intimate association
exists, therefore, between this formation and
the accumulations of ice and snow which characterize
the approaching glacial period.
There is very distinct evidence of two successive
deluges in our hemisphere during the Quaternary
epoch. The two may be distinguished as the European
Deluge and the Asiatic. The two European
deluges occurred prior to the appearance of man; the
Asiatic deluge happened after that event; and the
human race, then in the early days of its existence,
certainly suffered from this cataclysm.
The first occurred in the north of Europe, where
it was produced by the upheaval of the mountains of
Norway. Commencing in Scandinavia, the wave
spread and carried its ravages into those regions
which now constitute Sweden, Norway, European
Russia, and the north of Germany, sweeping before
it all the loose soil on the surface, and covering the
whole of Scandinavia—all the plains and valleys of
Northern Europe—with a mantle of transported soil.
As the regions in the midst of which this great mountainous
upheaval occurred—as the seas surrounding
these vast spaces were partly frozen and covered with
ice, from their elevation and neighborhood to the
pole—the wave which swept these countries carried
along with it enormous masses of ice.
The physical proof of this deluge of the north of
Europe exists in the accumulation of unstratified deposits
which covers all the plains and low grounds of
Northern Europe. On and in this deposit are found
numerous blocks which have received the characteristic
and significant name of erratic blocks, and
which are frequently of considerable size. These become
more characteristic as we ascend to higher
latitudes, as in Norway, Sweden, and Denmark, the
southern borders of the Baltic, and in the British Islands
generally, in all of which countries deposits of
marine fossil shells occur, which prove the submergence
of large areas of Scandinavia, of the British
Isles, and other regions during parts of the glacial
period. Some of these rocks, characterized as erratic,
are of very considerable volume; such, for instance,
is the granite block which forms the pedestal of the
statue of Peter the Great at St. Petersburg. This
block was found in the interior of Russia, where the
whole formation is Permian, and its presence there
can only be explained by supposing it to have been
transported by some vast iceberg, carried by a diluvial
current. This hypothesis alone enables us to
account for another block of granite, weighing about
340 tons, which was found on the sandy plains in the
north of Prussia, an immense model of which was
made for the Berlin Museum. The last of these
erratic blocks deposited in Germany covers the grave
of King Gustavus Adolphus, of Sweden, killed at the
battle of Lutzen, in 1632. He was interred beneath
the rock. Another similar block has been raised in
Germany into a monument to the geologist Leopold
von Buch.
These erratic blocks, which are met with in the
plains of Russia, Poland, and Prussia, and in the eastern
parts of England, are composed of rocks entirely
foreign to the region where they are found. They
belong to the primary rocks of Norway; they have
been transported to their present sites, protected
by a covering of ice, by the waters of the northern
deluge.
The second European deluge is supposed to have
been the result of the formation and upheaval of the
Alps. It has filled with débris and transported material
the valleys of France, Germany, and Italy over
a circumference which has the Alps for its centre.
The proofs of a great convulsion at a comparatively
recent geological date are numerous. The Alps may
be from eighty to one hundred miles across, and the
probabilities are that their existence is due, as Sir
Charles Lyell supposes, to a succession of unequal
movements of upheaval and subsidence; that the Alpine
region had been exposed for countless ages to the
action of rain and rivers, and that the larger valleys
were of pre-glacial times, is highly probable. In the
eastern part of the chain some of the Primary fossiliferous
rocks, as well as Oolitic and Cretaceous rocks,
and even Tertiary deposits, are observable; but in the
central Alps these disappear, and more recent rocks,
in some places even Eocene strata, graduate into
metamorphic rocks, in which Oolitic, Cretaceous,
and Eocene strata have been altered into granular
marble, gneiss, and other metamorphic schists; showing
that eruptions continued after the deposit of the
Middle Eocene formations. Again, in the Swiss and
Savoy Alps, Oolitic and Cretaceous formations have
been elevated to the height of 12,000 feet, and Eocene
strata 10,000 feet above the level of the sea; while
in the Rothal, in the Bernese Alps, occurs a mass of
gneiss 1,000 feet thick between two strata containing
Oolitic fossils.
Besides these proofs of recent upheaval, we can
trace effects of two different kinds, resulting from the
powerful action of masses of water violently displaced
by this gigantic upheaval. At first broad
tracks have been hollowed out by the diluvial waves,
which have, at these points, formed deep valleys.
Afterward these valleys have been filled up by materials
derived from the mountain and transported into
the valley, these materials consisting of rounded pebbles,
argillaceous and sandy mud, generally calcareous
and ferriferous. This double effect is exhibited,
with more or less distinctness, in all the great valleys
of the centre and south of France. The valley of the
Garonne is, in respect to these phenomena, classic
ground, as it were.
The small valleys, tributary to the principal valley,
would appear to have been excavated secondarily,
partly out of diluvial deposits, and their alluvium,
essentially earthy, has been formed at the
expense of the Tertiary formation, and even of the
diluvium itself. Among other celebrated sites, the
diluvial formation is largely developed in Sicily.
The ancient temple of the Parthenon at Athens is
built on an eminence formed of diluvial earth.
In the valley of the Rhine, in Alsace, and in many
isolated parts of Europe, a particular sort of diluvium
forms thick beds; it consists of a yellowish-gray
mud, composed of argillaceous matter mixed with
carbonate of lime, quartzose and micaceous sand, and
oxide of iron. This mud, termed by geologists loess,
attains in some places considerable thickness. It is
recognizable in the neighborhood of Paris. It rises
a little both on the right and left, above the base of the
mountains of the Black Forest and of the Vosges;
and forms thick beds on the banks of the Rhine.
The fossils contained in diluvial deposits consist,
generally, of terrestrial, lacustrine, or fluviatile shells,
for the most part belonging to species still living.
In parts of the valley of the Rhine, between Bingen
and Basle, the fluviatile loam or loess, now under consideration,
is seen forming hills several hundred feet
thick, and containing, here and there, throughout
that thickness, land and fresh-water shells; from
which it seems necessary to suppose, according to
Lyell, first, a time when the loess was slowly accumulated,
then a later period, when large portions of
it were removed—and followed by movements of
oscillation, consisting, first, of a general depression,
and then of a gradual re-elevation of the land.
The Asiatic deluge—of which sacred history has
transmitted to us the few particulars we know—was
the result of the upheaval of a part of the long chain
of mountains which are a prolongation of the
Caucasus. The earth opening by one of the fissures
made in its crust, in course of cooling, an eruption
of volcanic matter escaped through the enormous
crater so produced. Volumes of watery vapor or
steam accompanied the lava discharged from the interior
of the globe, which, being first dissipated in
clouds and afterward condensing, descended in torrents
of rain, and the plains were drowned with the
volcanic mud. The inundation of the plains over
an extensive radius was the immediate effect of this
upheaval, and the formation of the volcanic cone of
Mount Ararat, with the vast plateau on which it
rests, altogether 17,323 feet above the sea, the permanent
result. The event is graphically detailed in
the seventh chapter of Genesis.
All the particulars of the Biblical narrative here
recited are only to be explained by the volcanic and
muddy eruption which preceded the formation of
Mount Ararat. The waters which produced the
inundation of these countries proceeded from a volcanic
eruption accompanied by enormous volumes
of vapor, which in due course became condensed
and descended on the earth, inundating the extensive
plains which now stretch away from the foot of
Ararat. The expression, “the earth,” or “all the
earth,” as it is translated in the Vulgate, which might
be implied to mean the entire globe, is explained by
Marcel de Serres, in a learned book entitled La
Cosmogonie de Moïse, and other philologists, as
being an inaccurate translation. He has proved that
the Hebrew word haarets, incorrectly translated “all
the earth,” is often used in the sense of region or
country, and that, in this instance, Moses used it to
express only the part of the globe which was then
peopled, and not its entire surface. In the same manner
“the mountains” (rendered “all the mountains”
in the Vulgate) only implies all the mountains known
to Moses.
Of this deluge many races besides the Jews have
preserved a tradition. Moses dates it from 1,500 to
1,800 years before the epoch in which he wrote. Berosus,
the Chaldean historian, who wrote at Babylon
in the time of Alexander, speaks of a universal deluge,
the date of which he places immediately before
the reign of Belus, the father of Ninus.
The Vedas, or sacred books of the Hindus, supposed
to have been composed about the same time
as Genesis, that is, about 3,300 years ago, make out
that the deluge occurred 1,500 years before their
time. The Guebers speak of the same event as having
occurred about the same date.
Confucius, the celebrated Chinese philosopher and
lawgiver, born toward the year 551 before Christ,
begins his history of China by speaking of the Emperor
named Jas, whom he represents as making
the waters flow back, which, being raised to the
heavens, washed the feet of the highest mountains,
covered the less elevated hills, and inundated the
plains. Thus the Biblical deluge is confirmed in
many respects; but it was local, like all phenomena
of the kind, and was the result of the upheaval of the
mountains of western Asia.
GLACIERS — Louis Agassiz
The long summer was over. For ages a tropical
climate had prevailed over a great part of
the earth, and animals whose home is now beneath
the equator roamed over the world from the far
south to the very borders of the Arctics. The
gigantic quadrupeds, the mastodons, elephants,
tigers, lions, hyenas, bears, whose remains are found
in Europe from its southern promontories to the
northernmost limits of Siberia and Scandinavia,
and in America from the Southern States to Greenland
and the Melville Islands, may indeed be said
to have possessed the earth in those days. But their
reign was over. A sudden intense winter, that was
also to last for ages, fell upon our globe; it spread
over the very countries where these tropical animals
had their homes, and so suddenly did it come upon
them that they were embalmed beneath masses of
snow and ice, without time even for the decay which
follows death. The elephant was by no means a
solitary specimen; upon further investigation it was
found that the disinterment of these large tropical
animals in Northern Russia and Asia was no unusual
occurrence. Indeed, their frequent discoveries of
this kind had given rise among the ignorant inhabitants
to the singular superstition that gigantic
moles lived under the earth which crumbled away
and turned to dust as soon as they came to the upper
air. This tradition, no doubt, arose from the fact
that, when in digging they came upon the bodies
of these animals, they often found them perfectly
preserved under the frozen ground, but the moment
they were exposed to heat and light they decayed
and fell to pieces at once. Admiral Wrangell, whose
Arctic explorations have been so valuable to science,
tells us that the remains of these animals are heaped
up in such quantities in certain parts of Siberia that
he and his men climbed over ridges and mounds
consisting entirely of the bones of elephants, rhinoceroses,
etc.
We have as yet no clew to the source of this great
and sudden change of climate. Various suggestions
have been made, among others that formerly the inclination
of the earth’s axis was greater, or that the
submersion of the continents under water might have
produced a decided increase of cold; but none of
these explanations is satisfactory, and science has
yet to find any cause which accounts for all the
phenomena connected with it. It seems, however,
unquestionable that, since the opening of the Tertiary
age, a cosmic summer and winter have succeeded
each other, during which a tropical heat
and an Arctic cold have alternately prevailed
over a great portion of the present temperate zone.
In the so-called drift (a superficial deposit subsequent
to the Tertiaries) there are found far to the
south of their present abode the remains of animals
whose home now is in the Arctics or the coldest parts
of the Temperate Zones. Among them are the musk-ox,
the reindeer, the walrus, the seal, and many kinds
of shells characteristic of the Arctic regions. The
northernmost part of Norway and Sweden is at this
day the southern limit of the reindeer in Europe;
but their fossil remains are found in large quantities
in the drift about the neighborhood of Paris,
and they have been traced even to the foot
of the Pyrenees, where their presence would, of
course, indicate a climate similar to the one now prevailing
in Northern Scandinavia. Side by side with
the remains of the reindeer are found those of the
European marmot, whose present home is in the
mountains, about six thousand feet above the level of
the sea. The occurrence of these animals in the
superficial deposits of the plains of Central Europe,
one of which is now confined to the high north and
the other to mountain heights, certainly indicates an
entire change of climatic conditions since the time
of their existence. European shells now confined to
the Northern Ocean are found as fossils in Italy,
showing that, while the present Arctic climate prevailed
in the Temperate Zone, that of the Temperate
Zone extended much further south to the regions we
now call sub-tropical. In America there is abundant
evidence of the same kind; throughout the recent
marine deposits of the Temperate Zone, covering
the low lands above tide water on this Continent,
are found fossil shells whose present home is on the
shores of Greenland. It is not only in the Northern
Hemisphere that these remains occur, but in Africa
and in South America, wherever there has been an
opportunity for investigation, the drift is found to
contain the traces of animals whose presence indicates
a climate many degrees colder than that now
prevailing there.
But these organic remains are not the only evidence
of the geological winter. There are a number
of phenomena indicating that during this period
two vast caps of ice stretched from the northern pole
southward and from the southern pole northward,
extending in each case far toward the equator, and
that ice fields, such as now spread over the Arctics,
covered a great part of the Temperate Zones, while
the line of perpetual ice and snow in the tropical
mountain ranges descended far below its present
limits.
The first essential condition for the formation of
glaciers in mountain ranges is the shape of their valleys.
Glaciers are by no means in proportion to the
height and extent of mountains. There are many
mountain chains as high or higher than the Alps
which can boast of but few and small glaciers, if,
indeed, they have any. In the Andes, the Rocky
Mountains, the Pyrenees, the Caucasus, the few
glaciers remaining from the great ice period are insignificant
in size. The volcanic, cone-like shape of
the Andes gives, indeed, but little chance for the
formation of glaciers, though their summits are
capped with snow. The glaciers of the Rocky
Mountains have been little explored, but it is known
that they are by no means extensive. In the Pyrenees
there is but one great glacier, though the height of
these mountains is such that, were the shape of their
valleys favorable to the accumulation of snow, they
might present beautiful glaciers. In the Tyrol, on
the contrary, as well as in Norway and Sweden, we
find glaciers as fine as those of Switzerland in
mountain ranges much lower than either of the
above-mentioned chains. But they are of diversified
forms, and have valleys widening upward on the
slope of long crests. The glaciers on the Caucasus
are very small in proportion to the height of the
range; but on the northern side of the Himalayas
there are large and beautiful ones, while the southern
slope is almost destitute of them. Spitzbergen and
Greenland are famous for their extensive glaciers,
coming down to the seashore, where huge masses of
ice, many hundred feet in thickness, break off and
float away into the ocean as icebergs.
At the Aletsch in Switzerland, where a little lake
lies in a deep cup between the mountains, with the
glacier coming down to its brink, we have these
Arctic phenomena on a small scale; a miniature iceberg
may often be seen to break off from the edge
of the larger mass and float out upon the surface of
the water. Icebergs were first traced back to their
true origin by the nature of the land ice of which
they were always composed, and which is quite distinct
in structure and consistency from the marine ice
produced by frozen sea water, and called “ice flow”
by the Arctic explorers, as well as from the pond or
river ice, resulting from the simple congelation of
fresh water, the laminated structure of which is in
striking contrast to the granular structure of glacier
ice.
Land ice, of which both the ice fields of the Arctics
and the glaciers consist, is produced by slow and
gradual transformation of snow into ice; and though
the ice thus formed may eventually be as clear and
transparent as the present pond or river ice, its
structure is, nevertheless, entirely distinct.
We may compare these different processes during
any moderately cold winter in the ponds and snow
meadows immediately about us. We need not join
an Arctic exploring expedition, or even undertake
a more tempting trip to the Alps, in order to investigate
these phenomena ourselves, if we have any
curiosity to do so. The first warm day after a thick
fall of light, dry snow, such as occurs in the coldest
of our winter weather, is sufficient to melt its
surface.
As this snow is porous, the water readily penetrates
it, having also a tendency to sink by its own
weight, so that the whole mass becomes more or
less filled with moisture in the course of the day.
During the lower temperature of the night, however,
the water is frozen again, and the snow is now filled
with new ice particles. Let this process be continued
long enough and the mass of snow is changed into a
kind of ice gravel, or, if the grains adhere together,
to something like what we call pudding-stone, allowing,
of course, for the difference of material; the
snow, which has been rendered cohesive by the
process of partial melting and regelation, holding
the ice globules together, just as the loose materials
of the pudding-stone are held together by the cement
which unites them.
Within this mass air is intercepted and held inclosed
between the particles of ice. The process by
which snowflakes or snow crystals are transformed
into grains of ice, more or less compact, is easily
understood. It is the result of a partial thawing
under a temperature maintained very nearly at
thirty-two degrees, falling sometimes a little below
and then rising a little above the freezing-point, and
thus producing constant alternations of freezing and
thawing in the same mass of snow. This process
amounts to a kind of kneading of the snow, and when
combined with the cohesion among the particles more
closely held together in one snowflake, it produces
granular ice. Of course, the change takes place
gradually, and is unequal in its progress at different
depths in the same bed of fallen snow. It depends
greatly on the amount of moisture infiltrating the
mass, whether derived from the melting of its own
surface, or from the accumulation of dew, or the
falling of rain or mist upon it.
The amount of water retained within the mass
will also be greatly affected by the bottom on which
it rests and by the state of the atmosphere. Under
a certain temperature the snow may only be glazed
at the surface by the formation of a thin, icy crust,
an outer membrane, as it were, protecting the mass
below from a deeper transformation into ice; or it
may be rapidly soaked throughout its whole bulk,
the snow being thus changed into a kind of soft pulp,
what we commonly call slush, which, upon freezing,
becomes at once compact ice; or, the water
sinking rapidly, the lower layers only may be soaked,
while the upper portion remains comparatively dry.
But, under all these various circumstances, frost will
transform the crystalline snow into more or less compact
ice, the mass of which will be composed of an
infinite number of aggregated snow particles, very
unequal in regularity of outline, and cemented by
ice of another kind, derived from the freezing of
the infiltrated moisture, the whole being interspersed
with air.
Let the temperature rise, and such a mass, rigid before,
will resolve itself again into disconnected ice
particles, like grains more or less rounded. The
process may be repeated till the whole mass is transformed
into very compact, almost uniformly transparent
and blue ice, broken only by the intervening
air-bubbles. Such a mass of ice, when exposed to a
temperature sufficiently high to dissolve it, does not
melt from the surface and disappear by a gradual
diminution of its bulk, like pond ice, but crumbles
into its original granular fragments, each one of
which melts separately. This accounts for the sudden
disappearance of icebergs, which, instead of
slowly dissolving into the ocean, are often seen to
fall to pieces and vanish at once.
Ice of this kind may be seen forming every winter
on our sidewalks, on the edge of the little ditches
which drain them, or on the summits of broad gate
posts when capped with snow. Of such ice glaciers
are composed; but, in the glacier, another element
comes in which we have not considered as yet—that
of immense pressure in consequence of the vast
accumulations of snow within circumscribed spaces.
We see the same effects produced on a small scale
when snow is transformed into a snowball between
the hands. Every boy who balls a mass of snow in his
hands illustrates one side of glacial phenomena.
Loose snow, light and porous, and pure white from
the amount of air contained in it, is in this way presently
converted into hard, compact, almost transparent,
ice. This change will take place sooner if
the snow be damp at first, but if dry, the action of
the hand will presently produce moisture enough to
complete the process. In this case, mere pressure
produces the same effect which, in the cases we have
been considering above, was brought about by alternate
thawing and freezing, only that, in the latter,
the ice is distinctly granular, instead of being uniform
throughout, as when formed under pressure.
In the glaciers, we have the two processes combined.
But the investigators of glacial phenomena have
considered too exclusively one or the other: some of
them attributing glacial motion wholly to the dilatation
produced by the freezing of infiltrated moisture
in the mass of snow; others accounting for it
entirely by weight and pressure. There is yet a
third class, who, disregarding the real properties
of ice, would have us believe that, because tar, for
instance, is viscid when it moves, therefore ice is
viscid because it moves.
There is no chain of mountains in which the shape
of the valleys is more favorable to the formation of
glaciers than the Alps. Contracted at their lower
extremity, these valleys widen upward, spreading
into deep, broad, trough-like depressions. Take, for
instance, the valley of Hassli, which is not more than
half a mile wide where you enter it above Meyringen;
it opens gradually upward till, above the Grimsel,
at the foot of the Finster-Aarhorn, it measures
several miles across. These huge mountain-troughs
form admirable cradles for the snow, which collects
in immense quantities within them, and as it moves
slowly down from the upper ranges is transformed
into ice on its way, and compactly crowded into the
narrower space below. At the lower extremity of the
glacier the ice is pure blue and transparent, but as
we ascend it appears less compact, more porous and
granular, assuming gradually the character of snow,
till in the higher regions the snow is as light, as
shifting, as incoherent as the sand of the desert. A
snowstorm on a mountain summit is very different
from a snowstorm on the plain on account of the
different degrees of moisture in the atmosphere.
At great heights there is never dampness enough to
allow the fine snow crystals to coalesce and form
what are called snowflakes. I have even stood on
the summit of the Jungfrau when a frozen cloud
filled the air with ice-needles, while I could see the
same cloud poring down sheets of rain upon Lauterbrunnen
below. I remember this spectacle as one
of the most impressive I have ever witnessed in my
long experience of Alpine scenery. The air immediately
about me seemed filled with rainbow dust,
for the ice-needles glittered with a thousand hues
under the decomposition of light upon them, while
the dark storm in the valley below offered a strange
contrast to the brilliancy of the upper region in
which I stood. One wonders where even so much
vapor as may be transformed into the finest snow
should come from at such heights. But the warm
winds creeping up the sides of the valley, the walls
of which become heated during the middle of the
day, come laden with moisture which is changed
to a dry snow like dust as soon as it comes into contact
with the intense cold above.
Currents of warm air affect the extent of the
glaciers and influence also the line of perpetual snow,
which is by no means at the same level, even in
neighboring localities. The size of glaciers, of
course, determines to a great degree the height at
which they terminate, simply because a small mass
of ice will melt more rapidly, and at a lower temperature,
than a larger one. Thus the small glaciers,
such as those of the Rothhorn or of Trift, above the
Grimsel, terminate at a considerable height above
the plain, while the Mer de Glace, fed from the great
snow-caldrons of Mont Blanc, forces its way down
to the bottom of the Valley of Chamouni, and the
glacier of Grindelwald, constantly renewed from the
deep reservoirs where the Jungfrau hoards her vast
supplies of snow, descends to about four thousand
feet above the sea level. But the glacier of the
Aar, though also very large, comes to a pause at
about six thousand feet above the level of the sea;
for the south wind from the other side of the Alps,
the warm sirocco of Italy, blows across it, and it
consequently melts at a higher level than either the
Mer de Glace or the Grindelwald. It is a curious
fact that, in the Valley of Hassli, the temperature
frequently rises instead of falling, as you ascend;
at the Grimsel the temperature is at times higher
than at Meyringen, below, where the warmer winds
are not felt so directly. The glacier of Aletsch, on
the southern slope of the Jungfrau, and into which
many other glaciers enter, terminates also at a considerable
height, because it turns into the Valley
of the Rhone, through which the southern winds
blow constantly. Under ordinary conditions, vegetation
fades in these mountains at the height of six
thousand feet, but, in consequence of prevailing
winds and the sheltering influence of the mountain
walls, there is no uniformity in the limit of perpetual
snow and ice. Where currents of warm air
are very constant, glaciers do not occur at all, even
where other circumstances are favorable to their
formation.
There are valleys in the Alps far above six
thousand feet which have no glaciers, and where
perpetual snow is seen only on their northern sides.
These contrasts in the temperature lead to the most
wonderful contrasts in the aspect of the soil; summer
and winter lie side by side, and bright flowers
look out from the edge of snows that never melt.
Where the warm winds prevail there may be
sheltered spots at a height of ten or eleven thousand
feet, isolated nooks opening southward where the
most exquisite flowers bloom in the midst of perpetual
snow and ice; and occasionally I have seen
a bright little flower with a cap of snow over it that
seems to be its shelter. The flowers give, indeed,
a most peculiar charm to these high Alpine regions.
Occurring often in beds of the same kind, forming
green, blue, or yellow patches, they seem nestled
close together in sheltered spots, or even in fissures
and chasms of the rock, where they gather in dense
quantities.
Even in the sternest scenery of the Alps some sign
of vegetation lingers; and I remember to have found a
tuft of lichen growing on the only rock which pierced
through the ice on the summit of the Jungfrau. It
was a species then unknown to botanists, since described
under the name of Umbelicarus Higinis.
The absolute solitude, the intense stillness of the
upper Alps is most impressive; no cattle, no pasturage,
no bird, nor any sound of life—and, indeed, even
if there were, the rarity of the air in these high
regions is such that sound is hardly transmissible.
The deep repose, the purity of aspect of every object,
the snow, broken only by ridges of angular rocks, produce
an effect no less beautiful than solemn. Sometimes,
in the midst of the wide expanse, one comes
upon a patch of the so-called red snow of the Alps.
At a distance one would say that such a spot marked
some terrible scene of blood, but as you come nearer
the hues are so tender and delicate, as they fade from
deep red to rose, and so die into the pure colorless
snow around, that the first impression is completely
dispelled. This red snow is an organic growth, a
plant springing up in such abundance that it colors
extensive surfaces, just as the microscopic plants dye
our pools with green in the spring. It is an Alga
(Protocoites nivalis), well known in the Arctics,
where it forms wide fields in the summer.
In ordinary times, layers from six to eight feet deep
are regularly added annually to the accumulation of
snow in the higher regions—not taking into account,
of course, the heavy drifts heaped up in particular
localities, but estimating the uniform average over
wide fields. This snow is gradually transformed into
more or less compact ice, passing through an intermediate
condition analogous to the slush of our roads,
and in that condition chiefly occupies the upper part
of the extensive troughs into which these masses descend
from the loftier heights. This region is called
the region of the névé. It is properly the birthplace
of the glaciers, for it is here that the transformation
of the snow into ice begins. The névé ice,
though varying in the degree of its compactness and
solidity, is always very porous and whitish in color,
resembling somewhat frozen slush, while lower down
in the region of the glacier proper the ice is close,
solid, transparent, and of a bluish tint.
In consequence of the greater or less rapidity in
the movement of certain portions of the mass, its centre
progressing faster than its sides, and the upper,
middle and lower regions of the same glacier advancing
at different rates, the strata, which in the
higher ranges of the snow fields were evenly spread
over wide expanses, become bent and folded to such
a degree that the primitive stratification is nearly
obliterated, while the internal mass of the ice has
also assumed new features under these new circumstances.
There is, indeed, as much difference between
the newly formed beds of snow in the upper
region and the condition of the ice at the lower end
of a glacier as between a recent deposit of coral sand
or a mud bed in an estuary and the metamorphic
limestone or clay slate twisted and broken as they
are seen in the very chains of mountains from which
the glacier descended.
VOLCANIC ACTION — Sir Archibald Geikie
Large quantities of water accompany many volcanic
eruptions. In some cases, where ancient
crater-lakes or internal reservoirs, shaken by repeated
detonations, have been finally disrupted, the
mud which has thereby been liberated has issued
from the mountain. Such “mud-lava” (lava d’aqua),
on account of its liquidity and swiftness of motion,
is more dreaded for destructiveness than even the
true melted lavas. On the other hand, rain or melted
snow or ice, rushing down the cone and taking up
loose volcanic dust, is converted into a kind of mud
that grows more and more pasty as it descends.
The mere sudden rush of such large bodies of water
down the steep declivity of a volcanic cone can not
fail to effect much geological change. Deep trenches
are cut out of the loose volcanic slopes, and sometimes
large areas of woodland are swept away, the débris
being strewn over the plains below.
One of these mud-lavas invaded Herculaneum
during the great eruption of 79, and by quickly
enveloping the houses and their contents, has preserved
for us so many precious and perishable monuments
of antiquity. In the same district, during the
eruption of 1622, a torrent of this kind poured down
upon the villages of Ottajano and Massa, overthrowing
walls, filling up streets, and even burying houses
with their inhabitants. During the great eruption
of Cotopaxi, in June, 1877, enormous torrents of
water and mud, produced by the melting of the snow
and ice of the cone, rushed down from the mountain.
Huge portions of the glaciers of the mountain
were detached by the heat of the rocks below them
and rushed down bodily, breaking up into blocks.
The villages all round the mountain to a distance
of sometimes more than ten geographical miles were
left deeply buried under a deposit of mud mixed
with blocks of lava, ashes, pieces of wood, lumps of
ice, etc. Many of the volcanoes of Central and
South America discharge large quantities of mud
directly from their craters. Thus, in the year 1691,
Imbaburu, one of the Andes of Quito, emitted floods
of mud so largely charged with dead fish that pestilential
fevers arose from the subsequent effluvia.
Seven years later (1698), during an explosion of another
of the same range of lofty mountains, Carguairazo
(14,706 feet), the summit of the cone is said
to have fallen in, while torrents of mud containing
immense numbers of the fish Pymelodus Cyclopum
poured forth and covered the ground over a space
of four square leagues. The carbonaceous mud
(locally called moya) emitted by the Quito volcanoes
sometimes escapes from lateral fissures, sometimes
from the craters. Its organic contents, and
notably its siluroid fish, which are the same as those
found living in the streams above ground, prove that
the water is derived from the surface, and accumulates
in craters or underground cavities until discharged
by volcanic action. Similar but even more
stupendous and destructive outpourings have taken
place from the volcanoes of Java, where wide tracts
of luxuriant vegetation have at different times been
buried under masses of dark gray mud, sometimes
100 feet thick, with a rough hillocky surface from
which the top of a submerged palm-tree would here
and there protrude.
A volcano, as its activity wanes, may pass into the
Solfatara stage, when only volatile emanations are
discharged. The well-known Solfatara near Naples,
since its last eruption in 1198, has constantly discharged
steam and sulphurous vapors. The island
of Volcano has now passed also into this phase,
though giving vent to occasional explosions. Numerous
other examples occur among the old volcanic
tracts of Italy, where they have been termed
soffioni.
Another class of gaseous emanations betokens a
condition of volcanic activity further advanced toward
final extinction. In these, the gas is carbon-dioxide,
either issuing directly from the rock or
bubbling up with water which is often quite cold.
The old volcanic districts of Europe furnish many
examples. Thus on the shores of the Laacher See—an
ancient crater-lake of the Eifel—the gas issues
from numerous openings called moffette, round
which dead insects, and occasionally mice and birds,
may be found. In the same region occur hundreds
of springs more or less charged with this gas. The
famous Valley of Death in Java contains one of the
most remarkable gas-springs in the world. It is a
deep, bosky hollow, from one small space on the
bottom of which carbon-dioxide issues so copiously
as to form the lower stratum of the atmosphere.
Tigers, deer, and wild boars, enticed by the shelter
of the spot, descend and are speedily suffocated.
Many skeletons, including those of man himself,
have been observed.
As a distinct class of gas-springs, we may group
and describe here the emanations of volatile hydrocarbons
which, when they take fire, are known as
Fire-wells. These are not of volcanic origin, but
arise from changes within the solid rocks underneath.
They occur in many of the districts where
mud-volcanoes appear, as in northern Italy, on the
Caspian, in Mesopotamia, in southern Kurdistan,
and in many parts of the United States.
In the oil regions of Pennsylvania, certain sandy
strata occur at various geological horizons whence
large quantities of petroleum and gas are obtained.
In making the borings for oil-wells, reservoirs of
gas as well as subterranean courses or springs of
water are met with. When the supply of oil is
limited, but that of gas is large, a contest for possession
of the bore-hole sometimes takes place between
the gas and water. When the machinery is removed
and the boring is abandoned, the contest is allowed
to proceed unimpeded, and results in the intermittent
discharge of columns of water and gas to heights
of 130 feet or more. At night, when the gas has been
lighted, the spectacle of one of these “fire-geysers”
is inconceivably grand.
Eruptive fountains of hot water and steam, to
which the general name of Geysers (i. e., gushers)
is given, from the examples in Iceland, which were
the first to be seen and described, mark a declining
phase of volcanic activity. The Great and Little
Geysers, the Strokkr, and other minor springs of hot
water in Iceland, have long been celebrated examples.
More recently another series has been discovered
in New Zealand. But probably the most
remarkable and numerous assemblage is that which
has been brought to light in the northwest part of
the Territory of Wyoming, and which has been included
within the “Yellowstone National Park.” In
this singular region the ground in certain tracts is
honeycombed with passages which communicate
with the surface by hundreds of openings, whence
boiling water and steam are emitted. In most cases,
the water remains clear, tranquil, and of a deep
green-blue tint, though many of the otherwise quiet
pools are marked by patches of rapid ebullition.
These pools lie on mounds or sheets of sinter, and are
usually edged round with a raised rim of the same
substance, often beautifully fretted and streaked
with brilliant colors. The eruptive openings usually
appear on small, low, conical elevations of sinter,
from each of which one or more tubular projections
rise. It is from these irregular tube-like excrescences
that the eruptions take place.
The term geyser is restricted to active openings
whence columns of hot water and steam are from
time to time ejected; the non-eruptive pools are only
hot springs. A true geyser should thus possess an
underground pipe or passage, terminating at the surface
in an opening built round with deposits of
sinter.
At more or less regular intervals, rumblings and
sharp detonations in the pipe are followed by an
agitation of the water in the basin, and then by the
violent expulsion of a column of water and steam to
a considerable height in the air. In the Upper Fire
Hole basin of the Yellowstone Park, one of the
geysers, named “Old Faithful,” has ever since the
discovery of the region sent out a column of mingled
water and steam every sixty-three minutes or
thereabout. The column rushes up with a loud
roar to a height of more than 100 feet, the whole
eruption not occupying more than about five or six
minutes. The other geysers of the same district are
more capricious in their movements, and some of
them more stupendous in the volume of their discharge.
The eruptions of the Castle, Giant, and Beehive
vents are marvelously impressive.
In course of time, the network of underground
passages undergoes alteration. Orifices that were
once active cease to erupt, and even the water fails
to overflow them. Sinter is no longer formed round
them, and their surfaces, exposed to the weather,
crack into fine shaly rubbish like comminuted oyster-shells.
Or the cylinder of sinter grows upward until,
by the continued deposit of sinter and the failing
force of the geyser, the tube is finally filled up, and
then a dry and crumbling white pillar is left to mark
the site of the extinct geyser.
Mud-Volcanoes are of two kinds: 1st, where the
chief source of movement is the escape of gaseous
discharges; 2d, where the active agent is steam.
Although not volcanic in the proper sense of the
term, certain remarkable orifices of eruption may be
noticed here, to which the names of mud-volcanoes,
salses, air-volcanoes, and maccalubas have been applied
(Sicily, the Apennines, Caucasus, Kertch,
Tamar). These are conical hills formed by the
accumulation of fine and usually saline mud, which,
with various gases, is continuously or intermittently
given out from the orifice or crater in the centre.
They occur in groups, each hillock being sometimes
less than a yard in height, but ranging up to elevations
of 100 feet or more. Like true volcanoes, they
have their periods of repose, when either no discharge
takes place at all, or mud oozes out tranquilly
from the crater, and their epochs of activity,
when large volumes of gas, and sometimes columns
of flame, rush out with considerable violence and explosion,
and throw up mud and stones to a height
of several hundred feet. The gases play much the
same part, therefore, in these phenomena that steam
does in those of true volcanoes. They consist of
marsh-gas and other hydrocarbons, carbon-dioxide,
sulphureted hydrogen, and nitrogen, with petroleum
vapors. The mud is usually cold. In the water
occur various saline ingredients, among which common
salt generally appears; hence the names Salses.
Naphtha is likewise frequently present. Large
pieces of stone, differing from those in the neighborhood,
have been observed among the ejections, indicative
doubtless of a somewhat deeper source than
in ordinary cases. Heavy rains may wash down the
minor mud-cones and spread out the material over
the ground; but gas-bubbles again appear through
the sheet of mud, and by degrees a new series of
mounds is once more thrown up.
The second class of mud-volcano presents itself
in true volcanic regions, and is due to the escape of
hot water and steam through beds of tuff or some
other friable kind of rock. The mud is kept in ebullition
by the rise of steam through it. As it becomes
more pasty and the steam meets with greater resistance,
large bubbles are formed which burst, and
the more liquid mud from below oozes out from the
vent. In this way, small cones are built up, many
of which have perfect craters atop. In the Geyser
tracts of the Yellowstone region, there are instructive
examples of such active and extinct mud-vents. Some
of the extinct cones there are not more than a foot
high, and might be carefully removed as museum
specimens.
Mud-volcanoes occur in Iceland, Sicily (Maccaluba),
in many districts of northern Italy, at
Tamar and Kertch, at Baku on the Caspian, near the
mouth of the Indus, and in other parts of the globe.
It is not only on the surface of the land that volcanic
action shows itself. It takes place likewise under
the sea, and as the geological records of the earth’s
past history are chiefly marine formations, the characteristics
of submarine volcanic action have no small
interest for the geologist. In a few instances, the
actual outbreak of a submarine eruption has been
witnessed. Thus, in the early summer of 1783, a
volcanic eruption took place about thirty miles from
Cape Reykjanaes on the west coast of Iceland. An
island was built up, from which fire and smoke continued
to issue, but in less than a year the waves had
washed the loose pumice away, leaving a submerged
reef from five to thirty fathoms below sea-level.
About a month after this eruption, the frightful
outbreak of Skaptar-Jökull began, the distance of
this mountain from the submarine vent being nearly
200 miles. A century afterward, viz., in July, 1884,
another volcanic island is said to have been thrown
up near the same spot, having at first the form of a
flattened cone, but soon yielding to the power of the
breakers. Many submarine eruptions have taken
place within historic times in the Mediterranean.
The most noted of these occurred in the year 1831,
when a new volcanic island (Graham’s Island, Ile
Julia) was thrown up, with abundant discharge
of steam and showers of scoriæ, between Sicily and
the coast of Africa. It reached an extreme height
of 200 feet or more above the sea-level (800 feet
above sea-bottom), with a circumference of 3 miles,
but on the cessation of the eruptions was attacked
by the waves and soon demolished, leaving only a
shoal to mark its site. In the year 1811, another
island was formed by submarine eruption of the
coast off St. Michael’s in the Azores. Consisting,
like the Mediterranean example, of loose cinders,
it rose to a height of about three hundred feet, with a
circumference of about a mile, but subsequently disappeared.
In the year 1796 the island of Johanna
Bogoslawa, in Alaska, appeared above the water,
and in four years had grown into a large volcanic
cone, the summit of which was 3,000 feet above
sea-level.
Ideal Landscape of the Carboniferous Period Showing Lepidodendra and other Giant Ferns and Mosses whose remains
are found in the Coal-Measures
Unfortunately, the phenomena of recent volcanic
eruptions under the sea are for the most part inaccessible.
Here and there, as in the Bay of Naples,
at Etna, among the islands of the Greek Archipelago,
and at Tahiti, elevation of the sea-bed has
taken place, and brought to the surface beds of tuff
or of lava, which have consolidated under water.
Both Vesuvius and Etna began their career as submarine
volcanoes. The Islands of Santorin and
Therasia form the unsubmerged portions of a great
crater-rim rising round a crater which descends
1,278 feet below sea-level.
Confining attention to vents now active, of which
the total number may be about 300, the chief facts
regarding their distribution over the globe may be
thus summarized. (1) Volcanoes occur along the
margins of the ocean-basins, particularly along lines
of dominant mountain ranges, which either form
part of the mainland of the continents or extend as
adjacent lines of islands. The vast hollow of the
Pacific is girdled with a wide ring of volcanic foci.
(2) Volcanoes rise, as a striking feature, from the
submarine ridges that traverse the ocean basins. All
the oceanic islands are either volcanic or formed of
coral, and the scattered coral-islands have in all
likelihood been built upon the tops of submarine
volcanic cones. (3) Volcanoes are situated not far
from the sea. The only exceptions to this rule are
certain vents in Manchuria and in the tract lying
between Tibet and Siberia; but of the actual nature
of these vents very little is yet known. (4) The
dominant arrangement of volcanoes is in series along
subterranean lines of weakness, as in the chain of
the Andes, the Aleutian Islands, and the Malay
Archipelago. A remarkable zone of volcanic vents
girdles the globe from Central America eastward
by the Azores and Canary Islands to the Mediterranean,
thence to the Red Sea, and through the chains
of islands from the south of Asia to New Zealand
and the heart of the Pacific. (5) On a smaller scale
the linear arrangement gives place to one in groups,
as in Italy, Iceland, and the volcanic islands of the
great oceans.
In the European area there are six active volcanoes—Vesuvius,
Etna, Stromboli, Volcano, Santorin,
and Nisyros. Asia contains twenty-four,
Africa ten, North America twenty, Central America
twenty-five, and South America thirty-seven. By
much the larger number, however, occur on islands
in the ocean. In the Arctic Ocean rises the solitary
Jan Mayen. On the ridge separating the Arctic and
Atlantic basins, the group of Icelandic volcanoes is
found. Along the great central ridge of the Atlantic
bottom, numerous volcanic vents have risen above
the surface of the sea—the Azores, Canary Islands,
and the extinct degraded volcanoes of St. Helena,
Ascension, and Tristan d’Acunha. On the eastern
border lie the volcanic vents of the islands off the
African coast, and to the west those of the West
Indian Islands. Still more remarkable is the development
of volcanic energy in the Pacific area.
From the Aleutian Islands southward, a long line of
volcanoes, numbering upward of a hundred active
vents, extends through Kamtchatka and the Kurile
Islands to Japan, whence another numerous series
carries the volcanic band far south toward the Malay
Archipelago, which must be regarded as the chief
centre of the present volcanic activity of our planet.
In Sumatra, Java, and adjoining islands, no fewer
than fifty active vents occur. The chain is continued
through New Guinea and the groups of
islands to New Zealand. Even in the Antarctic
regions, Mounts Erebus and Terror are cited as
active vents; while in the centre of the Pacific Ocean
rise the great lava cones of the Sandwich Islands.
In the Indian Ocean, the Red Sea, and off the east
coast of Africa a few scattered vents appear.
THOUGHTS ABOUT KRAKATOA — Sir Robert S. Ball
Midway between Sumatra and Java lies a
group of small islands, which, prior to 1883,
were beautified by the dense forests and glorious
vegetation of the tropics. Of these islands Krakatoa
was the chief, though even of it but little was known.
Its appearance from the sea must, indeed, have been
familiar to the crews of the many vessels that navigated
the Straits of Sunda, but it was not regularly
inhabited. Glowing with tropical verdure, such an
island seemed an unlikely theatre for the display of
an unparalleled effect of plutonic energy, but yet
there were certain circumstances which may tend to
lessen our surprise at the outbreak. In the first place,
as Professor Judd has so clearly pointed out, not only
is Krakatoa situated in a region famous, or perhaps
infamous, for volcanoes and earthquakes, but it actually
happens to lie at the intersection of two main
lines, along which volcanic phenomena are, in some
degree, perennial. In the second place, history records
that there have been previous eruptions at Krakatoa.
The last of these appears to have occurred in
May, 1680, but unfortunately only imperfect accounts
of it have been preserved. It seems, however,
to have annihilated the forests of the island, and to
have ejected vast quantities of pumice, which cumbered
the seas around. Krakatoa then remained active
for a year and a half, after which the mighty
fires subsided. The irrepressible tropical vegetation
again resumed possession. The desolated islet again
became clothed with beauty, and for a couple of centuries
reposed in peace.
It was one o’clock in the afternoon of Sunday, August
26, 1883, when Krakatoa commenced a series of
gigantic volcanic efforts. Detonations were heard
which succeeded each other at intervals of about ten
minutes. These were loud enough to penetrate as far
as Batavia and Buitenzorg, distant 96 and 100 miles
respectively from the volcano. A vast column of
steam, smoke, and ashes ascended to a prodigious elevation.
It was measured at two P. M. from a ship 76
miles away, and was then judged to be 17 miles high—that
is, three times the height of the loftiest mountain
in the world. As the Sunday afternoon wore on,
the volcanic manifestations became ever fiercer. At
3 P. M. the sounds were loudly heard in a town 150
miles away. At 5 P. M. every ear in the island of
Java was engaged in listening to volcanic explosions,
which were considered to be of quite unusual intensity
even in that part of the world. These phenomena
were, however, only introductory. Krakatoa was
gathering strength. Between 5 and 6 P. M. the British
ship Charles Bal, commanded by Captain Watson,
was about ten miles south of the volcano. The
ship had to shorten sail in the darkness, and a rain of
pumice, in large pieces and quite warm, fell upon her
decks. At 7 P. M. the mighty column of smoke is described
as having the shape of a pine tree, and as
being brilliantly illuminated by electric flashes. The
sulphurous air is laden with fine dust, while the lead
dropped from a ship in its anxious navigation astounds
the leadsman by coming up hot from the bottom
of the sea. From sunset on Sunday till midnight
the tremendous detonations followed each other so
quickly that a continuous roar may be said to have issued
from the island. The full terrors of the eruption
were now approaching. The distance of 96
miles between Krakatoa and Batavia was not sufficient
to permit the inhabitants of the town to enjoy
their night’s sleep. All night long the thunders of
the volcano sounded like the discharges of artillery
at their very doors, while the windows rattled with
aerial vibrations.
On Monday morning, August 27, the eruption culminated
in four terrific explosions, of which the third,
shortly after 10 A. M. Krakatoa time, was by far the
most violent. The quantity of material ejected was
now so great that darkness prevailed even as far as
Batavia soon after 11 A. M., and there was a rain of
dust until three in the afternoon. The explosions
continued with more or less intensity all the afternoon
of Monday and throughout Monday night.
They finally ceased at about 2:30 A. M. on Tuesday,
August 28. The entire series of grand phenomena
thus occupied a little more than thirty-six hours.
It seems to be certain that if all the materials
poured forth from Krakatoa during the critical period
could be collected together, the mass they would
form would be considerably over a cubic mile in volume.
It is in the other standards of comparison that
the importance of the explosion of Krakatoa is to be
sought. The intensity of this outbreak in its last
throes was such that mighty sounds were heard and
mighty waves arose in the sea for which we can find
no parallel. Every part of our globe’s surface felt
the pulse of the air-waves, and beautiful optical phenomena
made the circuit of the globe even more than
once or twice. In these last respects the eruption of
Krakatoa is unique.
It appears to me that the most remarkable incident
connected with the eruption of Krakatoa was the
production of the great air-wave by that particular
explosion that occurred at ten o’clock on the morning
of Monday, August 27. The great air-wave was
truly of cosmical importance, affecting as it did
every particle of the atmosphere on our globe.
The comprehensive series of phenomena wherein
the atmosphere of the entire globe participates in an
organized vibration has, so far as we know, only once
been witnessed, and that was after the greatest outbreak
at Krakatoa, at ten o’clock on the morning of
August 27. But the ebb and the flow of these mighty
undulations are not immediately appreciable to the
senses. The great wave, for instance, passed and re-passed
and passed again over London, and no inhabitant
was conscious of the fact. But the automatic records
of the barometer at Greenwich show that the vibration
from Krakatoa to its antipodes, and from the
antipodes back to Krakatoa, was distinctly perceptible
over London not less than six or seven times.
From all parts of Europe, from Berlin to Palermo,
from St. Petersburg to Valencia, we obtain the same
indications. Fortunately self-recording barometric
instruments are now to be found all over the world.
Almost all the instruments show distinctly the first
great wave from Krakatoa to its antipodes in Central
America, and the return wave from the antipodes to
Krakatoa. They also all show the second great wave
which sped from Krakatoa, as well as the second
great wave which returned from the antipodes.
Thus, the first four of the oscillations are depicted
on upward of forty of the barograms. The fifth and
sixth oscillations are also to be distinguished on several
of the curves, and even the seventh is certainly
established at some few places, of which Kew is one.
Then the gradually increasing faintness of the indications
renders them unrecognizable, from which we
conclude that after seven pulsations our atmosphere
had sensibly regained its former condition ere it was
disturbed by Krakatoa.
In the whole annals of noise there is nothing
which can be compared to the records. Lloyd’s
agent at Batavia, 94 miles distant, says that on the
morning of August 27 the reports and concussions
were deafening. At Carimon, Java Island, reports
were heard which led to the belief that some vessel
offshore was making signals of distress, and boats
were accordingly put out to render succor, but no
vessel was found, as the reports were from Krakatoa,
at a distance of 355 miles. At Macassar, in Celebes,
explosions were heard all over the province. Two
steamers were sent out to discover the cause, for the
authorities did not then know that what they heard
came from Krakatoa, 969 miles away. But mere
hundreds of miles will not suffice to exemplify the
range of this stupendous siren. In St. Lucia Bay, in
Borneo, a number of natives, who had been guilty
of murder, thought they heard the sounds of vengeance
in the approach of an attacking force. They
fled from their village, little fancying that what
alarmed them really came from Krakatoa, 1,116
miles distant. All over the island of Timor alarming
sounds were heard, and so urgent did the situation
appear that the government was aroused, and sent off
a steamer to ascertain the cause. The sounds had,
however, come 1,351 miles, all the way from Krakatoa.
In the Victoria Plains of West Australia the
inhabitants were startled by the discharge of artillery—an
unwonted noise in that peaceful district—but
the artillery was at Krakatoa, 1,700 miles distant.
The inhabitants of Daly Waters, in South Australia,
were rudely awakened at midnight on Sunday, August
26, by an explosion resembling the blasting of
a rock, which lasted for a few minutes. The time and
other circumstances show that here again was Krakatoa
heard, this time at the monstrous distance of
2,023 miles. But there is undoubted testimony that
to distances even greater than 2,023 miles the waves
of sound conveyed tidings of the mighty convulsion.
Diego Garcia, in the Chagos Islands, is 2,267 miles
from Krakatoa, but the thunders traversed even this
distance, and created the belief that there must be
some ship in distress, for which a diligent but necessarily
ineffectual search was made. To pass at once
to the most remarkable case of all, we have a report
from Mr. James Wallis, chief of police in Rodriguez,
that “several times during the night of August
26-27, 1883, reports were heard coming from the
eastward, like the distant roar of heavy guns. These
reports continued at intervals of between three and
four hours.” We have thus the astounding fact that
almost across the whole wide extent of the Indian
Ocean, that is, to a distance of nearly 3,000 miles
(2,968), the sound of the throes of Krakatoa was
propagated.
I shall content myself with the mention of three
facts in illustration of the great sea waves which accompanied
the eruption of Krakatoa. Of these, probably
the most unusual is the magnitude of the area
over which the undulations were perceived. Thus,
to mention but a single instance, and that not by any
means an extreme one, we find that the tide gauge
at Table Bay reveals waves which, notwithstanding
that they have traveled 5,100 miles from Krakatoa,
have still a range of eighteen inches when they arrive
at the southern coast of Africa. The second fact that
I mention illustrates the magnitude of the seismic
waves by the extraordinary inundations that they produced
on the shores of the Straits of Sunda. Captain
Wharton shows that the waves, as they deluged the
land, must have been fifty feet, or, in one well authenticated
case, seventy-two feet high. It was, of course,
these vast floods which caused the fearful loss of life.
The third illustrative fact concerns the fate of a
man-of-war, the Berouw. This unhappy vessel was
borne from its normal element and left high and dry
in Sumatra, a mile and three-quarters inland, and
thirty feet above the level of the sea.
During the crisis on August 26-27, the volume of
material blown into the air was sufficiently dense to
obscure the coasts of Sumatra to such a degree that
at 10 A. M. the darkness there is stated to have been
more intense than it is even in the blackest of nights.
The fire-dust ascended to an elevation which, as we
have already mentioned, is estimated to have been as
much as seventeen miles. Borne aloft into these
higher regions of our atmosphere, the clouds of dust
at once became the sport of the winds and the currents
which may be found there. If we had not previously
known the prevailing tendency of the winds
at these elevations and in these latitudes, the journey
of the Krakatoa dust would have taught us.
It seems certain that, having attained their lofty
elevation, the mighty clouds of dust were seized by
easterly winds, and were swept along with a velocity
which may not improbably be normal at a height of
twenty miles above the earth’s surface.
It appears that this cloud of dust started immediately
from Krakatoa for a series of voyages round the
world. The highway which it at first pursued may,
for our present purpose, be sufficiently defined by the
Tropic of Cancer and the Tropic of Capricorn,
though it hardly approached these margins at first.
Westward the dust of Krakatoa takes its way. In
three days it had crossed the Indian Ocean and was
rapidly flying over the heart of Equatorial Africa;
for another couple of days it was making a transatlantic
journey; and then it might be found, for still
a couple of days more, over the forests of Brazil
ere it commenced the great Pacific voyage which
brought it back to the East Indies. The dust of
Krakatoa had put a girdle round the earth in thirteen
days! The shape of the cloud appears to have been
elongated, so that it took two or three days to complete
the passage over any stated place.
It remains to give some brief account of the optical
phenomena due to the presence of dust, unusual
both in quantity and in character, in the upper atmosphere.
Beautiful pictures show the twilight and
after-glow effects as seen by Mr. W. Ascroft on
the bank of the Thames a little west of London,
on the evening of November 26, 1883. Analogous
phenomena were seen almost universally during
November and December in the same year. Who
is there that does not remember the wondrous loveliness
of the twilights and the after-glows during
that remarkable winter! These appearances at sunrise
and sunset are only the more generally recognized
of a whole system of strange optical phenomena.
One of the most striking indications of the
presence of the dust-stream in its first voyage round
the earth was given by the strange blue hue it imparted
to the sun. The dust-stream was also visible
in its rapid voyages as a lofty haze or extensive
cloud of cirro-stratus. Then, too, strange halos were
often seen, there were occasional blue or green moons,
and the sun was sometimes glorified by a corona that
had its origin in our atmosphere. Everywhere in
the world there were remarkable features in the sky
that winter: from Tierra del Fuego to Lake Superior;
from China to the Gulf of Guinea; from Panama
to Australia. Wherever on land there were inhabitants
with sufficient intelligence to note the unusual,
wherever on the sea there were mariners who
kept a careful log, from all such observers we learn
that in the autumn and winter months following the
great eruption of Krakatoa, there were extraordinary
manifestations witnessed in the heavens.
VOLCANOES — Sir Archibald Geikie
The term volcanic action (volcanism or volcanicity)
embraces all the phenomena connected
with the expulsion of heated materials from the interior
of the earth to the surface. Among these phenomena,
some possess an evanescent character, while
others leave permanent proofs of their existence. It
is naturally to the latter that the geologist gives chief
attention, for it is by their means that he can trace
former phases of volcanic activity in regions where,
for many ages, there have been no volcanic eruptions.
In the operations of existing volcanoes, he can observe
only superficial manifestations of volcanic action.
But examining the rocks of the earth’s crust, he
discovers that amid the many terrestrial revolutions
which geology reveals, the very roots of former volcanoes
have been laid bare, displaying subterranean
phases of volcanism which could not be studied in
any modern volcano. Hence an acquaintance only
with active volcanoes will not afford a complete
knowledge of volcanic action. It must be supplemented
and enlarged by an investigation of the traces
of ancient volcanoes preserved in the crust of the
earth.
The word “volcano” is applied to a conical hill or
mountain (composed mainly or wholly of erupted
materials), from the summit and often also from
the sides of which hot vapors issue, and ashes and
streams of molten rock are intermittently expelled.
The term “volcanic” designates all the phenomena
essentially connected with one of these channels of
communication between the surface and the heated
interior of the globe. Yet there is good reason to
believe that the active volcanoes of the present day do
not afford by any means a complete type of volcanic
action. The first effort in the formation of a new volcano
is to establish a fissure in the earth’s crust. A
volcano is only one vent or group of vents established
along the line of such a fissure. But in many
parts of the earth, alike in the Old World and the
New, there have been periods in the earth’s history
when the crust was rent into innumerable fissures
over areas thousands of square miles in extent, and
when the molten rock, instead of issuing, as it does at
a modern volcano, in narrow streams from a central
elevated cone, welled out from numerous small vents
along the rents, and flooded enormous tracts of country
without forming any mountain or conspicuous
volcanic cone in the usual sense of these terms. Of
these “fissure-eruptions,” apart from central volcanic
cones, no examples appear to have occurred within
the times of human history, except in Iceland, where
vast lava-floods issued from a fissure in 1783. They
can best be studied from the remains of former convulsions.
The materials erupted from volcanic vents may be
classed as (1) gases and vapors, (2) water, (3) lava,
(4) fragmentary substances.
Gases and vapors exist dissolved in the molten
magma within the earth’s crust. They play an important
part in volcanic activity, showing themselves
in the earliest stages of a volcano’s history, and continuing
to appear for centuries after all other subterranean
action has ceased. By much the most
abundant of them all is water-gas, which, ultimately
escaping as steam, has been estimated to form 999-1000ths
of the whole cloud that hangs over an active
volcano. In great eruptions, steam rises in prodigious
quantities, and is rapidly condensed into a heavy
rainfall. M. Fouqué calculated that, during 100
days, one of the parasitic cones on Etna had ejected
vapor enough to form, if condensed, 2,100,000 cubic
metres (462,000,000 gallons) of water. But even
from volcanoes which, like the Solfatara of Naples,
have been dormant for centuries, steam sometimes
still rises without intermission and in considerable
volume. Jets of vapor rush out from clefts in the
sides and bottom of a crater with a noise like that
made by the steam blown off by a locomotive. The
number of these funnels or “fumaroles” is often so
large, and the amount of vapor so abundant, that
only now and then, when the wind blows the dense
cloud aside, can a momentary glimpse be had of a
part of the bottom of the crater; while at the same
time the rush and roar of the escaping steam remind
one of the din of some vast factory. Aqueous vapor
rises likewise from rents on the outside of the volcanic
cone. It issues so copiously from some flowing
lavas that the stream of rock may be almost concealed
from view by the cloud; and it continues to escape
from fissures of the lava, far below the point of exit,
for a long time after the rock has solidified and come
to rest.
Abundant discharges of water accompany some
volcanic explosions. Three sources of this water may
be assigned: (1) from the melting of snow by a rapid
accession of temperature previous to or during an
eruption; this takes place from time to time on Etna,
in Iceland, and among the snowy ranges of the Andes,
where the cone of Cotopaxi is said to have been entirely
divested of its snow in a single night by the
heating of the mountain; (2) from the condensation
of the vast clouds of steam which are discharged during
an eruption; this undoubtedly is the chief source
of the destructive torrents so frequently observed to
form part of the phenomena of a great volcanic explosion;
and (3) from the disruption of reservoirs of
water filling subterranean cavities, or of lakes occupying
crater-basins; this has several times been observed
among the South American volcanoes, where
immense quantities of dead fish, which inhabited the
water, have been swept down with the escaping torrents.
The volcano of Agua in Guatemala received
its name from the disruption of a crater-lake at its
summit by an earthquake in 1540, whereby a vast and
destructive debacle of water was discharged down
the slopes of the mountain. In the beginning of the
year 1817, an eruption took place at the large crater
of Idjèn, one of the volcanoes of Java, whereby a
steaming lake of hot acid water was discharged with
frightful destruction down the slopes of the mountain.
After the explosion, the basin filled again with
water, but its temperature was no longer high.
The term lava is applied generally to all the molten
rocks of volcanoes. The use of the word in this
broad sense is of great convenience in geological descriptions,
by directing attention to the leading character
of the rocks as molten products of volcanic action,
and obviating the confusion and errors which
are apt to arise from an ill-defined or incorrect
lithological terminology.
While still flowing or not yet cooled, lavas differ
from each other in the extent to which they are impregnated
with gases and vapors. Some appear to be
saturated, others contain a much smaller gaseous impregnation;
and hence arise important distinctions in
their behavior. After solidification, lavas present
some noticeable characters, then easily ascertainable.
(1) Their average specific gravity may be taken as
ranging between 2.37 and 3.22. (2) The heavier
varieties contain much magnetic or titaniferous iron,
with augite and olivine, their composition being
basic, and their proportion of silica averaging about
45 to 55 per cent. (3) Lavas differ much in structure
and texture. (4) Lavas vary greatly in color
and general external aspect. The heavy basic kinds
are usually dark gray, or almost black, though, on
exposure to the weather, they acquire a brown tint
from the oxidation and hydration of their iron.
Their surface is commonly rough and ragged, until
it has been sufficiently decomposed by the atmosphere
to crumble into soil which, under favorable circumstances,
supports a luxuriant vegetation. The less
dense lavas, such as phonolites and trachytes, are frequently
paler in color, sometimes yellow or buff, and
decompose into light soils; but the obsidians present
rugged black sheets of rock, roughened with ridges
and heaps of gray froth-like pumice. Some of the
most brilliant surfaces of color in any rock-scenery
on the globe are to be found among volcanic rocks.
The walls of active craters glow with endless hues of
red and yellow. The Grand Cañon of the Yellowstone
River has been dug out of the most marvelously
tinted lavas and tuffs.
Volcanic action may be either constant or periodic.
Stromboli, in the Mediterranean, so far as we know,
has been uninterruptedly emitting hot stones and
steam, from a basin of molten lava, since the earliest
period of history. Among the Moluccas, the volcano
Sioa, and in the Friendly Islands, that of Tofua, have
never ceased to be in eruption since their first discovery.
The lofty cone of Sangay, among the Andes of
Quito, is always giving off hot vapors; Cotopaxi, too,
is ever constantly active. But, though examples of
unceasing action may thus be cited from widely different
quarters of the globe, they are nevertheless exceptional.
The general rule is that a volcano breaks
out from time to time with varying vigor, and after
longer or shorter intervals of quiescence.
It is usual to class volcanoes as active, dormant,
and extinct. This arrangement, however, often presents
considerable difficulty in its application. An
active volcano can not of course be mistaken, for even
when not in eruption, it shows by its discharge of
steam and hot vapors that it might break out into
activity at any moment. But in many cases it is impossible
to decide whether a volcano should be called
extinct or only dormant. The volcanoes of Silurian
age in Wales, of Carboniferous age in Ireland, of
Permian age in the Harz, of Miocene age in the
Hebrides, of younger Tertiary age in the Western
States and Territories of North America, are certainly
all extinct. But the older Tertiary volcanoes
of Iceland are still represented there by Skaptar-Jökull,
Hecla, and their neighbors. Somma, in the
First Century of the Christian era, would have been
naturally regarded as an extinct volcano. Its fires
had never been known to have been kindled; its vast
crater was a wilderness of wild vines and brushwood,
haunted, no doubt, by wolf and wild boar. Yet in a
few days, during the autumn of the year 79, the half
of the crater walls was blown out by a terrific series
of explosions, the present Vesuvius was then formed
within the limits of the earlier crater, and since that
time volcanic action has been intermittently exhibited
up to the present day. Some of the intervals of
quietude, however, have been so considerable that the
mountain might then again have been claimed as an
extinct volcano. Thus, in the 131 years between 1500
and 1631, so completely had eruptions ceased that the
crater had once more become choked with copse-wood.
A few pools and springs of very salt and hot
water remained as memorials of the former condition
of the mountain. But this period of quiescence
closed with the eruption of 1631—the most powerful
of all the known explosions of Vesuvius, except the
great one of 79.
In short, no essential distinction can be drawn between
dormant and extinct volcanoes. Volcanic action
is apt to show itself again and again, even at vast
intervals, within the same regions and over the same
sites. The dormant or waning condition of a volcano,
when only steam and various gases and sublimates
are given off, is sometimes called the Solfatara
phase, from the well-known dormant crater of that
name near Naples.
The interval between two eruptions of an active
volcano shows a gradual augmentation of energy.
The crater, emptied by the last discharge, has its floor
slowly upraised by the expansive force of the lava-column
underneath. Vapors rise in constant outflow,
accompanied sometimes by discharges of dust or
stones. Through rents in the crater-floor red-hot
lava may be seen only a few feet down. Where the
lava is maintained at or above its fusion-point and
possesses great liquidity, it may form boiling lakes,
as in the great crater of Kilauea, where acres of seething
lava may be watched throwing up fountains of
molten rock, surging against the walls and re-fusing
large masses that fall into the burning flood. The
lava-column inside the pipe of a volcano is all this
time gradually rising, until some weak part of the
wall allows it to escape, or until the pressure of the
accumulated vapors becomes great enough to burst
through the hardened crust of the crater-floor and
give rise to the phenomena of an eruption.
Kluge has sought to trace a connection between the
years of maximum and minimum sun-spots and those
of greatest and feeblest volcanic activity, and has
constructed lists to show that years which have been
specially characterized by terrestrial eruptions have
coincided with those marked by few sun-spots and
diminished magnetic disturbance. Such a connection
can not be regarded as having yet been satisfactorily
established. Again, the same author has called attention
to the frequency and vigor of volcanic explosions
at or near the time of the August meteoric
shower. But in this case, likewise, the cited examples
can hardly yet be looked upon as more than coincidences.
At many volcanic vents the eruptive energy manifests
itself with more or less regularity. At Stromboli,
which is constantly in an active state, the explosions
occur at intervals varying from three or four
to ten minutes and upward. A similar rhythmical
movement has been often observed during the eruptions
at other vents which are not constantly active.
Volcano, for example, during its eruption of September,
1873, displayed a succession of explosions
which followed each other at intervals of from
twenty to thirty minutes. At Etna and Vesuvius a
similar rhythmical series of convulsive efforts has
often been observed during the course of an eruption.
Among the volcanoes of the Andes a periodic
discharge of steam has been observed; Mr. Whymper
noticed outrushes of steam to proceed at intervals
of from twenty to thirty minutes from the summit
of Sangai, while during his inspection of the great
crater of Cotopaxi, this volcano was seen to blow off
steam at intervals of about half an hour. At the eruption
of the Japanese volcano, Oshima, in 1877, Mr.
Milne observed that the explosions occurred nearly
every two seconds, with occasional pauses of 15 or 20
seconds. Kilauea, in Hawaii, seems to show a regular
system of grand eruptive periods. Dana has
pointed out that outbreaks of lava have taken place
from that volcano at intervals of from eight to nine
years, this being the time required to fill the crater
up to the point of outbreak, or to a depth of 400 or
500 feet.
The approach of an eruption is not always indicated
by any premonitory symptoms, for many tremendous
explosions are recorded to have taken place
in different parts of the world without perceptible
warning. Much in this respect would appear to depend
upon the condition of liquidity of the lava, and
the amount of resistance offered by it to the passage
of the escaping vapors through its mass. In Hawaii,
where the lavas are remarkably liquid, vast outpourings
of them have taken place quietly without earthquakes
during the present century. But even there
the great eruption of 1868 was accompanied by violent
earthquakes.
The eruptions of Vesuvius are often preceded by
failure or diminution of wells and springs. But more
frequent indications of an approaching outburst are
conveyed by sympathetic movements of the ground.
Subterranean rumblings and groanings are heard;
slight tremors succeed, increasing in frequency and
violence till they become distinct earthquake shocks.
The vapors from the crater grow more abundant as
the lava-column in the pipe or funnel of the volcano
ascends, forced upward and kept in perpetual agitation
by the passage of elastic vapors through its mass.
After a long previous interval of quiescence, there
may be much solidified lava toward the top of the
funnel, which will restrain the ascent of the still
molten portion underneath. A vast pressure is thus
exercised on the sides of the cone, which, if too weak
to resist, will open in one or more rents, and the
liquid lava will issue from the outer slope of the
mountain; or the energies of the volcano will be directed
toward clearing the obstruction in the chief
throat, until with tremendous explosions, and the rise
of a vast cloud of dust and fragments, the bottom and
sides of the crater are finally blown out, and the top
of the cone disappears. The lava may now escape
from the lowest part of the lip of the crater, while,
at the same time, immense numbers of red-hot bombs,
scoriæ, and stones are shot up into the air. The lava
at first rushes down like one or more rivers of melted
iron, but, as it cools, its rate of motion lessens. Clouds
of steam rise from its surface, as well as from the
central crater. Indeed, every successive paroxysmal
convulsion of the mountain is marked, even at a distance,
by the rise of huge ball-like wreaths or clouds
of steam, mixed with dust and stones, forming a column
which towers sometimes a couple of miles or
more above the summit of the cone. By degrees these
eructations diminish in frequency and intensity. The
lava ceases to issue, the showers of stones and dust decrease,
and after a time, which may vary from hours
to days or months, even in the régime of the same
mountain, the volcano becomes once more tranquil.
The convulsions which culminate in the formation
of a volcano usually split open the terrestrial crust by
a more or less nearly rectilinear fissure, or by a system
of fissures. In the subsequent progress of the mountain,
the ground at and around the focus of action is
liable to be again and again rent open by other fissures.
These tend to diverge from the focus; but
around the vent where the rocks have been most exposed
to concussion, the fissures sometimes intersect
each other in all directions. In the great eruption of
Etna, in the year 1669, a series of six parallel fissures
opened on the side of the mountain. One of these,
with a width of two yards, ran for a distance of 12
miles, in a somewhat winding course, to within a mile
of the top of the cone.
In the deeper portions of a volcanic vent the convulsive
efforts of the lava-column to force its way
upward must often produce lateral as well as vertical
rifts, and into these the molten material will rush,
exerting as it goes an enormous upward pressure on
the mass of rock overlying it. At a modern volcano
these subterranean manifestations can not be seen,
but among the volcanoes of Tertiary and older times
they have been revealed by the progress of denudation.
Though lava very commonly issues from the lateral
fissures on a volcanic cone, it may sometimes approach
the surface in them without actually flowing
out. The great fissure on Etna in 1669, for example,
was visible even from a distance, by the long line of
vivid light which rose from the incandescent lava
within. Again, it frequently happens that minor
volcanic cones are thrown up on the line of a fissure,
either from the congelation of the lava round the
point of emission, or from the accumulation of
ejected scoriæ round the fissure-vent. One of the
most remarkable examples of this kind is that of the
Laki fissure in Iceland, the whole length of which
(12 miles) bristles with small cones and craters
almost touching each other.
Apart from the appearance of visible fissures, volcanic
energy may be, as it were, concentrated on a
given point, which will usually be the weakest in the
structure of that part of the terrestrial crust, and
from which the solid rock, shattered into pieces, is
hurled into the air by the enormous expansive energy
of the volcanic vapors. The history of the cone of
Vesuvius brings before us a long series of such explosions,
beginning with that of A. D. 79, and coming
down to the present day. Even now, in spite
of all the lava and ashes poured out during the
last eighteen centuries, it is easy to see how stupendous
must have been that earliest explosion by which
the southern half of the ancient crater was blown out.
At every successive important eruption, a similar but
minor operation takes place within the present cone.
The hardened cake of lava forming the floor is burst
open, and with it there usually disappears much of
the upper part of the cone, and sometimes, as in 1872,
a large segment of the crater-wall. The islands of
Santorin bring before us evidence of a prehistoric
catastrophe of a similar nature, by which a large volcanic
cone was blown up. The existing outer islands
are a chain of fragments of the periphery of the
cone, the centre of which is now occupied by the sea.
In the year 1538 a new volcano, Monte Nuovo, was
formed in twenty-four hours on the margin of the
Bay of Naples. An opening was drilled by successive
explosions, and such quantities of stones, scoriæ,
and ashes were thrown out from it as to form a hill
that rose 440 English feet above the sea-level, and
was more than a mile and a half in circumference.
A communication having been opened, either by
fissuring or explosion, between the heated interior
and the surface, fragmentary materials are commonly
ejected from it, consisting at first mainly of
the rocks through which the orifice has been opened,
afterward of volcanic substances. In a great eruption,
vast numbers of red-hot stones are shot up into
the air, and fall back partly into the crater and
partly on the outer slopes of the cone. According
to Sir W. Hamilton, cinders were thrown by Vesuvius,
during the eruption of 1779, to a height of
10,000 feet. Instances are known where large stones,
ejected obliquely, have described huge parabolic
curves in the air, and fallen at a great distance.
Stones eight pounds in weight occur among the ashes
which buried Pompeii. The volcano of Antuco in
Chili is said to send stones flying to a distance of
thirty-six miles, Cotopaxi is reported to have hurled
a 200-ton block nine miles, and the Japanese volcano,
Asama, is said to have ejected many blocks of stone
measuring from 40 to more than 100 feet in diameter.
But in many great eruptions, besides a constant
shower of stones and scoriæ, a vast column of exceedingly
fine dust rises out of the crater, sometimes
to a height of several miles, and then spreads outward
like a sheet of cloud. The remarkable fineness
of this dust may be understood from the fact that
during great volcanic explosions no boxes, watches,
or close-fitting joints have been found to be able to
exclude it. Mr. Whymper collected some dust that
fell sixty-five miles away from Cotopaxi, and which
was so fine that from 4,000 to 25,000 particles were
required to weigh a grain. So dense is the dust-cloud
as to obscure the sun, and for days together the darkness
of night may reign for miles around the volcano.
The eruption of Cotopaxi, on 26th June, 1877,
began by an explosion that sent up a column of fine
ashes to a prodigious height into the air, where it
rapidly spread out and formed so dense a canopy as
to throw the region below it into total darkness. So
quickly did it diffuse itself, that in an hour and a
half a previously bright morning became at Quito,
thirty-three miles distant, a dim twilight, which in
the afternoon passed into such darkness that the hand
placed before the eye could not be seen. At Guayaquil,
on the coast, 150 miles distant, the shower of
ashes continued till the 1st of July. Dr. Wolf collected
the ashes daily, and estimated that at that place
there fell 315 kilogrammes on every square kilometre
during the first thirty hours, and on the 30th
of June, 209 kilogrammes in twelve hours.
One of the most stupendous outpourings of volcanic
ashes on record took place, after a quiescence
of twenty-six years, from the volcano Coseguina, in
Nicaragua, during the early part of the year 1835.
On that occasion, utter darkness prevailed over a
circle of thirty-five miles radius, the ashes falling so
thickly that, even eight leagues from the mountain,
they covered the ground to a depth of about ten
feet. It was estimated that the rain of dust and sand
fell over an area at least 270 geographical miles in
diameter. Some of the finer materials, thrown so
high as to come within the influence of an upper air-current,
were borne away eastward, and fell, four
days afterward, at Kingston, in Jamaica—a distance
of 700 miles. During the great eruption of Sumbawa,
in 1815, the dust and stones fell over an area of
nearly one million square miles, and were estimated
by Zollinger to amount to fully fifty cubic miles
of material, and by Junghuhn to be equal to one
hundred and eighty-five mountains like Vesuvius.
Toward the end of the Eighteenth Century, during a
time of great disturbance among the Japanese volcanoes,
one of them, Sakurajima, threw out so much
pumiceous material that it was possible to walk a distance
of twenty-three miles upon the floating débris
in the sea.
The varying degree of liquidity or viscosity of the
lava probably modifies the force of explosions, owing
to the different amounts of resistance offered to the
upward passage of the absorbed gases and vapors.
Thus explosions and accompanying scoriæ are abundant
at Vesuvius, where the lavas are comparatively
viscid; they are almost unknown at Kilauea, where
the lava is remarkably liquid.
In tranquil conditions of a volcano, the steam,
whether collecting into larger or smaller vesicles,
works its way upward through the substance of the
molten lava, and as the elasticity of this compressed
vapor overcomes the pressure of the overlying
lava, it escapes at the surface, and there the
lava is thus kept in ebullition. But this comparatively
quiet operation, which may be watched within
the craters of many active volcanoes, does not produce
clouds of fine dust. The collision or friction
of millions of stones ascending and descending in the
dark column above the crater must doubtless cause
much dust and sand. But the explosive action of
steam is probably also an immediate cause of much
trituration. The aqueous vapor or water-gas which
is so largely dissolved in many lavas must exist within
the lava-column, under an enormous pressure, at a
temperature far above its critical point, even at a
white heat, and therefore possibly in a state of dissociation.
The sudden ascent of lava so constituted
relieves the pressure rapidly without sensibly affecting
the temperature of the mass. Consequently, the
white-hot gases or vapors at length explode, and
reduce the molten mass to the finest powder, like
water shot out of a gun.
As every shower of dust and sand adds to the
height of the ground on which it falls, thick volcanic
accumulations may be formed far beyond the base
of the mountain. The volcano of Sangay, in Ecuador,
for instance, has buried the country around it
to a depth of 4,000 feet under its ashes. In such loose
deposits are entombed trees and other kinds of vegetation,
together with the bodies of animals, as well as
the works of man. In some cases, where the layer
of volcanic dust is thin, it may merely add to the
height of the soil, without sensibly interfering with
the vegetation. But it has been observed at Santorin
that though this is true in dry weather, the fall of
rain with the dust at once acts detrimentally. On the
3d of June, 1866, the vines were there withered up,
as if they had been burned, along the track of the
smoke cloud. By the gradual accumulation of volcanic
ashes, new geological formations arise which,
in their component materials, not only bear witness to
the volcanic eruptions that produced them, but preserve
a record of the land-surfaces over which they
spread. In the third place, besides the distance to
which the fragments may be hurled by volcanic explosions,
or to which they may be diffused by the
ordinary aerial movements, we have to take into
account the vast spaces across which the finer dust is
sometimes borne by upper air-currents. In the instance
already cited, ashes from Coseguina fell 700
miles away, having been carried all that long distance
by a high counter-current of air, moving apparently
at the rate of about seven miles an hour in
an opposite direction to that of the wind which blew
at the surface. By the Sumbawa eruption, also
referred to above, the sea west of Sumatra was covered
with a layer of ashes two feet thick. On several
occasions ashes from the Icelandic volcanoes have
fallen so thickly between the Orkney and Shetland
Islands, that vessels passing there have had the unwonted
deposit shoveled off their decks in the
morning. In the year 1783, during the memorable
eruption of Skaptar-Jökull, so vast an amount of fine
dust was ejected that the atmosphere over Iceland
continued loaded with it for months afterward. It
fell in such quantities over parts of Caithness—a
distance of 600 miles—as to destroy the crops; that
year is still spoken of by the inhabitants as the year
of “the ashie.” Traces of the same deposit have
been observed in Norway, and even as far as Holland.
Hence it is evident that volcanic accumulations
may take place in regions many hundreds of
miles distant from any active volcano. A single thin
layer of volcanic detritus in a group of sedimentary
strata would not thus of itself prove the existence of
contemporaneous volcanic action in its neighborhood.
At its exit from the side of a volcano, lava glows
with a white heat, and flows with a motion which
has been compared to that of honey or of melted iron.
It soon becomes red, and like a coal fallen from a
hot fireplace rapidly grows dull as it moves along,
until it assumes a black, cindery aspect. At the same
time the surface congeals, and soon becomes solid
enough to support a heavy block of stone. The
aspect of the stream varies with the composition and
fluidity of the lava, form of the ground, angle of
slope, and rapidity of flow. Viscous lavas, like those
of Vesuvius, break up along the surface into rough
brown or black cinder-like slags and irregular
ragged cakes, bristling with jagged points, which, in
their onward motion, grind and grate against each
other with a harsh, metallic sound, sometimes rising
into rugged mounds or becoming seamed with rents
and gashes, at the bottom of which the red-hot
glowing lava may be seen. In lavas possessing
somewhat greater fluidity, the surface presents froth-like,
curving lines, as in the scum of a slowly flowing
river, or is arranged in curious ropy folds, as the
layers have successively flowed over each other and
congealed. A large area which has been flooded
with lava is perhaps the most hideous and appalling
scene of desolation anywhere to be found on the
surface of the globe.
A lava-stream usually spreads out as it descends
from its point of escape, and moves more slowly.
Its sides look like huge embankments, or like some
of the long mounds of “clinkers” in a great manufacturing
district. The advancing end is often much
steeper, creeping onward like a great wall or rampart,
down the face of which the rough blocks of
hardened lava are ever rattling.
In a lofty volcano, lava occasionally rises to the
lip of the crater and flows out there; but more
frequently it escapes from some fissure or orifice in a
weak part of the cone. In minor volcanoes, on the
other hand, where the explosions are less violent,
and where the thickness of the cone in proportion to
the diameter of the funnel is often greater, the lava
very commonly rises into the crater. Should the
crater-walls be too weak to resist the pressure of the
molten mass, they give way, and the lava rushes out
from the breach. This is seen to have happened in
several of the puys of Auvergne. But if the crater be
massive enough to withstand the pressure, the lava
may at last flow out from the lowest part of the rim.
As soon as the molten rock reaches the surface, the
superheated water-vapor or gas dissolved within its
mass escapes copiously, and hangs as a dense white
cloud over the moving current. The lava-streams
of Vesuvius sometimes appear with as dense a steam-cloud
at their lower ends as that which escapes at
the same time from the main crater. Even after the
molten mass has flowed several miles, steam continues
to rise abundantly both from its end and from numerous
points along its surface, and continues to do
so for many weeks, months, or it may be for several
years.
Should the point of escape of a lava-stream lie
well down on the cone, far below the summit of the
lava-column in the funnel, the molten rock, on its
first escape, driven by hydrostatic pressure, will
sometimes spout up high into the air—a fountain of
molten rock. This was observed in 1794 on Vesuvius,
and in 1832 on Etna. In the eruption of 1852
at Mauna Loa, an unbroken fountain of lava, from
200 to 700 feet in height and 1,000 feet broad, burst
out at the base of the cone. Similar “geysers” of
molten rock have subsequently been noticed in the
same region. Thus in March and April, 1868, four
fiery fountains, throwing lava to heights varying
from 500 to 1,000 feet, continued to play for several
weeks. According to Mr. Coan, such outbursts take
place from the bottom of a column of lava 3,000 feet
high. The volcano of Mauna Loa strikingly illustrates
another feature of volcanic dynamics in the
position and outflow of lava. It bears upon its flanks
at a distance of 20 miles, but 10,000 feet lower, the
huge crater Kilauea. As Dana has pointed out, these
orifices form part of one mountain, yet the column
of lava stands 10,000 feet higher in one conduit than
in the other. On a far smaller scale the same independence
occurs among the several pipes of some of
the geysers in the Yellowstone region of North
America.
The rate of movement is regulated by the fluidity
of the lava, by its volume, and by the form and inclination
of the ground. Hence, as a rule, a lava-stream
moves faster at first than afterward, because
it has not had time to stiffen, and its slope of descent
is usually steeper than further down the mountain.
One of the most fluid and swiftly flowing lava-streams
ever observed on Vesuvius was that erupted
on 12th August, 1805. It is said to have rushed down
a space of 3 Italian (3⅔ English) miles in the
first four minutes, but to have widened out and
moved more slowly as it descended, yet finally to
have reached Torre del Greco in three hours. A
lava erupted by Mauna Loa in 1852 went as fast as
an ordinary stage-coach, or fifteen miles in two
hours; but some of the lavas from that mountain
have in parts of their course moved with double that
rapidity.
In some cases, lava escaping from craters or fissures
comes to rest before reaching the base of the
slopes, like the obsidian current which has congealed
on the side of the little volcanic island of Volcano.
In other instances, the molten rock not only reaches
the plains, but flows for many miles away from the
point of eruption. Sartorius von Waltershausen computed
the lava emitted by Etna in 1865 at 92 millions
of cubic metres, that of 1852 at 420 millions, that of
1669 at 980 millions, and that of a prehistoric lava-stream
near Randazzo at more than 1,000 millions.
The most stupendous outpouring of lava on record
was that which took place in Iceland in the year
1783. Successive streams issued from a fissure about
12 miles long, filling up river gorges which were
sometimes 600 feet deep and 200 feet broad, and advancing
into the alluvial plains in lakes of molten
rock 12 to 15 miles wide and 100 feet deep. Two
currents of lava which, filling up the valley of the
Skapta, escaped in nearly opposite directions, extended
for 45 and 50 miles respectively, their usual
thickness being 100 feet. Bischof estimated that the
total amount of lava poured forth during this single
eruption “surpassed in magnitude the bulk of Mont
Blanc.”
The varying degrees of liquidity are manifested in
a characteristic way on the surface of lava. Thus,
in the great lava-pools of Hawaii, the rock exhibits
a remarkable liquidity, throwing up fountains of
molten rock to a height of 300 feet or more. During
its ebullition in the crater-pools, jets and driblets
a quarter of an inch in diameter are tossed up, and,
falling back on one another, make “a column of
hardened tears of lava,” one of which was found
to have attained a height of 40 feet, while in other
places the jets thrown up and blown aside by the
wind give rise to long threads of glass which lie
thickly together like mown grass, and are known
by the natives under the name of “Pele’s Hair,” after
one of their divinities. Yet, although the ebullition is
caused by the uprise and escape of highly heated
vapors, there is no cloud over the boiling lake itself,
heavy white vapor only escaping at different points
along the edge.
EARTHQUAKES — William Hughes
It appears, from the accurate records of such phenomena
which have been kept within recent periods,
that earthquakes are of much more frequent
occurrence than is commonly supposed. Upward of
three thousand earthquakes are recorded as having
occurred within the first half of the Nineteenth Century—an
average of more than one for every week
throughout the entire period. But not more than one
in forty is of considerable importance, by far the
greater number consisting of such slight shocks as
are occasionally experienced in Great Britain and
other countries favored with a like immunity in this
regard. An important earthquake, however, in some
part of the world or other, appears, from the above
average, to occur once in every eight months. In Europe
alone, where a more complete record of such
occurrences is obtainable than in other parts of the
world, as many as 320 distinct earthquakes are recorded
to have occurred within a period of ten years
(1833-42)—an average of thirty-two annually, and
of one such shock for every ten days throughout the
period.
[The geographical area within which shocks of earthquakes
are experienced is a widely spread one, and does not appear to
undergo any material change (if, indeed, any change whatever)
as to its limits. At any rate, the regions in which violent
earthquakes are recorded to have occurred in former times are
those in which such disturbances are of most frequent recurrence
at the present day. One of the most striking evidences
in favor of the supposition that the volcanic eruption is due to
the same deeply seated cause which produces the shock of the
earthquake, is afforded by the fact, that all the volcanoes which
have been in eruption within the modern period of geology
are found within regions liable to earthquakes, and, for the
most part, to violent shocks.]
Regarding the earthquake and the volcanic eruption
as the manifestation, under different conditions,
of the earth’s internal fires, we readily mark out upon
the globe the great regions of geographical distribution
in the case of such phenomena. The most
widely extended of these coincides with the circuit
of the Pacific Ocean. Along the entire western coast
of the New World, from Tierra del Fuego to the
peninsula of Alaska and the neighborhood of the
Aleutian Islands, shocks of earthquakes are known
to occur; and, within a large portion of the space,
vents of active eruption are found. The subterranean
igneous force is, indeed, much more powerfully displayed
in the southern than in the northern half of
the American continent, and the active volcanoes that
occur within the limits referred to are nearly all
found amid the cordilleras of the Andes, or upon the
plateaus of the Mexican isthmus. One of the Mexican
volcanoes—Jorullo—is especially deserving of
notice, from the circumstance of its having first risen
above the surrounding plain by the accumulation of
volcanic matter during an eruption in the year 1759.
The Aleutian Islands connect the volcanic region
of the eastern Pacific with that which extends along
its western shores. In the latter case, however, it is
upon the peninsular regions, or in the chains of islands
that adjoin the mainland, that the igneous force
is displayed. Kamtchatka, the Kurile Islands, Yesso,
the Japanese group, and the entire region of the
Malay Archipelago, exhibit the presence of igneous
force below the ground. Seven active volcanoes occur
in Kamtchatka. The Japanese Archipelago is
said to contain at least twenty-seven active volcanoes,
eight of them upon Yesso and the adjacent islets. Between
Japan and the Loo-choo group is Sulphur Island,
an insular volcano, from which smoke is constantly
emitted.
The Philippine Islands, in which earthquakes are
of frequent occurrence, prolong the volcanic chain
to the southward. Thence it is traced, at intervals,
along the northern shores of New Guinea, and
through the prolonged chains of the Solomon Islands,
and the New Hebrides, to the North Island
of New Zealand. Slight shocks of earthquake have
also been experienced within the southern and eastwardly
portions of the Australian mainland.
The numerous volcanoes of the Malay Archipelago,
the whole area of which is liable to frequent
earthquake shocks, often of the most destructive violence,
belong to the eastern portion of this region, and
display the agency of subterranean heat on the grandest
scale. The island of Java alone contains forty-three
active volcanoes, ranging in a linear direction
throughout its length. The volcanic chain of Java
is prolonged to the eastward through the Lesser
Sunda Islands (Sumbawa, etc.), in which direction
it is united with that which borders the Pacific
waters. There are active volcanoes on an island in
the Gulf of Siam, besides the well-known crater of
Barren Island, in the Bay of Bengal. The region adjoining
the last-named body of water, together with
the whole of northern India, is of frequent liability
to earthquakes, some of them (as that of Cutch, in
1819) of the most destructive violence. The volcanic
island of Mayotta (Comoro group), the active Piton
of Réunion or Bourbon Island, and the hot springs
and extinct craters of St. Paul and Amsterdam Islands,
in a high southern latitude of the Indian Ocean,
constitute points which indicate, at distant intervals,
the continuity of the volcanic chain.
The southwestern portion of Asia, the southern
shores of Europe, and the northwestwardly portion
of the African mainland, fall within this region on
the one side, as the islands of the West Indies do
upon the other. The entire breadth of the Atlantic
Ocean, as well as the circuit of the Mediterranean, is
thus included within its limits. To the northward,
the numerous volcanoes of Iceland, and the more distant
cone of Jan Mayen Island, lying within the Arctic
circle, must be regarded as within its area; together
with, in an opposite direction, the still-burning
peak of the Cameroon Mountains, adjoining the
upper extremity of the Gulf of Guinea. The volcanic
peaks found within the widely detached groups
of the Azores and the Cape Verde Islands, with Tenerife,
in the Canary group, are among its outlying
members.
Throughout the wide region thus indicated, earthquakes
are of frequent occurrence. There are fewer
active vents of eruption than in the case of the Pacific
circuit. But the cones of Etna and Vesuvius, with
the island of Santorin, in the Mediterranean, and the
numerous volcanoes of Iceland, attest the destructive
violence of the subterranean fires. Western Asia,
from the Caspian to the shores of the Archipelago
(including Armenia, Syria, and the Lesser Asia),
Greece, southern Italy, the Spanish peninsula, and
the region of Mount Atlas, in Northwestern Africa,
are all liable to the frequent repetition of such convulsions.
The only portion of the Mediterranean
coasts exempt from such disturbing phenomena is on
its southern shores, embracing that part of the North
African coast which stretches from the Lesser Syrtis
to the valley of the Nile. We have no record of the
experience of any shocks of earthquake in Egypt.
Had it been otherwise, perhaps the pyramids of that
land of wonders might have proved less enduring
monuments of the past.
The movement imparted to the ground during an
earthquake may be either horizontal or vertical. In
the former case, the phenomenon consists in an undulating,
wave-like movement; in the latter, in an upheaval
or subsidence of land. The vertical shock
affects most the relative levels of adjacent objects,
and produces the most striking permanent changes in
the natural aspect of the region in which it is experienced.
But the undulatory movement is attended
by more serious consequences to man, since it at once
shakes the foundations of the strongest edifices, and
may overthrow in the space of a few seconds the accumulated
labors of prior ages. Whole tracts of
land, with their cities or villages, may be elevated
or depressed with comparatively little injury to life;
but nothing can withstand the force of a motion
which rocks the solid strata of the earth itself.
The most solidly constructed buildings are not proof
against the earthquake any more than the weakest.
Indeed, it has in many instances been observed that
those erections which displayed the strongest masonry
have suffered more from the effects of an earthquake
than buildings of slighter structure. The
cracking of walls, the falling-in of roofs, and the
crash of tumbling houses on every side, burying their
inmates beneath the ruins, are among the characteristics
of the earthquake in its most violent and frightful
form.
It has been asserted that a third kind of movement—viz.,
in a rotatory direction—sometimes occurs,
and certain phenomena by which earthquakes have
been attended have favored this belief. Thus, isolated
columns or statues have been found, after such
an occurrence, to face a different quarter from that
which they previously did. This, however, would be
sufficiently accounted for by a vibratory movement,
acting upon a column which was unequally attached
to its base; i.e., the fastening of which was of unequal
strength relatively to the central point of junction.
During the Chilian earthquake of 1835, vessels
moored alongside of one another in the harbor
of Concepcion were afterward found with their cables
twisted together.
The duration of any single earthquake shock is
seldom more than a few seconds, though the terror
which it inspires naturally tends to make it seem of
longer continuance; but in the case of the more violent
movements, even a few moments serve to destroy
the work of ages. In the Chilian earthquake of 1835,
the great shock which destroyed the city of Concepcion
was preceded by several tremulous movements
of minor intensity. During the first half-minute,
many persons remained in their houses; but the convulsive
motion of the earth then became so strong
that all rushed into the open streets for safety. The
horrid motion (writes an eye-witness of the scene)
increased; people could hardly stand; buildings
waved and tottered; suddenly an awful and overpowering
shock caused universal destruction. In less
than six seconds the city was in ruins!
The earthquake is propagated to enormous distances
from the region in which the shock originates,
the rate at which the motion travels varying not
merely with the violence of the originating impulse,
but also with the nature of the formations through
which it passes. Rocks of solid and homogeneous
texture, as granite, favor the transmission of the
shock; while formations of loose texture, such as
sand, most retard its speed. The well-known Lisbon
earthquake of 1755, by which sixty thousand persons
are said to have perished within the brief space of six
minutes, was felt in the British Islands, as well as
upon the coast of Barbary, and even among the islands
of the West Indies, on the opposite side of the
Atlantic.
MOUNTAINS — A. Keith Johnston
The number and altitude of the mountains of
the globe are so great that they form almost
everywhere prominent objects, and operate to a large
extent in modifying the climatic conditions of every
country in the world. Yet the amount of solid material
so raised above the ordinary level of the land is
not so much as might be expected. Remembering
that elevated plateaus of great extent occur in several
regions, and that the general surface of the earth is
considerably higher than the sea-level, it has been
estimated that were the whole dry land reduced to
a uniform level, it would form a plain having an
elevation of 1,800 feet above the sea. And were these
solid materials scattered over the whole surface of
the globe, so as to fill up the bed of the ocean, the
resulting level would be considerably below the present
surface of the sea, inasmuch as the main height
of the dry land most probably does not exceed one-fifteenth
of the mean depth of the bed of the ocean.
Mountains, and especially mountain-chains, subserve
important uses in the economy of nature, especially
in connection with the water system of the
world. They are at once the great collectors and
distributers of water. In the passage of moisture-charged
winds across them the moisture is precipitated
as rain or snow. When mountain ranges intersect
the course of constant winds by thus abstracting
the moisture, they produce a moist country on
the windward side, and a comparatively dry and arid
one on the leeward. This is exemplified in the Andes,
the precipitous western surface of which has a different
aspect from the sloping eastern plains; and so
also the greater supply of moisture on the southern
sides of the Himalayas brings the snow-line 5,000
feet lower than on the northern side.
Above a certain height the moisture falls as snow,
and a range of snow-clad summits would form a
more effectual separation between the plains on
either side than would the widest ocean, were it not
that transverse valleys are of frequent occurrence,
which open up a pass, or way of transit, at a level
below the snow-line. But even these would not prevent
the range being an impassable barrier, if the
temperate regions contained as lofty mountains as
the tropics. Mountain ranges, however, decrease in
height from the equator to the poles in relation to
the snow-line.
The numerous attempts that have been made to
generalize on the distribution of mountains on the
globe have hitherto been almost unsuccessful. In
America, the mountains take a general direction
more or less parallel to the meridian, and for a distance
of 8,280 miles, from Patagonia to the Arctic
Ocean, form a vast and precipitous range of lofty
mountains, which follow the coast-line in South
America, and spread somewhat out in North America,
presenting everywhere throughout their course
a tendency to separate into two or more parallel
ridges, and giving to the whole continent the character
of a precipitous and lofty western border,
gradually lowering into an immense expanse of
eastern lowlands. In the Old World, on the other
hand, there is no single well-defined continuous chain
connected with the coast-line. The principal ranges
are grouped together in a Y-shaped form, the general
direction of which is at right angles to the New
World chain. The centre of the system in the Himalayas
is the highest land in the hemisphere. From
this, one arm radiates in a northeast direction, and
terminates in the high land at Behring Strait: the
other two take a westerly course; the one a little to
the north, through the Caucasus, Carpathians, and
Alps, to the Pyrenees; the other more to the south,
through the immense chain of Central African
mountains, and terminating at Sierra Leone. Most
of the principal secondary ranges have generally a
direction more or less at right angles to this great
mountain tract.
The inquiry into the origin of mountains is one
that has received not a little attention. Geologists
have shown that the principal agents in altering the
surface of the globe are denudation, which is always
abrading and carrying to a lower level the exposed
surfaces, and an internal force which is rising or
depressing the existing strata, or bringing unstratified
rocks to the surface. Whether the changes are the
small and almost imperceptible alterations now taking
place, or those recorded in the mighty mountains
and deep valleys everywhere existing, denudation
and internal force are the great producing causes.
These give us two great classes of mountains.
The extent to which denudation has altered the surface
of the globe can scarcely be imagined. All the
stratified rocks are produced by its action; but these
do not measure its full amount, for many of these
beds have been deposited and denuded, not once or
twice, but repeatedly, before they reach their present
state. Masses of rock more indurated, or better defended
from the wasting currents than those around,
serve as indices of the extent of denudation. The
most remarkable case of this kind with which we
are acquainted is that of the three insulated mountains
in Ross-shire—Suil Veinn, Coul Beg, and Coul
More—which are about 3,000 feet high. The
strata of the mountains are horizontal, like the
courses of masonry in a pyramid, and their deep red
color is in striking contrast with the cold bluish hue
of the gneiss which forms the plain, and on whose
upturned edges the mountain-beds rest. It seems
very probable, as Hugh Miller suggests, that when
the formation of which these are relics (at one time
considered as Old Red Sandstone, but now determined
by Sir Roderick Murchison as being older
than Silurian) was first raised above the waves, it
covered with an amazing thickness the whole surface
of the Highlands of Scotland, from Ben Lomond to
the Maiden Paps of Caithness, but that subsequent
denudation swept it all away, except in circumscribed
districts, and in detached localities like these
pyramidal hills.
Mountains produced by internal force are of
several kinds. (a) Mountains of ejection, in
which the internal force is confined to a point,
so to speak, having the means of exhausting itself
through an opening in the surface. The lava,
scoriæ, and stones ejected at this opening form a
conical projection which, at least on the surface, is
composed of strata sloping away from the crater.
Volcanoes are mostly isolated conical hills, yet they
chiefly occur in a somewhat tortuous linear series,
on the mainland and islands which inclose the great
Pacific Ocean. Vesuvius and the other European
volcanoes are unconnected with this immense volcanic
tract. (b) But the internal force may be
diffused under a large tract or zone, which, if it obtain
no relief from an opening, will be elevated in
the mass. When the upheaval occurs to any extent,
the strata are subjected to great tension. If they can
bear it, a soft rounded mountain-chain is the result;
but generally one or more series of cracks are formed,
and into them igneous rocks are pushed, which,
rising up into mountain-chains, elevate the stratified
rocks on their flanks, and perhaps as parallel ridges.
Thus, the Andes consist of the stratified rocks of
various ages, lying in order on the granite and
porphyry of which the mass of the range is composed.
The position of the strata on such mountains
supplies the means of determining, within definite
limits, the period of upheaval. The newest strata
that have been elevated on the sides of the mountain
when it was formed, give a date antecedent to that
at which the elevation took place, while the horizontal
strata at the base of the mountains supply one
subsequent to that event. Thus, the principal chain
of the Alps was raised during the period between
the deposition of the Tertiary and that of the older
recent deposits. (c) But there is yet another way in
which the upheaving internal force operates, viz.,
where it does not act at right angles to the surface,
but rather obliquely, and, as it were, pushes the solid
strata forward, causing them to rise in huge folds,
which, becoming permanent, form parallel ranges
of mountains.
The crust of the earth, in its present solid and brittle
condition, is thus curved, in a greater or less degree,
by the shock of every earthquake; it is well
known that the trembling of the earth is produced by
the progress of a wave of the solid crust; that the
destruction of buildings is caused by the undulation;
and that the wave has been so evident that it has been
described as producing a sickening feeling on the observer,
as if the land were but thin ice heaving over
water. The Appalachians were thus formed. Many
other ranges have had a similar origin, as some in
Belgium and in the Southern Highlands of Scotland,
as has been suggested by Mr. Carruthers.
It is evident that in the last two classes the parallel
ridges were produced at the same time. Elie de
Beaumont generalized this, maintaining that all
parallel ridges or fissures are synchronous; and
on this he based a system of mountain structure
which is too universal and too geometrical to be
true. The synchronism of parallel fissures had
been noticed by Werner, and it is now received
as a first principle in mining. The converse
is also held to be generally true, that fissures
differing in direction differ also in age; yet divergence
from a centre, and consequent want of
parallelism, as in the case of volcanoes, may be an
essential characteristic of contemporaneity. Nevertheless,
Elie de Beaumont classified the mountains
of the world according to this parallelism, holding
that the various groups are synchronous. The parallelism
does not consist in having the same relations
to the points of the compass—for these, as regards
north and south, would be far from parallel—but is
estimated in its relation to some imaginary great
circle, which being drawn round the globe would
divide it into equal hemispheres. Such circles he
calls Great Circles of Reference. But beyond this,
he went a step further, and proposed a more refined
classification, depending on a principle of geometrical
symmetry, which he believed he had discovered
among his great circles of reference. It is to be
feared, however, that his geometrical speculations
have little foundation in nature.
LAKES—FRESH, SALT, AND BITTER — Sir Archibald Geikie
Depressions filled with water on the surface
of the land, and known as Lakes, occur abundantly
in the northern parts of both hemispheres, and
more sparingly, but often of large size, in warmer
latitudes. For the most part, they do not belong to
the normal system of erosion in which running water
is the prime agent, and to which the excavation of
valleys and ravines must be attributed. On the contrary,
they are exceptional to that system, for the
constant tendency of running water is to fill them up.
Their origin, therefore, must be sought among some
of the other geological processes.
Lakes are conveniently classed as fresh or salt.
Those which possess an outlet contain in almost all
cases fresh water; those which have none are usually
salt.
In the northern parts of Europe and America, as
first emphasized by Sir Andrew C. Ramsay, lakes are
prodigiously abundant on ice-worn rock-surfaces, irrespective
of dominant lines of drainage. They seem
to be distributed as it were at random, being found
now on the summits of ridges, now on the sides of
hills, and now over broad plains. They lie for the
most part in rock-basins, but many of them have barriers
of detritus. In the mountainous regions of
temperate and polar latitudes, lakes abound in valleys,
and are connected with main drainage-lines.
In North America and in Equatorial Africa, vast
sheets of fresh water occur in depressions of the
land, and are rather inland seas than lakes.
The water of many lakes has been observed to rise
above its normal level for a few minutes or for more
than an hour, then to descend beneath that level, and
to continue this vibration for some time. In the Lake
of Geneva, where these movements, locally known
there as Seiches, have long been noticed, the amplitude
of the oscillation ranges up to a metre or even
sometimes to two metres. These disturbances may
sometimes be due to subterranean movements; but
probably they are mainly the effect of atmospheric
perturbations, and, in particular, of local storms with
a vertical descending movement.
Among the geological functions discharged by
lakes the following may be noticed:
1st. Lakes equalize the temperature of the localities
in which they lie, preventing it from falling as
much in winter and rising as much in summer as it
would otherwise do.[1] The mean annual temperature
of the surface water at the outflow of the Lake of Geneva
is nearly 4° warmer than that of the air.
2d. Lakes regulate the drainage of the area below
their outfall, thereby preventing or lessening the destructive
effects of floods.
3d. Lakes filter river-water and permit the undisturbed
accumulation of new deposits, which in some
modern cases may cover thousands of square miles
of surface, and may attain a thickness of nearly 3,000
feet (Lake Superior has an area of 32,000 square
miles; Lago Maggiore is 2,800 feet deep). How
thoroughly lakes can filter river-water is typically
displayed by the contrast between the muddy river
which flows in at the head of the Lake of Geneva,
and the “blue rushing of the arrowy Rhone,” which
escapes at the foot. The mouths of small brooks entering
lakes afford excellent materials for studying
the behavior of silt-bearing streams when they reach
still water. Each rivulet may be observed pushing
forward its delta composed of successive sloping
layers of sediment. On a shelving bank, the coarser
detritus may repose directly upon the solid rock of
the district. But as it advances into the lake, it may
come to rest upon some older lacustrine deposit. The
river Linth since 1860 has annually discharged into
Lake Wallenstadt some 62,000 cubic metres of detritus.
A river which flows through a succession of lakes
can not carry much sediment to the sea, unless it has
a long course to run after it has passed the lowest
lake, and receives one or more muddy tributaries.
Let us suppose, for example, that, in a hilly region,
a stream passes through a series of lakes. As the
highest lake will intercept much, perhaps all, of this
sediment, the next in succession will receive little or
none until the first is either filled up or has been
drained by the cutting of a gorge through the intervening
rock. The same process will be repeated
until the lakes are effaced, and their places are taken
by alluvial meadows. Examples of this sequence
of events are of frequent occurrence in Britain.
Besides the detrital accumulations due to the influx
of streams, there are some which may properly be
regarded as the work of lakes themselves. Even on
small sheets of water, the eroding influence of wind-waves
may be observed; but on large lakes the wind
throws the water into waves which almost rival those
of the ocean in size and destructive power. Beaches,
sand-dunes, shore-cliffs, and other familiar features
of the meeting-line between land and sea, reappear
along the margins of such great fresh-water seas as
Lake Superior. Beneath the level of the water a terrace
or platform is formed, of which the distance
from shore and depth vary with the energy of the
waves by which it is produced. This platform is well
developed in the Lake of Geneva.
Some of the distinctive features of the erosion and
deposition that take place in lake-basins have been
admirably laid open for study in those basins of vanished
lakes which have been so well described by
Gilbert, Dutton, Russell, and Upham in the Western
Territories of the United States. They have been
treated of in a masterly way by Gilbert in his essay
on The Topographic Features of Lake-Shores.
4th. Lakes serve as basins in which chemical deposits
may take place. Of these the most interesting
and extensive are those of iron-ore, which chiefly
occur in northern latitudes.
5th. Lakes furnish an abode for a lacustrine fauna
and flora, receive the remains of the plants and animals
washed down from the surrounding country,
and entomb these organisms in the growing deposits,
so as to preserve a record of the lacustrine and terrestrial
life of the period during which they continue.
Besides the more familiar pond-snails and
fishes, lakes possess a peculiar pelagic fauna, consisting
in large measure of entomostracous crustaceans,
distinguished more especially by their transparency.
These, as well as the organisms of shallower water,
doubtless furnish calcareous materials for the mud or
marl of the lake bottoms. But it is as receptacles of
sediment from the land, and as localities for the preservation
of a portion of the terrestrial fauna and
flora, that lakes present their chief interest to a geologist.
Their deposits consist of alternations of sand,
silt, mud, gravel, and occasional irregular seams of
vegetable matter, together with layers of calcareous
marl formed of lacustrine shells, Entomostraca, etc.
In lakes receiving much sediment, little or no marl
can accumulate during the time when sediment is
being deposited. In small, clear, and not very deep
lakes, on the other hand, where there is little sediment,
or where it only comes occasionally at intervals
of flood, thick beds of white marl, formed entirely
of organic remains, may gather on the bottom, as
has happened in numerous districts of Scotland
and Ireland. The fresh-water limestones and clays
of some old lake-basins (those of Miocene time in
Auvergne and Switzerland, and of Eocene age in
Wyoming, for example) cover areas occasionally
hundreds of square miles in extent, and attain a thickness
of hundreds, sometimes even thousands, of feet.
Existing lakes are of geologically recent origin.
Their disappearance is continually in progress by
infilling and erosion. Besides the displacement of
their water by alluvial accumulations, they are lowered
and eventually drained by the cutting down of
the barrier at their outlets. Where they are effaced
merely by erosion, it must be an excessively slow
process, owing to the filtered character of the water;
but where it is performed by the retrocession of a
waterfall at the head of an advancing gorge, it may
be relatively rapid after it has once begun. In a
river-course it is usual to find a lake-like expansion
of alluvial land above each gorge. These plains
may be regarded as old lake-bottoms, which have
been drained by the cutting out of the ravines. Successive
terraces often fringe a lake and mark former
levels of its waters. It is when we reflect upon the
continued operation of the agencies which tend to
efface them, that we can best realize why the lakes
now extant must necessarily be of comparatively
modern date.
Saline lakes, considered chemically, may be
grouped as salt lakes, where the chief constituents
are sodium and magnesium chlorides with magnesium
and calcium sulphates; and bitter lakes, which
are usually distinguished by their large percentage of
sodium carbonate as well as chloride and sulphate
(natron-lakes), sometimes by their proportion of
borax (borax lakes). From a geological point of
view they may be divided into two classes—(1) those
which owe their saltness to the evaporation and concentration
of water poured into them by their feeders;
and (2) those which were originally parts of
the ocean.
Salt and bitter lakes of terrestrial origin are abundantly
scattered over inland areas of drainage in the
heart of continents, as in Utah and adjacent territories
of North America, and on the great plateau of
Central Asia. These sheets of water were doubtless
fresh at first, but they have progressively increased in
salinity, because, though the water is evaporated,
there is no escape for its dissolved salts, which consequently
remain in the increasing concentrated
liquid. In Ladâkh, extensive lakes formed by the
ponding back of valley waters by alluvial fans have
grown saline and bitter, and have become the site of
deposits of rock-salt and soda.
The Great Salt Lake of Utah, which has now been
so carefully studied by Gilbert and other geologists,
may be taken as a typical example of an inland basin,
formed by unequal subterranean movement that has
intercepted the drainage of a large area, wherein
rainfall and evaporation on the whole balance each
other, and where the water becomes increasingly salt
from evaporation, but is liable to fluctuations in level,
according to oscillations of meteorological conditions.
The present lake occupies an area of rather
more than 2,000 square miles, its surface being at a
height of 4,250 feet above the sea. It is, however,
merely the shrunk remnant of a once far more extensive
sheet of water, to which the name of Lake
Bonneville has been given by Gilbert. It is partly
surrounded with mountains, along the sides of which
well-defined lines of terrace mark former levels of
the water. The highest of these terraces lies about
940 feet above the present surface of the lake, so that
when at its greatest dimensions this vast sheet of
water must have stood at a level of about 5,200 feet
above the sea, and covered an area of 300 miles from
north to south, and 180 miles in extreme width from
east to west. It was then certainly fresh, for, having
an outlet to the north, it drained into the Pacific
Ocean, and in its stratified deposits an abundant lacustrine
molluscan fauna has been found. According
to Gilbert there are proofs that, previous to the
great extension of Lake Bonneville, there was a dry
period, during which considerable accumulations of
subaerial detritus were formed along the slopes of the
mountains. A great meteorological change then took
place, and the whole vast basin, not only that termed
Lake Bonneville, but a second large basin, Lake Lahontan
of King, lying to the west and hardly inferior
in area, was gradually filled with fresh water.
Again, another meteorological revolution supervened
and the climate once more became dry. The waters
shrank back, and in so doing, when they had sunk
below the level of their outlet, began to grow increasingly
saline. The decrease of the water and the increase
of salinity were in direct relation to each
other until the present degree of concentration has
been reached. The Great Salt Lake, at present having
an extreme depth of less than 50 feet, is still subject
to oscillations of level. When surveyed by the
Stansbury Expedition in 1849, its level was 11 feet
lower than in 1877, when the Survey of the 40th
Parallel examined the ground. From 1866, however,
a slow subsidence of the lake has been in progress,
consequent upon a diminution of the rainfall.
Large tracts of flat land, formerly under water, are
being laid bare. As the water recedes from them and
they are exposed to the remarkably dry atmosphere
of these regions, they soon become crusted with a
white saliferous and alkaline deposition, which likewise
permeates the dried mud underneath. So
strongly saline are the waters of the lake, and so
rapid the evaporation, as I found on trial, that one
floats in spite of one’s self, and the under surfaces of
the wooden steps leading into the water at the bathing-places
are hung with short stalactites of salt from
the evaporation of the drip of the emergent bathers.
Some of the smaller lakes in the great arid basin
of North America are intensely bitter, and contain
large quantities of carbonate and sulphate as well as
chloride of sodium. The Big Soda Lake near Ragtown
in Nevada contains 129.015 grammes of salts
in the litre of water. These salts consist largely of
chloride of sodium (55.42 per cent of the whole),
sulphate of soda (14.86 per cent), carbonate of soda
(12.96 per cent), and chloride of potassium (3.73 per
cent). Soda is obtained from this lake for commercial
purposes.
Salt lakes of oceanic origin are comparatively few
in number. In their case, portions of the sea have
been isolated by movements of the earth’s crust, and
these detached areas, exposed to evaporation, which
is only partially compensated by inflowing rivers,
have shrunk in level, and at the same time have sometimes
grown much salter than the parent ocean.
The Caspian Sea, 180,000 square miles in extent,
and with a maximum depth of from 2,000 to 3,000
feet, is a magnificent example. The shells living in
its waters are chiefly the same as those of the Black
Sea. Banks of them may be traced between the two
seas, with salt lakes, marshes, and other evidences to
prove that the Caspian was once joined to the Black
Sea, and had thus communication with the main
ocean. In this case, also, there are proofs of considerable
changes of water-level. At present the surface
of the Caspian is 85½ feet below that of the Black
Sea. The Sea of Aral, also sensibly salt to the taste,
was once probably united with the Caspian, but now
rests at a level of 242.7 feet above that sheet of water.
The steppes of southeastern Russia are a vast depression
with numerous salt lakes and abundant saline
and alkaline deposits. It has been supposed that this
depression continued far to the north, and that a great
firth, running up between Europe and Asia, stretched
completely across what are now the steppes and
plains of the Tundras, till it merged into the Arctic
Sea. Seals of a species (Phoca caspica) which may
be only a variety of the common northern form (Ph.
fætida) abound in the Caspian, which is the scene
of one of the chief seal-fisheries of the world.[2] On
the west side of the Ural chain, even at present, by
means of canals connecting the rivers Volga and
Dwina, vessels can pass from the Caspian into the
White Sea.[3]
The cause of the isolation of the Caspian and the
other saline basins of that region is to be sought in
underground movements which, according to Helmersen,
are still in progress, but partly, and, in the
case of the smaller basins, probably chiefly in a general
diminution of the water supply all over Central
Asia and the neighboring regions. The rivers that
flow from the north toward Lake Balkash, and that
once doubtless emptied into it, now lose themselves in
the wastes and are evaporated before reaching that
sheet of water, which is fed only from the mountains
to the south. The channels of the Amur Darya, Syr
Darya, and other streams bear witness also to the
same general desiccation. At present, the amount of
water supplied by rivers to the Caspian Sea appears
on the whole to balance that removed by evaporation,
though there are slight yearly or seasonal fluctuations.
In the Aral basin, however, there can be
no doubt that the waters are progressively diminishing.
Owing to the enormous volume of fresh water
poured into it by its rivers, the Caspian Sea is not
as a whole so salt as the main ocean, and still less so
than the Mediterranean Sea. Nevertheless the inevitable
result of evaporation is there manifested.
Along the shallow pools which border this sea, a
constant deposition of salt is taking place, forming
sometimes a pan or layer of rose-colored crystals on
the bottom, or gradually getting dry and covered
with drift-sand. This concentration of the water is
particularly marked in the great offshoot called the
Karaboghaz, which is connected with the middle
basin of the Caspian Sea by a channel 150 yards wide
and 5 feet deep. Through this narrow mouth there
flows from the main sea a constant current, which
Von Baer estimated to carry daily into the Karaboghaz
350,000 tons of salt. An appreciable increase of
the saltness of that gulf has been noticed; seals, which
once frequented it, have forsaken its barren shores.
Layers of salt are gathering on the mud at the bottom,
where they have formed a salt bed of unknown extent,
and the sounding-line, when scarcely out of the
water, is covered with saline crystals.
The study of the precipitations which take place
on the floors of modern salt lakes is important in
throwing light upon the history of a number of chemically
formed rocks. The salts in these waters accumulate
until their point of saturation is reached, or
until by chemical reactions they are thrown down.
The least soluble are naturally the first to appear,
the water becoming progressively more and more saline
till it reaches a condition like that of the mother-liquor
of a salt work. Gypsum begins to be thrown
down from sea-water, when 37 per cent of water has
been evaporated, but 93 per cent of water must be
driven off before chloride of sodium can begin to be
deposited. Hence the concentration and evaporation
of the water of a salt lake having a composition like
that of the sea would give rise first to a layer or sole
of gypsum, followed by one of rock-salt. This has
been found to be the normal order among the various
saliferous formations in the earth’s crust. But gypsum
may be precipitated without rock-salt, either because
the water was diluted before the point of saturation
for rock-salt was reached, or because the salt,
if deposited, has been subsequently dissolved and removed.
In every case where an alternation of layers
of gypsum and rock-salt occurs, there must have been
repeated renewals of the water-supply, each gypsum
zone marking the commencement of a new series of
precipitates.
But from what has now been adduced it is obvious
that the composition of many existing saline lakes is
strikingly unlike that of the sea in the proportions of
the different constituents. Some of them contain carbonate
of sodium; in others the chloride of magnesium
is enormously in excess of the less soluble chloride
of sodium. These variations modify the effects
of evaporation of additional supplies of water now
poured into the lakes. The presence of the sodium-carbonate
causes the decomposition of lime salts, with
the consequent precipitation of calcium-carbonate
accompanied with a slight admixture of magnesium-carbonate,
while by further addition of the sodium-carbonate
a hydrated magnesium-carbonate may be
eventually precipitated. Hunt has shown that solutions
of bicarbonate of lime decompose sulphate of
magnesia with the consequent precipitation of gypsum,
and eventually also of hydrated carbonate of
magnesia, which, mingling with carbonate of lime,
may give rise to dolomite. By such processes the
marls or clays deposited on the floors of inland seas
and salt lakes may conceivably be impregnated and
interstratified with gypseous and dolomitic matter,
though in the Trias and other ancient formations
which have been formed in inclosed saline waters,
the magnesium-chloride has probably been the chief
agent in the production of dolomite.
The Dead Sea, Elton Lake, and other very salt
waters of the Aralo-Caspian depression, are interesting
examples of salt lakes far advanced in the process
of concentration. The great excess of the magnesium-chloride
shows, as Bischof pointed out, that the
waters of these basins are a kind of mother-liquor,
from which most of the sodium-chloride has already
been deposited. The greater the proportion of the
magnesium-chloride, the less sodium-chloride can be
held in solution. Hence, as soon as the waters of the
Jordan and other streams enter the Dead Sea, their
proportion of sodium-chloride (which in the Jordan
water amounts to from .0525 to .0603 per cent) is at
once precipitated. With it gypsum in crystals goes
down, also the carbonate of lime which, though
present in the tributary streams, is not found in the
waters of the Dead Sea. In spring, the rains bring
large quantities of muddy water into this sea. Owing
to dilution and diminished evaporation, a check must
be given to the deposition of common salt, and a
layer of mud is formed over the bottom. As the
summer advances and the supply of water and mud
decreases, while evaporation increases, the deposition
of salt and gypsum begins anew. As the level of the
Dead Sea is liable to variations, parts of the bottom
are from time to time exposed, and show a surface
of bluish-gray clay or marl full of crystals of common
salt and gypsum. Beds of similar saliferous and
gypsiferous clays, with bands of gypsum, rise along
the slopes for some height above the present surface
of the water, and mark the deposits left when the
Dead Sea covered a larger area than it now does.
Save occasional impressions of drifted terrestrial
plants, these strata contain no organic remains. Interesting
details regarding saliferous deposits of recent
origin, on the site of the Bitter Lakes, were
obtained during the construction of the Suez Canal.
Beds of salt, interleaved with laminæ of clay and
gypsum-crystals, were found to form a deposit upward
of 30 feet thick extending along 21 miles in
length by about 8 miles in breadth. No fewer than
42 layers of salt, from 3 to 18 centimetres thick, could
be counted in a depth of 2.46 metres. A deposit of
earthy gypsum and clay was ascertained to have a
thickness of 367 feet (112 metres), and another bed
of nearly pure crumbling gypsum to be about 230 feet
(70 metres) deep.
The desiccated floors of the great saline lakes of
Utah and Nevada have revealed some interesting
facts in the history of saliferous deposits. The ancient
terraces marking former levels of these lakes
are cemented by tufa, which appears to have been
abundantly formed along the shores where the
brooks, on mingling with the lake, immediately
parted with their lime. Even at present, oolitic
grains of carbonate of lime are to be found in course
of formation along the margin of Great Salt Lake,
though carbonate of lime has not been detected in the
water of the lake, being at once precipitated in the
saline solution. The site of the ancient salt lake
which has been termed Lake Lahontan displays areas
several square miles in extent covered with deposits
of calcareous tufa, 20 to 60 and even 150 feet thick.
This tufa, however, presents a remarkable peculiarity.
It is sometimes almost wholly composed of
what have been determined to be calcareous pseudo-morphs
after gaylussite (a mineral composed of carbonates
of calcium and sodium with water)—the
sodium of the mineral having been replaced by calcium.
When this variety of tufa, distinguished by
the name of thïnolite, was originally formed, the
waters of the vast lake must have been bitter, like
those of the little soda-lakes which now lie on its
site—a dense solution in which carbonate of soda predominated.
On the margin of one of the present
Soda Lakes, crystals of gaylussite now form in the
drier season of the year. Yet no trace of carbonate
of lime has been detected in the water. The carbonate
of lime in the crystals must be derived from
water which on entering the saline lakes is at once
deprived of its lime.
UNDERGROUND WATER: SPRINGS, CAVES, RIVERS, AND LAKES — Élisée Reclus
If all soils were absolutely impervious, there
would be no springs, and the whole of the liquid
mass furnished by rain and snow would flow away
over the surface of the ground like the torrents and
flood-waters of the mountains. The greater part,
however, of the water which falls upon the ground
sinks in the first place into the depths of the earth.
There it becomes more or less perfectly purified
from the foreign bodies with which it was charged,
gradually rising to the temperature of the strata
through which it passes, and becoming impregnated
with the soluble salts which it meets with. Ultimately,
when the water, in sinking down, encounters
impervious beds, it can penetrate no further, and,
flowing laterally to the outcrop of the beds, makes
its escape in the form of springs.
The absorption of the rain and melted snow takes
place in various ways, according to the nature of
the soil. Ordinary vegetable earth only allows the
water to penetrate to a very slight depth, especially
when the rain falls in showers and the slope of the
ground is favorable for drainage. As mould is
capable of absorbing a very large quantity—indeed,
more than half its own weight, it prevents the strata
beneath from receiving its due share of moisture,
retaining almost the whole of it for the use of the
vegetation which it nourishes. In fact, it requires an
altogether exceptional rainfall to saturate any ordinary
arable soil to the extent of a yard below the surface.
Water passes with much more facility through
sandy and gravelly beds; but compact loams and clay
will not allow it to penetrate through them, retaining
it in the form of pools or ponds on the surface of the
ground.
The action of vegetation is not confined merely
to imbibing the water falling from the clouds; it
often, also, assists the superabundant moisture in
penetrating the interior of the ground. Trees, after
they have received the water upon their foliage, let
it trickle down drop by drop on the gradually softened
earth, and thus facilitate the gentle permeation
of the moisture into the substratum; another part of
the rain-water, running down the trunk and along
the roots, at once finds its way to lower strata. On
mountain slopes, the mosses and the freshly growing
carpet of Alpine plants swell like sponges when they
are watered with rain or melted snow, and retain the
moisture in the interstices of their leaves and stalks
until the vegetable mass is thoroughly saturated and
the liquid surplus flows away. Peat-mosses especially
absorb a very considerable quantity of water,
and form great feeding reservoirs for the springs
which gush out at a lower level. The immense fields
of peat which cover hundreds and thousands of acres
on the mountain slopes of Ireland and Scotland may,
notwithstanding their elevation and inclined position,
be considered as actual lacustrine basins containing
millions of tons of water dispersed among
their innumerable leaflets. The superabundant
water of these tracts of peat-mosses issues forth in
springs in the plains below.
Rocks, like vegetable earth, also absorb water in
greater or less quantities, according to their fissures
and the density of their particles. If the soil is
formed of volcanic scoriæ, or porous beds of pebbles,
gravel, or sand, the water rapidly descends toward
the underlying strata. Some of the harder rocks,
especially certain kinds of granite, absorb but a very
small quantity of water, on account of the small number
of their clefts; others, on the contrary, as most
of the calcareous masses, imbibe every drop of water
which falls on their surface. There are some rocks
which have their layers broken and cracked to such
an extent that they resemble enormous walls of
rubble-work; the rain instantly disappears on them
as if it had fallen into a sieve. But the greater part of
the calcareous rocks belonging to various geological
periods are formed of thick and regular strata, cleft
at intervals by long vertical crevices. Below the
surface-beds, perhaps, are layers of soft marl, which
the water penetrates with difficulty, although it can
soften and carry away its particles. Here are
formed, rill by rill, the subterranean rivulets which
ultimately spread all over the substratum of marl,
following the general slope of the bed. After a more
or less considerable lapse of time, the stratum of
marl ultimately becomes saturated, and the water
then flows out through caverns which are variously
modified by subsidences—faults in the strata and the
perpetual action of the streams. The springs which
proceed from calcareous rocks of this nature are in
general the most abundant, owing to the length of
their subterranean course. The water which falls on
vast areas on the surface of plateaus is ultimately
united in one bed. A liquid mass of this kind, which
springs up suddenly into sight, just as if it merely
issued from the soil, drains perhaps an extent of country
of many hundreds or thousands of square miles.
Thus, according to the nature of the rock on which
the rain falls, the latter finds its way again to the surface,
either at a considerable distance from the spot
where it fell, or else springs out in little rivulets immediately
below the place where its drops were first
gathered. On a great many mountains we are surprised
to meet with springs gushing out at a few
yards from the summit. These jets have, indeed,
often been considered as the evidence of some miraculous
intervention. Among others, we may mention
the “Sorcerers’ Spring,” which gushes out on
one of the highest points of the Brocken, the culminating
peak in the Hartz Mountains.
The springs which cause the most astonishment are
those which for a time flow plentifully, and then all
at once cease running, but, after an uncertain lapse of
time, again make their appearance. One might almost
fancy that some invisible hand alternately
opened and shut the secret flood-gate which gave an
outlet to the subterranean stream. The cause for this
phenomenon of intermission is easily explained.
When the water brought by the underground stream
is collected in a capacious cavity in the rock, which
communicates with the exterior surface through a
siphon-shaped channel, the liquid mass gradually
rises in the stone reservoir before it rushes out into
the air. It is necessary that the reservoir should be
filled up to the level of the siphon, in order that the
latter should be primed, and that the water should
flow out as a spring into the external basin. If the
water in the reservoir is not replenished with sufficient
rapidity, and is unable to keep at least on a
level with the external outlet, the jet of water will
immediately cease, and can not recommence until
the upper part of the liquid mass has again risen up
to the highest point of the siphon. After an indefinite
period of repose, the spring then enters on a new
phase of activity.
There are many of these subterranean streams
which, before they break forth in springs, do not flow
over beds continuously sloping in the direction of
their current, as in the case with the water-courses
on the surface of the ground. There are some indeed
which first descend into the bowels of the earth,
either by a uniform declivity or by a series of cascades
or rapids, and ultimately reascend from the depths
toward the surface, or jet out vertically from the
ground.
In obedience to the law which compels liquids to
seek the same level in all connected reservoirs, a
rivulet of water will never fail to dart forth as a
spring as soon as it finds an outlet below the caverns
in which the water is collected from which it proceeds.
Likewise, if the spot where the gushing out
takes place is on a much lower level than that of the
feeding reservoirs situated above, the liquid jet must
necessarily shoot up in a column above the surface
of the ground. This is the case at Châtagna, in the
department of the Jura, where a natural jet d’eau
springs up to a height of 10 or 12 feet. In the grotto
of Male-Mort, near Saint-Etienne, in Dauphiné,
the jet of water is not less than 23 to 26 feet in height.
But the water of the fountains being always more
or less charged with sediment, the deposit accumulates
in the form of a circular hillock around the
orifice, thus almost always ultimately raising it to the
level of the top of the liquid column. As an instance
of these rising fountains, we may mention the famous
springs of Moses (Aïn Musa), which gush out
in a charming oasis not far from the shores of the
Gulf of Suez. These springs, the temperature of
which varies from 70° to 84° (Fahr.), now flow from
the top of several small cones of sandy and slimy
débris which they have gradually thrown up above
the level of the plain. They are also shaded by olive
and tamarind trees.
In the innumerable multitude of springs, either
cold or thermal, which rise from the earth, we may
observe the whole range of possible temperatures
from freezing-point up to the boiling-point. A
spring which flows from the side of the Hangerer,
in the Oetzthal, at a height of 6,742 feet, is only 1°
warmer than ice. On the Alps, the Pyrenees, and
all the other chains of snow-clad mountains, near
the summits small rills of water are very frequently
met with, the temperature of which is scarcely
higher than that of melting snow. Even at the bases
of mountains, and especially those of a calcareous
nature, a great number of springs are found which
are much colder than the surrounding soil. This is
so because, in addition to the water, the air also
enters the subterranean channels and circulates in all
the network of clefts and crevices, and, by incessantly
gliding over the wet sides of the channels, produces
a rapid evaporation of moisture, and, in consequence,
refrigerates the surface of the rocks and
even the stream itself. The temperature, therefore,
of springs which proceed from the interior of cavernous
mountains is always several degrees lower than
the normal temperature of the soil.
Springs which have a higher temperature than the
soil are called thermal springs.
It is to be remarked that nearly all thermal springs
which do not owe their high temperature to the
vicinity of volcanoes issue forth from faults which
open on the surface of masses of a crystalline nature,
and principally at the side of modern eruptive rocks
which have been thrust up through older strata.
The influence of rains and seasons has much less
effect upon thermal waters than upon cold springs
which proceed from the upper layers of the soil.
A great number of warm springs, however, undergo
certain changes in their yield of water, which
must be without doubt attributable, at least partially,
to the same causes as the variations in the discharges
of superficial streams. In Auvergne, in the
Pyrenees, and in Switzerland, several springs, perfectly
protected against any infiltration of rain-water,
flow in much greater abundance at the very same period
when the adjacent torrents become swollen. At
Brig-Baden, in the Valais, the water, the mean
temperature of which is in autumn and winter from
71° to 72° (Fahr.), rises to 113° and 122° (Fahr.)
when the breath of spring melts the ice on the Jungfrau.
Most thermal springs contain mineral substances
in solution; there are, however, a certain number
which are almost as pure as rain-water—such as,
for instance, the celebrated waters of Plombières,
also that of Gastein, Pfeffers, Wildbad, and Badenweiler.
The springs of Chaudes-Aigues—those in
France which have the highest temperature, 158°
to 176° (Fahr.)—contain only a small amount of
mineral substances. The inhabitants of Chaudes-Aigues
use the water to prepare their food, to wash
their linen, and to warm their houses. Wooden conduits,
erected in all the streets of the town, supply,
on the ground floor of each house, a reservoir which
serves to heat it during cold weather, and thus dispenses
with fires and chimneys.
Among the various substances which spring-water
brings to the surface, those which are most
common proceed from the strata which serve to constitute
the very framework of the globe. Chalk, especially,
occurs in different proportions in most springs,
either under the form of sulphate of lime, or, more
often, as carbonate of lime. Water which contains
carbonic acid in solution is charged with calcareous
matter dissolved away from the sides of the rocks
through which it passes; then, by means of evaporation,
it redeposits the stony substances which it previously
held in solution. Hence arise all those calcareous
concretions which form around so many
springs; also the stalactites in caverns.
The Giant’s Causeway, Antrim, Ireland
Nearly all countries of the world possess some of
these curious springs, which cover with a calcareous
crust any object placed in their waters. Among these
incrusting springs, those of Saint Allyre, near Clermont,
Rivoli, and San Filippo, not far from Rome,
have justly become celebrated. These latter have,
in a space of twenty years, filled up a pond with a
bed of travertin 30 feet thick, and, in the neighborhood,
entire strata of this same rock may be seen
having a depth of more than 328 feet. The springs
of Hammam-Mes-Khoutine, in the province of
Constantine, are also very remarkable on account
of the considerable amount of their deposits. This
water, which rises at a temperature of 203° (Fahr.),
and from which a high column of steam always rises,
is frequently compelled to change its point of issue on
account of the dense beds of travertin which are
gradually deposited upon the soil. Most of these deposits
are of a dazzling white hue, striped here and
there with bright colors, and are developed in mammillated
strata; other concretions, accumulating
gradually round an orifice, have taken the form of
cones, and are like small craters near a volcano, some
of them rising to a height of as much as 33 feet; lastly,
there are masses of travertin which stretch out in a
kind of wall below the flow which deposits them.
One of these walls, which is interrupted at intervals
by heaps of earth upon which large trees grow, is
not less than 4,921 feet long, 66 feet high, and, on an
average, from 33 to 49 feet wide.
The thermal waters of Algeria are, however, surpassed
in grandeur and beauty by the springs of the
ancient Ionian city of Hierapolis (holy city), which
at the present time flow in the solitary plateau called
Panbouk-Kelessi (Castle of Cotton), on account of
the cotton-like aspect of the white masses of travertin
of which it is composed. On reaching this
spot from Smyrna, something like an immense cataract
may be seen in the distance, 328 feet high and
2½ miles wide; this is formed by the walls which the
water has gradually constructed, column after column,
and layer after layer, by flowing over the edges
of the plateau and gushing out on the slopes. Here
and there, real cascades glitter in the sun, and their
sparkling surfaces light up the dead whiteness of the
crystal walls. As a spectator ascends the declivities,
the masses deposited and carved out by the water
appear in all their strange beauty; one might fancy
that they were colonnades, groups of figures, and
rude bas-reliefs which the chisel had not yet perfectly
set free from their rough coverings of stone.
And all these calcareous deposits which have been
fashioned by the cascades during a succession of ages
open a multitude of cup-like hollows with fluted
edges fringed with stalactites; these graceful reservoirs—some
of which are shaded with yellow or
veined with red, brown, and violet, like jasper or
agate—are filled with pure water. Higher still follow
two steps of the plateau on which stood the ancient
thermal edifice and the Necropolis of Hierapolis.
There whitish masses cover the ancient tombstones
and fill up the conduits. The ground is crossed
in various directions by the former beds of rivulets,
which have gradually stopped up their own courses
by depositing concretions upon them. Above one
of the widest of these dried-up channels, the magnificent
span of a natural bridge displays its graceful
form, like an arch of alabaster, streaming with
innumerable stalactites. At what date did this majestic
structure take its rise, and how many years and
centuries did the process of its formation last? No
one knows. According to Strabo, the channels of
the baths of Hierapolis were soon filled up by solid
masses, and if Vitruvius can be believed, when the
proprietors of the environs wished to inclose their
domain, they caused a current of water to run along
the boundary-line, and in the space of a year the
walls had risen.
Silica, which is still more important than chalk in
the formation of terrestrial rocks, is also sometimes
deposited on the edge of springs, but in very small
quantities.
The various dislocations of the terrestrial strata,
the cooling of the waters, and, perhaps, in many instances,
the obstruction of channels by deposits of ore,
explain why, in the present period, so small a number
of thermal springs issue from metalliferous beds.
Nevertheless, many localities might be mentioned
where these phenomena take place at the present time.
A spring at Badenweiler, in the Black Forest, issues
forth at a few yards from a vein of sulphuret of lead.
In the granitic plateau of central France other
springs are likewise found to be associated with this
metal. Various thermal waters in the Black Forest,
like those of Carlsbad and Marienbad, are in close
connection with veins of iron and manganese.
Oligiste iron is found in the fissures of the springs
of Plombières and Chaude-Fontaine. In Tuscany
sulphureous fumaroles proceed from the veins of
antimony. In France and Algeria the waters of
Sylvanès and Hammam R’ira issue forth from beds
of copper. Lastly, near Freyberg, a voluminous
thermal spring has been discovered in a vein of
silver.
Among the mineral substances which some springs
bring to the surface of the soil, the most important,
in an economical point of view, is common salt. This
substance, being one of those which dissolve most
readily in water, all the liquid veins which pass over
saline beds become saturated with salt; therefore
springs of this kind, which flow in great abundance,
give rise to salt-works of more or less importance.
The masses of common salt which make their way
every year from the interior of the earth may be
estimated at thousands of tons. The springs of
Halle, which rise on the northern slope of the Alps
of Salzburg (Salt Town), and are managed with the
greatest care, annually produce 15,000 tons of this
mineral. The salt springs of Halle, in Prussia,
which have been worked from time immemorial by
a company, furnish 10,000 tons of salt every year.
Other parts of Germany also yield for consumption
thousands of tons of white salt, which is produced
by the evaporation of saline springs. The mass of
salt furnished by the single artesian well of Neusalzwerk,
near Minden, in Prussia, represents every
year a cube measuring 78 feet on each side.
Though not so rich as Germany in saline springs
thus turned to account, most of the civilized countries
of the world possess salt-works which are also very
important. France enjoys the springs of Dieuze,
Salins, and Salies; Switzerland, those of Bex; Italy
has the springs in the environs of Modena, and many
others besides. In England, near Chester, there are
some mines of rock-salt in which numerous liquid
veins issue forth which are impregnated with salt.
Lastly, the United States have the celebrated springs
of Syracuse.
Not far from the “spot where Troy once stood” is
the valley of Touzla-sou, which owes its name (Salt
Water) to its numerous salt springs. The mountains
which rise around its circumference are variously
shaded with blue, red, and yellow, and the
rocks are incessantly decomposing under the action
of the liquid salt which oozes out from and trickles
down their sides. The plain itself is covered with a
variegated crust, while jets of boiling water, saturated
with salt, burst forth in every direction. Here
and there pools are found, the moisture of which,
by evaporating in the sun, leaves upon the soil beds
of salt as white as snow. Near the mouth of the valley
springs become more and more numerous.
Lastly, in the place where the cliffs approach near
together, so as to form a defile, a magnificent spout
of water jets out from one side of the rock. This jet
is not less than a foot in diameter at the orifice, and
falls again after having described a parabola of more
than a yard and a half. Other springs shoot out on
both sides, the constant temperature of which is more
than 212° (Fahr.); these, together with the principal
jet, form a rivulet of boiling and steaming water.
Springs of salt water are used for the treatment
of diseases as well as for the extraction of salt. They
constitute one of the most important groups of medicinal
waters, according to the various substances
which they contain in solution. The other springs
made use of, on account of their healing virtues, have
been classed under ferruginous, sulphureous, and
acidulous springs. These waters also contain, in
different proportions, a variable quantity of gases
and salts which they have dissolved in their passage
over subterranean beds of every kind.
Mineral springs are most numerous and abundant
in mountain valleys, and there, consequently, the
great thermal institutions are established. In Europe
the chain of the Pyrenees is probably the richest in
mineral, sulphureous, saline, ferruginous, and acidulous
springs. According to Francis, the engineer,
in 1860 more than 550 mineral springs, 187 of which
are used, flowed upon the French slopes of the
Pyrenees. These waters supplied 83 hot baths in
53 localities, the principal of which are Bagnères de
Bigorre, Luchon, Eaux-Bonnes, and Cauterets. The
most abundant springs, those of Graus d’Olette,
form a sort of mineral stream, yielding more than
four gallons a second, or 2,322 cubic yards a day. In
Algeria the spring of Hammam-Mes Khoutine
yields 6 gallons a second.
There are regions, some volcanic and some not,
in which nearly all the springs are thermal and mineral;
springs of pure and fresh water being so rare,
they are there considered to be most precious treasures.
One of these regions comprehends a large part
of the plateau of Utah. In this place numerous thermal
springs issue forth, to which have been given the
vulgar names of the Beer, Steamboat, Whistle
Springs, etc., and into one of which the Mormons
plunge their neophytes. The springs which are not
thermal are loaded with saline and calcareous matter.
It is only in spring, at the time when the snow melts,
that the springs, which then become very abundant,
yield comparatively pure water. During the dry
season, salt and carbonate of lime become concentrated
in the nearly exhausted springs, and give to the
liquid flow an unpalatable taste. Palgrave, the
traveler, informs us that all the springs of the country
of Hasa, in Arabia, are also thermal.
It can readily be understood that when all these
substances escape from the interior of the rocks, together
with the water which holds them in solution,
they must leave empty spaces in the earth. During
the course of long centuries whole strata are dissolved,
and, under a form more or less chemically
modified, are brought up from the depths and distributed
on the surface of the soil. The thermal
waters of Bath, which are far from being remarkable
for the proportion of mineral substances they
contain, bring to the surface of the earth an annual
amount of sulphates of lime and soda, and chlorides
of sodium and magnesium, the cubic mass of which
is not less than 554 cubic yards. It has also been
calculated that one of the springs of Louèche, that of
Saint Laurent, brings every year to the surface
8,822,400 pounds of gypsum, or about 2,122 cubic
yards; this quantity is enough to lower a bed of
gypsum a square mile in extent, more than five feet
in one century. But this is only one spring, and we
have reckoned one century only; if we think of the
thousands of mineral springs which gush from the
soil, and of the immensity of time during which their
waters have flowed, some idea may be formed of the
importance of the alterations caused by springs. In
time they lower the whole mass of mountains, and,
no doubt, after these sinkings, violent oscillations of
the earth may often have taken place.
In regions where the strata are pierced with wide
and deep caverns, and especially in calcareous countries,
the waters sometimes accumulate in sufficient
quantities to form perfect streams with long subterranean
courses. At their issue from the caverns,
these waters form a contrast with the rocks and hills
around, all the more striking because the latter are
completely devoid of moisture, and fearfully sterile,
while on the brink of the limpid stream the fresh
verdure of plants and trees is at once developed.
Like a captive, joyous at seeing the light once more,
the water which shoots forth from the sombre grotto
of rocks sparkles in the sun, and careers along with
a light murmur between its flowery banks.
Among these subterranean streams, the most celebrated,
and doubtless one of the most beautiful, is
the Sorgues of Vaucluse. The vaulted grotto from
which the mighty mass of water escapes opens at
the mouth of an amphitheatre of calcareous rocks
with perpendicular sides. Above the spring rises a
high white cliff, bearing on its summit a ruined
tower of the Middle Ages; the rock is everywhere
sterile and bare; there is nothing but a miserable
fig-tree, clinging to the stone like a parasitical plant
to the bark of a tree, which has plunged its roots
into the fissure of the cave, and greedily absorbs
with its leaves the moisture which floats like a mist
above the cascades of the spring. After heavy rains,
the liquid mass, which is then estimated at 26 or even
32 cubic yards a second, flows in a wide sheet high
above the entrance to the cavern, which is then altogether
inaccessible. When the waters are low, they
flow bubbling across the barrier of rocky débris
which obstructs the entrance; at that time it is quite
possible to penetrate under the arch, and to contemplate
the vast basin in which the blue waters of
the subterranean stream spread out before they leap
into the open air. Soon after its issue from the cave
and amphitheatre of Vaucluse, the Sorgues is divided
into numerous irrigation channels, which spread
fertility in the country over an area of more than 77
square miles. The subterranean course of the affluents
which form the stream is not ascertained; but it
is known that most of them commence 12 or 15 miles
to the east, in the plateaus of Saint Christol and
Lagarde, which are pierced all over with avens or
chasms, into which the rain-water sinks and disappears.
In another part of France there is a second important
subterranean stream, which is much less
known but no less remarkable than that of Vaucluse;
this is the Touvre of Angoulême, continuing the
course of the Bandiat, the waters of which, like those
of the Tardoire, are swallowed up in several abysses
at distances varying from 3 to 7 miles to the east and
northeast. The three principal springs of the Touvre
flow slowly out of a deep cave, hollowed out at the
base of an escarped cliff; another spring bubbles up
in a basin of rock; the third emerges from a sort of
boggy meadow intersected by drains. At the outlet
of their subterranean courses these three enormous
springs immediately form three streams, which reunite,
leaving between them two long peninsulas of
reeds and other aquatic plants. Below the junction,
the Touvre, which is here more than 100 yards wide,
passes round a rugged hill, and, dividing into several
branches, turns the numerous mill-wheels of the important
gun-foundry of Ruelle; then, after a course
of five miles, it flows into the Charente at a small
distance above Angoulême. Among the hundreds
and thousands of travelers whom steam annually conveys
over the bridge of the Touvre, there are few who
are aware of the curious nature of the source of the
river of limpid water over which the train passes in
its noisy career.
Omitting to mention the streams which accidentally
pass under the strata of rocks during a small
part of their course, or of the subterranean outlets of
certain lakes, a multitude of other instances might
be brought forward of masses of water, more or less
abundant, which appear above ground after having
traversed a considerable distance under the earth.
Of this kind is the graceful spring of Nîmes, the blue
transparent water of which, reflecting the foliage of
pines and chestnut trees, glides in its gentle ripples
over the semicircular steps of an old Roman staircase.
Of this kind, too, is the spring of Vénéran,
near Saintes: this spring, which was formerly sacred
to the Goddess of Love, gushes from the ground in a
gorge of rocks, and, passing through a mill, the wheel
of which it turns, it suddenly disappears, being swallowed
up in an abyss; thus it appears on the earth to
work but for an instant.
Numbers of water-courses do not reappear on
the surface of the soil after being swallowed
up in the earth, but flow straight to the sea by
means of subterranean channels. On nearly the
whole extent of the continental shores, and principally
in localities where the coasts are of a
calcareous nature, the outlets of submarine tributaries
may be noticed, some of which are perfect
rivers. Most of the springs of the department of
Bouches du Rhone jet up from the bottom of the sea,
but at various distances from the shore. One of them,
that of Porte Miou, near Cassis, forms on the surface
of the sea a considerable current, which drifts
any floating bodies to a great distance. At Saint
Nazaire, Ciotat, Cannes, San Remo, and Spezzia,
other streams also issue from the midst of the salt
waves, and attempts have even been made to measure
approximately their discharge. M. Villeneuve-Flayosc
estimates at 24 cubic yards a second the
quantity of water discharged into the sea by all the
hidden affluents of the Mediterranean between Nice
and Genoa. Some of the submarine springs of
Provence and Liguria proceed from enormous
depths. The orifice of the spring of Cannes is 531
feet below the level of the sea; that of San Remo rises
from a depth of 954 feet; lastly, at four miles to the
south of Cape Saint Martin, between Monaco and
Mentone, another stream of fresh water empties itself
under a bed of salt water, near 2,296 feet deep.
The coasts of Algeria, Istria, Dalmatia, and the
Herzegovina also present numerous instances of submarine
streams; on the eastern shores of the Adriatic
the traveler may even have the pleasure of contemplating
the delta of a considerable river, the Trebintchitza,
visible through the sea-water at the depth
of a yard. The abundant springs of fresh water
which pour out into the open sea to the southwest of
the Cuban port of Batabano are well known, since
Humboldt described them, and it is observed that
the lamantins, or sea-cows, which dread salt water,
delight in frequenting these parts. Lastly, the Red
Sea, which does not throughout its immense circumference
receive a single permanent stream flowing
on the surface of the ground, nevertheless receives
some which spring from the bottom of its bed. The
shores of the United States, the calcareous soil of
which is probably pierced with caverns from the
very centre of the continent, perhaps are the coasts
which pour into the sea the most abundant subterranean
rivers. Near the mouth of the stream of St.
John, a submarine stream of perfectly pure water
spouts in bubbles as far as one to two yards above
the level of the sea. Off the Carolinas, and Florida,
salt water has been known to change into brackish
water under the influence of the sudden increase of
its subterranean affluents. In the month of January,
1857, all that part of the sea which is adjacent to the
southern point of Florida was the scene of an immense
eruption of fresh water. Muddy and yellowish
water furrowed the straits, and myriads of dead
fish floated on the surface and accumulated on the
shores. Even in the open sea the saltness diminished
by one-half, and in some places the fishermen drew
their drinking-water from the surface of the sea as if
from a well. It is affirmed by all those who witnessed
this remarkable inundation of the subterranean river
that, during more than a month, it discharged at least
as much water as the Mississippi itself, and spread
over all the strait, 31 miles wide, which separates
Key West from Florida.
On the coasts of Yucatan, the fresh waters which
take a subterranean course down to the sea do not
appear to flow like rivers which have a narrow bed
and attain considerable speed, but more in the form
of a wide sheet of liquid with a nearly imperceptible
current. Cenotes open here and there over the surface
of the country; they are a kind of natural draining-well
or hole, not very deep, into which the inhabitants
descend to draw spring water. At Merida
and in the environs the subterranean water is found
at a depth of 26 to 30 feet; but the nearer we approach
to the sea the thinner the layer of rock becomes
which covers the liquid veins; on the seashore
fresh water is found nearly on a level with the soil.
The height of the veins varies several inches, according
to the quantity of rain; but in every season the
mass of water descending from the plateau of Yucatan
is poured into the sea through innumerable outlets.
Over a great extent of the shore of the peninsula,
these hidden springs furnish collectively a mass
sufficiently large to counterpoise the waters of the
sea. Under the pressure of the marine current which
runs along the coast, there is formed, between the
open sea and the liquid mass which has made its way
from the land, a littoral bank like those barriers
which the waves construct before the mouths of
rivers. This embankment, which protects the coasts
of Yucatan like a breakwater, is not less than 171
miles long, and is cut through by the sea at two or
three points. The channel, which stretches like a
wide river between the bank of alluvium and the
Yucatan coast, is, not without reason, designated by
the inhabitants by the name of stream or rio.
Among the remarkable phenomena which perhaps
owe their existence to subterranean water-courses, we
must mention the sudden or gradual appearance of
those hillocks of clay (“mud-lumps”) which rise, to
the great danger of navigators, either in the middle
of the bar of the Mississippi, or in the immediate
vicinity. Like small volcanoes of mud, the “mud-lumps”
generally appear under the form of isolated
cones, allowing a rill of dirty water to escape from
their summits. Some of them are irregular on their
surface, on which lateral orifices here and there
show themselves, some in full activity, others abandoned
by the springs which formerly gushed from
them. The water of some “mud-lumps” is loaded
with oxide of iron or carbonate of lime, which, with
the agglutinated sands, form hard masses, having the
consistence of perfect rocks. These hillocks vary
both in their height and shape. The greater part
remain hidden at the bottom of the water, and even
their summits do not reach the level of the river or
sea; others hardly raise their heads above the waves;
the most considerable, however, rise to a height of
6, 9, or even 19 feet, and their base covers an area
of several acres. The sudden way in which most of
these water-volcanoes make their appearance, the
anchors of vessels, and the remains of cargoes which
have been found on their surface, their conical form,
their terminal craters, and all the springs, “which
seem to spout out as if from a subterranean sieve,”
indicate the existence of a subterranean force always
at work to upheave this band of hillocks.
M. Thomassy is of opinion that the hillocks of
these bars are the orifices of regular artesian wells
naturally formed by a sheet of subterranean water
descending from the plateaus of the interior and
flowing below the Mississippi and the clayey levels
of Louisiana. However this may be, the mode in
which these mud hillocks are formed is well enough
known to render it easy to clear them away from the
mouths of the Mississippi and to protect the interests
of navigation. When a cone of clay makes its appearance
on the bar, a charge of powder is introduced
into it and explodes it. Thus, in the year
1858, the southwest passage was cleared of a “mud-lump”
which formed a considerable island; a single
charge was sufficient to annihilate the whole. The
island suddenly sunk; in its place a wide depression
was formed, the circumference of which resembled
that of a volcanic crater; at the same time an enormous
quantity of hydrogen gas was discharged into
the atmosphere.
Above the springs the course of subterranean rivulets
is generally indicated by a series of chasms or
natural wells, which disclose the stream beneath.
The arches of caves not being always strong enough
to support the weight of the superincumbent masses,
they necessarily fall in some places, leaving above
them other spaces into which the upper beds successively
sink. The débris of the ruin is afterward
cleared away by the water, or dissolved, atom by
atom, by the carbonic acid contained in the stream,
and gradually all the loose rubbish is carried away.
In this manner, above the subterranean rivulets, a
kind of well is formed, which is designated in various
countries by very different names.
By means of these natural gulfs it is possible to reach
the subterranean streams, and to give some account
of their system, which is exactly like that of rivulets
and rivers flowing in the open air. These streams
also have their cascades, their windings, and their
islands; they also erode or cover with alluvium the
rocks which compose their bed, and they are subject
to all the fluctuations of high and low water. The
only important difference which superficial waters
and subterranean currents present in their phenomena
is that these streams in some places fill the whole
section of the cave, and are thus kept back by the
upper sides, which compress the liquid mass. In
fact, the spaces hollowed out by the waters in the
interior of the earth are only in a few places formed
into regular avenues, which might be compared to
our railway tunnels. Where beds of hard stone oppose
the flow of the rivulet, all it has done during the
course of centuries has been to hew out one narrow
aperture. This succession of widenings and contractions,
similar to those of the valleys on the surface,
forms a series of chambers, separated one from
the other by partitions of rock. The water spreads
widely in large cavities, then, contracting its stream,
rushes through each defile as if through a sluice.
On account of these partitions, it is very difficult,
or even impossible, to navigate the course of subterranean
rivers to any considerable distance, even at the
time the water is low. When it is high, the liquid
mass, detained by the partitions, rises to a very high
level in the large interior cavities, and often reaches
the roof above. Sometimes when, through the clefts
of the rocks, a communication exists between the cave
and some hollow above, the surplus water from the
subterranean streams makes its appearance there.
Thus the Recca, which flows beneath the adjacent
plateau of Trieste, does not always find space enough
to flow freely in its lower channels, and Schmidt
has seen it ascend in the chasms of Trebich to a
height of 341 feet. It may be understood that the
pressure of such a column of water often shatters
enormous pieces of rock, and thus modifies the course
of underground streams.
When the water, impelled by force of gravitation,
seeks a new bed in the cavernous depths of the earth,
and disappears from its former channels, these are at
first much easier of access than they formerly were;
but ere long, in most caves, a new agent intervenes,
which seeks to contract or even completely obstruct
them. This agent is the snow-water, or rain, which
percolates, drop by drop, through the enormous filter
of the upper strata. In passing through the calcareous
mass, each one of these drops dissolves a certain
quantity of carbonate of lime, which is afterward set
free on the arch or the sides of the cave. When the
drop of water falls, it leaves attached to the stone a
small ring of a whitish substance; this is the commencement
of a stalactite. Another drop trickles
down, and, trembling on this ring, lengthens it
slightly by adding to its edges a thin circular deposit
of lime, and then falls. Thus drop succeeds
drop in an infinite series, each depositing the particles
of lime which it contains, and forming ultimately
a number of frail tubes, round which the
calcareous deposit slowly accumulates. But the
water which drops from the stalactites has not yet
lost all the lime which it held in solution; it still
retains sufficient to enable it to elevate the stalagmites
and all the mammillated concretions which
roughen or cover the floor of the grotto. It is well
known what fairy-like decorations some caverns owe
to this continuous oozing through the vaults of their
roofs. There are few sights in the world more astonishing
than that of these subterranean galleries, with
their dead-white columns, their innumerable pendants
and multiform groups, like veiled statues, all
yet unstained by the smoke of the visitor’s torch.
When the action of the water is not disturbed, the
needles and other deposits of the calcareous sediment
continue to increase with considerable regularity.
In some cases each new layer which is added
to the concretions may be studied as a kind of time-measurer,
indicating the date when the running
water abandoned the cave. At length, however, the
soft concentric layers disappear, and are replaced
by forms of a more or less crystalline character; for
in every case where solid particles exist, subject to
constant conditions of imbibition by water, crystals
are readily produced. Sooner or later, the stalactites,
increasing gradually in a downward direction,
meet and unite with the needles rising from the
surface of the ground, and, forming by their number
a kind of barrier, obstruct the narrower passages
and close up the defiles separating the cavern into
distinct chambers.
One of these Kentucky caves, called the “Mammoth
Cave,” is the largest which is at present known.
The whole of its extent has not been as yet fully explored,
for it may be almost called a subterranean
world, having a system of lakes and rivers, and a network
of galleries and passages without number,
which cross and recross one another, going down to
an immense depth. From the chief entrance to the
further recesses of the cave, the distance is reckoned
to be not less than 9¼ miles, and the whole length of
the two hundred alleys that have been traced out
in this enormous labyrinth is 217 miles in extent.
This “Mammoth Cave” once served as a retreat for
savage tribes, for skeletons of men of an unknown
race have been found buried in it under layers of
stalactite.
The district which is the most remarkable among
all the calcareous countries of Europe for its caves,
its subterranean streams, and its abysses is unquestionably
the region of the Carniolan and Istrian Alps,
which extends to the east of the Adriatic, between
Laibach and Fiume. The whole surface of the country,
as in certain plateaus of the Jura in France, is
everywhere pierced with deep boat-shaped cavities,
at the bottom of which the water forms a kind of
whirlpool, like the water flowing out of the hold of
a stranded ship. Many mountains are penetrated
in every direction with caverns and passages, just
as if the whole rocky mass was nothing more than
an accumulation of cells. On one steep cliff-side
may be noticed all kinds of perforations at different
heights—arched portals and orifices of fantastic
shape; on another there are numbers of springs of
blue water gushing from the caves, or from the rocks
heaped up at the foot of the cliff, and forming rivulets
which disappear a little further on in the fissures
of the ground, as if through the holes of a sieve. The
whole surface of the plateaus, whether bare or covered
with forests, is scattered over with wells, or
funnel-shaped holes communicating with subterranean
reservoirs.
One of the Istrian rivers, the subterranean course
of which, although still unknown as regards a great
number of points, has given rise to a most continuous
course of investigations, is the celebrated Timavus
(Timavo), which falls into the sea near Duino,
about twelve miles to the north of Trieste. Virgil’s
description no longer applies to the mouths of the
Timavo; at present they do not reach the number
of nine, because the extermination of the woods of
the Carso has diminished the mass of the water, or
the action of the stream and the alluvium of the
delta have modified the form of the shore. But still
it is a magnificent spectacle to see the outlet of the
three principal torrents of water which rush foaming
out of the heart of the rocks, and are navigable
from their mouths to their very source. A river of
this importance must certainly receive the drainage
of a vast basin, and yet all the neighboring valleys
seem perfectly devoid of rivulets, and their surface
presents little else but the bare rock; in fact, the
whole of the rain and snow-water runs away through
underground caverns.
The most remarkable network of caverns in this
region of the Alps is that which spreads out from
the southwest to the northeast across the Adelsberg
group of mountains, between Fiume and Laibach.
The principal cave is especially curious on account
of its size, the variety of its calcareous concretions,
and the torrent which runs roaring through it.
North of the town of Adelsberg the traveler passes
along the base of a hill with steep and bare sides,
bringing into view the sharp edges of its highly
pitched calcareous beds. On the right the stream of
the Poik winds peaceably in the valley; and then, its
course being arrested by a headland, turning suddenly,
it flows into the interior of the mountain
through a kind of high portal, opening between two
parallel beds of rocks. Unless the water in the
stream is very low, it is impossible to follow it over
the accumulation of rocks upon its bed; but on the
right, at a height of a few yards, there is another
entry, through which the traveler may descend dry-shod
into a vast cavity or chamber, where the Poik
again appears issuing from its narrow passage of
rocks.
At this point the cave divides; on the north
the stream, the depth of which varies, according to
the season, from a few inches to 30 or 33 feet, buries
itself in a winding avenue, which has been traversed
in a boat as far as a point 1,027 yards from the entrance;
on the northeast, a higher avenue, discovered
only in 1818, pushes its way far into the heart of the
mountain, branching out in various directions into
narrow passages and wide compartments. This portion
of the grotto, which appears to have been the
former bed of the Poik, is the most curious part of
the Adelsberg labyrinth; it affords wonderful groups
of stalactites, especially in the Salle du Calvaire, the
vaulted roof of which, having the enormous span of
210 yards, has dropped upon a hillock of débris a
perfect forest of stalagmitic columns and white
needles. The full length of the principal cave is not
less than 2,575 yards; but very probably some other
and still longer avenues may yet be discovered.
Although it is impossible to go in a boat along the
subterranean portion of the Poik for a greater distance
than 1,027 yards, by traversing the surface of
the calcareous plateaus we can at all events trace out
the subterranean stream by means of the funnel-shaped
holes which open above its course. One of
these gulfs, the Piuka-Jama, is situated about a mile
and a half to the north of the entrance of the Adelsberg
caves; the only way to descend into this is by
clinging to the branches of the shrubs and sliding
down by the assistance of a cord fastened to the top
of the rocks. By these means the entrance to a kind
of air-hole may be reached, from which the Poik
is visible foaming over its bed of rocks, and only a
slope of débris is to be descended to reach the edge
of the stream. It can only be followed in the downstream
direction for about 275 yards; but it can
easily be ascended for a distance of 495 yards by
passing under a high portal with lofty pillars, and in
this way a point can be reached which is less than a
mile from the place where the stream disappeared
in the cave of Adelsberg.
Further down the stream the Poik is not visible
again until it emerges from the mountain, where it
is known under the name of the Planina; it rushes
out through a circular arch at the base of a perpendicular
bluff crowned with fir-trees. It really is
the Poik, as is proved by the equal temperature of
water and the sudden increase of its liquid mass after
a storm has burst at Adelsberg; but the stream always
issues from the cave much more considerable
in bulk than it is when it enters, owing to the tributaries
which pour into it on both sides during its
subterranean course of five to six miles. One of these
rivulets, which comes down from the plateaus of
Kaltenfeld, joins the Poik at a little distance from
its outlet. Above the confluence the principal stream
can be ascended in a boat to a distance of more than
3,500 yards, which, with the other explored parts of
the subterranean river, makes about three miles. Below
the point of outlet the stream is partially lost in
the fissures of its bed, and then, joining the Unz,
goes on and empties itself into the Danubian Save.
About a dozen miles to the southeast of the Adelsberg
and Planina caves extends a large plain surrounded
on all sides by high calcareous cliffs, at the
base of which nestle seven villages. In this hollow,
the most elevated portion of which is under cultivation,
the remainder being covered with rushes
and other marsh-plants, there are to be found more
than 400 funnel-shaped holes resembling those in
other parts of Carniola. These dolinas, the average
depth of which is from 40 to 60 feet, have each their
special name, such as the “Grand Crible” (great
sieve), the “Crible-à-froment” (corn sieve), the
“Tambour” (drum), the “Cuve” (tub), the “Tonneau”
(cask), pointing out the form or some remarkable
peculiarity of each abyss. During extremely dry
seasons there is only one of these cavities which contains
any water; but after continuous and heavy rain,
the water of a stream which is swallowed up in the
rocks a little above the plain rises with a roaring
noise in each of these wells. Torrents escaping from
all these open “cribles” form in the wide space
hemmed in by the cliffs a sea of blue and transparent
water. This is the lake of Jessero or Zirknitz, the
lacus Lugens of the Romans. The surface of the
sheet of water extends over an area of 14,826 acres;
at the time of great inundations, this extraordinary
temporary lake, thus vomited out by the underground
river, is not less than 24,711 acres. The water runs
away through a subterranean channel, and, further
on, empties itself into the Unz, below the Planina.
Lacustrine basins of this sort, first emitted, and
then again absorbed by a subterranean water-course,
are rather rare; there are, however, some other remarkable
instances of them in Europe. Thus, in the
Oriental Hartz, in the midst of a beautiful spot surrounded
by fir-trees, the charming lake called Bauerngraben
(Peasants’ Ditch), or sometimes Hungersee
(Lake of Famine), sometimes makes its appearance;
but when this mass of blue water has filled but
for a few days its basin of gypsum rock, it is suddenly
swallowed up, and flows away by subterranean channels
into the stream of the Helme. The celebrated
lake of Copaïs, in Bœotia, may likewise be compared
to the Zirknitz lake, at least as regards certain portions
of its basin.
RIVERS — A. Keith Johnston
Rivers are the result of the natural tendency of
water, as of all other bodies, to obey the law of
gravitation by moving downward to the lowest position
it can reach. The supply of water for the formation
of rivers, though apparently derived from various
sources, as from rain-clouds, springs, lakes, or
from the melting of snow, is really due only to atmospheric
precipitation; for springs are merely collections
of rain-water; lakes are collections of rain
or spring water in natural hollows, and snow is
merely rain in a state of congelation. The rills issuing
from springs and from surface-drainage unite
during their downward course with other streams,
forming rivulets; these, after a further course, unite
to form rivers, which, receiving fresh accessions in
their course from tributaries (subordinate rivers or
rivulets) and their feeders (the tributaries of tributaries),
sweep onward through ravines, and over
precipices, or crawl with almost imperceptible motion
across wide, flat plains, till they reach their lowest
level in ocean, sea, or lake. The path of a river
is called its course; the hollow channel along which
it flows, its bed; and the tract of country from which
it and its subordinates draw their supplies of water,
its basin, or drainage-area. The basin of a river is
bounded by an elevated ridge, part of which is generally
mountainous, the crest forming the watershed;
and the size of the basin, and the altitude of
its watershed, determine, cæteris paribus, the volume
of the river. The greater or less degree of uniformity
in the volume of a river in the course of a year is
one of its chief physical features, and depends very
much on the mode in which its supply of water is
obtained.
In temperate regions, where the mountains do not
reach the limit of perpetual snow, the rivers depend
for their increase wholly on the rains, which, occurring
frequently, and at no fixed periods, and discharging
only comparatively small quantities of
water at a time, preserve a moderate degree of uniformity
in the volume of the rivers—a uniformity
which is aided by the circumstance that in these
ones only about one-third of the rainfall finds its
way directly over the surface to the rivers; the remaining
two-thirds sinking into the ground, and finding
its way to spring-reservoirs, or gradually oozing
through at a lower level in little rills which continue
to flow till the saturated soil becomes drained of its
surplus moisture, a process which continues for
weeks, and helps greatly to maintain the volume of
the river till the next rainfall. This process, it is evident,
is only possible where the temperature is mild,
the climate moist, evaporation small, and the soil sufficiently
porous; and under these circumstances great
fluctuations can only occur from long-continued and
excessive rains or droughts. In the hotter tracts of
the temperate zones, where little rain falls in summer,
we occasionally find small rivers and mountain
torrents becoming completely exhausted; such is
often the case in Spain, Italy, Greece, and with
the Orange, one of the largest rivers of South
Africa.
In tropical and semi-tropical countries, on the
other hand, the year is divisible into one dry and one
wet season; and in consequence the rivers have also
a periodicity of rise and fall, the former taking place
first near the source, and, on account of the great
length of course of some of the tropical rivers, and
the excessive evaporation to which they are subjected
(which has necessarily most effect where the current
is slow), not making itself felt in the lower part of
their course till a considerable time afterward.
Thus, the rise of the Nile occurs in Abyssinia in
April, and is not observed at Cairo till about mid-summer.
The fluctuations of this river were a subject
of perpetual wonderment to the ancient civilized
world, and were of course attributed to superhuman
agency; but modern travel and investigation have
not only laid bare the reason of this phenomenon,
but discovered other instances of it, before which
this one shrinks into insignificance.
The maximum rise of the Nile, which is about 40
feet, floods 2,100 square miles of ground; while that
of the Orinoco, in Guiana, which is from 30 to 36
feet, lays 45,000 square miles of savannah under water;
the Brahmaputra at flood covers the whole of
Upper Assam to a depth of 10 feet, and the mighty
Amazon converts a great portion of its 500,000 square
miles of selvas into one extensive lake. But the fluctuations
in the rise of the flood-waters are surpassed
by some of the comparatively small rivers of Australia,
one of which, the Hawkesbury, has been
known to rise 100 feet above its usual level. This,
however, is owing to the river-beds being hemmed in
by lofty abrupt cliffs, which resist the free passage
of a swollen stream.
The increase from the melting of snow in summer
most frequently occurs during the rainy season, so
that it is somewhat difficult to determine, with anything
like accuracy, the share of each in producing
the floods; but in some rivers, as the Ganges and
Brahmaputra, the increase from this cause is distinctly
observable, as it occurs some time after the
rains have commenced, while in the case of the Indus
it is the principal source of flood. When the increase
from melted snow does not occur during the rainy
season, we have the phenomenon of flooding occurring
twice a year, as in the case of the Tigris, Euphrates,
Mississippi, and others; but in most of these
cases the grand flood is that due to the melting of
the snow or ice about the source.
The advantages of this periodical flooding in
bringing down abundance of rich fertile silt—the
Nile bringing down, it is said, no less than 140 millions
of tons, and the Irrawadi 110 millions of tons
annually—are too well known to need exposition
here. Islands are thus frequently formed, especially
at a river’s mouth. Permanent and capacious lakes
in a river’s course have a modifying effect owing to
their acting as reservoirs, as is seen in the St. Lawrence;
while the Red River (North) and others in
the same tract inundate the districts surrounding
their banks for miles. In tropical countries, owing
to the powerful action of the sun, all rivers whose
source is in the regions of perpetual snow experience
a daily augmentation of their volume; while
some in Peru and Chili, being fed only by snow-water,
are dried up regularly during the night.
The course of a river is necessarily the line of lowest
level from its starting-point, and as most rivers
have their sources high up a mountain slope the velocity
of their current is much greater at the commencement.
The courses of rivers seem to be partially
regulated by geological conditions of the
country, as in the case of the San Francisco of Brazil,
which forms with the most perfect accuracy the
boundary-line between the granitic and the tertiary
and alluvial formations in that country; and many
instances are known of rivers changing their course
from the action of earthquakes, as well as from the
silting up of the old bed. The inclination of a river’s
course is also connected with the geological character
of the country; in primary and transition formations,
the streams are bold and rapid, with deep channels,
frequent waterfalls and rapids, and pure waters,
while secondary and alluvial districts present slow
and powerful currents, sloping banks, winding
courses, and tinted waters; the incline of a river is,
however, in general very gentle—the average inclination
of the Amazon throughout its whole course
being estimated at little more than six inches per
mile, that of the Lower Nile less than seven inches,
and of the Lower Ganges about four inches per mile.
The average slope of the Mississippi throughout
its whole length is more than seventeen inches per
mile, while the Rhone is, with the exception of some
much smaller rivers and torrents, the most rapid
river in the world, its fall from Geneva to Lyons
being eighty inches per mile, and thirty-two inches
from Lyons to its mouth.
The velocity of rivers does not depend wholly on
their slope; much is owing to their depth and volume
(the latter being fully proved by the fact that
the beds of many rivers remain unaltered in size and
slope after their streams have received considerable
accessions, owing to the greater rapidity with which
the water runs off); while bends in the course, jutting
peaks of rock or other obstacles, whether at the sides
or bottom, and even the friction of the aqueous particles,
which, though slight, is productive of perceptible
effect, are retarding agencies. In consequence,
the water of a river flows with different velocities
at different parts of its bed; it moves slower
at the bottom than at the surface, and at the sides than
the middle. The line of quickest velocity is the line
drawn along the centre of the current, and in cases
where this line is free from sudden bends or sharp
turns, it also represents the deepest part of the channel.
The average velocity of a river may be estimated
approximately by finding the surface-velocity
in the centre of the current by means of a float which
swims just below the surface, and taking four-fifths
of this quantity as a mean. If the mean velocity in
feet per minute be multiplied by the area of the
transverse section of the stream in square feet, the
product is the amount of water discharged in cubic
feet per minute. According to Sir Charles Lyell,
a velocity of 40 feet per minute will sweep along
coarse sand; one of 60 feet, fine gravel; one of 120
feet, rounded pebbles; one of 180 feet (a little more
than two miles per hour), angular stones the size of
an egg.
“Rivers are the irrigators of the earth’s surface,
adding alike to the beauty of the landscape and the
fertility of the soil; they carry off impurities and
every sort of waste débris; and when of sufficient
volume, they form the most available of all channels
of communication with the interior of continents....
They have ever been things of vitality and
beauty to the poet, silent monitors to the moralist,
and agents of comfort and civilization to all mankind.”
By far the greater portion of them find their
way to the ocean, either directly or by means of semi-lacustrine
seas; but others, as the Volga, Sir-Daria
(Jaxartes), Amu-Daria (Oxus), and Kur (Araxes),
pour their waters into inland seas; while many in the
interior of Asia and Africa—as the Murghab in Turkestan,
and the Gir in the south of Morocco—“lose
themselves in the sands,” partly, doubtless, owing to
the porous nature of their bed, but much more to the
excessive evaporation which goes on in those regions.
SWAMPS AND MARSHES — Élisée Reclus
Marshes proper are shallow lakes, the waters
of which are either stagnant or actuated by
a very feeble current; they are, at least in the temperate
zone, filled with rushes, reeds, and sedge, and
are often bordered by trees, which love to plunge
their roots into the muddy soil. In the tropical zone a
large number of marshes are completely hidden by
multitudes of plants or forests of trees, between the
crowded trunks of which the black and stagnant
water can only here and there be seen. Marshes of
this kind are inaccessible to travelers, except where
some deep channel, winding in the midst of the
chaos of verdure, allows boats to attempt a passage
between the water-lilies, or under some avenue of
great trees with their long garlands of creepers waving
in the shade. Whatever may be the climate, it
would, however, be impossible to draw any distinction,
even the most vague, between lakes and
marshes, as the level of these sheets of water oscillates
according to the seasons and years, and as the greater
number of lakes, principally those of the plains, terminate
in shallow bays which are perfect marshes.
Some very important lacustral basins, among others
Lake Tchad, one of the most considerable in all
Africa, are entirely surrounded by swamps and inundated
ground, which prohibit access to the lake
itself, and prevent its true dimensions from being
known.
In like manner, a portion of the course of many
rivers traverses low regions in which marshes are
formed, either temporary or permanent, the uncertain
limits of which change incessantly with the level
of the current. The borders of great water-courses,
when left in their natural state, are the localities in
which these marshy reservoirs principally exist.
The most remarkable marshes of this kind are perhaps
those crossed by the Paraguay and several of its
tributaries; they consist of wet prairies and interminable
sheets of water, which stretch away like a sea
from one horizon to the other. They have received
the names of Lakes Xarayes, Pantanal, etc. Further
south, certain tributaries of the Parana, the Maloya,
the Batel, and the Sarandi, which cross the State of
Corrientes from northeast to southwest, are nothing
but wide marshes, the water of which overflows
slowly across the grass on the imperceptible slope of
the territory. There is, indeed, one of these marshes,
the Laguna Bera, which drains simultaneously into
the two great rivers of Parana and Uruguay.
In the same way as the low river-shores are frequently
converted into marshes, vast extents of the
seacoasts when but slightly inclined are also covered
over by marshes, which are generally separated from
the main sea by tongues of sand gradually thrown up
by the waves. In these marshes, most of which once
formed a part of the sea and still mark its ancient
outline, the water presents the most varied proportions
of saline admixture. These half dried-up bays
are rarely deep enough to allow of large vessels sailing
in them, and their banks are generally overrun by
the most luxuriant vegetation. The shore constantly
keeps gaining upon them, and thus tends to the increase
of the mainland.
The coasts which surround the Caribbean Sea and
the Gulf of Mexico, and also the Atlantic shores of
North America from the point of Florida to the
mouth of the Chesapeake, are bordered by a very
large number of marine marshes, forming a continued
series over hundreds and thousands of miles in
length. In this immense series of coast-marshes all
kinds of vegetation seem to flourish, and threaten to
get the better of the mud and water, and to convert
them into terra firma. To the south, upon the shores
of Colombia and Central America, the mangroves
and other trees of like species plunge the terminal
points of their aerial roots deep into the mud, crossing
and recrossing in an arch-like form, and retaining
all the débris of plants and animals under the
inextricable network of their natural scaffoldings.
The shores of the Gulf of Mexico, in Louisiana,
Georgia, and Florida, are bordered by cypress
swamps, or forests of cypress (Cupressus disticha);
these strange trees, the roots of which, entirely
buried, throw out above the layer of water which
covers the soil multitudes of little cones, the business
of which is to absorb the air. For millions of
acres nearly all the marshy belt along the seashore
is nothing but an immense cypress swamp, with trees
bare of leaves, and fluttering in the wind their long
hair-like fibres of moss. Here and there the trees
and muddy soil give place to bays, lakes, or quaking-meadows,
formed by a carpet of grass lying upon a
soil of wet mud, or even upon the hidden water. In
Brazil these buoyant beds of vegetation are frequently
met with, and the significant name of tremendal
has been given to them: in Ireland these
are called “quaking-bogs.” The least movement of
the traveler who ventures upon them makes the
soil tremble to some yards’ distance.
To the north of Florida, in the Carolinas and
Virginia, the belt of cypress swamps continues; but in
consequence of the change of climate and vegetation,
the quaking-meadows are gradually converted into
peat-mosses. The surface of the marsh is incessantly
renewed by a carpet of green vegetation, while below,
the dead plants, deprived of air, carbonize slowly
in the moisture which surrounds them: these are
the beds of peat which form upon the ground just
as the layers of coal were formed in previous geological
epochs.
On the southern side, the first great peat-bog of a
well-defined character is the “Dismal Swamp,”
which extends along the frontiers of North Carolina
and Virginia. This spongy mass of vegetation rises
ten feet above the surrounding land. In the centre,
and, so to speak, upon the summit of the marsh, lies
Lake Drummond, the clear water of which is colored
reddish-brown by the tannin of the plants. A canal,
which crosses the Dismal Swamp to connect it with
the adjacent streams, is obliged to make its way along
the marsh by means of locks. To the north of Virginia
peat-bogs proper become more and more numerous;
and in Canada, Labrador, etc., they cover
vast expanses of country. All the interior of the
island of Newfoundland, inside the inclosure formed
by the forests on the shore, is nothing but a labyrinth—a
great part of which is still unknown—of
lakes and peat-bogs; even on the sides of the hills
there are marshes on so steep an incline that the
water from them would disappear and run off in
a stream if it was not stopped by the thick carpet of
plants which it saturates. Many a large peat-bog
which may be crossed dry-shod contains more water
than many lakes filling a hollow of the valley with
deep water.
Opposite Newfoundland, on the other side of the
Atlantic, Ireland is hardly less remarkable for the
enormous development of its peat-mosses or bogs.
These tracts of saturated vegetation, in which Sphagnum
palustre predominates, comprehend nearly two
and a half millions of acres—the seventh part of the
whole island. The inhabitants continue to extract
from them, every year, immense quantities of fuel.
The spaces left by the spade in the vegetable mass
are gradually filled up again by new layers. After
a certain number of years, which vary according to
the abundance of rain, the depth of the bed of water,
the force of vegetation, and the slope of the soil, the
turf “quarry” is formed anew. In Ireland it generally
takes about ten years to entirely fill up again
the trenches, measuring from nine to thirteen feet in
depth, which are made in the bogs on the plains,
when a fresh digging of turf may be commenced.
In Holland, crops of this fuel may be gathered, on
an average, every thirty years. In other peat-moss
districts the period of regeneration last forty, fifty,
and even a hundred years. In France, on the borders
of the Seugne (Charente-Inférieure), it has
been ascertained that ditches five feet deep and nearly
seven feet wide are completely obstructed by
vegetation after the lapse of twenty years. As for the
beds of peat which carpet the sides of mountains,
they take centuries to form afresh.
In Ireland, the Low Countries, the north of Germany
and Russia, heaps of trunks of former forest-trees—oaks,
beech, alder, and other trees—are frequently
discovered, which by their decay have made
way for the peat-mosses. The Sphagnum, too, often
takes possession of ground of which man had previously
made himself master, and in many places
roads, remains of buildings, and other vestiges of
human labor are found below the modern bed of
vegetation by which they are now covered. Certain
peat-bogs in Denmark and Sweden may be considered,
on account of the curiosities which have been
found in them, as perfect natural museums, in which
the relics of the civilization of ancient nations have
been preserved for the savants of our own day.
The air above the peat-mosses of Ireland and other
countries in the world is not often unhealthy, either
because the heat is not sufficient to develop miasma,
or else because the vegetation, by absorbing the water
into its spongy mass, impedes the corruption of the
liquid, and produces a considerable quantity of oxygen.
Further south, the peat-mosses, which are intermixed
with pools of stagnant water, and especially
marshes properly so-called, generate an impure air,
which spreads fever and death over the surrounding
country. Unless marshes are surrounded with dense
forests, which arrest the dispersion of the gases, the
latter exercise a most injurious influence on the general
salubrity of the district; for during dry weather,
a vast area of the bed of the marshes becomes exposed,
and the heaps of organic débris lying on the
bottom decompose in the heat and infect the whole
atmosphere. The average of life is much shorter in
all marshy countries than in the adjacent regions
which are invigorated by running water. In Brescia,
Poland, in the marshes of Tuscany, and in the
Roman plains, the wan and livid complexion of the
inhabitants, their hollow eyes, and their feverish
skin, announce at first sight the vicinity of some
centre of infection. There are some marshes in the
torrid zone where the decomposition of organic remains
goes on with a much greater rapidity than in
temperate climates; no one can venture on the edges
of these districts without peril to his life. As Frœbel
ascertained in his journey across Central America,
the miasma is occasionally produced in such abundance
that not only can it be smelt, but a distinct
impression of it is left upon the palate.
LOWLAND PLAINS — William Hughes
The plateaus and mountain regions of the globe
occupy a large portion of its surface—perhaps
more than half of the whole extent of the land—and
their influence over its climate and other natural conditions
affecting mankind is very great. The highlands
of the Old World—fitted by their physical
attributes to be the home of pastoral and nomad
races—were among the regions earliest occupied by
mankind. From the banks of the Euphrates and the
primeval cities of the Assyrian plain, the course of
the shepherd-warrior—whether directed to the east
or the west—led toward some of the elevated regions
which stretched thence within the same (or nearly
the same) degrees of latitude, and which, at least in
a general sense, are under like conditions of climate.
The highlands of Persia and Afghanistan, in the one
direction, of Syria and the Lesser Asia, in the other,
display abundant evidence, both in traditional and
monumental records, of their early occupation by
man. From the one, the natural order of advance
leads to the fertile plains of India; from the other,
to the shores of the Mediterranean, whence is easy
transit to the peninsula and islands that lie beyond.
But if the highlands of the earth were early the
dwelling-place of the shepherd-warrior, it was within
the adjoining lowland plains and fertile river
basins that the arts of civilization were first called
into being, that towns were built, that population
became numerous, and that systems of social polity
were developed. The lowland plains of Asia and
Europe constitute, in the present day, the most populous
regions of the globe, and include by far the more
numerous portion of the human race. The like regions
in the New World are fast filling with inhabitants,
as the redundant population of older lands is
directed, in an ever-flowing stream, across the waters
of the Atlantic.
The most important and extensive among the lowland
plains of the Old World are the following:
In Asia.—Plain of the Euphrates and Tigris (the
ancient Mesopotamia and Babylonia); Plain of Hindustan,
or Northern India; Plain of China, embracing
the northeast part of that country; Plain of
Siberia; Plain of Turkestan. Among lowland regions
of less importance are the plains of Pegu,
Siam, and Tonquin, all within the Indo-Chinese
peninsula, or India beyond the Ganges.
In Europe.—The Great Eastern Plain, embracing
nearly the whole of Russia; Plain of Hungary,
embracing the middle portion of the valley of the
Danube; Plain of Wallachia and Bulgaria, or the
Lower Danube; Plain of Lombardy, or Northern
Italy; Plain of Languedoc, in the south of France;
Plain of Andalusia, in the south of Spain; Plain of
Bohemia, or basin of the Upper Elbe.
The limits and direction of these regions may be
traced upon any ordinary map, by means of their
coincidence with the great river basins of the Eastern
Hemisphere. They include the longer slopes of the
land, which are directed toward the north and northwest,
as well as the less extensive low grounds which
border the Indian and Pacific Oceans. The Siberian
plain alone comprehends an area equal to that of
Europe, and the rivers by which it is watered are
among the most considerable in the Old World. So
vast an area, under other conditions of climate, might
have become the home of populous nations, the seat
of civilization and empire. But its high latitudes,
which involve the rigor of an Arctic sky, condemn a
large portion of Siberia to the condition of a sterile
wilderness, and must prevent even its more favored
districts from being other than thinly inhabited.
The dreary swamps and morasses of the tundras,
which replace, during the brief summer of those
latitudes, the plain of ice and snow, stretch along the
shores of the Arctic Sea through a vast extent of this
widespread region.
Conditions hardly more favorable belong to the
extreme northern portion of the great plain of
Europe, the slope of which is directed toward the
White Sea and the Arctic basin. But a large portion
of Eastern Europe is inclined toward a southerly
sky, and is watered by rivers which have their outfall
into the Black and Caspian Seas. The Volga, the
longest of European rivers, belongs to the Caspian
basin, the most depressed portion of the entire
region.
The southeastern division of the European lowland,
and the adjacent portions of Asia, constitute the
region of the steppes. These occupy an immense
portion of the empire of Russia, and are among the
most characteristic of the physical features of the Old
World. The steppes are grassy plains—prairies, or
meadows, they would be called in the New World—which
occupy a vast belt of the European and Asiatic
continents. They stretch eastward from the banks of
the Dnieper far into the heart of Asia—along the
shores of the Caspian and Aral Seas, and as far as
the banks of the great river Obi. Indeed, in so far
as their grassy covering and general level expanse—among
the prime characteristics of the steppe-land—are
concerned, a like region may be said to extend to
the eastward through Central Asia, as far as the
Great Wall of China and the valley of the Amour.
This is the “land of grass” of the Mongol shepherd,
the true home of the Tartar nations, whose descendants
yet preserve in their songs the memory of their
famous leader Timour—the Tamerlane of historic
record. So vast is the extent of this grass-covered
region, that a mounted horseman, it has been said,
setting out from one of its extremities at the beginning
of the year, and traveling day and night at his
utmost speed, would find the season of spring elapse
ere he reached its further limits.
The southwestern portion only of the steppe-land
falls within the limits of Europe. This exhibits an
unbroken expanse of level plain—fatiguing to the
eye from its perfect uniformity—dry and burned up
by excessive heat in summer, a pathless expanse of
snow during the opposite season of the year. The
steppe is only productive during the brief time that
the thirsty soil is refreshed by the rains of spring and
early summer. Its aspect is then, for a time, glowing
and verdant; grass and wild flowers cover the earth
with a carpet of varied and attractive hues, and the
wild cattle and horses luxuriate in the abundant pasture.
In the autumn, when the herbage has become
dry and withered, the steppe sometimes exhibits a
vast sheet of rolling flame, the grass being occasionally
fired by accident, at other times intentionally,
for the sake of the young crop which springs up
through the ashes. The illusive phenomena of
mirage—the result of atmospheric refraction, engendered
by the intense dryness of the air—are of frequent
occurrence in the steppe. Sometimes the eye is
cheated by the semblage of a lake, which vanishes on
approach. In other instances, the traveler over these
wild regions appears to see rising before him, and
glittering through the dense mist which often
prevails during the hours of midday heat, the towers
and other buildings of a distant city. Spires, trees,
bridges, rivers, all appear in picturesque combination,
only to sink into confusion as they are approached.
When the spot where the city of enchantment
had seemed to stand is actually reached, there is
found only the long, dry grass, waving as elsewhere
in the surrounding waste. The vast accumulation of
dry sand on the surface gives rise to another phenomenon,
of frequent occurrence on the steppe, resembling
waterspouts upon the sea, excepting that
the column is filled with dust instead of water.
“Suppose the great flat steppe stretched out beneath
the blue sky—nothing visible—no breath of air apparently
stirring—the whole plain an embodiment
of sultriness, silence, and calmness—when gradually
rise in the distance six or eight columns of dust, like
inverted cones, two or three hundred feet high, gliding
and gliding along the plain in solemn company;
they approach, they pass, and vanish again in the distance,
like huge genii on some preternatural errand.”
Such is the region over which the semi-nomad
tribes of Tartar shepherds, who constitute a fraction
of the vast population of the Russian Empire, pasture
their herds. It is only here, within the limits of
Europe, that the camel is successfully reared.
Odessa, the great outport of southern Russia, stands
almost on the edge of the steppe, and the whirlwinds
of dust that pass through its streets, and constitute,
during a portion of the year, one of its chief drawbacks
as a place of residence, furnish obvious evidence
of this proximity. The steppe includes two-thirds
of the Crimean peninsula, the extreme south
of which, however, is traversed by a hill-range of
considerable elevation, and exhibits widely different
features.
Beyond the Dnieper, the Don, and even the Volga,
the same region of alternate grassy plain and sandy
waste stretches far into the Asiatic continent. To the
east and north of the Caspian and the Aral are the
steppes over which roam the hordes of the Khirghiz.
The names of Kara-kum and Kizil-kum, given respectively
to the sandy wastes which extend upon
either side of the river Syr, or Jaxartes, are strikingly
indicative of the general character of the tracts
to which they are applied.
Mr. T. W. Atkinson, in his Travels in Regions
on the Upper and Lower Amour, thus describes the
journey through these wild regions: “For many
miles the sand was hard, like a floor, over which we
pushed on at a rapid pace. After this we found it
soft in places, and raised into thousands of little
mounds by the wind. Our horses were now changed,
and in an hour these mounds were passed, when we
were again on a good surface, riding hard....
Hour after hour went by, and our steeds had been
changed a second time.... In our route there was
no change visible—it was still the same plain; there
was not so much as a cloud floating in the air, that,
by casting a shadow over the steppe, could give a
slight variation to the scene.... The whole horizon
was swept with my glass, but neither man, animal,
nor bird could be seen.... We rode on for several
hours, but there was no change of scene. One spot
was so like another that we seemed to make no progress....
No landmark was visible, no rock protruded
through the sterile soil; neither thorny shrub
nor flowering plant appeared, to indicate the approach
to a habitable region; all around was ‘kizil-kum’
(red sand).”
The perfect solitude and unbroken silence of the
desert are not less characteristic than its wearisome
monotony of surface. No sound of bird or animal
breaks the solemn stillness which reigns around; no
trees expose their foliage to the influence of the
wind. The course of the traveler is still onward,
through the same apparently interminable waste.
“Fourteen hours had passed, and still a desert was
before us. The sun was just sinking below the horizon.
The Kirghiz assured me that two hours more
would take us to pastures and to water.... It had
now become quite dark, and the stars were shining
brilliantly in the deep blue vault. My guides altered
their course, going more to the south. On inquiring
why they made this change, one of them pointed to a
star, intimating that by that they must direct their
course.
“We traveled onward, sometimes glancing at the
planets above, and then anxiously scanning the gloom
around, in the hope of discovering the fire of some
dwelling that would furnish food and water for our
animals. Having ridden on in this manner for many
miles, one of our men stopped suddenly, sprang from
his horse, and discovered that we had reached vegetation.
The horses became more lively and increased
their speed, by which the Kirghiz knew that
water was not far off. In less than half an hour they
plunged with us into a stream, and eagerly began to
quench their terrible thirst, after their long and toilsome
journey.”
The features above described are those of the
steppe region, regarded as a whole. But this aspect
undergoes considerable variation in particular localities.
The Lower Steppes, as those portions of the
great plains which immediately border the Caspian
are termed, exhibit a soil largely impregnated with
saline particles, and contain numerous salt-water
lakes. Some of these lakes furnish a large quantity
of salt, derived by means of evaporation. This region
resembles in aspect the dried-up bed of a sea.
The Caspian, upon which it borders, occupies the
lowest part of a depression below the general level
of the earth’s surface, its waters being 81 feet lower
than those of the Black Sea. The extent of the Caspian
appears to be gradually diminishing.
The features of the steppe-land, however, are exceptional
to the general characteristics of the European
plain, regarded as a whole. Large portions
of its middle and western divisions possess a rich
arable soil, and exhibit annually a waving sea of corn.
The geographical limits of the lowland region are
marked, in the direction of north and south, by the
Black Sea and the Arctic Ocean. The eastwardly portions
of this vast level expanse stretch into the heart
of Asia. In the west it reaches the shores of the Baltic,
and is thence prolonged, with narrower dimensions,
through northern Germany and the low flats
of Holland, until it subsides beneath the water of the
German Ocean. Throughout this vast extent, tertiary
and recent formations prevail, and the abundant
clays, sands, and gravels give their character
to the surface-soil. The plain lying to the south of
the Baltic consists principally of sandy heaths, and
contains, toward the seashore, a vast number of small
lakes or meers.
The low shores of Holland—conquered from the
sea by the persevering industry of the Dutch nation—furnish
a conspicuous example of the sand-hills,
or dunes, which are often found on low sandy coasts,
and which owe their origin to the action of prevailing
winds upon the loose drift-sand. Where no
means are adopted to fix them to the soil, the sand-hills
become agents of destruction, sometimes overwhelming
whole villages in their slow but steady advance
inland. But this is not the case in Holland,
where the ingenuity of the Dutch has converted them
from instruments of destruction into a means of national
preservation. In some of the provinces of the
Netherlands, a large portion of the land is actually
lower than the level of high-water mark, and is therefore
exposed (it might appear) to the ravages of the
adjoining ocean. But from the channel of the Helder
southward, the coast is protected by a line of
broad dunes, or sand-hills, which are partially covered
with grass or heath, and are in some places from
forty to fifty feet in height. These have been formed
by the natural process above adverted to, and still in
operation; the prevalent sea-winds raise banks or
ridges of sand at a short distance from the coast,
which the inhabitants prevent from proceeding
further inland by sowing them with a kind of grass
(arundo arenaria), the long roots of which bind the
whole mass firmly together.
The district of the Landes, in the southwest corner
of France, offers an example of the combined action
of sand and sea which is widely different from the
above in its results. The coast here exhibits a line of
shifting sand, backed toward the interior by a belt of
pine-forest. For a length of nearly two hundred
miles, from the mouth of the Garonne to that of the
Adour, there stretches along the extreme edge of the
sea a range of hills composed of white sand, as fine as
though it had been sifted for an hour-glass. Every
gale changes the shape of these rolling masses of
drift-sand. A strong wind from the land flings millions
of tons of sand per hour into the sea, to be again
washed up by the surf, flung upon the beach, and
with the first Biscay gale blown in whirlwinds inland.
A water hurricane from the west has been
known to fill up with sand many square miles of
shallow lake, driving the displaced waters inland,
dispersing them among the pine-woods, flooding and
frequently destroying the scattered hamlets of the
people, and burying forever their fields of millet and
rye. The shepherds of the Landes pursue their avocation
mounted upon stilts, which raise them above
the reach of the sand-blasts. The pine-forests yield
annually a large supply of resin, the only harvest of
this wild region. Intermixed with the pine-forests,
a chain of shallow and marshy lakes stretches in a
direction parallel to the coast, and at a few miles
inland.
Great Barrier Coral Reef, Queensland, Australia This Reef is composed entirely of Stag’s Horn Coral (Madrepora Hebes)
The lowland plains of the New World are on a
scale of vast magnitude, and, if not superior in extent
to those of the Eastern Hemisphere, yet bear a much
larger proportion to the entire area of the land.
They are watered, moreover, by the longest rivers of
the globe, and enjoy, for the most part, conditions of
situation and climate in the highest degree favorable
to man. Both in North and South America, the
whole central expanse of the continent exhibits a vast
succession of lowland plains, the only division between
the different portions of which is that formed
by the watersheds of its longer rivers—not always
to be traced without difficulty, owing to the generally
level nature of the entire plain. In North
America, the prairies; in South America, the tracts
known as llanos, selvas, and pampas, are included
within the lowland region, and exhibit some of the
most characteristic among the aspects of nature in
the Western world.
The prairies coincide, in a general sense, with the
middle and upper portion of the Mississippi Valley,
embracing the vast region which extends from the
Great Lakes to the base of the Rocky Mountains.
They are covered in their natural state with a rich
herbage, and exhibit a waving sea of grass several
feet high. At intervals, toward the banks of the
rivers, patches of forest vegetation break the uniformity
of the prospect, but the prairie itself is destitute
of trees, and (as the name implies) is merely
a grassy plain, or meadow. Alternate forest and
prairie constitute the great features of natural scenery
in the New World. When the rich soil of the
prairie-land is broken up by the plow—an operation
which is rapidly progressing, year by year, within the
Western States of America—it yields abundant crops
of corn. There are, however, within the vast extent
of the North American continent, immense regions
which yet retain the aspect of the wilderness. It was
within these regions that the buffalo roamed, in vast
herds, and that the native Indian hunter pursued his
game ere the advancing footsteps of the white man
had driven him from his haunts.
The llanos, or savannahs, are vast grassy plains,
which occupy nearly the whole basin of the Orinoco
River, excepting only toward its highest portion,
when they are succeeded by wooded plains. The
llanos resemble in general features the prairies of the
Mississippi Valley, but have for the most part a
lower level, and (owing to the abundant rains of the
torrid zone) are annually inundated by the rivers to
an immense extent. Whole districts, embracing thousands
of square miles, are annually converted, within
the interior plains of South America, into lakes, or
temporary seas of fresh water, to be rapidly evaporated
under the burning rays of a vertical sun. At
the close of the rainy season the llanos are covered
with grass, and form rich natural pasture-grounds.
During the prolonged season of drought which ensues,
the verdure is entirely destroyed, and the
parched earth opens in wide and deep crevices—again
to be laid under water with the recommencement
of the rains.
The selvas, or forest-plains, belong to the valley of
the Amazon, and include an immense area of Brazil,
watered by the lower portion of the great stream and
its chief tributary, the Madera. Vast regions are
here covered by an uninterrupted forest, composed
of trees of giant growth, their boughs interlaced by
immense creeping plants, and the ground beneath
thickly covered with a dense growth of underwood.
To the southward of the forest region are vast grassy
plains, which stretch in that direction into the valley
of the Paraguay.
The pampas, or plains of the Paraguay and Paraná
valleys, exhibit the same luxuriant natural growth of
herbaceous plants as other lowland regions of the
New World. They include an immense region,
which stretches from the neighborhood of the southern
tropic far to the southward of the river Negro
(lat. 39° S.), and from the banks of the Paraná to
the eastern base of the Andes. The pampas are variously
covered with long coarse grass, mixed with
wild oats, clover, and other herbage. The tract of
country known by the name of El Gran Chaco, immediately
to the westward of the upper Paraguay—scarcely
tenanted excepting by wild beasts—exhibits
a luxuriant covering of grass, which springs from a
soil possessed of the highest natural capabilities.
Further south, the plains that extend from Buenos
Ayres to the foot of the Andes are covered, during a
great part of the year, with gigantic thistles, which
grow to the height of seven or eight feet, and are
so thick as to render the country almost impassable.
For nine months of the year the thistles are here the
predominant (and almost the sole) feature of the
vegetable kingdom; but with the heats of summer
they are burned up, and their tall leafless stems are
leveled to the ground by the powerful blast of the
pampero, or southwest wind, which blows from the
snowy ranges of the Andes, after which the ground
is covered for a brief season with herbage. This is
destined, with the returning spring, again to give
place to the stronger vegetation which it had succeeded,
and for a time supplanted.
THE SMELL OF EARTH — G. Clarke Nuttall
A bright fine evening after a day of rain is
one of Nature’s compensations. The air is
peculiarly sweet and fresh, as though the rain had
washed all evil out of it. The mind, relieved from
the depressing influence of continuous rain, is exhilarated,
and, above all, the strong smell of the earth
rises up with a scent more pleasing than many a
fragrant essence. In the town, indeed, this earthy
smell is often obscured by the bricks and mortar
which cover the land, and by the stronger, less wholesome,
odors of human life, but in the country it has
full sway, and fills the whole air with its presence.
Even a slight shower, particularly after drought, is
sufficient to bring out the sweet familiar smell of the
land and thrust it upon our notice.
The smell of freshly turned earth is often regarded
by country lovers as one of the panaceas for the ills
of the flesh, and “follow a plowshare and you will
find health at its tail” has proved a sound piece of
advice to many a weakly town-sick one, over whose
head the threatenings of consumption hung like the
sword of Damocles, though it is possible that it is
the fresh air, and more especially the sunshine, which
are the saving media, and not the mere smell.
But what do we know about this characteristic
smell of the soil? Can we regard it as the mere attribute
of the soil as a simple substance, such an
attribute as is, for instance, the peculiar smell of
leather, or the odor of india-rubber; or can we go
deeper and find that it is really an expression of complexity
below?
Strangely enough this is the case, for the smell of
damp earth is one of the latest sign-posts we have
found which lead us into a world which, until recently,
was altogether beyond our ken. It points
us to the presence, in the ground beneath us, of large
numbers of tiniest organisms, and not merely to
their presence only, but to their activity and life, and
reveals quite a new phase of this activity. A handful
of loose earth picked up in a field by the hedgerow,
or from a garden, no longer represents to us a
mere conglomeration of particles of inorganic mineral
matter, “simply that and nothing more”; we
realize now that it is the home of myriads of the
smallest possible members of the great kingdom of
plants, who are, in particular, members of the fungus
family in that kingdom, plants so excessively
minute that their very existence was undreamed of
until a few years ago.
Some faint idea of their relative size, and of the
numbers in which they inhabit the earth, may be
gleaned from the calculations of an Italian, Signor
A. Magiora, who, a short time ago, made a study of
the question. He took samples of earth from different
places round about Turin and examined them
carefully. In ordinary cultivated agricultural soil
he found there would be eleven millions of these
germs in the small quantity of a gramme, a quantity
whose smallness will be appreciated when it is remembered
that a thousand grammes only make up
about two and a quarter pounds of our English measure.
Thus, a shovelful of earth would be the home
of a thousand times eleven millions of bacteria—but
the finite mind can not grasp numbers of such
magnitude. In soil taken from the street, and, therefore,
presumably more infected with germs, he calculated
that there was the incredible number of
seventy-eight million bacteria to the gramme. Sandy
soil is comparatively free from them, only about one
thousand being discovered in the same amount taken
from sandy dunes outside Turin.
But though the workers were hidden yet their
works were known, for what they do is out of all
proportion to what they are; in fact they perform
the deeds of giants, not those of veriest dwarfs. “By
their works shall ye know them” might be a fitting
aphorism to describe the bacteria of the soil. And
the nature of their deeds is widely various, for though
the different groups are members of one great
family, yet, like the individuals of a human family
that is well organized, they have each of them their
special vocation. In the spring time, when the sun
warms the chilly earth, they act upon the husks that
have protected the seeds against the rigors of the
winter, and crumble them up so that the seedling is
free to grow; they break down the stony wall of the
cherry and plum which has hitherto imprisoned the
embryo; and then, when the young plant starts, they
attach themselves to its roots, assist it to take in all
sorts of nutriment from air and soil, and thus help
it in its fight through life, and when its course has
run they decently bury it. They turn the green
leaves and the woody stem and the dark root back
into the very elements from which they were
built up; they effect its decay and putrefaction,
and resolve it into earth again. “Dust to
dust, ashes to ashes,” is the great life work of the
earth bacteria.
But up to about 1898 the fresh smell of the earth,
the smell peculiar to it, had not been in any way
associated with these energetic organisms, and it was
quite a new revelation to find that it was a direct
outcome of their activity. Among the many bacteria
which inhabit the soil, a new one, hitherto unknown,
has been isolated and watched. It lives,
as is usual with them, massed into colonies, which
have a chalky-white appearance, and as it develops
and increases in numbers it manifests itself by the
familiar smell of damp earth, hence the name that
has been given it—Cladothrix odorifera. Taken
singly, it is a colorless thread-like body, which increases
numerically by continuous subdivisions into
two in the direction of its length. It derives its nutriment
from substances in the soil, which either are,
or have been, touched by the subtle influence of life,
and in the processes of growth and development it
evolves from these materials a compound whose
volatilizing gives the odor in question. This compound
has not yet been fully examined; it is not
named, nor have all its properties been satisfactorily
elucidated, but two facts concerning it stand out
clearly. One is that it is the true origin of the smell
that we have hitherto attributed to earth simply;
and the other, that it changes into vapor under the
same conditions as water does. Therefore, when the
sun, shining after the rain, draws up the water from
the earth in vapor form, it draws up, too, the odorous
atoms of this newly found compound, and these
atoms, floating in the air, strike on our olfactory
nerves, and it is then we exclaim so often, “How fresh
the earth smells after the rain!”
Though moisture, to a certain extent, is a necessary
condition of the active work of these bacteria, yet
the chief reason why the earthy smell should be
specially noticeable after the rain is probably because
this compound has been accumulating in the
soil during the wet period. We only smell substances
when they are in vapor form, and since the compound
under consideration has precisely the same
properties in this respect as water, it will only assume
gaseous form when the rain ceases. The bacteria
have, however, been hard at work all the time, and
when the sun shines and “drying” begins, then the
accumulated stores commence their transformation
into vapor, and the strong smell strikes upon our
senses. For the same reason we notice a similar
sort of smell, though in a lesser degree, from freshly
turned earth. This is more moist than the earth at
the surface, and hence, on exposing it, evaporation
immediately begins which quickly makes itself
known to us through our olfactory nerves.
It may also have been remarked that this particular
odor is always stronger after a warm day
than after a cold one, and is much more noticeable
in summer than in winter. This is because moderate
warmth is highly conducive to the greater increase
of these organisms, and, in fact, in the summer they
are present in far larger numbers and exhibit greater
vitality than in the winter, when they are often more
or less quiescent.
Two other characteristics of Cladothrix odorifera
are worthy of notice as showing the tenacity with
which it clings to life. It is capable of withstanding
extremely long periods of drought without injury; its
development may be completely arrested (for water
in some degree is a necessity with all living things,
from highest to lowest), but its vitality remains
latent, and with the advent of water comes back renewed
activity. But besides drought it is pretty
well proof against poisons. It can even withstand a
fairly large dose of that most harmful poison to the
vegetable world, corrosive sublimate. Hence any
noxious matter introduced into the soil would harm
it little ultimately; the utmost it could do would be
to retard it for a time.
This, then, is the history of the smell of earth as
scientists have declared it unto us, and its recital
serves to further point the moral that the most obvious,
the most commonplace things of everyday life—things
that we have always taken simply for
granted without question or interest—may yet have a
story hidden beneath them. Like sign-posts in a
foreign land, they may be speaking, though in a language
not always comprehended by us, of most fascinating
regions—regions we may altogether miss to
our great loss if we neglect ignorantly the directions
instead of learning to comprehend them.
DESERTS — Élisée Reclus
The most important group of deserts in the
world is that of the Sahara, which extends
across the African continent from the shores of the
Atlantic to the valley of the Nile. This immense
area is more than 3,100 miles from east to west, and
is, on an average, more than 600 miles in breadth;
it is, in fact, equal in size to two-thirds of Europe.
In this region there is only one season, viz., summer,
burning and merciless. It is but rarely that rain
comes to refresh these regions, on which the solar
rays dart vertically down.
The mean altitude of the Sahara is estimated at
2,000 feet; but the level of the soil varies singularly
in the different districts. To the south of Algeria,
the surface of the Chott Mel-R’ir, the remains of an
ancient sea, which communicated with the Mediterranean,
is at the present time more than 165 feet
below the Gulf of Cabes; while to the south and east,
the ground rises into plateaus and mountains of sandstone
or granite to a height varying from 3,300 to
6,660 feet. In the centre of the Sahara stands the
Djebel-Hogger, the sides of which are covered with
snow during three months in the year; from December
to March, its picturesque defiles are traversed
by streams which flow some distance and lose themselves
beneath the surrounding plains. This group
of lofty mountains is the great landmark which forms
the boundary between the eastern deserts, or the Sahara
proper, and the group of western deserts, designated
under the general name of Sahel.
The Sahel is very sandy. Throughout the greater
part of its extent the soil is composed of gravel and
large-grained sand, which does not give way even
under the foot of the camel. Some of the ranges of
sand-hills which rise in this desert are chains of small
hills, composed of heavy sand which resists the influence
of the wind. But in many districts of the Sahel,
the arenaceous particles of the soil are fine and small.
The trade-winds which pass over the desert distribute
these sandy masses into long waves similar to
those of the ocean, and here and there raise them
into movable sand-hills, which overwhelm all the
oases which lie across their path. Traveling toward
the southwest, in which direction they are driven by
the wind, the sands reach the northern shores of the
Niger and Senegal at many points of their course,
and by their incessant deposits gradually drive the
waters of these rivers toward the south. To the west,
the sand of the desert encroaches also upon the ocean.
Off the coast which stretches between Cape Bojador
and Cape Blanco—pointed out from afar by the
highest dunes in the world—a line of sand-banks extends
far out into the sea. A current of sand is, therefore,
constantly passing across the desert from northeast
to southwest. The débris of rocks in a state of
decomposition, and the particles brought to the coast
of the Gulf of Cabes by the tide, which is very powerful
at this point, are driven before the wind into the
plains of the Sahel, and thence, after a journey lasting
hundreds and perhaps thousands of years, they
at last reach the seashore of the Atlantic, in order
to recommence in the oceanic currents another eventful
odyssey.
Some parts of the eastern Sahara are equally
sandy; but the principal parts of the surface of this
desert are occupied by plateaus of rock or clay, and
by groups of grayish or yellowish mountains. The
chains of sand-hills are numerous, and, like those of
the west, they travel incessantly under the impulse
of the wind in a south or southwest direction. The
rocky plateaus are crossed and recrossed here and
there by wide and deep clefts, which are gradually
filled by the drifted sand, and into which the traveler
runs the risk of sinking, like the mountaineer into
the crevasses of a glacier. In the hollows, patches of
salt take the place of the lakes which in more rainy
countries would be found there.
Those districts of the Sahara which are destitute of
oases present a truly formidable aspect, and are fearful
places to travel over. The path which the feet
of the camels have marked out in the immense solitude
points in a straight line toward the spot which
the caravan wishes to reach. Sometimes these faint
footmarks are again covered with sand, and the travelers
are obliged to consult the compass, or examine
the horizon; a distant sand-hill, a bush, a heap of
camels’ bones, or some other indications which the
practiced eye of the Touareg alone can understand,
are the means by which the road is recognized.
Terrible stories are told by the side of the watch-fires
of caravans being overtaken when amid the
sand-hills by a sudden storm of wind, and completely
buried under the moving masses; they also tell of
whole companies losing their way in the deserts of
sand or rocks, and dying of madness after having
undergone all the direst tortures of heat and thirst.
Happily such adventures are rare, even if the accounts
of them are at all authentic. Caravans, when
led by an experienced guide and protected by treaties
and tribute against the attacks of plundering
Arabs and Berbers, nearly always arrive at the end
of their journey without having undergone any other
sufferings than those caused by the intolerable heat,
the want of good water, and the coldness of the
nights; for the nights which follow the burning days
in the Sahara are in general very cold. In fact, the
air of these countries being entirely destitute of
aqueous vapor, the heat collected during the day on
the surface of the desert is, owing to the nocturnal
radiation, again lost in space. The sensation of cold
produced by this waste of heat is most acute, and
especially so to the chilly Arab. Not a year passes
without ice forming on the ground, and white frosts
are frequent.
In all those countries in the Sahara where the water
gushes out in springs or descends in streams from
some group of mountains, there is an oasis formed—a
little green island, the beauty of which contrasts
most strikingly with the barrenness of the surrounding
sands. These oases, compared by Strabo to the
spots dotted over the skin of the panther, are very
numerous, and perhaps comprehend altogether an
area equal in extent to one-third of the whole Sahara.
In the greater part of this region, the oases, far from
being scattered about irregularly, are, on the contrary,
arranged in long lines in the middle of the
desert. The cause of this is either the higher proportion
of moisture contained in the aerial currents
which pass in this direction, or, and perhaps principally,
the subterranean water which follows this
slope, and here and there rises to the surface.
The oases are, par excellence, the country of date-trees;
in the neighborhood of Mourzouk there are
no less than thirty-seven varieties. These trees form
the riches of the tribe, for their fruit supplies food
to man as well as to beast—to dromedaries, horses,
and dogs. Below the wide fan of leaves, which
quiver in the blue air, are thickly growing clumps of
apricot, peach, pomegranate, and orange trees, their
branches loaded with fruit, and vines intertwining
round the trunks; maize, wheat, and barley ripen
under the shade of this forest of fruit-trees, and,
lower still, the modest trefoil fills up the very smallest
intervals of the soil which is capable of irrigation.
To the east of Egypt, which may be considered as
a long oasis situated on the banks of the Nile, the
desert begins again, and borders the whole extent of
the Red Sea. A large part of Arabia presents nothing
but sands and rocks, and toward the southeast,
in the Dahna, there are solitudes which no traveler,
either Arab or Frank, seems yet to have crossed. To
the north and east stretch the Nefouds, or “daughters
of the great desert,” which are much smaller than
the Dahna, but are nevertheless formidable tracts to
travel over. One of these regions, which was crossed
by Palgrave, is that in which the mass of sand, formerly
deposited there by the marine currents, affords
the greatest depth; in certain places it is 330, 400, and
even 500 feet deep. It can be measured by the eye
by descending to the bottom of the funnel-shaped
cavities, which the springs of water, spouting out of
the adjacent granite or calcareous rock, have gradually
hollowed out in the bed of sand. This enormous
bed of material, which represents chains of pulverized
mountains, does not exhibit an even surface, as
one would expect, but, throughout its whole expanse,
presents long symmetrical undulations, similar to
those waves which roll in the Caribbean Sea under
the even influence of the trade-winds. These waves
stretch from north to south, parallel to the meridian;
it is probable that they are owing to the movement
of the earth round its axis. The solid rocks beneath
unresistingly obey the impelling force which carries
them toward the east, but the movable sands which
are above them do not allow themselves to be carried
away with an equal rapidity; each day an infinitesimal
quantity remains behind and seems to glide toward
the west, like the waves of the ocean, the atmospheric
currents, and everything that is movable
on the face of the globe. The parallel furrows of
sand in the Nefouds certainly rise to a greater height
than those of the other deserts, and differ much in
their aspect from the smaller waves of sand formed
by the wind; but the reason is, that the bed of sand
in this region is of a very great bulk, and because at
this point the swiftness of the globe nearly attains its
maximum on account of its vicinity to the equator.
To the east of the Arabian peninsula, the chain of
deserts is prolonged obliquely across Asia. The principal
part of the plateau of Iran, occupying a quadrilateral
space, surrounded by mountains which stop
the rains in their passage, consists of sterile solitudes,
some covered with saline beds, the remains of dried-up
lakes, others spread over with shifting sands,
which the wind blows up into eddies, or dotted over
with reddish-colored hills, which the mirage renders
either nearer or more distant to the eye than they
really are, incessantly modifying them according to
the undulations of the atmosphere. This plateau is
only separated from the steppes of Turkestan by the
Elburz Mountains, and is continued toward the east
by the deserts of Afghanistan and Beloochistan, which
are not so large, and much easier to travel over.
Even the rich peninsula of India is protected by a belt
of sterile tracts situated on the right and left of the
Indus. Between each of the five rivers (Punjaub),
which, by the union of their waters, form the great
river, stretches a line of steppes in which the torrent-waters
of the mountains are soon lost. The soil of
these steppes is nearly everywhere barren, except on
the edge of the irrigation canals constructed by the
inhabitants at a very heavy outlay.
Beyond the mighty central group, whence radiate
far and wide the mountain-chains of Asia, the steppes
and deserts, mutually alternating according to the
topographical conditions, and the abundance or
scarcity of water, extend over a space of more than
1,850 miles between Siberia and China Proper. The
eastern part of this belt is called, according to the
languages, Gobi or Chamo, that is to say, the desert
par excellence, and, from its enormous dimensions,
corresponds with the Sahara of Africa, situated exactly
at the opposite extremity of the long chain of
solitudes which stretches right across the Old World.
The mirage, the moving sand-hills blown up into
eddies, and many other phenomena described by African
travelers, are found in certain districts of the
Gobi, just the same as in all other deserts. But the
cold here is exceptionally intense, on account of the
great height of the plateaus, which is on an average
4,950 feet, and the vicinity of the plains of Siberia,
which are crossed by the polar wind. It freezes nearly
every night, and often during the day. The dryness
of the atmosphere is extreme; there is hardly
any vegetation, and a few grassy hollows are the only
oases of these regions. From Kiahkta to Pekin, there
are only five trees for a distance of 400 to 500 miles,
which is the width of the desert in this part of Mongolia.
The Gobi, however, like the Sahara, was formerly
covered by the waters of the ocean; even on the
elevated plateaus, old cliffs may be noticed, the bases
of which are worn away by the waves, and long
strands of round shingle stretch around the area
which was formerly occupied by a now vanished
gulf.
In North, as in South America, the deserts proper
lie to the west of the continent, and occupy the basins
commanded by the parallel or divergent walls of
the Rocky Mountains.
The most northerly of these American deserts occupies,
to the west of Lake Utah, a part of the space
called the “Great Basin,” and is comprised between
the principal chain of the Rocky Mountains and the
Sierra Nevada of California. The desert of Utah
is an immense surface of clay, dotted over with thin
tufts of artemisia; in certain places, however, it exhibits
no trace of vegetation, and resembles a causeway
of concrete, intersected by innumerable clefts,
forming nearly regular polygons. In the midst of
these solitudes no rivulet flows, and no water-spring
gushes forth; only after journeying for many a long
hour the traveler sometimes comes upon some field of
crystallized salt, a white expanse, on which the clouds
and blue sky are reflected as on the surface of a
lake. On the extreme horizon some volcanic rocks
may be seen, like great scoriæ, half veiled by warm
atmospheric columns, quivering like the air over the
flame of a hot brazier. Across these vast plains, inhabited
only by a prodigious quantity of extraordinarily
shaped lizards, the road employed by the emigrants
used to pass, which was so soon destined to be
supplanted by the Pacific Railway from New York
to San Francisco.
The deserts of North America, crossed here and
there by fertile valleys, extend eastward toward the
basins of the Red River and the Arkansas, where they
blend with the savannas, and to the south into the
Mexican states of Chihuahua, Sonora, and Sinaloa.
But in the tropical zone, which commences beyond
these points, the heavy summer rains and the much
smaller extent of the Mexican territory between the
two oceans, have prevented the formation of deserts.
Regions destitute of trees and verdure are only again
found on the coasts of Peru, to the south of the Gulf
of Guayaquil. The trade-winds, after having discharged
their moisture on the eastern slopes of the
Andes, pass away through the air far above the seashore
on the western side of the mountains, and then
sweep far out to sea over the surface of the Pacific.
The solitudes of the Andes most resembling the
desert regions of the Old World and of the United
States are the elongated plateaus which rise one above
another between the sea and the principal chain of
the Andes, in southern Peru and on the frontiers of
Bolivia and Chili; such as the pampas of Islay and
Tamarugal and the desert of Atacama. The pampa
of Tamarugal, so called from the Tamarugos, or
tamarisks, which grow in the hollows where some
moisture oozes out of the soil, has a mean altitude
of from 2,900 to 3,900 feet. It is a plain nearly covered
with beds of salt, or salares, which are worked
like rock quarries. The strata of salt are so thick,
and rain is so rare upon the plateau, that the houses
of the village of Noria, which are inhabited by the
workmen, are entirely constructed of blocks of salt.
Some deserts, situated to the east of the Tamarugal,
on more elevated plateaus, contain a still larger quantity
of salt. The pampa of Sal, which is overlooked
by the volcano of Isluga, has a mean altitude of not
less than 13,800 feet, and its whole extent, which is
125 miles long and from nine to twenty-four miles
wide, is perfectly white. The depth of salt deposited
upon this plateau varies from five to sixteen
inches, according to the undulations of the ground.
Whence do these enormous masses of salt proceed?
Doubtless from the sea or ancient lakes which formerly
covered these countries and have been gradually
emptied by the rising of the soil. Saline matter
saturates even the rocks and clays, for a film of salt
again forms by efflorescence on all the ground in the
desert from which crops have previously been taken.
The district of Santa-Rosa, which was completely
cleared of salt in 1827, was all white again and fit for
working after a lapse of twenty-three years. Sea-salt
is not the only production of these immense natural
laboratories; but nitrates, sulphates, carbonate of
soda, borates of soda and lime, are also found there
and increase every year in thickness, thanks to
the ephemeral torrents which sometimes descend
loaded with débris from the adjacent Cordilleras.
Saltpetre is also procured from the pampa of Tamarugal,
and is the article which, during all the wars of
Europe and America, gave such great commercial
importance to the town of Iquique.
The desert of Atacama, the largest of all those in
South America, occupies a wide belt of plateaus between
the shores of the Pacific and the high rampart
of the Andes, which separates Bolivia from the Argentine
Republic. This expanse of reddish-colored
rocks, and crescent-shaped shifting sand-hills, is so
repulsively desolate a place that the conquerors of
Chili, whether Incas or Spaniards, never made up
their minds to venture into it, in going along the sea-coast;
they have been obliged to pass far into the
interior, by the plateaus of Bolivia, and to twice cross
the Andes before entering the Chilian valleys. Not
long since, men of science were the only travelers
who dared to enter the desert of Atacama. Nevertheless
this formidable-looking country also possesses,
like the pampa of Tamarugal, great natural
riches, which will not fail to summon the labor of
man and all the progress of civilization to these desolate
regions. Besides salt and saltpetre, this desert
produces guano—that is, heaps of the almost exhaustless
droppings of all the sea-birds which settle
down in clouds upon the seashore. During the course
of centuries the ordure has accumulated into perfect
rocks which the sun dries up, and the surface
of which is but rarely softened by rain. These
masses of detritus, which are, to all appearance, useless
upon these barren shores, are life itself to the
countries of England, France, and Belgium, which
have become exhausted by the extent of cultivation;
and, consequently, this substance constitutes a most
important element of national commerce.
II.—THE SEA — THE PRIMITIVE OCEAN — G. Hartwig
The greatest of all histories, traced in mighty
characters by the Almighty Himself, is that
of the earth-rind. The leaves of this great volume
are the strata which have been successively deposited
in the bosom of the sea or raised by volcanic powers
from the depths of the earth; the wars which it
relates are the Titanic conflicts of two hostile elements,
water and fire, each anxious to destroy the
formations of its opponent; and the historic documents
which bear witness to that ancient strife lie
before us in the petrified or carbonified remains of
extinct forms of organic existence—the medals of
creation.
It is only since yesterday that science has attempted
to unriddle the hieroglyphics in which the past history
of our planet reveals itself to man, and it stands
to reason that in so difficult a study truth must often
be obscured by error; but although the geologist is
still a mere scholar, endeavoring to decipher the
first chapters of a voluminous work, yet even now
the study of the physical revolutions of our globe
distinctly points out a period when the molten earth
wandered, a ball of liquid fire, through the desert
realms of space. In those times, so distant from ours
that even the wildest flight of imagination is unable
to carry us over the intervening abyss, the waters
of the ocean were as yet mixed with the air, and
formed a thick and hazy atmosphere through which
no radiant sunbeams, no soft lunar light ever penetrated
to the fiery billows of molten rock, which at
that time covered the whole surface of the earth.
What pictures of desolation rise before our fancy
at the idea of yon boundless ocean of fluid stone
which rolled from pole to pole without meeting on
its wide way anything but itself. Ever and ever in
the dark-red clouds shone the reflection of that vast
conflagration, witnessed only by the eye of the Almighty,
for organic life could not exist on a globe
which exclusively obeyed the physical and chemical
laws of inorganic nature. But while the fiery
mass with its surrounding atmosphere was circling
through the icy region of ethereal space (the temperature
of which is computed to be lower than 60°
R. below freezing point) it gradually cooled, and
its hitherto fluid surface began to harden to a solid
crust. Who can tell how many countless ages may
have dropped one after the other into the abyss of
the past, ere thus much was accomplished; for the
dense atmosphere constantly threw back again upon
the fiery earth-ball the heat radiating from its surface,
and the caloric of the vast body could escape
but very slowly into vacant space?
Thus millions of years may have gone by before
the aqueous vapors, now no longer obstinately repelled
by the cooling earth-rind, condensed into
rain, and, falling in showers, gave birth to an incipient
ocean. But it must not be supposed that the
waters obtained at once a tranquil and undisturbed
possession of their new domain, for, as soon as they
descended upon the earth, those endless elementary
wars began, which, with various fortunes, have continued
to the present day.
As soon as the cooling earth-rind began to harden,
it naturally contracted, like all solid bodies when no
longer subject to the influence of expanding heat,
and thus in the thin crust enormous fissures and rents
were formed through which the fluid masses below
gushed forth, and, spreading in wide sheets over the
surface, once more converted into vapors the waters
they met with in their fiery path.
But after all these revolutions and vicissitudes
which opposed the birth of ocean, perpetually destroying
its perpetually renewed formation, we come
at last to a period when, in consequence of the constantly
decreasing temperature of the earth-rind and
its increasing thickness, the waters at last conquered a
permanent abode on its surface, and the oceanic empire
was definitely founded.
The scene has now changed; the sea of fire has disappeared,
and water covers the surface of the earth.
The rind is still too thin and the eruptions from below
are still too fluid to form higher elevations above
the general surface: all is flat and even, and land
nowhere rises above the mirror of a boundless ocean.
This new state of things still affords the same
spectacle of dreary uniformity and solitude in all
its horrors. The temperature of the waters is yet
too high, and they contain too many extraneous substances,
too many noxious vapors arise from the clefts
of the earth-rind, the dense atmosphere is still too
much impregnated with poisons to allow the hidden
germs of life anywhere to awaken. A strange and
awful primitive ocean rises and falls, rolls and rages,
but nowhere does it beat against a coast; no animal,
no plant grows and thrives in its bosom; no bird flies
over its expanse.
But, meanwhile, the hidden agency of Providence
is unremittingly active in preparing a new order of
things. The earth-rind increases in thickness, the
crevices become narrower, and the fluid or semi-fluid
masses escaping through the clefts ascend to a
more considerable height.
Thus the first islands are formed, and the first
separation between the dry land and the waters takes
place. At the same time no less remarkable changes
occur, as well in the constitution of the waters as in
that of the atmosphere. The further the glowing
internal heat of the planet retires from the surface,
the greater is the quantity of water which precipitates
itself upon it. The ocean, obliged to relinquish
part of its surface to the dry land, makes up for the
loss of extent by an increase of depth, and the clearer
atmosphere allows the enlivening sunbeam to gild
here the crest of a wave, there a naked rock.
And now also life awakens in the seas, but how
often has it changed its forms, and how often
has Neptune displaced his boundaries since that
primordial dawn?
Alternately rising or subsiding, what was once the
bottom of the ocean now forms the mountain crest,
and whole islands and continents have been gradually
worn away and whelmed beneath the waves of
the sea, to arise and to be whelmed again. In every
part of the world we are able to trace these repeated
changes in the fossil remains imbedded in the
strata that have been successively deposited in the
sea, and then raised again above its level by volcanic
agencies, and thus, by a wonderful transposition, the
history of the primitive ocean is revealed to us by
the tablets of the dry land. The indefatigable zeal
of the geologists has discovered no less than thirty-nine
distinct fossiliferous strata of different ages, and
as many of these are again subdivided into successive
layers, frequently of a thickness of several thousand
feet, and each of them characterized by its peculiar
organic remains, we may form some idea of the vast
spaces of time required for their formation.
The annals of the human race speak of the rise and
downfall of nations and dynasties, and stamp a couple
of thousand years with the mark of high antiquity;
but each stratum or each leaf in the records of our
globe has witnessed the birth and the extinction of
numerous families, genera, and species of plants and
animals, and shows us organic Nature as changeable
in time as she appears to us in space. As, when we
sail to the Southern Hemisphere, the stars of the
northern firmament gradually sink below the horizon,
until finally entirely new constellations blaze
upon us from the nightly heavens; thus in the organic
vestiges of the Palæozoic seas we find no form of life
resembling those of the actual times, but every class
“Seems to have undergone a change
Into something new and strange.”
Then spiral-armed Brachiopods were the chief representatives
of the mollusks; then crinoid star-fishes
paved the bottom of the ocean; then the fishes, covered
with large, thick rhomboidal scales, were
buckler-headed like the Cephalaspis, or furnished
with wing-like appendages like the Pterichthys;
and then the Trilobites, a crustacean tribe, thus
named from its three-lobed skeleton, swarmed in the
shallow littoral waters where the lesser sea-fry
afforded them abundant food. From a comparison
of their structures with recent analogies, it is supposed
that these strange creatures swam in an inverted
position close beneath the surface of the water,
the belly upward, and that they made use of their
power of rolling themselves into a ball as a defence
against attacks from above. The remains of seventeen
families of Trilobites, including forty-five
genera and 477 species, some of the size of a pea,
others two feet long, testify the once flourishing condition
of these remarkable crustaceans, yet but few
of their petrified remains, so numerous in the Silurian
and Devonian strata, are found in the carboniferous
or mountain limestone, and none whatever in
formations of more recent date.
Thus, long before the wind ever moaned through
the dense fronds of the tree ferns and calamites
which once covered the swampy lowlands, and long
before that rich vegetation began to which we are
indebted for our inexhaustible coal-fields, now frequently
buried thousands of feet below the surface
on which they originally grew, the Trilobites belonged
already to the things of the past.
In the seas of the Mesozoic or medieval period,
new forms of life appear upon the scene. A remarkable
change has taken place in the cephalopods;
for the chambered and straightened Orthoceratites
and many families of the order have passed
away, and the spiral Ammonites, branching out into
numerous genera, and more than 600 species, now
flourish in the seas, so that in some places the rocks
seem, as it were, composed of them alone. Some are
of small dimensions, others upward of three feet in
diameter. They are met with in the Alps, and have
been found in the Himalaya Mountains at elevations
of 16,000 feet, as eloquent witnesses of the vast
revolutions of which our earth has been the scene.
Carnivorous, and resembling in habits the Nautili,
their small and feeble representatives of the present
day, their immense multiplication proves how numerous
must have been the mollusks, crustaceans, and
annelides, on which they fed, all like them widely
different from those of the present day.
Then also flourished the Belemnites (Thunder-stones),
supposed by the ancients to be the thunder-bolts
of Jove, but now known to be the petrified
internal bones of a race of voracious ten-armed
cuttle-fishes, whose importance in the Oolitic or
cretaceous seas may be judged by the frequency of
their remains and the 120 species that have been
hitherto discovered. Belemnites two feet long have
been discovered, so that, to judge by analogies, the
animals to which they belonged as cuttle-bones must
have measured eighteen to twenty feet from end to
end, a size which reduces the rapacious Onychoteuthis
of the present seas to dwarfish dimensions.
But of all the denizens of the Mesozoic seas, none
were more formidable than the gigantic Saurians,
whose approach put even the voracious sharks to
flight. The first of these monsters that raises its
frightful head above the waters is the dreadful
Ichthyosaurus, a creature thirty or even fifty feet
long, half fish, half lizard, and combining in strange
assemblage the snout of the porpoise, the teeth of
the crocodile, and the paddles of the whale. Singular
above all is the enormous eye, in size surpassing a
man’s head. Woe to the fish that meets its appalling
glance! No rapidity of flight, no weapon, be it sword
or saw, avails, for the long-tailed, gigantic Saurian
darts like lightning through the water, and its dense
harness bids defiance to every attack. Not only have
fifteen distinct species of Ichthyosauri been distinguished,
but the remains of crushed and partially
digested fish-bones and scales which are found
within their skeleton indicate the precise nature of
their food. Their fossil remains abound along the
whole extent of the Lias formation, from the coast to
Dorset, through Somerset and Leicestershire to the
coast of Yorkshire, but the largest specimens have
been found in Franconia. Along with this monster,
another and still more singular deformity makes its
appearance, the Plesiosaurus, in which the fabulous
chimæras and hydras of antiquity seem to start
into existence. Fancy a crocodile twenty-seven feet
long, with the fins of a whale, the long and flexible
neck of a swan, and a comparatively small head.
With the appearance of this new tyrant, the last hope
of escape is taken from the trembling fishes; for into
the shallow waters inaccessible to the more bulky
Ichthyosaurus the slender Plesiosaurus penetrates
with ease.
A race of such colossal powers seemed destined
for an immortal reign, for where was the visible
enemy that could put an end to its tyranny? But
even the giant strength of the Saurians was obliged
to succumb to the still more formidable power of
all-changing time, which slowly but surely modified
the circumstances under which they were called into
being, and gave birth to higher and more beautiful
forms.
In the Tertiary period, the dreadful reptiles of the
Mesozoic seas have long since vanished from the
bosom of the ocean, and cetaceans, walruses, and
seals, unknown in the primitive deep, now wander
through the waters or bask on the sunny cliffs. With
them begins a new era in the life of the sea. Hitherto
it has only brought forth creatures of base and brutal
instinct, but now the Divine spark of parental affection
begins to ennoble its more perfect inhabitants
and to point out the dim outlines of the spiritual
world.
During all these successive changes the surface
of the earth has gradually cooled to its present temperature,
and many plants and animals that formerly
enjoyed the widest range must now rest satisfied
with narrower limits. The sea-animals of the North
find themselves forever severed from their brethren
of the South by the impassable zone of the tropical
ocean; and all the fishes, mollusks and zoophytes,
whose organization requires a greater warmth, confine
themselves to the equatorial regions.
As the Tertiary period advances toward the present
epoch, the species which flourished in its prime
become extinct, like the numberless races which preceded
them; new modifications of life, more and
more similar to those of the present day, start into
existence; and, finally, creation appears with increasing
beauty in her present rich attire.
Thus old Ocean, after having devoured so many
of his children, has transformed himself at last into
our contemporaneous seas, with their currents and
floods, and the various animals and plants, growing
and thriving in their bosom.
Who can tell when the last great revolutions of the
earth-rind took place, which, by the upheaving of
mighty mountains or the disruption of isthmuses,
drew the present boundaries of land and sea? or who
can pierce the deep mystery which veils the future
duration of the existing phase of planetary life?
So much is certain, that the ocean of the present
day will be transformed as the seas of the past have
been, and that “all that it inhabit” are doomed to
perish like the long line of animal and vegetable
forms which preceded them.
We know by too many signs that our earth is
slowly but unceasingly working out changes in her
external form. Here lands are rising, while other
areas are gradually sinking, here the breakers perpetually
gnaw the cliffs and hollow out their sides,
while in other places alluvial deposits encroach upon
the sea’s domain.
However slowly these changes may be going on,
they point to a time when a new ocean will encircle
new lands, and new animal and vegetable forms arise
within its bosom. Of what nature and how gifted
these races yet slumbering in the lap of time may be.
He only knows whose eye penetrates through all
eternity; but we can not doubt that they will be superior
to the present denizens of the ocean.
Hitherto the annals of the earth-rind have shown
us uninterrupted progress; why, then, should the future
be ruled by different laws? At first the sea only
produces weeds, shells, crustacea; then the fishes
and reptiles appear; and the cetaceans close the vista.
But is this the last word, the last manifestation of
oceanic life, or is it not to be expected that the future
seas will be peopled with beings ranking as high
above the whale or dolphin as these rank above the
giant Saurians of the past?
THE FLOOR OF THE OCEAN — John James Wild
If we wish to form a perfect idea of the distribution
of land and water, we must consider not only
the length and breadth of the areas occupied, but
also the height of the land and the depth of the
water; in other words, the volume of those portions
of the solid crust of the earth which are raised above
the level of the sea, and the volume of the masses
of water which fill up the depressed portions of the
earth’s crust. We are thus led to regard the surface
of the solid crust of our planet as composed of
heights and hollows, of areas of elevation and areas
of depression, and, as a next step, to discriminate between
these areas—not according to the usual standard
of the level of the sea, but according to their
relative distance from the centre of the earth. In
this sense we may conceive an area of elevation—i. e.,
a raised portion of the earth’s surface, which may be
partially or entirely covered with water, and an area
of depression—i. e., a hollow in the same surface,
which may be raised above the level of the sea,
and from dry land or the basin of an inland sea
or lake.
If we examine a chart of the world in the light
which has been thrown upon this question by all the
reliable soundings obtained up to the present, it will
be found that continents and islands which we have
been in the habit of considering as separated from
each other by wide seas and deep straits virtually
form part of the same area of elevation; and, in a
similar manner, that certain oceans and seas, which
we are accustomed to distinguish by separate names,
form part of the same area of depression. It will
also appear that, with the exception of the islands
scattered over the face of the ocean and of the Antarctic
region, all the dry land at present existing
may be reduced to one large area of elevation gravitating
to the North Pole, as the common centre of the
principal land masses; similarly, if we except the
Arctic region and other inland basins, all the oceans
and seas compose a single vast area of depression,
with the South Pole for common centre of the larger
accumulations of water on this globe. The Arctic
region forms a distinct area of depression placed
in the centre of the great area of elevation, and the
Antarctic region, according to the evidence we at
present possess, is an area of elevation, surrounded on
all sides by the above-described great area of depression.
The numerous small islands that crop up in
the middle of the oceanic basins are generally found
associated in groups, and they belong to areas of elevation
at the present time submerged, that is to say,
in the condition in which we know the dry land to
have been at an epoch more or less remote in the history
of our planet. In support of the above generalization,
we may point to the following facts as established
by recent soundings. The 100-fathom line, as
is well known, joins the whole of the British Islands,
including the Hebrides, Orkneys, and Shetland Islands,
to the continent of Europe. It forms a broad
band connecting the Asiatic and American continents
across Behring Strait. It unites Australia,
Papua, and Tasmania in a single area of elevation,
which, together with the intervening archipelago of
Java, Sumatra, Borneo, Celebes, the Moluccas, and
the Philippines, may be looked upon as a prolongation
of the continent of Asia. It joins Ceylon to Hindostan
and the Falkland Islands to the South American
continent. The 500-fathom line connects North
America, Greenland, Iceland, the Faroe Islands, and
the continent of Europe, the only unexplored space
being Denmark Strait, between Iceland and Greenland,
where the soundings may exceed the above
depth. The 1,000-fathom line unites New Zealand
with Australia, Madagascar with Africa, and nearly
exhausts the depth of the more or less landlocked
seas which lie between Australia and Asia, Africa
and Europe, South and North America, and of the
seas situated within the Arctic and Antarctic Circles.
The Cape de Verde Islands and the Canaries belong
to Africa, Madeira to Europe, and less than 500
fathoms divide Norway from Spitzbergen.
Depths from 100 to 1,000 fathoms may be considered
as shallow in comparison with the prevailing
depths from 2,000 to 3,000 fathoms of the principal
oceanic basins, and sufficient to establish a connection
between islands and continents, the more so as we
generally find one or more islands occupying the
intervening space, thus betraying the common link
between them.
The result of this examination is that all the larger
land masses compose an area of elevation which,
after nearly completing the circuit of the world in the
latitude of the Arctic Circle, subdivides itself into
two parts, an eastern and a western one—the former
embracing Europe, Africa, Asia, and Australia, the
latter North and South America. In a similar manner,
the different oceans combine into an area of depression
which, after making the circuit of the world
along the parallel of lat. 60° S. under the name
of the Southern Ocean, divides itself into three large
basins, respectively designated as the Pacific, the Atlantic,
and the Indian Oceans. Thus the two elements,
land and water, starting from opposite hemispheres,
extend their arms across the equator, holding
each other in close embrace, like two champions
wrestling for the mastery of the world.
A comparison of the deep-sea soundings obtained
up to the present date shows that, if we omit the seas
situated beyond the parallels of lat. 60° N. and lat.
60° S.—no depths exceeding 2,000 fathoms having as
yet been ascertained beyond these latitudes—the average
depth of the ocean between these parallels may
be estimated at about 2,500 fathoms, or more roughly
at three English miles, and the average depth of all
seas on the surface of the globe at probably two
miles.
Contrary to the ideas formerly entertained of the
enormous depth of the ocean, the soundings of
H.M.S. Challenger, S.M.S. Gazelle, and of the
U.S.S. Tuscarora and Gettysburg, indicate that
depths of five miles, or even 4,000 fathoms, are but
seldom met with, and are as exceptional as heights
of the same amount on land.
One of the greatest depths ascertained in the Atlantic
was found by H.M.S. Challenger, about eighty
miles north of the island of St. Thomas in the West
Indies. It is 3,875 fathoms, or about four and a half
miles. In May, 1876, the Gettysburg found 3,593
fathoms only eleven miles south of the Challenger
sounding. A depth of 3,370 fathoms obtained by the
American ship shows that the deepest area in the
Atlantic is placed to the northward of the Virgin
Islands, and extends over 400 miles along the meridian
of 65° W.
The greatest depth observed in the Indian Ocean
was discovered by the Gazelle in May, 1875. Two
soundings of 3,020 and 3,010 fathoms were taken in
the eastern extremity of this ocean between the northwest
coast of Australia and the line of islands extending
from Java to Timor.
The greatest of all depths of which we have reliable
evidence was found by the Challenger on the
23d March, 1875, in the comparatively narrow
channel which separates the Caroline Islands from
the Mariana or Ladrone Islands. This sounding
amounts to 4,575 fathoms, or about five miles and a
quarter. Several soundings exceeding 4,000 fathoms
were obtained by the Tuscarora to the eastward of
the islands of Nippon and Yesso, and another close to
the most westerly of the Aleutian Islands. Two of
these soundings are over 4,600 fathoms, but as it
appears that no sample of the bottom was brought up,
there is no evidence of the latter having been reached.
H.M.S. Challenger, shortly after her departure from
Yokohama, sounded 3,950 and 3,625 fathoms, and in
doing so seems to have just touched the southern border
of this deep but narrow area of depression, which
runs parallel to the eastern coasts of Japan and the
Kurile Archipelago as far as the entrance to the
Behring Sea.
It will be observed that the above exceptional
depths in the Atlantic, Indian, and Pacific Oceans
are not placed, as one might be inclined to conjecture,
in or near the centre of these oceanic basins, but,
on the contrary, upon their confines and in close
proximity to the land. This remarkable circumstance
suggests the idea that such areas of maximum
depression may be the effect of a sinking of the bottom
of the sea in compensation for an upward movement
of the land in their immediate vicinity.
Just as the results of the recent soundings have
rendered the existence of depths from six to nine
miles, as formerly reported, highly improbable, so
have they modified our ideas of the shape of the sea-bottom.
The latter was generally represented as a
repetition of the dry land with its combination of
mountain, valley, and plain. No doubt the sea-bottom
within a short distance of the shore naturally
forms a continuation of the leading features of the
adjoining land. Thus a large plain or a low-lying
country will, as a rule, continue its almost level slopes
to a considerable distance out to sea, while a range
of hills or a chain of mountains often extends its
steep inclines below the surface of the water.
The alteration of level in mid-ocean between two
points as much as a hundred miles apart is generally
so slight that to an observer standing at the bottom of
the sea, the latter would appear a perfect plain. Thus
the bottom of our oceanic basins is composed of gentle
undulations rising and falling from a few fathoms to
two or three miles, in distances extending over many
hundred miles. This view accords with the experience
of the geologist who finds that the bulk of the
dry land consists of sedimentary strata originally laid
down in a horizontal, or nearly horizontal, position
at the bottom of the sea, and there can be little doubt
but that the depths of the ocean are at the present
time the scene of the formation of sedimentary strata
which some day may be converted into dry land, and
contain imbedded in their folds traces of the animal
life with which they abound.
One of the most remarkable results in connection
with the exploration of the sea is the discovery of
several extensive submarine plateaus, which interrupt
what was until lately supposed to be an unbroken
waste of fathomless abyss. One of these plateaus
traverses the Atlantic Ocean in its whole length from
north to south, repeating in its form the S-shaped
contour of the eastern and western shores of this
ocean. After attaching itself by its northern end to
the plateau which connects Europe and Iceland, and
separates the Atlantic from the Arctic basin, it runs
southward toward the Azores, and, gradually contracting
in width, sweeps round toward St. Paul’s
Rocks. Reduced, comparatively speaking, to a narrow
ridge, it follows the line of the equator as far
as the meridian of Ascension Island, where, resuming
its southward course, it widens out until in lat.
30° S. it occupies nearly half the space between
South America and Africa, uniting the island of Ascension
with St. Helena in the east, Trinidad in the
west, and the group of Tristan d’Acunha and Gough
Island at its southern end.
Considerable portions of this plateau are within
1,500 fathoms, or a mile and a half, of the surface
of the sea, and most of the islands are of volcanic
origin. An extinct volcano, 8,300 feet in height,
forms the island of Tristan d’Acunha; Ascension Island
rises to 2,800 feet, and the summit of Pico in the
Azores to 7,600 feet above the level of the sea. The
northern end of the plateau joins the plateau of
Iceland with its still active focus of eruption.
By this central plateau, the Atlantic Ocean is divided
into two longitudinal areas of depression or
channels, one following the shores of North and
South America, the other the shores of Europe and
Africa. The depths vary from 2,000 to nearly 4,000
fathoms, the average depth being about three miles.
The deepest portion of the eastern channel is situated
to the westward of the Cape de Verde Islands,
and forms an area of depression of over 3,000 fathoms.
In the western channel there are two such
depressions, one placed between the Antilles, Bermudas,
and the Azores, the other between Cape St.
Roque, Ascension, and Trinidad. They are divided
from each other by a submarine elevation, which apparently
connects the central plateau with the South
American continent.
The soundings taken in the Indian Ocean prove the
existence of a submerged plateau on the limit between
the Indian Ocean and the Southern Ocean.
It rises in many parts to within 1,500 fathoms of the
sea-surface, and forms the common foundation of all
the islands situated in this part of the world—viz.,
Prince Edward Island, the Crozet Islands, the Kerguelen
group, the Heard Islands, and the islands of
St. Paul and Amsterdam. The origin of all these
islands is probably volcanic.
The main basin of the Indian Ocean with an average
depth of over 2,000 fathoms, stretches from the
meridian of the Cape of Good Hope toward the
angle between Java and northwestern Australia,
where it attains its greatest depths, forming a depression
of over 3,000 fathoms. It communicates with
the Arabian Sea by two narrow channels situated
north and south of the Chagos Archipelago, being
nearly cut off from that sea by a line of islands and
shallow soundings which connect Africa, Madagascar,
Bourbon, and Mauritius, the Chagos Islands and
the Maldive Islands with the Asiatic continent. The
2,500-fathom area of the Indian Ocean crosses the
parallel of lat. 40° S. between St. Paul and Amsterdam
Islands and Cape Leeuwin in Australia, and
forms, between the south coast of Australia and the
forty-fifth parallel, an area of depression which extends
beyond the southern end of Tasmania, includes
the deepest portion of the basin between New
South Wales and New Zealand, and probably communicates
with the depths of the Pacific by a channel
situated off the southern extremity of New Zealand.
If we divide the Pacific Ocean into an eastern and
a western half by a line passing from Honolulu to
Tahiti, or by the meridian of long. 150° W., we observe
a remarkable contrast between the two portions
thus formed. While the eastern half, extending toward
America, presents a vast unbroken sheet of
water, almost devoid of islands, the western half, toward
Asia and Australia, and inclosed between the
parallels of lat. 30° N. and lat. 30° S., is composed of
a labyrinth of seas, separated from each other by
chains of islands, the projecting points of numerous
submarine ridges. Although extensive tracts in the
Pacific Ocean remain as yet untouched by the sounding-line,
the observations made by the Challenger,
the Gazelle, and the Tuscarora enable us to form an
idea of the general contours of its bottom. From the
shores of North and South America, the depths of
the eastern half of the Pacific gradually increase until,
upon the line between Honolulu and Tahiti, they
attain 3,000 fathoms. The latter depth forms extensive
areas of depression in the western half of this
ocean, and increases to 4,000 fathoms in the already
described hollow extending along the Japanese and
Kurile Islands toward the entrance of the Behring
Sea. Thus the idea formerly entertained of the inferior
depths of the Pacific in comparison with the
Atlantic, founded apparently upon the large number
of islands scattered over its surface, is proved to be
erroneous. Many of these islands, especially in the
northwestern half, rise immediately from depths of
3,000 fathoms and more.
In the southeastern portion of the Pacific there are
indications of a submerged plateau connecting the
Society Islands, the Low Archipelago, the Marqueses,
and the intervening islands of Easter Island
and Juan Fernandez with the coast of Chili and
Patagonia. It seems as if an almost uninterrupted
area of elevation crossed the whole basin of the Pacific
in a northwesterly direction from Patagonia to
Japan. The tendency of most of the submerged
ridges of this ocean to follow the same direction has
been frequently commented upon, and, as is the case
with the submerged plateaus of the Indian and Atlantic
Oceans, their association with centres of volcanic
activity is equally evident.
A line passing from Kamtchatka over Japan, the
Ladrone, Caroline, Marshall, Gilbert, Ellice, Samoa,
Tonga, and Kermadec Islands to New Zealand,
divides the main basin of the Pacific, of an average
depth of 3,000 fathoms, from the much shallower
seas lying to the westward, and may possibly have
formed the coast-line of a large continent which existed
at a remote epoch in the history of the surface
of our planet, and the boundaries of which have since
been driven back to the present confines of Asia and
Australia.
The Southern Ocean, which makes the circuit of
the world along the parallel of 60° S., in length equal
to half the circumference of the earth at the equator,
may be considered as occupying the space between
the Antarctic Circle and the parallel of lat. 40° S.
Owing to the limited number of soundings as yet obtained
within its limits, we can only form a general
idea of the distribution of its depths.
The boundary-line of the fortieth parallel, which
separates the Southern Ocean from the Pacific, Atlantic,
and Indian Oceans, is occupied alternately by
areas of depression, with depths ranging from 2,500
to nearly 3,000 fathoms, and by areas of elevation,
or submarine plateaus, approaching to within 1,500
fathoms of the sea-surface. With regard to the general
distribution of depth in the Southern Ocean, its
bottom appears to rise gradually from nearly 3,000
fathoms at the fortieth parallel (with the exception
of the intervening plateaus) to little over 1,500 fathoms
at the Antarctic Circle. There are also indications
of an area of depression of an average depth
of 2,000 fathoms, making the circuit of the globe between
the parallels of 50° and 60° lat. S. The whole
surface of the Southern Ocean is strewn with masses
of floating ice, some of them forming islands many
miles in extent, and rising from 100 to 300 feet
above the level of the sea—an imposing spectacle,
but fraught with much danger to the navigator in
these regions. It is this central ocean which supplies
the masses of cold water that fill up nearly two-thirds
of the total depth of the Atlantic, Pacific, and Indian
Oceans.
We are indebted to Sir James Ross for the first
soundings procured within the Antarctic Circle.
They are situated in the wide inlet, discovered by
that illustrious navigator in the year 1840, which extends
along the meridian of New Zealand, and terminates
at the foot of Mount Erebus and Mount Terror.
These soundings, which are all under 500 fathoms,
viewed in combination with the above-mentioned
gradual rise of the bottom of the Southern
Ocean toward the Antarctic Circle, justify the assumption
that the seas included within the latter do
not exceed 1,500 fathoms in depth, their average
depth probably falling below this estimate. The extensive
formation of ice in this region, as well as the
numerous indications of land reported by the daring
sailors who have penetrated so far south, suggest
the hypothesis of the existence, if not of an Antarctic
continent, at all events of a considerable extent of
land, rising in the mountain ranges and volcanoes of
Victoria Land to 10,000 and 15,000 feet above the
level of the sea.
The region inclosed within the Arctic Circle forms
an area of depression, almost completely surrounded
by the land-masses of the great eastern and western
continents. A shallow strait of less than fifty fathoms
in depth connects it with the Pacific Ocean, and
it is separated from the depths of the Atlantic by the
plateau between the British Islands and Iceland,
which rises to within 500 fathoms of the sea-surface.
Greenland is probably the largest land-mass belonging
to this basin, and next in importance we have the
group of Spitsbergen, of Franz Joseph Land, discovered
by the Austrian expedition; Nova Zembla,
the Liakhov Islands, Kellett Land, off Behring
Strait, discovered by the Americans in 1867; and
finally the extensive archipelago, a continuation of
the American continent.
The few soundings taken within the Arctic Circle
leave much to conjecture, but we are tolerably safe
in stating that the average depth of the Arctic basin
is probably under 1,000 fathoms. The immense
plains of Northern Asia and America seem to continue
beneath the surface of the Arctic Sea, as indicated
by the numerous islands which skirt the coasts
of these continents, and the greatest depths to be
found inside the Arctic Circle are probably confined
to the basin situated between Greenland and
Norway, Iceland and Spitzbergen.
CORAL FORMATIONS — Charles Darwin
I will give a very brief account of the three great
classes of coral-reefs: namely, Atolls, Barrier,
and Fringing-Reefs, and will explain my views on
their formation. Almost every voyager who has
crossed the Pacific has expressed his unbounded astonishment
at the lagoon islands, or as I shall for the
future call them by their Indian name of atolls, and
has attempted some explanation. Even as long ago
as the year 1605, Pyrard de Laval well exclaimed,
“C’est une meruille de voir chacun de ces atollons,
enuironné d’un grand banc de pierre tout autour,
n’y avant point d’artifice humain.” The immensity of
the ocean, the fury of the breakers, contrasted with
the lowness of the land and the smoothness of the
bright green water within the lagoon, can hardly be
imagined without having been seen.
The earlier voyagers fancied that the coral-building
animals instinctively built up their great circles
to afford themselves protection in the inner parts;
but so far is this from the truth that those massive
kinds, to whose growth on the exposed outer shores
the very existence of the reef depends, can not live
within the lagoon, where other delicately branching
kinds flourish. Moreover, on this view, many species
of distinct genera and families are supposed to combine
for one end; and of such a combination, not a
single instance can be found in the whole of nature.
The theory that has been most generally received is,
that atolls are based on submarine craters; but when
we consider the form and size of some, the number,
proximity, and relative positions of others, this idea
loses its plausible character: thus, Suadiva atoll is
44 geographical miles in diameter in one line, by
34 miles in another line; Rimsky is 54 by 20 miles
across, and it has a strangely sinuous margin; Bow
atoll is 30 miles long and on an average only 6 in
width; Menchicoff atoll consists of three atolls united
or tied together. This theory, moreover, is totally
inapplicable to the northern Maldive atoll in the
Indian Ocean (one of which is 88 miles in length,
and between 10 and 20 in breadth), for they are not
bounded like ordinary atolls by narrow reefs, but
by a vast number of separate little atolls; other little
atolls rising out of the great central lagoon-like
spaces. A third and better theory was advanced by
Chamisso, who thought that from the corals growing
more vigorously where exposed to the open sea, as
undoubtedly is the case, the outer edges would grow
up from the general foundation before any other part,
and that this would account for the ring or cup-shaped
structure. But we shall immediately see
that in this, as well as in the crater theory, a most
important consideration has been overlooked; namely,
on what have the reef-building corals, which
can not live at a great depth, based their massive
structures?
Numerous soundings were carefully taken by
Captain Fitz Roy on the steep outside of Keeling
atoll, and it was found that within ten fathoms the
prepared tallow at the bottom of the lead invariably
came up marked with the impressions of living
corals, but as perfectly clean as if it had been dropped
on a carpet of turf; as the depth increased, the impressions
became less numerous, but the adhering
particles of sand more and more numerous, until
at last it was evident that the bottom consisted of a
smooth sandy layer: to carry on the analogy of the
turf, the blades of grass grew thinner and thinner,
till at last the soil was so sterile that nothing sprang
from it. From these observations, confirmed by
many others, it may be safely inferred that the utmost
depth at which corals can construct reefs is between
20 and 30 fathoms. Now there are enormous areas
in the Pacific and Indian Oceans in which every
single island is of coral formation, and is raised only
to that height to which the waves can throw up fragments
and the winds pile up sand. Thus the Radack
group of atolls is an irregular square, 520 miles long
and 240 broad; the Low Archipelago is elliptic-formed,
840 miles in its longer, and 420 in its shorter
axis: there are other small groups and single low
islands between these two archipelagoes, making a
linear space of ocean actually more than 4,000 miles
in length, in which not one single island rises above
the specified height. Again, in the Indian Ocean
there is a space of ocean 1,500 miles in length, including
three archipelagoes, in which every island
is low and of coral formation. From the fact of
the reef-building corals not living at great depths, it
is absolutely certain that throughout these vast areas,
wherever there is now an atoll, a foundation must
have originally existed within a depth of from 20 to
30 fathoms from the surface. It is improbable in
the highest degree that broad, lofty, isolated, steep-sided
banks of sediment, arranged in groups and
lines hundreds of leagues in length, could have been
deposited in the central and profoundest parts of the
Pacific and Indian Oceans, at an immense distance
from any continent, and where the water is perfectly
limpid. It is equally improbable that the elevatory
forces should have uplifted, throughout the above
vast areas, innumerable great rocky banks within 20
to 30 fathoms, or 120 to 180 feet, of the surface of
the sea, and not one single point above that level;
for where on the whole face of the globe can we find
a single chain of mountains, even a few hundred
miles in length, with their many summits rising within
a few feet of a given level, and not one pinnacle
above it? If then the foundations, whence the atoll-building
corals sprang, were not formed of sediment,
and if they were not lifted up to the required
level, they must of necessity have subsided into it; and
this at once solves the difficulty. For as mountain
after mountain, and island after island, slowly sank
beneath the water, fresh bases would be successively
afforded for the growth of the corals. It is impossible
here to enter into all the necessary details, but I
venture to defy any one to explain in any other manner
how it is possible that numerous islands should
be distributed throughout vast areas—all the islands
being low—all being built of corals.
Matterhorn, Valais Alps, Switzerland
Before explaining how atoll-formed reefs acquire
their peculiar structure, we must turn to the second
great class, namely, Barrier-reefs. These either extend
in straight lines in front of the shores of a continent
or of a large island, or they encircle smaller
islands; in both cases, being separated from the land
by a broad and rather deep channel of water, analogous
to the lagoon within an atoll. It is remarkable
how little attention has been paid to encircling barrier-reefs;
yet they are truly wonderful structures.
Encircling barrier-reefs are of all sizes, from three
miles to no less than forty-four miles in diameter;
and that which fronts one side, and encircles both
ends, of New Caledonia, is 400 miles long. Each
reef includes one, two, or several rocky islands of
various heights; and in one instance, even as many
as twelve separate islands. The reef runs at a greater
or less distance from the included land; in the Society
Archipelago generally from one to three or
four miles; but at Hogoleu the reef is 20 miles on
the southern side, and 14 miles on the opposite or
northern side, from the included islands. The depth
within the lagoon-channel also varies much; from 10
to 30 fathoms may be taken as an average; but at
Vanikoro there are spaces no less than 56 fathoms
or 336 feet deep. Internally the reef either slopes
gently into the lagoon-channel, or ends in a perpendicular
wall, sometimes between two and three hundred
feet under water in height; externally the reef
rises, like an atoll, with extreme abruptness out of
the profound depths of the ocean. What can be
more singular than these structures? We see an
island, which may be compared to a castle situated
on the summit of a lofty submarine mountain, protected
by a great wall of coral-rock, always steep
externally and sometimes internally, with a broad
level summit, here and there breached by narrow
gateways, through which the largest ships can enter
the wide and deep encircling moat.
As far as the actual reef of coral is concerned, there
is not the smallest difference, in general size, outline,
grouping, and even in quite trifling details of
structure, between a barrier and an atoll. The
geographer Balbi has well remarked that an encircled
island is an atoll with high land rising out
of its lagoon; remove the land from within, and a
perfect atoll is left.
But what has caused these reefs to spring up at
such great distances from the shores of the included
islands? It can not be that the corals will not grow
close to the land; for the shores within the lagoon-channel,
when not surrounded by alluvial soil, are
often fringed by living reefs; and we shall presently
see that there is a whole class, which I have called
fringing-reefs, from their close attachment to the
shores both of continents and of islands. Again, on
what have the reef-building corals, which can not live
at great depths, based their encircling structures?
This is a great apparent difficulty, analogous to that
in the case of atolls, which has generally been overlooked.
Are we to suppose that each island is surrounded
by a collar-like submarine ledge of rock,
or by a great bank of sediment, ending abruptly
where the reef ends? If the sea had formerly
eaten deeply into the islands, before they were
protected by the reefs, thus having left a shallow
ledge round them under water, the present shores
would have been invariably bounded by great precipices;
but this is most rarely the case. Moreover,
on this notion, it is not possible to explain why the
corals should have sprung up, like a wall, from the
extreme outer margin of the ledge, often leaving a
broad space of water within, too deep for the growth
of corals. The accumulation of a wide bank of
sediment all round these islands, and generally
widest where the included islands are smallest, is
highly improbable, considering their exposed positions
in the central and deepest parts of the ocean.
In the case of the barrier-reef of New Caledonia,
which extends for 150 miles beyond the northern
point of the island, in the same straight line with
which it fronts the west coast, it is hardly possible
to believe that a bank of sediment could thus have
been straightly deposited in front of a lofty island,
and so far beyond its termination in the open sea.
Finally, if we look to other oceanic islands of about
the same height and of similar geological constitution,
but not encircled by coral-reefs, we may in vain
search for so trifling a circumambient depth as 30
fathoms, except quite near to their shores; for usually
land that rises abruptly out of water, as do most
of the encircled and non-encircled oceanic islands,
plunges abruptly under it. On what then, I repeat,
are these barrier-reefs based? Why, with their wide
and deep moat-like channels, do they stand so far
from the included land? We shall soon see how
easily these difficulties disappear.
We come now to our third class of fringing-reefs,
which will require a very short notice. Where the
land slopes abruptly under water, these reefs are
only a few yards in width, forming a mere ribbon or
fringe round the shores: where the land slopes gently
under the water the reef extends further, sometimes
even as much as a mile from the land; but in such
cases the soundings outside the reef always show that
the submarine prolongation of the land is gently inclined.
In fact, the reefs extend only to that distance
from the shore at which a foundation within
the requisite depth from 20 to 30 fathoms is found.
As far as the actual reef is concerned, there is no
essential difference between it and that forming a
barrier or an atoll: it is, however, generally of less
width, and consequently few islets have been formed
on it. From the corals growing more vigorously on
the outside, and from the noxious effect of the sediment
washed inward, the outer edge of the reef is the
highest part, and between it and the land there is
generally a shallow sandy channel a few feet in
depth. Where banks of sediment have accumulated
near to the surface, as in parts of the West Indies,
they sometimes become fringed with corals, and
hence in some degree resemble lagoon-islands or
atolls; in the same manner as fringing-reefs, surrounding
gently sloping islands, in some degree resemble
barrier-reefs.
No theory on the formation of coral-reefs can be
considered satisfactory which does not include the
three great classes. We have seen that we are driven
to believe in the subsidence of those vast areas, interspersed
with low islands, of which not one rises
above the height to which the wind and waves can
throw up matter, and yet are constructed by animals
requiring a foundation, and that foundation to lie at
no great depth. Let us then take an island surrounded
by fringing-reefs, which offer no difficulty
in their structure; and let this island with its reef
slowly subside. Now as the island sinks down, either
a few feet at a time or quite insensibly, we may
safely infer, from what is known of the conditions
favorable to the growth of coral, that the living
masses, bathed by the surf on the margin of the reef,
will soon regain the surface. The water, however,
will encroach little by little on the shore, the island
becoming lower and smaller and the space between
the inner edge of the reef and the beach proportionally
broader. Coral islets are supposed to
have been formed on the reef; and a ship is anchored
in the lagoon-channel. This channel will
be more or less deep, according to the rate of subsidence,
to the amount of sediment accumulated in it,
and to the growth of the delicately branched corals
which can live there. We can now see why encircling
barrier-reefs stand so far from the shores which
they front. We can also perceive, that a line drawn
perpendicularly down from the outer edge of the
new reef, to the foundation of solid rock beneath the
old fringing-reef, will exceed, by as many feet as
there have been feet of subsidence, that small limit
of depth at which the effective corals can live: the
little architects having built up their great wall-like
mass, as the whole sank down, upon a basis formed
of other corals and their consolidated fragments.
Thus the difficulty on this head, which appeared
so great, disappears.
If, instead of an island, we had taken the shore
of a continent fringed with reefs, and had imagined
it to have subsided, a great straight barrier, like that
of Australia or New Caledonia, separated from the
land by a wide and deep channel, would evidently
have been the result.
As the barrier-reef slowly sinks down, the corals
will go on vigorously growing upward; but as the
island sinks, the water will gain inch by inch on
the shore—the separate mountains first forming separate
islands within one great reef—and finally, the
last and highest pinnacle disappearing. The instant
this takes place, a perfect atoll is formed: I have said,
remove the high land from within an encircling
barrier-reef, and an atoll is left, and the land has
been removed.
We can now perceive how it comes that atolls,
having sprung from encircling barrier-reefs, resemble
them in general size, form, in the manner
in which they are grouped together, and in their
arrangement in single or double lines; for they may
be called rude outline charts of the sunken islands
over which they stand. We can further see how it
arises that the atolls in the Pacific and Indian Oceans
extend in lines parallel to the generally prevailing
strike of the high islands and great coast-lines of
those oceans. I venture, therefore, to affirm that,
on the theory of the upward growth of the corals
during the sinking of the land, all the leading features
in those wonderful structures, the lagoon-islands
or atolls, which have so long excited the attention
of voyagers, as well as in the no less wonderful
barrier-reefs, whether encircling small islands or
stretching for hundreds of miles along the shores of
a continent, are simply explained.
It may be asked whether I can offer any direct
evidence of the subsidence of barrier-reefs or atolls;
but it must be borne in mind how difficult it must
ever be to detect a movement the tendency of which
is to hide under water the part affected. Nevertheless,
at Keeling atoll I observed on all sides of the
lagoon old cocoanut trees undermined and falling;
and in one place the foundation-posts of a shed,
which the inhabitants asserted had stood seven years
before just above high-water mark, but now was
daily washed by every tide: on inquiry I found that
three earthquakes, one of them severe, had been felt
here during the last ten years. At Vanikoro the
lagoon-channel is remarkably deep, scarcely any
alluvial soil has accumulated at the foot of the lofty
included mountains, and remarkably few islets have
been formed by the heaping of fragments and sand
on the wall-like barrier-reef; these facts, and some
analogous ones, led me to believe that this island must
lately have subsided and the reef grown upward:
here again earthquakes are frequent and very severe.
In the Society Archipelago, on the other hand,
where the lagoon-channels are almost choked up,
where much low alluvial land has accumulated,
and where in some cases long islets have been formed
on the barrier-reefs—facts all showing that the
islands have not very lately subsided—only feeble
shocks are most rarely felt. In these coral formations,
where the land and water seem struggling for
mastery, it must be ever difficult to decide between
the effects of a change in the set of the tides and of
a slight subsidence: that many of these reefs and
atolls are subject to changes of some kind is certain;
on some atolls the islets appear to have increased
greatly within a late period; on others they have been
partially or wholly washed away. The inhabitants
of parts of the Maldive Archipelago know the date
of the first formation of some islets; in other parts
the corals are now flourishing on water-washed reefs,
where holes made for graves attest the former existence
of inhabited land. It is difficult to believe
in frequent changes in the tidal currents of an open
ocean; whereas we have, in the earthquakes recorded
by the natives on some atolls and in the great fissures
observed on other atolls, plain evidence of changes
and disturbances in progress in the subterranean regions.
Not only the grand features in the structure of
barrier-reefs and of atolls, and of their likeness to
each other in form, size, and other characters, are
explained on the theory of subsidence—which theory
we are independently forced to admit in the very
areas in question, from the necessity of finding bases
for the corals within the requisite depth—but many
details in structure and exceptional cases can thus
also be simply explained. I will give only a few instances.
In barrier-reefs it has long been remarked
with surprise that the passages through the reef exactly
face valleys in the included land, even in cases
where the reef is separated from the land by a
lagoon-channel so wide and so much deeper than the
actual passage itself that it seems hardly possible
that the very small quantity of water or sediment
brought down could injure the corals on the reef.
Now, every reef of the fringing class is breached by
a narrow gateway in front of the smallest rivulet,
even if dry during the greater part of the year, for
the mud, sand, or gravel, occasionally washed down,
kills the corals on which it is deposited. Consequently,
when an island thus fringed subsides, though
most of the narrow gateways will probably become
closed by the outward and upward growth of the
corals, yet any that are not closed (and some must
always be kept open by the sediment and impure
water flowing out of the lagoon-channel) will still
continue to front exactly the upper parts of those valleys
at the mouths of which the original basal fringing-reef
was breached.
We can easily see how an island fronted only on
one side, or on one side with one end or both ends
encircled by barrier-reefs, might after long-continued
subsidence be converted either into a single
wall-like reef, or into an atoll with a great straight
spur projecting from it, or into two or three atolls tied
together by straight reefs—all of which exceptional
cases actually occur. As the reef-building corals
require food, are preyed upon by other animals, are
killed by sediment, can not adhere to a loose bottom,
and may be easily carried down to a depth whence
they can not spring up again, we need feel no surprise
at the reefs both of atolls and barriers becoming
in parts imperfect. The great barrier of New
Caledonia is thus imperfect and broken in many
parts; hence, after long subsidence, this great reef
would not produce one great atoll 400 miles in
length, but a chain or archipelago of atolls, of very
nearly the same dimensions with those in the Maldive
Archipelago. Moreover, in an atoll once
breached on opposite sides, from the likelihood of
the oceanic and tidal currents passing straight
through the breaches, it is extremely improbable that
the corals, especially during continued subsidence,
would ever be able again to unite the rims: if they
did not, as the whole sank downward one atoll would
be divided into two or more. In the Maldive
Archipelago there are distinct atolls so related to
each other in position, and separated by channels
either unfathomable or very deep (the channel between
Ross and Ari atolls is 150 fathoms, and that
between the north and south Nillandoo atolls is 200
fathoms in depth), that it is impossible to look at a
map of them without believing that they were once
more intimately related. And in this same archipelago,
Mahlos-Mahdoo atoll is divided by a bifurcating
channel from 100 to 132 fathoms in depth,
in such a manner that it is scarcely possible to say
whether it ought strictly to be called three separate
atolls or one great atoll not yet finally divided.
I will not enter on many more details; but I must
remark that the curious structure of the northern
Maldive atolls receives (taking into consideration
the free entrance of the sea through their broken
margins) a simple explanation in the upward and
outward growth of the corals, originally based both
on small detached reefs in their lagoons, such as
occur in common atolls, and on broken portions of
the linear marginal reef, such as bounds every atoll
of the ordinary form. I can not refrain from once
again remarking on the singularity of these complex
structures—a great sandy and generally concave
disk rises abruptly from the unfathomable ocean,
with its central expanse studded, and its edge symmetrically
bordered with oval basins of coral-rock
just lipping the surface of the sea, sometimes clothed
with vegetation, and each containing a lake of clear
water!
One more point in detail: as in two neighboring
archipelagoes corals flourish in one and not in the
other, and as so many conditions before enumerated
must affect their existence, it would be an inexplicable
fact if, during the changes to which earth,
air, and water are subjected, the reef-building corals
were to keep alive for perpetuity on any one spot or
area. And as by our theory the areas including
atolls and barrier-reefs are subsiding, we ought occasionally
to find reefs both dead and submerged.
In all reefs, owing to the sediment being washed out
of the lagoon or lagoon-channel to leeward, that
side is least favorable to the long-continued vigorous
growth of the corals; hence dead portions of reef
not infrequently occur on the leeward side; and these,
though still retaining their proper wall-like form,
are now in several instances sunk several fathoms beneath
the surface. The Chagos group appears from
some cause, possibly from the subsidence having been
too rapid, at present to be much less favorably circumstanced
for the growth of reefs than formerly:
one atoll has a portion of its marginal reef, nine
miles in length, dead and submerged; a second has
only a few quite small living points which rise to the
surface; a third and fourth are entirely dead and
submerged; a fifth is a mere wreck, with its structure
almost obliterated. It is remarkable that in all these
cases the dead reefs and portions of reefs lie at
nearly the same depth; namely, from six to eight
fathoms beneath the surface, as if they had been
carried down by one uniform movement. One of
these “half-drowned atolls,” so called by Captain
Scoresby (to whom I am indebted for much invaluable
information), is of vast size; namely,
ninety nautical miles across in one direction and
seventy miles in another line; and is in many respects
eminently curious. As by our theory it follows that
new atolls will generally be formed in each new
area of subsidence, two weighty objections might
have been raised; namely, that atolls must be increasing
indefinitely in number; and, secondly, that in old
areas of subsidence each separate atoll must be increasing
indefinitely in thickness, if proofs of their
occasional destruction could not have been adduced.
Thus have we traced the history of these great rings
of coral-rock, from their first origin through their
normal changes, and through the occasional accidents
of their existence, to their death and final
obliteration.
Authors have noticed with surprise that, although
atolls are the commonest coral structures throughout
some enormous oceanic tracts, they are entirely absent
in other seas, as in the West Indies: we can now
at once perceive the cause, for where there has not
been subsidence, atolls can not have been formed;
and in the case of the West Indies and parts of the
East Indies, these tracts are known to have been
rising within the recent period. The larger areas
are all elongated; and there is a degree of rude alternation,
as if the rising of one had balanced the sinking
of the other. Taking into consideration the
proofs of recent elevation both on the fringed coasts
and on some others (for instance, in South America)
where there are no reefs, we are led to conclude that
the great continents are for the most part rising
areas; and from the nature of the coral-reefs, that
the central parts of the great oceans are sinking
areas. The East Indian archipelago, the most
broken land in the world, is in most parts an area
of elevation, but surrounded and penetrated, probably
in more lines than one, by narrow areas of subsidence.
Bearing in mind the statements made with respect
to the upraised organic remains, we must feel astonished
at the vastness of the areas which have
suffered changes in level either downward or upward,
within a period not geologically remote. It
would appear, also, that the elevatory and subsiding
movements follow nearly the same laws. Throughout
the spaces interspersed with atolls, where not a
single peak of high land has been left above the level
of the sea, the sinking must have been immense in
amount. The sinking, moreover, whether continuous,
or recurrent with intervals sufficiently long for
the corals again to bring up their living edifices to
the surface, must necessarily have been extremely
slow. This conclusion is probably the most important
one which can be deduced from the study
of coral formations; and it is one which it is difficult
to imagine how otherwise could ever have been
arrived at. Nor can I quite pass over the probability
of the former existence of large archipelagoes of
lofty islands, where now only rings of coral-rock
scarcely break the open expanse of the sea, throwing
some light on the distribution of the inhabitants of the
other high islands, now left standing so immensely remote
from each other in the midst of the great oceans.
The reef-constructing corals have indeed reared and
preserved wonderful memorials of the subterranean
oscillations of level; we see in each barrier-reef a
proof that the land has there subsided, and in each
atoll a monument over an island now lost. We may
thus, like unto a geologist who had lived his ten thousand
years and kept a record of the passing changes,
gain some insight into the great system by which the
surface of this globe has been broken up, and land
and water interchanged.
MAGNITUDE AND COLOR OF THE SEA — G. Hartwig
Of all the gods that divide the empire of the earth,
Neptune rules over the widest realms. If a
giant hand were to uproot the Andes and cast them
into the sea, they would be engulfed in the abyss, and
scarcely raise the general level of the waters. The
South American Pampas, bounded on the north by
tropical palm-trees, and on the south by wintry firs,
are no doubt of magnificent dimensions, yet these
vast deserts seem insignificant when compared with
the boundless plains of earth-encircling ocean. Nay!
a whole continent, even America or Asia, appears
small against the immensity of the sea, which covers
with its rolling waves nearly three-fourths of the entire
surface of the globe.
The length of all the coasts which form the
boundary between sea and land can only be roughly
estimated, for who has accurately measured the numberless
windings of so many shores? The entire coast-line
of deeply indented Europe and her larger isles
measures about 21,600 miles, equal to the circumference
of the earth; while the shores of compact Africa
extend to a length of only 14,000 miles. The coasts
of America measure about 45,000 miles, those of
Asia 40,000, while those of Australia and Polynesia
may safely be estimated at 16,000. Thus the entire
coast-line of the globe amounts to about 136,000
miles, which it would take the best pedestrian to traverse
from end to end.
How different is the aspect of these shores, along
which the ever-restless sea continually rises or falls!
Here steep rock-walls tower up from the deep, while
there a low sandy beach extends its flat profile as far
as the eye can reach. While some coasts are scorched
by the vertical sunbeam, others are perpetually
blocked up with ice. Here the safe harbor bids welcome
to the weather-beaten sailor, the lighthouse
greets him from afar with friendly ray; the experienced
pilot hastens to guide him to the port, and
all along the smiling margin of the land rise the
peaceful dwellings of civilized man. There, on the
contrary, the roaring breakers burst upon the shore
of some dreary wilderness, the domain of the savage
or the brute. What a wonderful variety of scenes unrolls
itself before our fancy as it roams along the
coasts of ocean from zone to zone! What changes,
as it wanders from the palm-girt coral island of the
tropical seas to the melancholy strands where, verging
toward the poles, all vegetable life expires! And
how magnificently grand does the idea of ocean swell
out in our imagination, when we consider that its various
shores witness at one and the same time the rising
and setting of the sun, the darkness of night and
the full blaze of day, the rigor of winter and the
smiling cheerfulness of spring!
The sea is not colorless; its crystal mirror not only
reflects the bright sky or the passing cloud, but naturally
possesses a pure bluish tint, which is only rendered
visible to the eye when the light penetrates
through a stratum of water of considerable depth.
In the Gulf of Naples, we find the inherent color of
the water exhibited to us by Nature on a most magnificent
scale. The splendid “Azure Cave,” at
Capri, might almost be said to have been created for
the purpose.
All profound and clear seas are more or less of a
deep blue color, while, according to seamen, a green
color indicates soundings. The bright blue of the
Mediterranean, so often vaunted by poets, is found
over all the deep pure ocean, not only in tropical
and temperate zones, but also in the regions of eternal
frost. Scoresby speaks with enthusiasm of the
splendid blue of the Greenland seas, and all along
the great ice-barrier which under 77° S. lat. obstructed
the progress of Sir James Ross toward the
pole, that illustrious navigator found the waters of
as deep a blue as in the classical Mediterranean. The
North Sea is green, partly from its water not being
so clear, and partly from the reflection of its sandy
bottom mixing with the essentially blue tint of the
water. In the Bay of Loanga the sea has the color
of blood, and Captain Tuckey discovered that this
results from the reflection of the red ground-soil.
But the essential color of the sea undergoes much
more frequent changes over large spaces, from enormous
masses of minute algæ, and countless hosts
of small sea-worms, floating or swimming on its
surface.
“A few days after leaving Bahia,” says Mr. Darwin,
“not far from the Abrolhos islets, the whole surface
of the water, as it appeared under a weak lens,
seemed as if covered by bits of hay with their ends
jagged. Each bundle consisted of from twenty to
sixty filaments, divided at regular intervals by transverse
septa, containing a brownish-green flocculent
matter. The ship passed several bands of them, one
of which was about ten yards wide, and, judging
from the mud-like color of the water, at least two
and a half miles long. Similar masses of floating
vegetable matter are a very common appearance near
Australia. During two days preceding our arrival
at the Keeling Islands, I saw in many parts masses
of flocculent matter of a brownish-green color floating
in the ocean. They were from half to three
inches square, and consisted of two kinds of microscopical
confervæ. Minute cylindrical bodies, conical
at each extremity, were involved in large numbers
in a mass of fine threads.”
“On the coast of Chili,” says the same author, “a
few leagues north of Concepcion, the Beagle one day
passed through great bands of muddy water; and
again a degree south of Valparaiso, the same appearance
was still more extensive. Mr. Sullivan, having
drawn up some water in a glass, distinguished by the
aid of a lens moving points. The water was slightly
stained, as if by red dust, and after leaving it for some
time quiet a cloud collected at the bottom. With a
slightly magnifying lens, small hyaline points could be
seen darting about with great rapidity and frequently
exploding. Examined with a much higher power,
their shape was found to be oval, and contracted by
a ring round the middle, from which line curved little
setæ proceeded on all sides, and these were the
organs of motion. Their minuteness was such that
they were individually quite invisible to the naked
eye, each covering a space equal only to the one-thousandth
of an inch, and their number was infinite,
for the smallest drop of water contained very many.
In one day we passed through two spaces of water
thus stained, one of which alone must have extended
over several square miles. The color of the water
was like that of a river which has flowed through a
red clay district, and a strictly defined line separated
the red stream from the blue water.”
In the neighborhood of Callao, the Pacific has an
olive-green color, owing to a greenish matter which
is also found at the bottom of the sea in a depth of
800 feet. In its natural state it has no smell, but
when cast on the fire it emits the odor of burned
animal substances.
Near Cape Palmas, on the coast of Guinea, Captain
Tuckey’s ship seemed to sail through milk, a
phenomenon which was owing to an immense number
of little white animals swimming on the surface
and concealing the natural tint of the water.
The peculiar coloring of the Red Sea, from which
it has derived its name, is owing to the presence of
a microscopic alga, sui generis, floating at the surface
of the sea and even less remarkable for its beautiful
red color than for its prodigious fecundity.
I could add many more examples, where, either
from minute algæ, or from small animals, the deep
blue sea suddenly appeared in stripes of white, yellow,
blue, brown, orange, or red. For fear, however,
of tiring the reader’s patience, I shall merely mention
the olive green water which covers a considerable
part of the Greenland seas. It is found between 74°
and 80° N. lat., but its position varies with the currents,
often forming isolated stripes, and sometimes
spreading over two or three degrees of latitude.
Small yellowish Medusæ, of from one-thirtieth to
one-twentieth of an inch in diameter, are the principal
agents that change the pure ultramarine of the
Arctic Ocean into a muddy green.
When the sea is perfectly clear and transparent,
it allows the eye to distinguish objects at a very great
depth. Near Mindora, in the Indian Ocean, the
spotted corals are plainly visible under twenty-five
fathoms of water.
The crystalline clearness of the Caribbean Sea
excited the admiration of Columbus. “In passing
over these splendidly adorned grounds,” says Schöpf,
“where marine life shows itself in an endless variety
of forms, the boat, suspended over the purest crystal,
seems to float in the air, so that a person unaccustomed
to the scene easily becomes giddy. On the
clear sandy bottom appear thousands of sea-stars,
sea-urchins, mollusks, and fishes of a brilliancy of
color unknown in our temperate seas. Fiery red, intense
blue, lively green, and golden yellow perpetually
vary; the spectator floats over groves of sea-plants,
gorgonias, corals, alcyoniums, flabellums, and
sponges that afford no less delight to the eye, and are
no less gently agitated by the heaving waters, than
the most beautiful garden on earth when a gentle
breeze passes through the waving boughs.”
TIDAL ACTION — Sir Robert S. Ball
Every one is familiar with the fact that the moon
raises tides on the earth; these tides ebb and flow
along our coasts, and in virtue of them the satellite
exercises a certain control on the movements of our
globe. If the moon had liquid oceans on its surface
there can not be a doubt that the attraction of the
earth would generate tides in the oceans on the moon
just as the attraction of the moon generates tides in
the oceans of the earth. But there would be a fundamental
difference between the two cases; the shores
of the lunar seas would be periodically inundated
by tides far vaster than any tides which the moon can
create on the earth. But it may be said that as the
moon contains no water it seems idle to talk of the
tides that might have been produced in oceans if they
had existed. It is no doubt true that the moon contains
no visible liquid water on its surface at the present
time; it is, however, by no means certain that our
satellite was always void of water; it is not at all
impossible that spreading oceans may have once
occupied a large part of that surface now an arid
wilderness. The waters from those oceans have vanished,
but the basins they presumably filled are still
left as characteristic features on our satellite. For
our present argument, however, it is really not material
that the moon should ever have had oceans as
we understand them. The water at those remote
periods must have been suspended in the form of
vapor around the more solid parts of the glowing
globe. But tides can be manifested in other liquids
besides that which forms our seas. In fact if the
basins of our great oceans were filled with oil or with
mercury, or even with molten iron instead of water,
the moon would still cause tides to ebb and flow, no
matter what the material might be, so long as it possessed
to some extent the properties of a liquid. It
need not be a perfect liquid, for any material which
is in some degree viscous, like honey or treacle, would
still respond to tidal influence, though not, it may
be well believed, with the same alacrity and freedom
of movement as would a fluid of a more perfect character.
In the molten moon itself, throughout the very
body of our satellite, the tidal influence of the earth
must have been experienced in these primitive ages.
There can not be a doubt that in ancient days when
the moon was sufficiently fluid, the action of the tides
tended without ceasing to the establishment of such
an adjustment between the rotation of the moon
around its axis and the revolution of the moon around
the earth, that the two should be brought to have
equal periods. Friction would incessantly operate
until this adjustment had been effected, and owing
to the preponderating mass of the earth such strenuous
tides must have been evoked in the moon that
our satellite was brought under tidal control with
comparative facility. Hence it arose that in those
early days the habit of bending the same face incessantly
toward the earth around which it revolved was
established on our satellite.
Time passed on, the moon gradually dispensed its
excessive heat by radiation into space, and it gradually
became transformed from a molten globe to a
globe with a solid crust. It may be that the water
was condensed from vapor and then collected together
into oceans on the newly formed surface; if
so, these oceans would not have any ebbing tide or
flowing tide, for it would be constant high tide at
some places and constant low tide at others. Such a
state of things would at all events endure so long as
the adjustment of equality between the moon’s rotation
and its revolution continued. In fact, should
any departure from this adjustment have manifested
itself, corresponding tides would have begun to throb
in the lunar oceans, and their tendency would be to
restore the adjustment which was disturbed. This
arrangement between the two movements was necessarily
stable when tidal control was always at hand
to check any tendency to depart from it.
It may be that the moon has now cooled so thoroughly
that not only is it hard and congealed on the
exterior as we see, but it seems highly probable that
the heat may have so entirely forsaken even the interior
that there is no longer any fluid in the globe of
our satellite to respond to tidal impulse. There is,
therefore, in all probability, no longer any actual
tidal control. On the other hand, however, there is
nothing to disturb the adjustment. It was, as we have
seen, caused by the tides which have done their work;
the consequences of that work are still exhibited
in the constant face of the moon, which, now that
it has been brought about, seems likely to exist
permanently as a stable adaptation of the movement.
The tendency of the tides on a tide-disturbed globe
is to adjust the movements of that globe in such a
way that the tides shall no longer ebb or flow, but
that permanent high tide shall be established in
some places and permanent low tide in others. If
the rotation of the body be not fast enough the tide
will pull the body round in order to effect this object.
If the rotation of the body be too rapid, then the influence
of the tide will tend to check the movement
and bring down the speed of rotation until the desired
adjustment is obtained. At present the earth
is spinning too fast to permit the high tides to remain
at permanent localities, and consequently tides are
applied with the effect of checking the rotation. The
earth is, however, so vast, and the tides generated by
so small a body as the moon are relatively so impotent,
that their effects in reducing the speed of the
earth’s rotation are insignificant. Nevertheless, small
though they are, they unquestionably exist, and there
can not be a doubt that to some extent the earth is
affected by the unremitting action of the tides; the
consequence is that the rapidity with which the earth
rotates upon its axis is gradually declining.
One result of this can be stated in a very simple
manner. The length of the day must be increasing.
It is true that this gradual stretching of the day is
very slow; it is indeed quite inappreciable in so far
as our ordinary use of the day as a measure of time
is concerned. The alteration almost eludes any
means of measurement at our disposal. Even in a
thousand years the change is so small that the increase
in the length of the day is only a fraction of a
second. We can doubtless afford to disregard so
trifling a variation in our standard of time so far
as the period contemplated in mere human affairs
is concerned. In fact the change is absolutely devoid
of significance within such periods as are contemplated
since the erection of the Pyramids, or indeed
since any other human monument has been reared.
We must not, however, conclude that the change in
the length of the day has no significance in earth
history.
The significance of the gradual elongation of the
day by the tides arises from the circumstance that the
change always takes place in one direction. In this
form of effect the tide differs from other more familiar
astronomical phenomena which sometimes advance
in one direction and then after the lapse of
suitable periods return in the opposite direction, and
thus restore again the initial state of things. But the
alteration of the length of the day is not of this character,
it is not periodic, its motion is never reversed,
is never even arrested. Only one condition is therefore
necessary to enable it to obtain tremendous dimensions,
and that is sufficient time in which it can
operate.
There are many lines of reasoning which show the
extreme antiquity of our globe: the disclosures of
geology are specially instructive on this head. Think,
for instance, of that mighty reptile the Atlantosaurus,
which once roamed over the regions now known
as Colorado. The bones of this vast creature indicate
an animal surpassing in proportions the greatest
elephant ever known. No one can count the æons
of years that have elapsed since the Atlantosaurus
whose bones are now to be seen in the museum at
Yale University breathed its last. A still more striking
conception of time than even the antiquity of this
creature affords is derived from the consideration
that his mighty form was itself the product of a long
and immeasurable line of ancestry, extending to a
depth in the remote past far beyond the limits of
computation. I have mentioned this illustration of
the antiquity of the earth for the purpose of showing
the ample allowance of time that is available for
tides to accomplish great work in earlier stages of
our globe’s history.
As the evidence of the earth’s crust proves that
our globe has lasted for incalculable ages, it becomes
of interest to think how far the gradual elongation
of the day may have attained significant proportions
since very early time. It may be that even in
a thousand years the effect of the tides is not sufficient
to alter the length of the day by so much as a
single second. But the effect may be very appreciable
or even large in a million years, or ten million
years. We have the best reasons for knowing that
in intervals of time comparable with those I have
mentioned, the change in the length of the day may
have amounted not merely to seconds or minutes, but
even to hours. Looking into the remote past, there
was a time at which this globe spun round in twenty-three
hours instead of twenty-four; at a still earlier
period the rate must have been twenty hours, and the
further we look back the more and more rapidly
does the earth appear to be spinning. At last, as we
strain our gaze to some epoch so excessively remote
that it must have been long anterior to those changes
which geology recognizes, we see that our globe was
spinning round in a period of six hours or five hours,
or possibly even less. Here then is a lesson which the
tides have taught us: they have shown that if the
causes at present in operation have subsisted without
interruption for a sufficiently long period in the past,
the day must have gradually grown to its present
length from an initial condition in which the earth
seems to have spun round about four times as quickly
as it does at present.
We should, however, receive a very inadequate impression
of what tides are able to accomplish if we
merely contemplated this change in the length of the
day, striking and significant though it doubtless is.
The student of natural philosophy is well aware that
there is no action without a corresponding reaction,
and it is instructive to examine in this case the form
which the reaction assumes. Our reasoning has been
founded on the supposition that it is the attraction of
the moon on the waters of our globe that gives rise
to the tides. It is, therefore, the influence of the
moon which checks the speed of the earth’s rotation
and adds to the length of the day.
As the moon acts in this fashion on the earth, so,
by the general law that I have mentioned, the earth
reacts upon the moon. The form which this reaction
assumes expresses itself in a tendency to allow
the moon gradually to move further and further
away from the earth than the earth’s attraction
would permit if our globe were a solid mass void of
all liquid capable of being distracted by tides. It is,
therefore, certain that the distance of the moon,
which is at present about two hundred and forty
thousand miles, must be gradually increasing; but
we need not look for any appreciable change in the
moon’s distance arising from this cause when only an
interval of a few centuries is considered. We need not
expect to measure the difference due to tides between
the size of the moon’s orbit this month and the size
of the orbit last month. In fact, there are so many
periodic causes of change in the dimensions of the
moon’s orbit that it becomes impossible to detect the
tidal influence even in the course of centuries. Here,
again, we have to remember that in dealing with the
history of our earth we are to consider not merely the
thousands of years that include the human period, not
merely the millions of years that are required by the
necessities of geology, but also those unknown periods
anterior to geological phenomena to which we
have already referred.
In the course of such vast ages the reaction of the
earth on the moon’s orbit has not only become perceptible,
it has become conspicuous; it has not only
become conspicuous, but it has become the chief determining
agent in making the moon’s orbit as we find
it at the present day. We have seen that as we look
into the past the length of the day seems ever shorter
and shorter; and concurrent with this decline in the
day is the diminution in the moon’s distance from the
earth. There was a time when the moon, instead of
revolving at a distance of two hundred and forty
thousand miles, as it does at present, revolved at a
distance of only two hundred thousand miles. As
we think of epochs still earlier we discern the moon
ever closer and closer to the earth, until at last, at
that critical time in the history of the earth-moon
system, when the earth was quickly revolving in a
period of a few hours, our satellite seems to have been
quite close to the earth; in fact, the two bodies were
almost in contact
The study of the tides has therefore conducted us
to the knowledge of a remarkable configuration exhibited
in the primitive earth-moon system. The
earth was then spinning round rapidly in a day which
was only a few hours long, while close to the earth,
or almost in contact with it, the moon coursed around
our globe, the period of its revolution being shorter
to such an extent that the satellite completed its circuit
in the same time as the earth required for one
turn round its axis.
We must remember that the materials destined to
form the pair of allied planets did not then form
two solid bodies as they do at present; they were both,
in all probability, incandescent masses glowing with
fervor, and soft, if not actually molten, or incoherent,
or even gaseous. These aggregations were close
together, and one of them was whirling around the
other in a period of a few hours, the duration of that
period being equal to the time in which the larger
mass revolved on its axis. In fact, the two objects,
even though distinct, seem to have revolved the one
around the other as if they had been bound together
by rigid bonds. The rapid rotation with which they
were animated suggests a cause for this state of things.
It is well known that a fly-wheel, when driven at an
unduly high speed, is liable to break asunder in consequence
of its rapid motion. If a grindstone be
urged around with excessive velocity the force tending
to rend the stone into fragments may overcome
its cohesion, and it will fly into pieces, often projected
with such violence that fearful accidents have
been the consequence.
Viewing the earth as a rotating body, it must be
subject to the law that there is a speed which can not
be exceeded with safety. With the present period of
rotation of once in every twenty-four hours the tendency
to disruption is but small and consequently the
earth retains its integrity, though no doubt the protuberance
at the equator is the result of the accommodation
of the shape of the globe to the circumstances
attending its revolution. But let us suppose
that the length of the day was greatly diminished, or,
what comes to the same thing, that the speed with
which the earth rotates on its axis was greatly increased;
it is then conceivable that the tension thus
arising might be too great for the coherence of the
material to withstand. We believe that the earth
could turn round with double the speed that it has
at present before this tension approached the point
at which disruption would ensue. But supposing the
day were to be so much shortened that the period of
rotation was only a very few hours instead of twenty-four,
there is then good reason to know that the tension
in the earth arising from this rapid rotation
would be so great that a rupture of the globe would
be imminent.
Provided with this conception, let us think of the
initial stage when the moon was quite close to the
earth. Our globe was then, as we know, spinning
round so rapidly that its materials were almost on
the point of breaking up in consequence of the strain
produced by the rotation. It is interesting to note
that the tidal action of the sun would also conduce to
the rupture of our globe in the critical circumstances
we have supposed. It seems hardly possible to doubt
that such a separation of the glowing mass did actually
take place, a small fragment was discarded, and
gradually drew itself by the mutual attraction of its
particles into a globular form and thus became the
moon.
We have seen that at the present moment the day
is becoming gradually longer and the moon is steadily
receding further and further from the earth. At
present these changes take place with extreme slowness,
but in the primitive periods of which we have
already spoken, the changes in the length of the day,
and the changes in the distance of the moon, proceeded
at a rate far more rapid than at present. As the moon
has receded further from the earth its efficiency as a
tide-producer has declined, and consequently the rate
at which the consequences of tidal action have proceeded
is continually lessening. It must therefore be
expected that the progress of tidal evolution in the
future will be ever getting slower and slower, so that
the periods of time required for the further development
of the phenomena far exceed those which
have elapsed in the course of the history already
given. We can, however, foreshadow what is to
happen in the following manner. The length of the
day will slowly increase; and we can indicate a state
of things in the excessively remote future toward
which it may be said the system is tending. The day
will grow until it becomes not merely twenty-five or
twenty-six hours, but until it becomes as long as two
or three of our present days. In fact, as we stretch
our imagination through ages so inconceivable that I
forbear to specify any figures which might characterize
them, we seem to discern that the length of the
day may go on ever getting longer and longer until
at last a stage is reached when the day is about fifty
or sixty times as long as our present day.
All this time, in accordance with the general law
of action and reaction, the moon must be gradually
retreating. As the orbit of the moon is gradually
enlarging, the time that the moon takes to revolve
around the earth must be continually on the increase;
from the present month of twenty-seven days the
length of the month will gradually augment as the
ages roll by until at last when the moon has reached
a certain distance the period of its rotation will have
become double what it is at present, or indeed rather
more than double, and we shall have the day and the
month equal, each being about fourteen hundred
hours long. When this state of things is reached, the
earth will always turn the same face toward the
moon, just as the moon at present always turns the
same face toward the earth.
We have already explained how the constant face
of the moon can be accounted for by the action of
tides raised in the moon by the attraction of the earth.
Owing to the small size of the moon the tides have
already wrought all that they were capable of doing,
and have compelled the moon to succumb to the conditions
they imposed. Owing to the great mass of
the earth and the comparatively small mass of the
moon the tides on the earth raised by the moon have
required a much longer period wherein to accomplish
their effects than was the case when the earth
raised tides on the moon. But small though our
satellite may be, yet the tides raised on the earth have
incessantly tended to wear down the speed of our
globe and reduce it to conformity with the law that
the two bodies shall bear the same face toward each
other. At present the earth turns round twenty-seven
times while the moon goes round once, so
the tides have still a gigantic task to accomplish.
With unflagging energy, however, they are incessantly
engaged at the work, and they are constantly
tending to bring down the speed of the earth; constantly
tending toward that ultimate condition of
things in which the earth and moon are destined to
revolve in a period of fourteen hundred hours as if
they were connected with invisible bonds.
If such a state of things as this were established
then it is plain that tides would no longer ebb and
flow, that is, at least, if we exclude from our consideration
the intervention of any other body. High
tides must prevail at some parts of the earth, and
low tides at other parts, but the position of these
tides will remain fixed. Where it is high tide it will
always be high tide; where it is low tide it will
always be low tide. When this state of things is
reached, the moon will be constantly visible in the
same part of the sky from one half of our globe,
while the other half of our globe will never be turned
toward the moon. In fact, the moon would always
appear to us in a fixed position as the earth would
always appear to be if viewed by an observer stationed
on the moon. If there were any Lunarians
whose residence was confined to the opposite side of
the moon, they could never see this earth at all, while
those who lived on this side of our satellite would
always be able to see the earth apparently fixed in
the same part of the sky. An observatory located at
the middle of the moon’s disk, say near the crater
Ptolemy, would always have the earth in its zenith
or very near thereto, while the astronomer, let us say,
in the Mare Crisium, would always find the earth
low down near his horizon.
In order to facilitate our reasoning I have assumed
that the moon is the only tide-producing agent; this
is, however, not the case. No doubt the ebb and the
flow around our coasts is generated mainly by the attraction
of the moon. It must not, however, be forgotten
that a portion of the tide is originated by the
attraction of the sun. These solar tides will still continue
to ebb and flow quite independently of the lunar
tides, so that even if the accommodation between the
earth and the moon had been completed some further
tidal disturbance would not be wanting. The
effect of the solar tides will be to abate still further
the velocity with which the earth turns round on its
axis, and consequently a time must ultimately arrive
when the length of the day will be longer than the
time which the moon takes to revolve around our
earth.
THE GULF STREAM — Lord Kelvin
I mean by the Gulf Stream that mass of heated
water which pours from the Strait of Florida
across the North Atlantic, and likewise a wider but
less definite warm current, evidently forming part
of the same great movement of water, which curves
northward to the eastward of the West Indian Islands.
I am myself inclined, without hesitation, to
regard this stream as simply the reflux of the equatorial
current, added to no doubt during its northeasterly
course by the surface-drift of the anti-trades
which follows in the main the same direction.
The scope and limit of the Gulf Stream will be
better understood if we inquire in the first place into
its origin and cause. As is well known—in two
bands, one to the north and the other to the south
of the equator—the northeast and southeast trade-winds,
reduced to meridional directions by the eastward
frictional impulse of the earth’s rotation, drive
before them a magnificent surface current of hot
water 4,000 miles long by 450 miles broad at an
average rate of thirty miles a day. Off the coast of
Africa, near its starting-point to the south of the
Islands of St. Thomas and Anna Bon this “equatorial
current” has a speed of forty miles in the
twenty-four hours, and a temperature of 23° C.
Increasing quickly in bulk, and spreading out
more and more on both sides of the equator, it flows
rapidly due west toward the coast of South America.
At the eastern point of South America, Cape St.
Roque, the equatorial current splits into two, and
one portion trends southward to deflect the isotherms
of 21°, 15°.5, 10°, and 4°.5 C. into loops upon our
maps, thus carrying a scrap of comfort to the Falkland
Islands and Cape Horn; while the northern
portion follows the northeast coast of South America,
gaining continually in temperature under the
influence of the tropical sun. Its speed has now increased
to sixty-eight miles in twenty-four hours,
and by the union with it of the waters of the river
Amazon, it rises to one hundred miles (6.5 feet in
a second), but it soon falls off again when it gets into
the Caribbean Sea. Flowing slowly through the
whole length of this sea, it reaches the Gulf of
Mexico through the Strait of Yucatan, when a part
of it sweeps immediately round Cuba; but the main
stream, “having made the circuit of the Gulf of
Mexico, passes through the Strait of Florida; thence
it issues as the ‘Gulf Stream’ in a majestic current
upward of thirty miles broad, two thousand two
hundred feet deep, with an average velocity of four
miles an hour, and a temperature of 86° Fahr. (30°
C.).” The hot water pours from the strait with a
decided though slight northeasterly impulse on account
of its great initial velocity. Mr. Croll calculates
the Gulf Stream as equal to a stream of water
fifty miles broad and a thousand feet deep flowing
at a rate of four miles an hour; consequently conveying
5,575,680,000,000 cubic feet of water per
hour, or 133,816,320,000,000 cubic feet per day.
This mass of water has a mean temperature of 18° C.
as it passes out of the gulf, and on its northern journey
it is cooled down to 4°.5, thus losing heat to the
amount of 13°.5 C. The total quantity of heat therefore
transferred from the equatorial regions per
day amounts to something like 154,959,300,000,000,000,000
foot-pounds.
This is nearly equal to the whole of the heat
received from the sun by the Arctic regions, and,
reduced by a half to avoid all possibility of exaggeration,
it is still equal to one-fifth of the whole amount
received from the sun by the entire area of the North
Atlantic. The Gulf Stream, as it issues from the
Strait of Florida and expands into the ocean on its
northward course, is probably the most glorious natural
phenomenon on the face of the earth. The
water is of a clear crystalline transparency and an
intense blue, and long after it has passed into the
open sea it keeps itself apart, easily distinguished by
its warmth, its color, and its clearness; and with its
edges so sharply defined that a ship may have her
stem in the clear blue stream while her stern is still
in the common water of the ocean.
Setting aside the wider question of the possibility
of a general oceanic circulation arising from heat,
cold, and evaporation, I believe that Captain Maury
and Dr. Carpenter are the only authorities who of
late years have disputed this source of the current
which we see and can gauge and measure as it passes
out of the Strait of Florida; for it is scarcely necessary
to refer to the earlier speculations that it is
caused by the Mississippi River, or that it flows
downward by gravitation from a “head” of water
produced by the trade-winds in the Caribbean Sea.
Captain Maury writes that “the dynamical force
that calls forth the Gulf Stream is to be found in the
difference as to specific gravity of intertropical and
polar waters.” “The dynamical forces which are
expressed by the Gulf Stream may with as much propriety
be said to reside in those northern waters as
in the West India seas: for on one side we have the
Caribbean Sea and Gulf of Mexico with their waters
of brine; on the other the great polar basin, the
Baltic and the North Sea, the two latter with waters
which are little more than brackish. In one set of
these sea-basins the water is heavy; in the other it is
light. Between them the ocean intervenes; but water
is bound to seek and to maintain its level; and here,
therefore, we unmask one of those agents concerned
in causing the Gulf Stream. What is the power of
this agent? Is it greater than that of other agents?
and how much? We can not say how much; we only
know it is one of the chief agents concerned. Moreover,
speculate as we may as to all the agencies concerned
in collecting these waters, that have supplied
the trade-winds with vapor, into the Caribbean Sea,
and then in driving them across the Atlantic, we are
forced to conclude that the salt which the trade-wind
vapor leaves behind it in the tropics has to be conveyed
away from the trade-wind region, to be mixed
up again in due proportion with the other water of
the sea—the Baltic Sea and the Arctic Ocean included—and
that these are some of the waters, at
least, which we see running off through the Gulf
Stream. To convey them away is doubtless one of
the offices which in the economy of the ocean has
been assigned to it.”
Dr. Carpenter attributes all the great movements
of ocean water to a general convective circulation,
and of this general circulation he regards the Gulf
Stream as a peculiarly modified case. Dr. Carpenter
states that “the Gulf Stream constitutes a peculiar
case, modified by local conditions,” of “a great general
movement of equatorial water toward the polar
area.” I confess I feel myself compelled to take a
totally different view. It seems to me that the Gulf
Stream is the one natural physical phenomenon on
the surface of the earth whose origin and principal
cause, the drift of the trade-winds, can be most
clearly and easily traced.
The further progress and extension of the Gulf
Stream through the North Atlantic in relation to
influence upon climate has been, however, a fruitful
source of controversy. The first part of its course,
after leaving the strait, is sufficiently evident, for
its water long remains conspicuously different in
color and temperature from that of the ocean,
and a current having a marked effect on navigation
is long perceptible in the peculiar Gulf Stream
water. “Narrow at first, it flows round the peninsula
of Florida, and, with a speed of about 70 or
80 miles, follows the coast at first in a due north,
afterward in a northeast direction. At the latitude
of Washington it leaves the North American
coast altogether, keeping its northeastward course;
and to the south of the St. George’s and Newfoundland
banks it spreads its waters more and
more over the Atlantic Ocean, as far as the Azores.
At these islands a part of it turns southward again
toward the African coast. The Gulf Stream has,
so long as its waters are kept together along the
American coast, a temperature of 26°.6 C.; but,
even under north latitude 36°, Sabine found it
23°.3 C. at the beginning of December, while the
sea-water beyond the stream showed only 16°.9 C.
Under north latitude 40-41° the water is, according
to Humboldt, at 22°.5 C. within, and 17°.5 C.
without the stream.”
Opposite Tortugas, passing along the Cuban coast,
the stream is unbroken and the current feeble; the
temperature at the surface is about 26°.7 C. Issuing
from the Strait of Bemini the current is turned
nearly directly northward by the form of the land;
a little to the north of the strait, the rate is from
three to five miles an hour. The depth is only 325
fathoms, and the bottom, which in the Strait of
Florida was a simple slope and counter-slope, is
now corrugated. The surface temperature is about
26°.5 C., while the bottom temperature is 4°.5; so
that in the moderate depth of 325 fathoms the equatorial
current above and the polar counter-current
beneath have room to pass one another, the current
from the north being evidently tempered considerably
by mixture. North of Mosquito inlet the
stream trends to the eastward of north, and off St.
Augustine it has a decided set to the eastward.
Between St. Augustine and Cape Hatteras the set
of the stream and the trend of the coast differ but
little, making 5° of easting in 5° of northing. At
Hatteras it curves to the northward, and then runs
easterly. In the latitude of Cape Charles it turns
quite to the eastward, having a velocity of from a
mile to a mile and a half in the hour.
A brief account of one of the sections will best
explain the general phenomena of the stream off the
coast of America. I will take the section following
a line at right angles to the coast off Sandy Hook.
From the shore out, for a distance of about 250
miles, the surface temperature gradually rises from
21° to 24° C.; at 10 fathoms it rises from 19° to 22°
C.; and at 20 fathoms it maintains, with a few irregularities,
a temperature of 19° C. throughout the
whole space; while at 100, 200, 300, and 400 fathoms
it maintains in like manner the respective temperatures
of 8°.8, 5°.7, 4°.5, and 2°.5 C. This space
is, therefore, occupied by cold water, and observation
has sufficiently proved that the low temperature is
due to a branch of the Labrador current creeping
down along the coast in a direction opposite to that
of the Gulf Stream. In the Strait of Florida this
cold stream divides—one portion of it passing under
the hot Gulf Stream water into the Gulf of Mexico,
while the remainder courses round the western end
of Cuba. Two hundred and forty miles from the
shore the whole mass of water takes a sudden rise
of about 10° C. within 25 miles, a rise affecting nearly
equally the water at all depths, and thus producing
the singular phenomenon of two masses of water in
contact—one passing slowly southward and the other
more rapidly northward, at widely different temperatures
at the same levels. This abutting of the side
of the cold current against that of the Gulf Stream
is so abrupt that it has been aptly called by Lieutenant
George M. Bache the “cold wall.” Passing
the cold wall, we reach the Gulf Stream, presenting
all its special characters of color and transparency
and of temperature. In the section which we have
chosen as an example, upward of 300 miles in
length, the surface temperature is about 26°.5 C.,
but the heat is not uniform across the stream, for
we find that throughout its entire length, as far
south as the Cape Canaveral section, the stream is
broken up into longitudinal alternating bands of
warmer and cooler water. Off Sandy Hook, beyond
the cold wall, the stream rises to a maximum of
27°.8 C., and this warm band extends for about 60
miles. The temperature then falls to a minimum of
26°.5 C., which it retains for about 30 miles, when
a second maximum of 27°.4 succeeds, which includes
the axis of the Gulf Stream, and is about 170 miles
wide. This is followed by a second minimum of
25°.5 C., and this by a third maximum, when the
bands become indistinct. It is singular that the
minimum bands correspond with valley-like depressions
in the bottom, which follow in succession the
outline of the coast and lodge deep southward extensions
of the polar indraught.
The last section of the Gulf Stream surveyed by
the American hydrographers extends in a southeasterly
direction from Cape Cod, lat. 41° N., and
traces the Gulf Stream, still broken up by its bands
of unequal temperature, spreading directly eastward
across the Atlantic; its velocity has, however, now
become inconsiderable, and its limits are best traced
by the thermometer.
The course of the Gulf Stream beyond this point
has given rise to much discussion. I again quote
Professor Buff for what may be regarded as the
view most generally received among physical geographers:
“A great part of the warm water is carried partly
by its own motion, but chiefly by the prevailing west
and northwest winds, toward the coast of Europe
and even beyond Spitzbergen and Nova Zembla; and
thus a part of the heat of the south reaches far into
the Arctic Ocean. Hence, on the north coast of the
Old Continent, we always find driftwood from the
southern regions, and on this side the Arctic Ocean
remains free from ice during a great part of the
year, even as far up as 80° north latitude; while on
the opposite coast (of Greenland) the ice is not quite
thawed even in summer.” The two forces invoked
by Professor Buff to perform the work are thus the
vis à tergo of the trade-wind drift and the direct
driving power of the anti-trades, producing what
has been called the anti-trade drift. This is quite
in accordance with the views here advocated. The
proportion in which these two forces act, it is undoubtedly
impossible in the present state of our
knowledge to determine.
Mr. A. G. Findlay, a high authority on all hydrographic
matters, read a paper on the Gulf Stream
before the Royal Geographical Society, reported in
the 13th volume of the Proceedings of the Society.
Mr. Findlay, while admitting that the temperature
of Northern Europe is abnormally ameliorated by
a surface-current of the warm water of the Atlantic
which reaches it, contends that the Gulf Stream
proper—that is to say, the water injected, as it were,
into the Atlantic through the Strait of Florida by
the impulse of the trade-winds—becomes entirely
thinned out, dissipated, and lost opposite the Newfoundland
banks about lat. 45° N. The warm water
of the southern portion of the North Atlantic basin
is still carried northward; but Mr. Findlay attributes
this movement solely to the anti-trades—the
southwest winds—which by their prevalence keep up
a balance of progress in a northeasterly direction in
the surface layer of the water.
Dr. Carpenter entertains a very strong opinion that
the dispersion of the Gulf Stream may be affirmed to
be complete in about lat. 45° N. and long. 35° W.
Dr. Carpenter admits the accuracy of the projection
of the isotherms on the maps of Berghaus, Dove,
Petermann, and Keith Johnston, and he admits likewise
the conclusion that the abnormal mildness of the
climate on the northwestern coast of Europe is due
to a movement of equatorial water in a northeasterly
direction. “What I question is the correctness of
the doctrine that the northeast flow is an extension
or prolongation of the Gulf Stream, still driven on
by the vis à tergo of the trade-winds—a doctrine
which (greatly to my surprise) has been adopted and
defended by my colleague, Professor Wyville Thomson.
But while these authorities attribute the whole
or nearly the whole of this flow to the true Gulf
Stream, I regard a large part, if not the whole, of
that which takes place along our own western coast,
and passes north and northeast between Iceland and
Norway toward Spitzbergen, as quite independent
of that agency; so that it would continue if the
North and South American Continents were so completely
disunited that the equatorial currents would
be driven straight onward by the trade-winds into
the Pacific Ocean, instead of being embayed in the
Gulf of Mexico and driven out in a northeast direction
through the ‘narrows’ off Cape Florida.” Dr.
Carpenter does not mean by this to indorse Mr.
Findlay’s opinion that the movement beyond the
54th parallel of latitude is due solely to the drift of
the anti-trades; he says, “On the view I advocate,
the northeasterly flow is regarded as due to the
vis à fronte originating in the action of cold upon
the water of the polar area, whereby its level is
always tending to depression.” The amelioration
of the climate of northwestern Europe is thus
caused by a “modified case” of the general oceanic
circulation, and neither by the Gulf Stream nor by
the anti-trade drift.
Although there are, up to the present time, very
few trustworthy observations of deep-sea temperatures,
the surface temperature of the North Atlantic
has been investigated with considerable care. The
general character of the isothermal lines, with their
singular loop-like northern deflections, has long
been familiar through the temperature charts of the
geographers already quoted, and of late years a prodigious
amount of data have been accumulated.
In 1870, Dr. Petermann, of Gotha, published an
extremely valuable series of temperature charts,
embodying the results of the reduction of upward
of 100,000 observations.
Dr. Petermann has devoted the special attention
of a great part of his life to the distribution of heat
on the surface of the ocean, and the accuracy and
conscientiousness of his work in every detail are beyond
the shadow of a doubt.
In the North Atlantic every curve of equal temperature,
whether for the summer, for the winter,
for a single month, or for the whole year, instantly
declares itself as one of a system of curves which
are referred to the Strait of Florida as a source of
heat, and the flow of warm water may be traced in
a continuous stream—indicated when its movement
can no longer be observed by its form—fanning out
from the neighborhood of the Strait across the Atlantic,
skirting the coasts of France, Britain, and
Scandinavia, rounding the North Cape, and passing
the White Sea and the Sea of Kari, bathing the western
shores of Nova Zembla and Spitzbergen, and
finally coursing round the coast of Siberia, a trace
of it still remaining to find its way through the narrow
and shallow Behring’s Strait into the North
Pacific.
Now, it seems to me that if we had only these curves
upon the chart, deduced from an almost infinite
number of observations which are themselves merely
laboriously multiplied corroborations of many previous
ones, without having any clew to their rationale,
we should be compelled to admit that whatever
might be the amount and distribution of heat derived
from a general oceanic circulation—whether produced
by the prevailing winds of the region, by convection,
by unequal barometric pressure, by tropical
heat, or by arctic cold—the Gulf Stream, the majestic
stream of warm water whose course is indicated
by the deflections of the isothermal lines, is sufficiently
powerful to mask all the rest, and, broadly
speaking, to produce of itself all the abnormal thermal
phenomena.
The deep-sea temperatures taken in the Porcupine
have an important bearing upon this question,
since they give us the depth and volume of the mass
of water which is heated above its normal temperature,
and which we must regard as the softener of
the winds blowing on the coasts of Europe. In the
Bay of Biscay, after passing through a shallow band
superheated by direct radiation, a zone of warm
water extends to the depth of 800 fathoms, succeeded
by cold water to a depth of nearly two miles. In
the Rockall channel the warm layer has nearly the
same thickness, and the cold underlying water is
500 fathoms deep. Off the Butt of Lewis the bottom
temperature is 5°.2 C. at 767 fathoms, so that there
the warm layer evidently reaches to the bottom. In
the Faroe channel the warm water forms a surface
layer, and the cold water underlies it, commencing
at a depth of 200 fathoms—567 fathoms above the
level of the bottom of the warm water off the Butt
of Lewis. The cold water abuts against the warm—there
is no barrier between them. Part of the
warm water flows over the cold indraught, and forms
the upper layer in the Faroe channel. What prevents
the cold water from slipping, by virtue of its
greater weight, under the warm water of the Butt
of Lewis? It is quite evident that there must be
some force at work keeping the warm water in that
particular position, or, if it be moving, compelling
it to follow that particular course. The comparatively
high temperature from 100 fathoms to 900
fathoms I have always attributed to the northern accumulation
of the water of the Gulf Stream. The
amount of heat derived directly from the sun by the
water as it passes through any particular region, must
be regarded, as I have already said, as depending
almost entirely upon latitude. Taking this into account,
the surface temperatures in what we were in
the habit of calling the “warm area” coincided precisely
with Petermann’s curves indicating the northward
path of the Gulf Stream.
Typical Forms of Snowflakes Showing the Tendency to take the Form of Six-Pointed Figures
The North Atlantic and Arctic seas form together
a cul de sac closed to the northward, for there is
practically no passage for a body of water through
Behring’s Strait. While, therefore, a large portion
of the water, finding no free outlet toward the northeast,
turns southward at the Azores, the remainder,
instead of thinning off, has rather a tendency to accumulate
against the coasts bounding the northern
portions of the trough. We accordingly find that
it has a depth off the west coast of Iceland of at least
4,800 feet, with an unknown lateral extension. Dr.
Carpenter, discussing this opinion, says: “It is to me
physically inconceivable that this surface film of
lighter (because warmer) water should collect itself
together again—even supposing it still to retain any
excess of temperature—and should burrow downward
into the ‘trough,’ displacing colder and heavier
water, to a depth much greater than that which it
possesses at the point of its greatest ‘glory’—its passage
through the Florida Narrows. The upholders
of this hypothesis have to explain how such a recollection
and dipping-down of the Gulf Stream
water is to be accounted for on physical principles.”
I believe that, as a rule, experimental imitations on
a small scale are of little use in the illustration of
natural phenomena; a very simple experiment will,
however, show that such a process is possible. If we
put a tablespoonful of cochineal into a can of hot
water, so as to give it a red tint, and then run it
through a piece of India-rubber tube with a considerable
impulse along the surface of a quantity of
cold water in a bath, we see the red stream widening
out and becoming paler over the general surface of
the water till it reaches the opposite edge, and very
shortly the rapidly heightening color of a band along
the opposite wall indicates an accumulation of the
colored water where its current is arrested. If we
now dip the hand into the water of the centre of the
bath, a warm bracelet merely encircles the wrist;
while at the end of the bath opposite the warm influx,
the hot water, though considerably mixed, envelops
the whole hand.
The North Atlantic forms a basin closed to the
northward. Into the corner of this basin, as into a
bath—with a northeasterly direction given to it by its
initial velocity, as if the supply pipe of the bath were
turned so as to give the hot water a definite impulse—this
enormous flood is poured, day and night, winter
and summer. When the basin is full—and not till
then—overcoming its northern impulse, the surplus
water turns southward in a southern eddy, so that
there is a certain tendency for the hot water to accumulate
in the northern basin, to “bank down” along
the northeastern coasts.
It is scarcely necessary to say that for every unit
of water which enters the basin of the North Atlantic,
and which is not evaporated, an equivalent must
return. As cold water can gravitate into the deeper
parts of the ocean from all directions, it is only under
peculiar circumstances that any movement having
the character of a current is induced; these circumstances
occur, however, in the confined and contracted
communication between the North Atlantic
and the Arctic Sea. Between Cape Farewell and
North Cape there are only two channels of any considerable
depth, the one very narrow along the east
coast of Iceland, and the other along the east coast
of Greenland. The shallow part of the sea is entirely
occupied, at all events during summer, by the
warm water of the Gulf Stream, except at one point,
where a rapid current of cold water, very restricted
and very shallow, sweeps round the south of Spitzbergen
and then dips under the Gulf Stream water
at the northern entrance of the German Ocean.
This cold flow, at first a current, finally a mere
indraught, affects greatly the temperature of the
German Ocean; but it is entirely lost, for the slight
current which is again produced by the great contraction
at the Strait of Dover has a summer temperature
of 7°.5 C. The path of the cold indraught
from Spitzbergen may be readily traced by the depressions
in the surface isothermal lines, and in
dredging by the abundance of gigantic amphipodous
and isopodous crustaceans, and other well-known
Arctic animal forms.
From its low initial velocity the Arctic return current,
or indraught, must doubtless tend slightly in a
westerly direction, and the higher specific gravity of
the cold water may probably even more powerfully
lead it into the deepest channels; or possibly the two
causes may combine, and in the course of ages the
currents may hollow out deep southwesterly grooves.
The most marked is the Labrador current, which
passes down inside the Gulf Stream along the coasts
of Carolina and New Jersey, meeting it in the
strange abrupt “cold wall,” dipping under it as it
issues from the Gulf, coming to the surface again
on the other side, and a portion of it actually passing
under the Gulf Stream, as a cold counter-current,
into the Gulf of Mexico.
Fifty or sixty miles out from the west coast of
Scotland, I believe the Gulf Stream forms another,
though a very mitigated, “cold wall.” In 1868, after
our first investigation of the very remarkable cold
indraught into the channel between Shetland and
Faroe, I stated my belief that the current was entirely
banked up in the Faroe Channel by the Gulf
Stream passing its gorge. Since that time I have
been led to suspect that a part of the Arctic water
oozes down the Scottish coast, much mixed, and sufficiently
shallow to be affected throughout by solar
radiation. About sixty or seventy miles from shore
the isothermal lines have a slight but uniform deflection.
Within that line types characteristic of the
Scandinavian fauna are numerous in shallow water,
and in the course of many years’ use of the towing net
I have never met with any of the Gulf Stream pteropods,
or of the lovely Polycystina and Acanthometrina
which absolutely swarm beyond that limit.
The difference in mean temperature between the east
and west coasts of Scotland, amounting to about 1°
C., is almost somewhat less than might be expected
if the Gulf Stream came close to the western shore.
While the communication between the North Atlantic
and the Arctic Sea—itself a second cul de sac—is
thus restricted, limiting the interchange of warm
and cold water in the normal direction of the flow
of the Gulf Stream, and causing the diversion of a
large part of the stream to the southward, the communication
with the Antarctic basin is as open as the
day; a continuous and wide valley upward of 2,000
fathoms in depth stretching northward along the
western coasts of Africa and Europe.
That the southern water wells up into this valley
there could be little doubt from the form of the
ground; but here again we have curious corroborative
evidence in the remarkable reversal of the
curves of the isotherms. The temperature of
the bottom water at 1,230 fathoms off Rockall is
3°.22 C., exactly the same as that of water at the same
depth in the serial sounding, lat. 47° 38′ N., long.
12° 08′ W. in the Bay of Biscay, which affords a
strong presumption that the water in both cases is
derived from the same source; and the bottom water
off Rockall is warmer than the bottom water in the
Bay of Biscay (2°.5 C.), while a cordon of temperature
soundings drawn from the northwest of Scotland
to a point on the Iceland shallow gives no
temperature lower than 6°.5 C. This makes it very
improbable that the low temperature of the Bay of
Biscay is due to any considerable portion of the Spitzbergen
current passing down the west coast of Scotland;
and as the cold current to the east of Iceland
passes southward considerably to the westward, as
indicated by the successive depressions in the surface
isotherms, the balance of probability seems
to be in favor of the view that the conditions of
temperature and the slow movement of this vast mass
of moderately cold water, nearly two statute miles in
depth, are to be referred to an Antarctic rather than
to an Arctic origin.
The North Atlantic Ocean seems to consist first of
a great sheet of warm water, the general northerly
reflux of the equatorial current. Of this the greater
part passes through the Strait of Florida, and its
northeasterly flow is aided and maintained by the
anti-trades, the whole being generally called the
Gulf Stream. This layer is of varying depths, apparently
from the observations of Captain Chimmo
and others, thinning to a hundred fathoms or so in
the mid-Atlantic, but attaining a depth of 700 to 800
fathoms off the west coasts of Ireland and Spain.
Secondly, of a “stratum of intermixture” which extends
to about 200 fathoms in the Bay of Biscay,
through which the temperature falls rather rapidly;
and, thirdly, of an underlying mass of cold water,
in the Bay of Biscay 1,500 fathoms deep, derived as
an indraught falling in by gravitation from the deepest
available source, whether Arctic or Antarctic. It
seems at first sight a startling suggestion, that the cold
water filling deep ocean valleys in the Northern
Hemisphere may be partly derived from the southern;
but this difficulty, I believe, arises from the idea
that there is a kind of diaphragm at the equator between
the northern and southern ocean basins, one of
the many misconceptions which follow in the train
of a notion of a convective circulation in the sea
similar to that in the atmosphere. There is undoubtedly
a gradual elevation of an intertropical belt of
the underlying cold water, which is being raised by
the subsiding of still colder water into its bed to
supply the place of the water removed by the equatorial
current and by excessive evaporation; but
such a movement must be widely and irregularly diffused
and excessively slow, not in any sense comparable
with the diaphragm produced in the atmosphere
by the rushing upward of the northeast and southeast
trade-winds in the zone of calms. Perhaps one
of the most conclusive proofs of the extreme slowness
of the movement of the deep indraught is the
nature of the bottom. Over a great part of the floor
of the Atlantic a deposit is being formed of microscopic
shells. These with their living inhabitants
differ little in specific weight from the water itself,
and form a creamy flocculent layer, which must be
at once removed wherever there is a perceptible
movement. In water of moderate depth, in the
course of any of the currents, this deposit is entirely
absent, and is replaced by coarser or finer gravel.
It is only on the surface of the sea that a line is
drawn between the two hemispheres by the equatorial
current, whose effect in shedding a vast intertropical
drift of water on either side as it breaks
against the eastern shores of equatorial land may be
seen at a glance on the most elementary physical
chart.
The Gulf Stream loses an enormous amount of
heat in its northern tour. At a point 200 miles west
of Ushant, where observations at the greatest depths
were made on board the Porcupine, a section of the
water of the Atlantic shows three surfaces at which
interchange of temperature is taking place. First,
the surface of the sea—that is to say, the upper surface
of the Gulf Stream layer—is losing heat rapidly
by radiation, by contact with a layer of air which is
in constant motion and being perpetually cooled by
convection, and by the conversion of water into
vapor. As this cooling of the Gulf Stream layer takes
place principally at the surface, the temperature of
the mass is kept pretty uniform by convection. Secondly,
the band of contact of the lower surface of
the Gulf Stream water with the upper surface of
the cold indraught. Here the interchange of temperature
must be very slow, though that it does take
place is shown by the slight depression of the surface
isotherms over the principal paths of the indraught.
But there is a good deal of intermixture extending
through a considerable layer. The cold water being
beneath, convection in the ordinary sense can not
occur, and interchange of temperature must depend
mainly upon conduction and diffusion, causes which
in the case of masses of water must be almost secular
in their action, and probably to a much greater extent
upon mixture produced by local currents and by the
tides. The third surface is that of contact between
the cold indraught and the bottom of the sea. The
temperature of the crust of the earth has been variously
calculated at from 4° to 11° C., but it must be
completely cooled down by anything like a movement
and constant renewal of cold water. All we
can say, therefore, is that contact with the bottom
can never be a source of depression of temperature.
As a general result the Gulf Stream water is nearly
uniform in temperature throughout the greater part
of its depth; there is a marked zone of intermixture
at the junction between the warm water and the cold,
and the water of the cold indraught is regularly
stratified by gravitation; so that in deep water the
contour lines of the sea-bottom are, speaking generally,
lines of equal temperature. Keeping in view
the enormous influence which ocean currents exercise
in the distribution of climates at the present time, I
think it is scarcely going too far to suppose that such
currents—movements communicated to the water by
constant winds—existed at all geological periods as
the great means, I had almost said the sole means, of
producing a general oceanic circulation, and thus
distributing heat in the ocean. They must have existed,
in fact, wherever equatorial land interrupted
the path of the drift of the trade-winds. Wherever
a warm current was deflected to north or south from
the equatorial belt a polar indraught crept in beneath
to supply its place; and the ocean consequently
consisted, as in the Atlantic and doubtless in the Pacific
at the present day, of an upper warm stratum
and a lower layer of cold water becoming gradually
colder with increasing depth.
I must repeat that I have seen as yet no reason to
modify the opinion which I have consistently held
from the first, that the remarkable conditions of climate
on the coasts of Northern Europe are due in
a broad sense solely to the Gulf Stream. That is to
say that, although movements, some of them possibly
of considerable importance, must be produced by
differences of specific gravity, yet the influence of the
great current which we call the Gulf Stream, the reflux
of the great equatorial current, is so paramount
as to reduce all other causes to utter insignificance.
THE PHOSPHORESCENCE OF THE SEA — G. Hartwig
He who still lingers on the shore after the shades
of evening have descended not seldom enjoys
a most magnificent spectacle; for lucid flashes burst
from the bosom of the waters, as if the sea were anxious
to restore to the darkened heavens the light it
had received from them during the day. On approaching
the margin of the rising flood to examine
more closely the sparkling of the breaking wave, the
spreading waters seem to cover the beach with a sheet
of fire.
Each footstep over the moist sands elicits luminous
star-like points and a splash in the water resembles
the awakening of slumbering flames. The
same wonderful and beauteous aspect frequently
gladdens the eye of the navigator who plows his way
through the wide deserts of ocean, particularly if his
course leads him through the tropical seas.
“When a vessel,” says Humboldt, “driven along
by a fresh wind, divides the foaming waters, one
never wearies of the lovely spectacle their agitation
affords; for, whenever a wave makes the ship incline
sidewise, bluish or reddish flames seem to shoot upward
from the keel. Beautiful beyond description
is the sight of a troop of dolphins gamboling in
the phosphorescent sea. Every furrow they draw
through the waters is marked by streaks of intense
light. In the Gulf of Cariaco, between Cumana and
the peninsula of Maniquarez, this scene has often delighted
me for hours.”
But even in the colder oceanic regions the brilliant
phenomenon appears from time to time in its full
glory. During a dark and stormy September night,
on the way from the Sealion Island, Saint George, to
Unalashka, Chamisso admired as beautiful a phosphorescence
of the ocean as he had ever witnessed in
the tropical seas. Sparks of light, remaining attached
to the sails that had been wetted by the spray,
continued to glow in another element. Near the
south point of Kamtchatka, at a water temperature
hardly above freezing point, Ermann saw the sea no
less luminous than during a seven months’ sojourn
in the tropical ocean. This distinguished traveler
positively denies that warmth decidedly favors the
luminosity of the sea.
At Cape Colborn, one of the desolate promontories
of the desolate Victoria Land, the phosphoric gleaming
of the waves, when darkness closed in, was so
intense that Simpson assures us he had seldom seen
anything more brilliant. The boats seemed to cleave
a flood of molten silver, and the spray, dashed from
their bows before the fresh breeze, fell back in glittering
showers into the deep.
Mr. Charles Darwin paints in vivid colors the
magnificent spectacle presented by the sea while sailing
in the latitude of Cape Horn on a very dark
night. There was a fresh breeze, and every part of
the surface, which during the day is seen as foam,
now glowed with a pale light. The vessel drove
before her bows two billows of liquid phosphorus,
and in her wake she was followed by a milky train.
As far as the eye reached, the crest of every wave was
bright, and the sky above the horizon, from the reflected
glare of these livid flames, was not so utterly
obscure as over the rest of the heavens.
While La Venus was at anchor before Simon’s
Town, the breaking of the waves produced so strong
a light that the room in which the naturalists of the
expedition were seated was illumined as by sudden
flashes of lightning. Although more than fifty paces
from the beach when the phenomenon took place,
they tried to read by this wondrous oceanic light, but
the successive glimpses were of too short duration
to gratify their wishes.
Thus we see the same nocturnal splendor which
shines forth in the tropical seas and gleams along
our shores burst forth from the Arctic waters, and
from the waves that bathe the southern promontories
of the Old and the New Worlds.
But what is the cause of the beautiful phenomenon
so widely spread over the face of the ocean? How
comes it that at certain times flames issue from the
bosom of an element generally so hostile to their
appearance?
Without troubling the reader with the groundless
surmises of ancient naturalists, or repeating the useless
tales of the past, I shall at once place myself with
him on the stage of our actual knowledge of this interesting
and mysterious subject.
It is now no longer a matter of doubt that many of
the inferior marine animals possess the faculty of
secreting a luminous matter, and thus adding their
mite to the grand phenomenon. When we consider
their countless multitudes, we shall no longer wonder
at such magnificent effects being produced by creatures
individually so insignificant.
In our seas it is chiefly a minute gelatinous animal,
the Noctiluca miliaris, most probably an aberrant
member of the infusorial group, which, as it were,
repeats the splendid spectacle of the starry heavens
on the surface of the ocean. In form it is nearly
globular, presenting on one side a groove, from the
anterior extremity of which issues a peculiar curved
stalk or appendage marked by transverse lines, which
might seem to be made use of as an organ of locomotion.
Near the base of this tentacle is placed the
mouth, which passes into a dilatable digestive cavity,
leading, according to Mr. Huxley, to a distinct anal
orifice. From the rather firm external coat proceed
thread-like prolongations through the softer mass of
the body, so as to divide it into irregular chambers.
This little creature, which is just large enough to be
discerned by the naked eye when the water in which
it may be swimming is contained in a glass jar exposed
to the light, seems to feed on diatoms, as this
loricæ may frequently be detected in its interior. It
multiplies by spontaneous fission, and the rapidity
of this process may be inferred from the immensity of
its numbers. A single bucket of luminous sea-water
will often contain thousands, while for miles and
miles every wave breaking on the shore expands in
a sheet of living flame. It was first described by
Forster in the Pacific Ocean; it occurs on all the
shores of the Atlantic; and the Polar Seas are illumined
by its fairy light. “The nature of its luminosity,”
says Dr. Carpenter, “is found by microscopic
examination to be very peculiar; for what appears
to the eye to be a uniform glow is resolvable under
a sufficient power into a multitude of evanescent scintillations,
and these are given forth with increased
intensity whenever the body of the animal receives
any mechanical shock.”
The power of emitting a phosphorescent light is
widely diffused, both among the free-swimming and
the sessile Cœlenterata. Many of the Physophoridæ
are remarkable for its manifestation, and a great
number of the jelly-fishes are luminous. Our own
Thaumantias lucifera, a small and by no means rare
medusid, displays the phenomenon in a very beautiful
manner, for, when irritated by contact of fresh
water, it marks its position by a vivid circlet of tiny
stars, each shining from the base of a tentacle. A
remarkable greenish light, like that of burning silver,
may also be seen to glow from many of our Sertularians,
becoming much brighter under various
modes of excitation.
Among the Ctenophora the large Cestum Veneris
of the Mediterranean is specially distinguished for
its luminosity, and while moving beneath the surface
of the water gleams at night like a brilliant band
of flame.
The Sea-pens are eminently phosphorescent, shining
at night with a golden-green light of a most
wonderful softness. When touched, every branchlet
above the shock emits a phosphoric glow, while all
the polyps beneath remain in darkness. When
thrown into fresh water or alcohol, they scatter
sparks about in all directions, a most beautiful sight;
dying, as it were, in a halo of glory.
But of all the marine animals the Pyrosomas, doing
full justice to their name (fire bodies), seem to emit
the most vivid coruscations. Bibra relates in his
Travels to Chili that he once caught half a dozen of
these remarkable light-bearers, by whose phosphorescence
he could distinctly read their own description
in a naturalist’s vade-mecum. Although completely
dark when at rest, the slightest touch sufficed to elicit
their clear blue-green light. During a voyage to India,
Mr. Bennett had occasion to admire the magnificent
spectacle afforded by whole shoals of Pyrosomas.
The ship, proceeding at a rapid rate, continued
during an entire night to pass through distinct
but extensive fields of these mollusks, floating and
glowing as they floated on all sides of her course.
Enveloped in a flame of bright phosphorescent light,
and gleaming with a greenish lustre, the Pyrosomas,
in vast sheets, upward of a mile in breadth, and
stretching out till lost in the distance, presented a
sight the glory of which may be easily imagined.
The vessel, as it chased the gleaming mass, threw
up strong flashes of light, as if plowing through
liquid fire, which illuminated the hull, the sails, and
the ropes with a strange, unearthly radiance.
In his memoir on the Pyrosoma, M. Péron describes
with lively colors the circumstances under
which he first made its discovery, during a dark and
stormy night, in the tropical Atlantic. “The sky,”
says this distinguished naturalist, “was on all sides
loaded with heavy clouds; all around the obscurity
was profound; the wind blew violently; and the
ship cut her way with rapidity. Suddenly we discovered
at some distance a great phosphorescent band
stretched across the waves, and occupying an immense
tract in advance of the ship. Heightened by
the surrounding circumstances, the effect of this spectacle
was romantic, imposing, sublime, riveting the
attention of all on board. Soon we reached the illuminated
tract, and perceived that the prodigious
brightness was certainly and only attributable to the
presence of an innumerable multitude of largish
animals floating with the waves. From their swimming
at different depths they took apparently different
forms—those at the greatest depths were very indefinite,
presenting much the appearance of great
masses of fire, or rather enormous, red-hot cannon-balls;
while those more distinctly seen near the surface
perfectly resembled incandescent cylinders of
iron.
“Taken from the water, these animals entirely resembled
each other in form, color, substance, and the
property of phosphorescence, differing only in their
sizes, which varied from three to seven inches. The
large, longish tubercles with which the exterior of the
Pyrosomas was bristled were of a firmer substance,
and more transparent than the rest of the body, and
were brilliant and polished like diamonds. These
were the principal scene of phosphorescence. Between
these large tubercles, smaller ones, shorter and more
obtuse, could be distinguished; these also were phosphorescent.
Lastly, in the interior of the substance
of the animal, could be seen, by the aid of the transparency,
a number of little, elongated, narrow bodies
(viscera), which also participated in a high degree
in the possession of phosphoric light.”
In the Pholades or Lithodomes, that bore their
dwellings in hard stone, as other shell-fish do in the
loose sands, the whole mass of the body is permeated
with light. Pliny gives us a short but animated description
of the phenomenon in the edible date-shell
of the Mediterranean (Pholas dactylus):
“It is in the nature of the pholades to shine in
the darkness with their own light, which is the more
intense as the animal is more juicy. While eating
them, they shine in the mouth and on the hands, nay,
even the drops falling from them upon the ground
continue to emit light, a sure proof that the luminosity
we admire in them is associated with their
juice.”
Milne-Edwards found this observation perfectly
correct, for, wishing to place some living pholades in
alcohol, he saw a luminous matter exude from their
bodies, which, on account of its weight, sank in the
liquid, covering the bottom of the vessel, and there
forming a deposit as shining as when it was in contact
with the air.
Several kinds of fishes likewise possess the luminous
faculty. The sunfish, that strange deformity
emits a phosphoric gleam; and a species of Gunard
(Trigla lucerna) is said to sparkle in the night, so
as to form fiery streams through the water.
With regard to the luminosity of the larger marine
animals, Ermann, however, remarks that he so
often saw small luminous crustacea in the abdominal
cavity of the transparent Salpa pinnata that it may
well be asked whether the phosphorescence of the
larger creatures is not in reality owing to that of
their smaller companions.
According to Mr. Bennett—Whaling Voyage
Round the Globe—a species of shark first discovered
by himself is distinguished by an uncommonly
strong emission of light. When the specimen, taken
at night, was removed into a dark apartment, it
afforded a very interesting spectacle. The entire inferior
surface of the body and head emitted a vivid
and greenish phosphorescent gleam, imparting to the
creature by its own light a truly ghastly and terrific
appearance. The luminous effect was constant, and
not perceptibly increased by agitation or friction.
When the shark expired (which was not until it had
been out of the water more than three hours), the
luminous appearance faded entirely from the abdomen,
and more gradually from other parts, lingering
longest around the jaws and on the fins.
The only part of the under surface of the animal
which was free from luminosity was the black collar
round the throat; and while the inferior surface of
the pectoral, anal, and caudal fins shone with splendor,
their superior surface (including the upper lobe
of the tail fin) was in darkness, as were also the dorsal
fins and the back and summit of the head.
Mr. Bennett is inclined to believe that the luminous
power of this shark resides in a peculiar secretion
from the skin. It was his first impression that
the fish had accidentally contracted some phosphorescent
matter from the sea, or from the net in which
it was captured; but the most rigid investigation did
not confirm this suspicion, while the uniformity with
which the luminous gleam occupied certain portions
of the body and fins, its permanence during life,
and decline and cessation upon the approach and occurrence
of death, did not leave a doubt in his mind
but that it was a vital principle essential to the economy
of the animal. The small size of the fins would
seem to denote that this fish is not active in swimming;
and, since it is highly predaceous and evidently
of nocturnal habits, we may perhaps indulge in the
hypothesis that the phosphorescent power it possesses
is of use to attract its prey, upon the same principle
as the Polynesian islanders and others employ
torches in night-fishing.
Some of the lower sea-plants also appear to be
luminous. Thus, over a space of more than 600 miles
(between lat. 8° N. and 2° S.), Meyen saw the ocean
covered with phosphorescent Oscillatoria, grouped
together into small balls or globules, from the size
of a poppy-seed to that of a lentil.
But if the luminosity of the ocean generally proceeds
from living creatures, it sometimes also arises
from putrefying organic fibres and membranes, resulting
from the decomposition of these living light
bearers. “Sometimes,” says Humboldt, “even a
high magnifying power is unable to discover any
animals in the phosphorescent water, and yet light
gleams forth wherever a wave strikes against a hard
body and dissolves in foam. The cause of this
phenomenon lies then most likely in the putrefying
fibres of dead mollusks, which are mixed with the
waters in countless numbers.”
Summing up the foregoing in a few words, it is thus
an indisputable fact that the phosphorescence of the
sea is by no means an electrical or magnetic property
of the water, but exclusively bound to organic matter,
living or dead. But although thus much has been
ascertained, we have as yet only advanced one step
toward the unraveling of the mystery, and its prominent
cause remains an open question. Unfortunately,
science is still unable to give a positive answer, and
we are obliged to be content with a more or less
plausible hypothesis.
We know as little of what utility marine phosphorescence
may be. Why do the countless myriads of
Mammariæ gleam and sparkle along our coasts? Is
it to signify their presence to other animals, and direct
them to the spot where they may find abundance
of food? So much is certain, that so grand and widespread
a phenomenon must necessarily serve some
end equally grand and important.
As the phosphorescence of the sea is owing to living
creatures, it must naturally show itself in its
greatest brilliancy when the ocean is at rest; for during
the daytime we find the surface of the waters
most peopled with various animals when only a slight
zephyr glides over the sea. In stormy weather, the
fragile or gelatinous world of the lower marine creatures
generally seek a greater depth, until the elementary
strife has ceased, when it again loves to sport
in the warmer or more cheerful superficial waters.
In the tropical zone, Humboldt saw the sea most
brilliantly luminous before a storm, when the air was
sultry and the sky covered with clouds. In the North
Sea we observe the phenomenon most commonly during
fine, tranquil autumnal nights; but it may be seen
at every season of the year, even when the cold is most
intense. Its appearance is, however, extremely capricious;
for, under seemingly unaltered circumstances,
the sea may one night be very luminous and the next
quite dark. Often months, even years, pass by without
witnessing it in full perfection. Does this result
from a peculiar state of the atmosphere, or do the
little animals love to migrate from one part of the
coast to another?
It is remarkable that the ancients should have
taken so little notice of oceanic phosphorescence.
The Periplus of Hanno contains, perhaps, the only
passage in which the phenomenon is described.
To the south of Cerne the Carthaginian navigator
saw the sea burn, as it were, with streams of fire.
Pliny, in whom the miracle (miraculum, as he calls
it) of the date-shell excited so lively an admiration,
and who must often have seen the sea gleam with
phosphoric light, as the passage proves where he
mentions in a few dry words the luminous gurnard
(lucerna) stretching out a fiery tongue, has no exclamation
of delight for one of the most beautiful
sights of nature. Homer also, who has given us so
many charming descriptions of the sea in its ever-changing
aspects, and who so often leads us with
long-suffering Ulysses through the nocturnal floods,
never once makes them blaze or sparkle in his immortal
hexameters. Even modern poets mention the phenomenon
but rarely. Camoens himself, whom Humboldt,
on account of his beautiful oceanic descriptions,
calls, above all others, the “poet of the sea,”
forgets to sing it in his Lusiad. Byron in his Corsair
has a few lines on the subject:
“Flash’d the dipt oars, and, sparkling with the stroke,
Around the waves phosphoric brightness broke;”
but contents himself, as we see, with coldly mentioning
a phenomenon so worthy of all a poet’s enthusiasm.
In Coleridge’s wondrous ballad of The Ancient
Mariner we find a warmer description:
“Beyond the shadow of the ship
I watch’d the water-snakes:
They moved in tracks of shining white,
And, when they rear’d, the elfin light
Fell off in hoary flakes.
“Within the shadow of the ship
I watch’d their rich attire—
Blue, glossy green, and velvet black:
They coiled and swam, and every track
Was a flash of golden fire.”
These, indeed, are lines whose brilliancy emulates
the splendor of the phenomenon they depict, but,
even they are hardly more beautiful than Crabbe’s
admirable description:
“And now your view upon the ocean turn,
And there the splendor of the waves discern;
Cast but a stone, or strike them with an oar,
And you shall flames within the deep explore;
Or scoop the stream phosphoric as you stand,
And the cold flames shall flash along your hand;
When, lost in wonder, you shall walk and gaze
On weeds that sparkle, and on waves that blaze.”
Or the graphic numbers of Sir Walter Scott:
“Awak’d before the rushing prow,
The mimic fires of ocean glow,
Those lightnings of the wave;
Wild sparkles crest the broken tides,
And dashing round, the vessel’s sides
With elfish lustre lave;
While, far behind, their vivid light
To the dark billows of the night
A blooming splendor gave.”
THE SEASHORE — P. Martin Duncan
The seashore is the debatable ground where the
sea is constantly striving to wear away the
land. It is the present limit to the ocean and sea,
and a little beyond, for it reaches inland further than
the wildest waves and the highest tides can attain.
Where the seashore begins and ends is a matter of
opinion; but all of it is influenced in some way or
other by the sea. In some places, high cliffs or rocks
keep the sea from driving in upon the land; they are
lofty, and may reach for miles along the coast. The
high tide comes up their steep faces for many yards,
and when it retires a rocky strip is seen at their feet,
and thence a breadth of rock, shingle, or sand leads
down with a greater or less slope to the water’s edge.
Here there can be no doubt how far the shore reaches
inland, for the cliffs limit it. In other parts of our
own and other maritime countries, there may be no
high land on the coast; but marshes and low lands,
with, or without sand-hills, form barriers to the incursion
of the sea. The highest tides have their limit
in those places, but the wash of the sea and the spray,
together with the drainage of the sea into the land,
make the water saltish for some distance inland, and
the earth close by is sodden with salt. Then, long
stretches of mud or of sand form the slope, over
which the sea rolls up to the land, and which is exposed
and remains more or less wet at low tide.
In these low-lying parts of the coast the shore is
not very distinctly separated from the land, and often
miles of swamp, marsh, and sand-banks are invaded
by the sea during storms and very high tides. The
ditches near the sea contain salt or brackish water,
and the whole of this kind of coast-line has a peculiar
and desolate appearance. If these two kinds of
coast are taken as the extremes, all the varieties of
seashores will fit in between them; but still it will
appear that while in some the limit between the land
and the sea is very decided, in others it is not so.
Seaward the shore is very variable in its extent.
In some places it may barely exist, or may only be a
ledge of rock, between the cliff, the high land, and
the water; and in others, miles of sand, shingle, and
mud may be between the furthest reach of the waves
and the limit of the low tide. The commonest examples
of shores are those which are between these
extremes. Some seashores slope very gradually to
the sea and their extent is then usually great; and
others, which are limited in their breadth, are more
precipitous. Perhaps it is best to say that a seashore
is the part of a coast which, at some time or other, is
covered or uncovered by the sea; and that it has an
extension inland, where the spray and wind are felt
and act on the land, and also seaward, where some
shore is only uncovered during excessively low tides.
According to this view, it is possible to portion out a
seashore into a greater or less number of breadths,
which may be placed, side by side, from the land
to the sea. First, a breadth will pass along the coast,
and will contain the marshy, swampy land, or the
hard rock down to the edge of the highest tide-mark.
It may be miles across, or only a few feet in extent.
Secondly, a breadth will be found between this last
and the sea, where it is highest during common tides
and storms. Thirdly, a breadth will exist four times
in the twenty-four hours as dry land, and for the
rest of the time it will be beneath the waves, and this
is situated between ordinary high and low-tide marks.
Finally, a breadth will be between this last and the
everlasting sea; it is narrow, and is only uncovered
for a few hours, in the months of the year when there
are what are called “low spring tides.” These four
breadths are termed zones, or belts; and in common
language the first is the beach and coast-line, the
second is the shore, the third is the tide-shore, and
the fourth is “low spring shore.”
Differing in their extent, and in the nature of their
surface, in every few miles of the coast of a maritime
country like Great Britain, the zones have their peculiar
animals and plants, and waifs and strays—the
wreckage of the sea, of its floor, and of the coast-line.
When the whole of the shore slopes very rapidly
to the sea, the third and fourth zones are small
in extent, but when the slope is gradual, they are
large. And when the tide rises much and falls correspondingly,
the third zone is usually uncovered but
for a short time. The tide usually moves along the
shore, and does not simply come in on to the land and
recede; for one tide moves in one direction and the
next in the opposite. Thus floating substances are
carried along the coast for miles by the rising tide,
and come back again, more or less, with the falling
tide.
Tide, wind, and wave forever act on the surface
of the zones, but their action is the greatest on
those which are landward. There are other wreckers
of the coast; for the heat of the sun, the winter’s
frost, the rain, and the chemical action of the air,
one and all crumble and break off pieces of rock or
earth. These fall on to the tidal shore, and are rolled
here and there, and up and down, to be turned into
mud, sand, and pebbles. The cliffs and bold headlands
are worn year by year, and during centuries
they lose much, and retire landward. Needles and
“no man’s lands” stand out on the shore, or out at
sea, testifying to the former extension of the land; and
shore exists where there was once high solid rock.
The shore consists of the worn surface of the old
land, rock, or earth, and this is usually hidden by
stone or stuff which has fallen from the cliffs, and
by sand, or mud, or pebble and stone, which the tide
has swept along. But often the jagged or rounded
remains of the former rock project out of the sand,
mud, and stone on the shore, and they may be bare,
or covered with sea-weed. In other spots, the hard
rock is hollowed out into places which let the water
stand in them like so many puddles, pools, and ponds,
when the tide has gone down. These are often
crowded with marine plants and animals of the shore.
The rolling stones, the wash of the tide, and the rush
and drawback of the waves, are ever wearing off the
surface of the shore and grooving it, or planing it
flat, and in some places where the stones do not
collect, this is very evident; but where they form
great masses of pebbles or shingle, it can not be
readily seen.
There are many shores around Great Britain,
where the rock is hard, which are rarely covered
with pebbles, bowlders, and sand; and the sea-weed
grows on them and protects them against the sea.
But usually the rock is only exposed here and there,
and the stones which collect and cover much of it
come from a distance, and are on the move at every
tide. In some places, where the coast is composed of
clay or soft sandstone, the shore is muddy, soft, and
may be uncovered or covered by stones.
The wear of the sea is but little seen in such places
as this, and still less so where the coast is low and
flat, and the shore is very extensive and the water is
shallow for a long distance. In fact, on many of these
flat shores, instead of erosion taking place, the sea is
adding to the land by depositing. This is particularly
the case at the entrance of great, and of many
small rivers. Their mud collects in the shallows at
their mouths, and is added to by sand and shingle, so
that land grows seaward, instead of the reverse. The
seashore is then, usually, uninviting and often consists
of large mud flats. Again, in some localities, where
much sand collects on the surface of the rock forming
the seashore, it may be “quick” in many places. The
rising tide gets under the sand, which suddenly becomes
like so much sand and water, and the falling
tide leaves it hard for a while. The ordinary condition
of a sandy shore is either that of a number of
very slightly rounded stretches of sand, with drainage-streams
between them, or it is pretty hard, readily
dug into, and marked on the surface by ripples.
The ripple-mark on sand always strikes the observer;
it represents little ripple-like waves, wonderfully
regular, and each has a ridge and a valley. They are
very lasting, but disappear on the slightest movement
of the wet sand as the tide comes in. These little
ridges and valleys are not found when the water
covers the sand at a considerable depth, but they are
especially seen between high and low spring-tide
limit. Such marks can be made, artificially, with
sand, for instance, on the bottom of a large basin. If
some sand is placed on the bottom, and water be
poured in, and the edge of the basin be pushed, a to-and-fro
movement of the water will occur, and it will
be continued down to the sand. As the motion
ceases, the sand will be seen to collect in ridges, side
by side, and they will be perfect when the motion
stops. Motion of the sea-water in one direction over
soft sand will not produce ripple-mark well, but a
slight to-and-fro movement will do it to perfection.
Infinitely more wonderful than these ripples are the
pebble beaches, for they often extend for many miles,
and have a very considerable thickness. Worn, in
the first instance, from distant rocks, born of huge
bowlders, which the mighty waves laden with rolling
stones have broken down, the pebble is formed by
rolling against others, and the result of its wear and
tear is carried off in the form of sand. They travel
miles and miles along the coast with the tide, and
therefore it is very common to find one kind of rock
forming the coast-line, and the shore close by having
pebbles made up of stone which is not known to be
near at hand. Thus, on the coast of South Devon, the
red rocks form the coast-line; they are sandy, and are
covered in some places by a beautiful green vegetation.
The sea is often of the brightest blue, or gray,
when the sky is not much tinted with color. But the
sea covering the shore at high tide looks whitish, and
this is produced by the white and light slate-colored
pebbles which reach up close to the red rocks. They
are not made up of red sand; on the contrary, they
are of gray and bluish limestone, and come from
rocks which are situated miles to the west. Further
east, the Chesil Bank is seen, and it is an enormous
shore of pebbles, which have been carried along the
coast and have found an uncertain resting-place
there. Every tide makes more sand out of the hardest
pebbles, as they knock one against the other and wear
away, and the sand already made scrubs them as it
is hurried hither and thither by the waves. In some
places where the sea is giving up rather than taking
off land, the sand which is cast up may be the result
of the wear of distant pebble-making, or it may be
composed of myriads of broken tiny shells which
once lived in shallow water.
It has been already stated that the sea is encroaching
on the land in some parts of England, and that
it does not do so in others, while it appears to be
giving place to land elsewhere. In the first instance,
the seashore must grow, as it were, must increase
landward, and it really does so at different rates, in
different parts of the country. In some parts of the
coast a yard is lost every year and the sea comes in
on the land so much the more. But all the space once
occupied by cliff and rock is soon worn by the sea
and is covered gradually by the tide, and after years
have elapsed this fore-shore is deepened seaward by
the rolling stone and rushing waves, so that the visible
beach or shore diminishes in size, unless a corresponding
landward extension takes place. Although
the cliffs and rocks fall, and their remains are swept
away from the level of the shore, by currents, tides,
and waves; yet, as has already been noticed, much of
the ruined surface, leveled down as it has been, is
covered up by relics of their wear and tear or by
stone brought from a distance. It is only after some
severe gale of wind, accompanied by a very high
tide, that these stones and covering-up relics are
swept away and the old rock-surface comes in view.
All these matters are of importance, for the living
creatures of the seashore depend upon the state of
things, in each of the zones, for their ability to exist
and flourish.
Where the coast has been low and the sea has
gradually encroached, the remains of stumps of trees
are often exposed after a gale. Then what is called
part of a submarine forest is opened to the sight.
There are many of them around England and especially
on the coast of Norfolk and Essex, on the east;
in many places on the south coast as far as Torbay;
and on the west they are found in the Bristol Channel,
and about Holyhead and the river Mersey.
Sometimes it appears that the sunken forest has not
been altogether produced by the encroachment of the
sea on the land, and that sinking of the coast, or
slipping of part of it, has caused the event. When
the sea comes in on the land, it wears everything before
it, and any forest land would in most instances
be completely wrecked and the roots of the great
trees would be worn and torn out of the soft earth
and carried off to sea by the waves, tides, and currents.
On looking at some smaller forests which
are laid bare at very low tides, it is found that they
consist of stumps of trees of great size, whose roots
are still in the clay in which they grew, and a quantity
of mud and sand is between the stumps and protects
them from the usual action of water on submerged
land. It appears that some movement of the
earth’s crust had caused the coast to sink down, and
then the sea invaded without wearing off the land.
The trees were ruined by the sea-water, and broken
off, and the mud, sand, and stone collected around
the stumps.
It is not uncommon to see collections of stone and
shells high up on the face of a rock or cliff, and when
they are carefully examined they are found to resemble
a bit of a shore or a piece of the beach,
hoisted up many feet above the present line of the
waves and tides.
They are called raised beaches, and they were
formed by an upheaval of part of the coast with its
shore during movements in the crust of the globe.
There was a shore and a cliff, as there may be now,
and the whole was pushed up some twenty, thirty,
or more than a hundred feet beyond the reach of the
highest tides and waves. In years past the waves
broke upon the cliff beneath the upraised portion,
and wore it away bit by bit; and then the air and
sun acted with the rain in wearing it, and now only
a portion remains.
Every coast-line is subject to these sinkings-down
and upheavals, and of course a seashore is produced
rapidly, and is made broad and shallow during the
first kind of occurrence, and is stopped and has to
be formed afresh during the last. As these remarkable
movements of the outside of the globe are not
universal, and affect some parts of a coast more than
others, they will tend to give great variety to the seashores
of a country. Together with the varying action
of the tides, waves, and currents upon cliffs and
rocks of different stones and earths, and of many
hardnesses, these movements have made the shores
of Great Britain very curiously varied in their size
and character.
It must be remembered that as new shores are
formed, or old ones are extended, the zones are
kept within their bounds, and that as one zone creeps
in on the land, those to the seaward move up also;
so that where there was once a between-tide zone
there may now be deep water. This change in the
position of zones is very important; for certain animals
and plants of the shore only live in certain zones,
and their increase or decrease in numbers depends
upon the corresponding state of their special locality.
III.—THE ATMOSPHERE — THE OCEAN OF AIR — Agnes Giberne
Our earth has many robes. Closely-fitting garments
come first, of brown soil or gray rock
and green grass, with wide liquid underskirts of
deep blue filling up the spaces between. Outside
these are coverings more wonderful still; fragile, yet
strong, transparent, almost invisible, folded around
layer upon layer, or, as one might say, veil upon veil,
each more gossamer-like than the last. These form
earth’s surrounding atmosphere—a substance pervading
everything, found everywhere. One may
travel from the equator to the poles, one may journey
by sea or by land, one may soar high in a balloon
or descend deep into a mine, but one can never
in this world go to a place where the atmosphere
is not.
A substance—for air can be felt; air has weight;
air occupies space; air, like any other body, can be
made hot or cold; air is composed of particles of
substantial matter. Air has a faint bluish tint, which
on a sunshiny day becomes in the sky a very pure
and deep blue. This tint is not believed to be the
natural color of the atmosphere. Were it so, the air
would merely act the part of a blue pane of glass,
rendering the white light of the sun blue as it reaches
our eyes; but the blue of the atmosphere is known to
be a reflected blue.
If reflected, there must be something in the atmosphere
to reflect it; and such indeed is the case. Perfectly
pure air would doubtless be without color, but
perfectly pure air we do not find. The whole atmosphere
is full of multitudinous minute specks, so small
as to be in themselves invisible, so light as to remain
aloft. To the presence of these the blue tint is believed
to be due. They scatter the light of the sun,
and produce the blue effect.
A beam of strong white light, caused to pass
through a liquid which contains a large supply of
minute floating particles, is affected by them in a like
manner. The short blue waves are more abundantly
reflected than the long red waves; and so the water
seems to be blue. This explanation serves for the
deep-blue color of the ocean, as well as for the blue
of the atmosphere.
The whole earth is surrounded by this marvelous
air-ocean; an ocean of gaseous matter, at least one
hundred times as deep as the water-ocean. At the
bottom of the gaseous ocean we small human creatures
crawl about, commonly on flat lower levels—the
ocean bottom, in fact. Sometimes, with much
toil and trouble, we climb the little ridges and
mounds called “mountains”; little compared with
the depth of the atmosphere, though not little compared
with ourselves. The highest mountain-peaks
of even the vast Himalayas lie low down near the
bottom of the ocean of air.
But the very extent of the ocean of air adds to our
difficulty in studying its nature. All observations
that we can make must be limited by the state of
the atmosphere just around ourselves. We can never
get out of and beyond the atmosphere, so as to see it
as a whole. At any time a slight local fog is enough
to put a stop altogether to such observations, beyond
the unpleasant experience of the fog itself.
It used to be supposed that the atmosphere reached
only to a height of about fifty miles above earth’s
surface. Of late years the opinion has gained ground
that the atmosphere reaches to a height certainly of
two or three hundred miles, probably of four or five
hundred, possibly a good deal more. But the condition
of the air far above is different from that of the
air in lower levels, where we live and breathe. The
higher we ascend, the more thin or “rare” becomes
the air. A less quantity fills a certain space up there
than down here. The particles float further apart
one from another.
This difference in the density of the air is chiefly
due to attraction. Each separate air-particle is
drawn steadily earthward by the force of gravitation,
and that force is stronger on the surface of earth
than at a distance. The closer to earth, the heavier
the pull; the further from earth, the less the pull.
Besides the actual attraction of the earth drawing the
air-particles downward, there is the great weight of
the whole atmosphere above, caused by the same attraction.
Miles and miles of air overhead press
mightily downward, packing tightly together the
lower layers of air near to earth’s surface.
Without this pressure of the overlying atmosphere,
the air down here would not be nearly so dense as it
is; and, indeed, would not be fitted to support life.
A man ascending a mountain or rising in a balloon
leaves heavy layers of air below, and has an ever-lightening
weight above, so that the atmosphere
around him becomes constantly more thin, more difficult
to breathe.
In the beginning of the last century Humboldt
made a vigorous attempt to scale Chimborazo, one of
the loftiest of the Andes. He and his party suffered
severely from sickness, giddiness, and difficulty in
breathing, and the attempt proved a failure. Not
till over seventy years later was the ascent actually
accomplished by Mr. Whymper.
Carried upward passively in a balloon, without
effort, men have risen higher than the highest mountains.
Mr. Coxwell and Mr. Glaisher in their celebrated
aerial voyage of 1862 are believed to have
mounted seven miles above the sea. No little peril
and suffering were involved, alike from the extreme
thinness of the air, and from the bitter cold.
The voyagers suffered from severe “sea-sickness,”
though not from bleeding of the nose or singing in
the ears, popularly expected on such occasions. They
had enough to bear without these additions. Mr.
Glaisher held manfully to his task, observing and
noting down the state of the atmosphere minute by
minute, despite sickness, brain-pressure, violent
headache, and a pulse at 108 per minute, all due to
the rarity of the air.
In those lofty regions of the air-ocean no living
creatures exist. The voyagers passed through boundless
silent solitudes—silent except for the hurried
beating of their own hearts, the sound of their own
panting breath, the sharp ticking of their watches,
and the “clang of the valve door.”
On leaving earth the thermometer stood at 59°.
Soon afterward the balloon passed through masses
of cloud, thousands of feet in depth, then came out
into dazzling sunshine, with deep-blue sky above and
countless mountain masses of billowy cloud below.
As they rose, they released at intervals a captive
pigeon. One set free at a height of nearly five miles
“fell downward like a stone.” Of two others taken
higher, one died of the cold and the other was stupefied.
When they reached five miles above the sea,
the temperature was below zero.
Still upward, further upward, rose the resolute
pair. Then blinding darkness and insensibility
seized Mr. Glaisher. Had he been alone, he could
never have revived. With no one to open the valve,
the balloon must have carried him onward into yet
higher and deathlier regions, where for lack of air
he would have perished. Even then Mr. Coxwell
did not at once give in; but he was strictly on the
watch. At the seven miles’ level, a tremendous
height, he too felt signs of failing consciousness. In
a few minutes more all would have been over with
them both, and at last he yielded. It was indeed
time that he should. His hands were powerless to
act, but he seized the valve rope in his teeth and
pulled. The gas rushed out; the balloon steadily
sank. Both lives were saved, and a mighty feat had
been accomplished.
Yes, a mighty feat, and a tremendous height—in
consideration of human powers! Seven miles high
would seem to be the outside limit at which animals
generally can exist for even a short time. Birds may
be to some extent an exception. Certain birds are
believed to soar occasionally two or three miles
higher still.
But what are seven miles—what are even ten miles—compared
with the four or five hundred miles of
atmosphere-depth? With all our utmost efforts, we
and the birds still find ourselves only able to creep
and flutter on or near the floor of the ocean of air.
What earth would be without her surrounding
ocean of air, we can scarcely imagine. The atmosphere
plays so extraordinary and essential a part in
all around, that to picture its entire absence is not
easy. We see faintly on the moon something of what
an airless world must be. Yet since we only “see”
from a distance of two hundred and forty thousand
miles, that does not mean much. Imagination has to
come in, and imagination is apt to play us curious
tricks when running after affairs which lie outside
the range of human experience.
Without air, man and beast can not breathe.
Without air, plants and trees can not grow. Without
air, life as we know it—the lower animal life
common to man and beast—is a thing impossible.
Without air, our world would be, as we suppose the
moon to be, a world of lifelessness.
Air is earth’s outer robe, “for use and for beauty”—for
use in modes uncountable; for beauty, not so
much in itself as in the softening, the diffusing, the
controlling effects of its presence. Air is a mighty
ocean, in which all things living must dwell. Even
the living things of the sea are not exceptions to this
rule, for water itself is pervaded by air. A man,
going into and under water, does not get beyond the
touch of air; only, not being provided, like fishes,
with breathing gills, he can not make use of what
is there—he can not separate the air from the water,
and so keep himself alive by breathing it.
Some animals living in the water-ocean are as
dependent upon the air-ocean as man himself for
“the breath of life.” Whales are a remarkable example
of this. They are not fishes, though often
mistakenly called so, but belong to the same “family”
of creatures as men and land-quadrupeds generally.
A whale is warm-blooded, has no gills, and breathes
atmospheric air, coming to the surface for it. A
whale kept forcibly for a long while under water
would be drowned exactly as a man would be. If a
whale is thrown upon the shore, it does not die of
suffocation, but of inanition. A fish’s gills are no
more fitted to breathe air in bulk than a man’s lungs
are fitted to breathe air diffused in minute particles
through water. The fish out of water is suffocated
by getting air too rapidly: the man under water by
exactly the reverse. A whale breathes like a man,
and on land it simply starves fast from lack of the
incessant food required by such a huge carcass.
There is a difference certainly between man and
whale in the matter of breathing. A man has to take
in fresh supplies of air constantly, and if he is beyond
reach of air for more than a few minutes he dies. A
whale comes to the surface for about ten minutes,
spouting out enormous supplies of used-up air and
taking in enormous supplies of fresh air, after which
it can remain under water for half an hour or more:
some say an hour. Then a fresh bout of noisy breathing
becomes an absolute necessity. This, however, is
merely a matter of internal arrangement. The whale
has an immense reservoir of blood, which, being
thoroughly purified by the air during ten minutes of
vigorous breathing, serves slowly to supply the creature’s
requirements while below. But the need for
air, and the effect of that air upon the blood, are
much the same in man and whale.
Small creatures, as well as big ones, spending
much time under water, and yet breathing air, have
to come regularly to the surface.
If our world had no ocean of air, there could be
on earth no men, no quadrupeds, no whales or fishes,
no birds or insects, no forms of life.
Like the ocean of water, the ocean of air knows
no repose or stagnation. What we call stillness on
the most sultry of summer days does not mean absolute
stillness. Though not enough wind may stir to
lift a feather, yet the air is in ceaseless motion, to
and fro, hither and thither. The whole atmosphere
is a vast and complicated system of air-currents, and
each lesser portion of air has its own lesser circulation.
You can not lift your hand without causing a
tiny breeze; you can not turn a wheel without making
a minute whirlwind; and every separate air-movement
draws other movements in its train.
There is water enough on earth for all needed
purposes; but we should find ourselves in direful
straits if the whole water-carrying from lakes and
rivers for men and animals had to be performed by
human agencies.
Far from this, a mighty apparatus is provided.
The scanty aid that man can give only shows how
little he is capable of. The entire atmosphere is a
tremendous pumping engine, an enormous watering
machine, always at work; always receiving supplies
of liquid from the ocean, from seas, lakes, rivers; always
showering this water down again upon the
land, as needful drink for plants and animals, as
needful cleansing for all things.
Air, the great carrier of water, in its wonderful
strength and restlessness, bears vast layers of cloud
to and fro, wafts away superfluous damp, drenches
the dry and thirsty earth, fills ponds and lakes, feeds—nay,
actually makes—the rivers, never flags in its
ceaseless energy. If clouds hang low or fogs arise,
we are glad of the moving air which sweeps them
elsewhere. If the soil is caked and plants droop, we
are glad of the moving air which brings rain. Thus
our wants are supplied, and the wide water circulation
of earth is carried on. Without circulation,
without motion, stir, change, there can not be life.
Stagnation must mean death. Our earth, without
her ocean of moving air, would be a world of death.
Without air, earth would be in great measure a
soundless world. Silence would reign here, as probably
it does reign on the moon. Sound, as it commonly
reaches our ears, depends for its very existence
upon air. Let the concussion of two bodies be
ever so mighty, if there were no air to bear away the
vibrations of that concussion, there could be no crash
of sound. True, sound-waves can be conveyed
through a liquid or through a solid as well as
through air; and we might be conscious of the
ground’s vibrations, but our ears would hear no
noise.
So an airless world would be a silent world. Without
air, supposing we could ourselves exist, we
should hear no trickling brooks, no rush of waterfalls,
no breaking ocean waves, no sighing of the
wind, no whisper of leaves, no singing of birds, no
voices of men, no music, no thunder, no one of the
thousand concomitant sound-waves which together
make up the babble and murmur of country and
town. Those only who are perfectly deaf can know
what such silence means.
Without air our world would not be in darkness;
for light does not, like sound, depend mainly upon
air for its transmission. Light travels through regions
where air is not; and if light is communicated
by waves, they are not waves of air. But though the
absence of air would not deprive the earth of light,
it would make a very great difference in the kind
and degree of light received.
Without air the blue sky would be black as ink;
stars would glitter coldly in the daytime beside a
glaring sun; deep shadows would alternate with
blinding dazzle, and all the soft tints of sunrise and
sunset would be wanting. Earth would be like the
almost airless moon—all fierce whiteness and utter
blackness—with no gray shades, no rosy gleams, no
golden evening clouds; nay, without air there could
be no clouds. On the moon is no twilight; for no
air-particles float about, reflecting the sunlight from
one to another, and forming a soft veil of brightness,
to reach further than the direct sunlight alone can
reach.
Sunbeams travel straight to earth, unbending as
arrows in their flight, and unaided they can not creep
any distance round a solid body, though they may be
reflected or turned back from it. But the air breaks
up the sunbeams, bends them, diffuses them, spreads
them about, surrounds us with a delicate lacework
of woven light. A sunbeam traveling through
space is invisible till it strikes upon some object. If
that object is solid, the light of the sunbeam is partly
absorbed, partly reflected; if the object is transparent,
the sunbeam passes through and onward. Few
substances, if any, are perfectly transparent. We call
air transparent, yet it is so only in a measure. Each
sunbeam passing through the atmosphere loses part
of its brightness by the way, and so the great glare of
the sun is softened before it reaches the lower depths
of the air-ocean.
The sun’s rays are rays of heat as well as of light.
While the atmosphere softens the glare, giving us
shade and twilight, it also modifies the extremes of
temperature, from which, without air, we should
suffer.
When the sun goes down, although we are often
conscious of a chill, it is not the instant and overwhelming
chill which we should feel but for the
atmosphere. All day long the sun has been warming
the earth and air. When his direct rays are withdrawn,
the warm air for a while keeps its warmth,
and gives over of that warmth to us.
WEATHER — Sir Ralph Abercromby
The earliest records of weather among every
nation are to be found in those myths, or popular
tales, which, while describing rain, cloud, wind,
and other natural phenomena in highly figurative
language, refer them to some supernatural or personal
agency by way of explanation.
The most interesting thing about these mythical
stories is the remarkable fidelity with which they
reflect the climate of the country that gave them
birth. For example, from the mythologies of Greece
and Scandinavia we can almost construct an account
of the climate of those two countries by simply translating
the figurative phraseology of their legends into
the language of modern meteorology.
Many survivals of mystic speech are still found
among popular prognostics, and especially in cloud
names.
In England and Sweden “Noah’s Ark” is still seen
in the sky, while in Germany the “Sea-Ship” still
turns its head to the wind before rain. In Scotland
the “Wind-Dog” and the “Boar’s Head” are still
the dread of the fisherman, while such names as
“Goat’s Hair” and “Mare’s Tails” recall some of the
shaggy monsters of antiquity.
At a rather later period of intellectual development,
the premonitory signs of good or bad weather
become formulated into short sayings, or popular
prognostics. A large number of these are still current
in every part of the world, but their quality and
value are very varied. Some represent the astrological
attitude of mind, by referring weather changes
to the influence of the stars or phases of the moon;
others, on the contrary, are very valuable, and, in
conjunction with other aids to weather forecasting,
prognostics will never be entirely superseded, especially
for use on board ship. Till within a very
recent period, their science and explanation had
hardly advanced since they were first recorded. In
many cases the prognostics came true; when they
failed, no explanation could be suggested why they
did so; neither could any reason be given why the
same weather was not always preceded by the same
signs. A halo sometimes precedes a storm; why does
it not always do so? Why is rain sometimes
preceded by a soft sky and sometimes by hard
clouds?
About one hundred and fifty years ago the barometer
was invented. Very soon after that discovery,
observation showed that, in a general way, the mercury
fell before rain and wind, and rose for finer
weather. Also that bad weather was more common
when the whole level of the barometer was low, independent
of its motion one way or the other, than
when the level was high. But as with prognostics,
so with these indications, many failures occurred.
Sometimes rain would fall with a high or rising
barometer, and sometimes there would be a fine day
with a very low or falling glass. No reason could
be given for these apparent exceptions, and the whole
science of barometric readings seemed to be shrouded
in mystery.
The science of probabilities came into existence
about the commencement of the Nineteenth Century,
and developed the science of statistics. By this method
the average readings of meteorological instruments,
such as the height of the barometer or thermometer,
or the mean direction and force of the
wind, at any number of places were calculated, and
the results were sometimes plotted on charts so as
to show the distribution of mean pressure, temperature,
etc., over the world.
By this means a great advance was made. Besides
giving a numerical value to many abstract quantities,
the plotting of such lines as the isothermals of
Dove conclusively showed that many meteorological
elements hitherto considered capricious were really
controlled by general causes, such as the distribution
of land and sea.
Still more fruitful were these charts as the parents
of the more modern methods of plotting the readings
of the barometer over large areas at a given moment,
instead of the mean value for a month or year. Then
by tabulating statistics the relative frequency of
different winds at sea, many ocean voyages—notably
those across the “doldrums,” or belt of calms near
the equator—were materially shortened.
Statistics also of the annual amount of rainfall
became of commercial value as bearing on questions
of the economic supply of water for large towns, and
much valuable information was acquired as to the
dependence of mortality on different kinds of
weather. Of more purely scientific interest were the
variations of pressure, temperature, wind, etc., depending
on the time of day, or what are technically
known as diurnal variations, which were
brought to light by these comparisons.
This branch of the subject is known as “Statistical
Meteorology,” and has advanced very little since it
was first developed by Dove and Kaemtz.
When the attempt was made to apply statistics to
weather changes from day to day, it was found that
average results were useless. The mean temperature
for any particular day of the year might be 50°,
if deduced from the returns of a great many years,
but in any particular year it might be as low as 40°,
or as high as 60°. The first application of the method
was made by the great Napoleon, who requested
Laplace to calculate when the cold set in severely
over Russia. The latter found that on an average it
did not set in hard till January. The emperor made
his plans accordingly; a sharp spell of cold came in
December, and the army was lost.
It has now been thoroughly recognized that statistics
give a numerical representation of climate,
but little or none of weather, and that large masses
of figures have been accumulated, to which it is difficult
to attach any physical significance. The misuse
of statistics has done much to bring the science of
meteorology into disrepute.
But within the last thirty years a new treatment
of weather problems has been introduced, known as
the synoptic method, by which the whole aspect of
meteorology has been changed. By this method, a
chart of a large area of the earth’s surface is taken,
and after marking on the map the height of the
barometer at each place, lines are drawn through all
stations at which the barometer marks a particular
height. Thus a line would be drawn through all
places where the pressure was 30.0 inches, another
through all where it was 29.8 inches, and so on at any
intervals which were considered necessary. These
lines are called “isobars,” because they mark out lines
of equal pressure. When these charts were first introduced,
the estimation of the value of the mean
pressure was so great that, instead of drawing lines
where pressure was equal at the moment, they were
drawn through those places where the pressure was
equally distant from the mean of the day for each
place. These lines were called “is-abnormals”; that
is, equal from the mean. After the isobars have been
put in, lines are usually drawn through all places
where the temperature is equal at the moment. These
are called “isotherms,” or lines of equal temperature.
Then arrows to mark the velocity and direction of
the wind are inserted; and finally letters, or other
symbols, to denote the appearance of the sky, the
amount of cloud, or the occurrence of rain or snow.
Such a chart is called a “synoptic chart,” because it
enables the meteorologist to take a general view, as it
were, over a large area. Sometimes they are called
“synchronous charts,” because they are compiled
from observations taken at the same moment of time.
Typical Forms of Clouds 1, Squall Cumulus; 2, Pillar Cumulus; 3, Cirrus; 4, High Stratus and Cumulus
When these came to be examined, the following
important generalizations were discovered:
1. That in general the configuration of the isobars
assumed one of seven well-defined forms.
2. That, independent of the shape of the isobars,
the wind always took a definite direction relative to
the trend of those lines, and the position of the nearest
area of low pressure.
3. That the velocity of the wind was always nearly
proportional to the closeness of the isobars.
4. That the weather—that is to say, the kind of
cloud, rain, fog, etc.—at any moment was related
to the shape, and not the closeness, of the isobars,
some shapes inclosing areas of fine, others of bad,
weather.
5. That the regions thus mapped out by isobars
were constantly shifting their position, so that
changes of weather were caused by the drifting past
of these areas of good or bad weather, just as on a
small scale rain falls as a squall drives by. The
motion of these areas was found to follow certain
laws, so that forecasting weather changes in advance
became possible.
6. That sometimes in the temperate zone, and
habitually in the tropics, rain fell without any
appreciable change in the isobars, though the
wind conformed to the general law of these
lines.
Observation also showed that, though the same
shapes of isobars appear all over the world, the details
of weather within them, and the nature of their
motion, are modified by numerous local, diurnal, and
annual variations. Hence modern weather science
consists in working out for each country the details
of the character and motion of the isobars which are
usually found over it; just as the geologist finds
crumplings and denudation all over the world, and
works out the history of the physical appearance
of his own scenery by studying the local development
of these agencies.
So far the science rests on pure observation—that
such and such wind or weather comes with such and
such a shape of isobars. But it has been found, still
further, that the seven fundamental shapes of isobars
are, as it were, the product of so many various ways
in which an atmosphere circulating from the equator
to the poles may move. Just as the motion of a river
sometimes forms descending eddies or whirlpools,
sometimes back-waters in which the water is rising
upward, or yet at other times ripples in which the
circulation is very complex, so it now appears that
the general movement of the atmosphere from the
equator to the pole sometimes breaks up into a rotating
and descending movement round that configuration
of isobars known as an anticyclone, sometimes
into a rotating and ascending movement round that
known as a cyclone, or at other times quite in a different
way during certain kinds of squalls and thunderstorms.
Isobars, therefore, represent the effect on our barometers
of the movements of the air above us, so
that by means of isobars we trace the circulation and
eddies of the atmosphere.
By carrying the general laws of physics into the
conception of a circulating gas, we find that a cold
mixed atmosphere of air and vapor descending into
a warmer soil would remain clear and bright; while
a similar atmosphere rising into cooler strata would
condense some of its vapor into rain or cloud. It is
by reasoning of this nature that the origin of some
of the most beautiful and complex forms of clouds
has been discovered.
Following out these lines of research, a new science
of meteorology has grown up, which entirely
alters the attitude of mind with which we regard
weather changes, and gives rise to an entirely new
method of weather forecasting that far surpasses all
previous efforts, and which explains and develops all
that was known before.
On the one hand, the new method not only explains
why certain prognostics are usually signs of
good or bad weather, and the reason why the indications
sometimes fail; but also the reason why rain,
for instance, is sometimes foretold by one prognostic
and sometimes by a totally different one.
On the other hand, it not only gives a more extended
meaning to all the statistics which partially
represent the climate of a place, and to the relation
of the diurnal to the general changes of weather; but
it also enables new inferences to be drawn, which had
hitherto been impossible from some observations, and
explains why other sets of figures must always remain
without any physical significance.
We may notice here an attempt which has been
made by one school of meteorologists to deduce all
weather à priori from changes in the radiative energy
of the sun; that is to say, that from a knowledge of
greater or less heat being emitted by the sun, they
would treat the consequent alteration of weather as
a direct hydrodynamical problem. Given an earth
surrounded by fifty miles of damp air, and a sun at
varying altitude, and of varying radiative energy,
deduce from that all the diverse changes of weather.
This is doubtless a very tempting ideal, for there is no
doubt that the sun’s heat is the prime mover of all
atmospheric circulation; but when we have explained
what the nature of weather changes is, we
see that there is little hope that this method will
ever lead to satisfactory results.
Other meteorologists, who lay less stress on the
varying power of the sun, have taken up the indications
of synoptic charts, and endeavored to construct
a mathematical theory of cyclones and the general
circulation of the atmosphere. Ferrel, Mohn, Gulberg,
Sprung, and others have all started with the
analysis of the motion of a free mass of air on the
earth’s surface, first given by Professor Ferrel, and
worked out, from that and other general principles,
schemes of the nature and propagation of cyclones,
and of the general distribution of pressure over the
world.
THE ROMANCE OF A RAINDROP — Arthur H. Bell
Depth of rainfall is, of course, ascertained by
means of a rain-gauge, which measures the
amount of water precipitated from the atmosphere
during certain definite periods—usually twenty-four
hours. Sir Christopher Wren has the credit of constructing
the first rain-gauge; but they have been
made in various shapes and sizes since his time; and
perhaps none of the instruments in the meteorologist’s
armory is so familiar to the general public as
the rain-gauge. The methods of using the instrument
and the meaning of rainfall statistics are also
thoroughly understood nowadays. However, behind
these statistics and the methods of obtaining them,
there are questions of great interest that obtrude
themselves when we are watching the falling rain,
and we desire to learn about the history of the raindrop—for
example: Why is a raindrop round?
How are raindrops formed? At what particular
time does vapor become visible as mist? And what
are the causes which change this mist into cloud and
subsequently into rain?
The two prime causes of rain are, of course, the
sun and the ocean; and since these two factors do
not appreciably vary from year to year, it follows
that the annual rainfall on the earth as a whole, if it
could be measured, would also be found to be invariable.
It is obvious, however, that the rainfall at
all places is not equal. In London, for instance, the
average yearly rainfall is twenty-two inches; but
on the Khasi Hills in India it is no less than six
hundred inches. Similar contrasts are observable in
other parts of the world, the differences being due
to local geographical conditions.
The starting points in the history of rain are,
therefore, heat and moisture. From the surface of
land and water tiny globules or vesicles of moisture
are continually rising into the atmosphere by the
force of the sun’s heat; and the warmer the air the
greater the number of these globules of water the
atmosphere is able to absorb. In this respect the
atmosphere may be likened to a sponge, for it is from
the moisture thus retained that the subsequent raindrops
are formed. Most persons are well acquainted
with the very familiar phenomenon which is to be
noticed when a glass of very cold water is brought
into a warm room: the drops of moisture which form
on the outside of the glass being among the commonest
phenomena in what may be termed domestic
meteorology. There is a similar transformation in
the outside atmosphere; so that when the warm,
moist currents of air flow against the sides of a cold
mountain, or it may be against a body of cold air,
there is a reduction in temperature, the atmosphere
is squeezed like a sponge, and the particles of moisture
are forced out of it. The particles then assume
the form of cloud, fog, mist, rain, snow, and hail, as
the case may be.
Now, as regards the globules of moisture, the most
recent experiments and observations point to the
conclusion that before the drops of vapor can form,
there must be a tiny nucleus of dust upon which the
condensed water may settle. At the centre of every
drop of vapor in a cloud there is probably a little
core of dust; and without these little atoms there
could be no rain. These atoms of dust are visible
only under the strongest microscopes; and so minute
are they that in a cubic foot of saturated air it
has been calculated that they number one thousand
millions, their total weight being only three grains.
It is commonly considered that the particles of
moisture within a cloud are quite motionless; and
when looking at a huge cloud floating serenely in a
summer sky it is difficult not to think of its constituent
parts as being quite at rest. The apparently
stationary cloud is all commotion and movement,
the particles within it being always on the move,
some going up and others down. The particles of
moisture, moreover, being probably only about the
four-thousandth part of an inch in diameter, the resistance
offered by the air to their movement is very
slight; indeed, as soon as they are condensed they
immediately begin to fall downward, and were it
not for the atoms of dust waiting to catch them the
particles would at once fall to the ground. It is
often asked why the vapor, if so readily condensed
in the atmosphere, does not continually fall to the
earth. The answer to this question, it will be seen, is
that the moisture, instead of always pouring down on
the earth, settles on the surface of the atoms of dust.
Thus the first downward movement of the incipient
raindrop is arrested by the dust-nuclei which swarm
in all parts of the atmosphere; so that instead of
being destroyed as soon as it is formed, the particle of
moisture is preserved and stored for future use. In
realizing the fact that a cloud is always in motion,
the first step has been taken in discovering how a
raindrop is formed.
It might be supposed that the raindrops would
evaporate as quickly as they were condensed; but
observation of the drops of moisture running down a
window-pane and forming larger drops gives a good
idea of what occurs in the clouds; as also does the
fact that in a bottle of soda-water the bubbles of air
overtake one another and, colliding, make larger
bubbles.
One of the principal causes of the manufacture
of a raindrop is to be found in the circumstance
that there is a similar process of amalgamation
at work in every part of the atmosphere. It
often happens that a drop of moisture falls downward
through a cloud for a distance of a mile or
more; and although it may pass through strata of
very warm air, thus running a great risk of being
evaporated and destroyed, it has also many collisions
by which its bulk is considerably increased, and
eventually becomes so heavy that its rate of progress
is very much accelerated. Then, no longer able to
float in the air, it plumps down to the earth as a full-grown
raindrop.
Supposing it were possible for an observer to occupy
a position immediately below a cloud, and close
enough to see all that was taking place, he would
notice raindrops of all sizes leaping from the under
side of the cloud and plunging toward the earth.
The simplest experiment to get some idea concerning
the variation in raindrops is to expose an ordinary
slate for a few minutes during a shower of rain, and
it will be seen by the different-shaped blotches on
the slate that, although the raindrops have all made
a similar journey, they have, nevertheless, contrived
to acquire an individuality during their downward
passage. That the raindrops are round admits of a
very simple explanation. They are this shape owing
to the action of capillarity, which in the case of the
raindrop acts equally in all directions.
In many parts of the world the very curious phenomenon
of colored rain sometimes occurs, and in
many instances it is due to very simple causes. In
some cases the coloring matter is found to be nothing
but the pollen-dust shaken out of the flowers on certain
trees at such times as a strong wind happened
to be blowing over them. Fir trees and cypress trees,
when grouped together in large forests, at certain
seasons of the year give off enormous quantities of
pollen, and this vegetable dust is often carried many
miles through the atmosphere by the wind, and frequently
falls to earth during a shower of rain. The
microscope clearly reveals the origin of such colored
rain, which has on more than one occasion
puzzled and mystified the inexperienced. Pollen is,
therefore, very largely responsible for the reports
sent from different parts of the world of golden,
black, and red rain. Fish and insects also descend to
earth during showers of rain; but since it is probable
that these and other unwonted visitors to the atmosphere
were originally drawn up into the air during
the passage across the country of a whirling storm,
with powerfully ascending currents of air, there is
no need to look for any far-fetched explanation of
what, after all, is a very simple occurrence.
The history of a raindrop, then, has some very romantic
and interesting episodes connected with it;
but, wonderful as are the incidents in what is really
a very remarkable career, it is not until the raindrops
fall on the earth that the full purport of the
work they do is wholly realized. Contemplated
by itself, a raindrop seems a very insignificant thing;
but when the drops combine in a heavy downpour of
rain the result is truly wonderful. The information
that one inch of rain has fallen over a certain area
is not very impressive; the amount does not seem
very great. A fall of one inch of rain means, however,
that no less than one hundred tons of water
have fallen on each acre of surface, or no less than
sixty thousand tons on each square mile. Instead of
expressing the amount of water in tons, it may be
thus stated in gallons, taking the Thames basin as a
convenient area for reference: a rainfall of three
inches over that area means that one hundred and
sixty thousand million gallons of water have been
precipitated from the atmosphere. At times, too,
when the rainfall is still heavier, rivers overflow their
banks and floods occur, and still further evidence is
then forthcoming of the power and the might of the
raindrops working toward one common end. Sooner
or later the raindrop, whether it runs off the surface
of the earth in a river or in a disastrous flood, finds
its way, under the influence of evaporation, back
into the atmosphere, and is then ready to start on another
journey, which, like all its predecessors, will
be full of incident from start to finish.
THE RAINBOW — John Tyndall
The oldest historic reference to the rainbow
is known to all: “I do set my bow in the
clouds, and it shall be for a token of a covenant between
me and the earth.... And the bow shall
be in the cloud; and I will look upon it, that I may
remember the everlasting covenant between God and
every living creature of all flesh that is upon the
earth.”
To the sublime conceptions of the theologian succeeded
the desire for exact knowledge characteristic
of the man of science. Whatever its ultimate cause
might have been, the proximate cause of the rainbow
was physical, and the aim of science was to account
for the bow on physical principles. Progress toward
this consummation was very slow. Slowly the ancients
mastered the principles of reflection. Still
more slowly were the laws of refraction dug from
the quarries in which Nature had imbedded them.
I use this language because the laws were incorporate
in Nature before they were discovered by
man. Until the time of Alhazan, an Arabian
mathematician, who lived at the beginning of the
Twelfth Century, the views entertained regarding
refraction were utterly vague and incorrect. After
Alhazan came Roger Bacon and Vitellio, who made
and recorded many observations and measurements
on the subject of refraction. To them succeeded
Kepler, who, taking the results tabulated by his predecessors,
applied his amazing industry to extract
from them their meaning—that is to say, to discover
the physical principles which lay at their root. In
this attempt he was less successful than in his astronomical
labors. In 1604, Kepler published his Supplement
to Vitellio, in which he virtually acknowledged
his defeat by enunciating an approximate
rule, instead of an all-satisfying natural law. The
discovery of such a law, which constitutes one of the
chief corner-stones of optical science, was made by
Willebrod Snell, about 1621.
A ray of light may, for our purposes, be presented
to the mind as a luminous straight line. Let such a
ray be supposed to fall vertically upon a perfectly
calm water-surface. The incidence, as it is called,
is then perpendicular, and the ray goes through the
water without deviation to the right or left. In
other words, the ray in the air and the ray in the
water form one continuous straight line. But the
least deviation from the perpendicular causes the
ray to be broken, or “refracted,” at the point of incidence.
What, then, is the law of refraction discovered
by Snell? It is this, that no matter how the
angle of incidence and with it the angle of refraction
may vary, the relative magnitude of two lines, dependent
on these angles, and called their sines, remains,
for the same medium, perfectly unchanged.
Measure, in other words, for various angles, each of
these two lines with a scale, and divide the length
of the longer one by that of the shorter; then, however
the lines individually vary in length, the
quotient yielded by this division remains absolutely
the same. It is, in fact, what is called “the index of
refraction” of the medium.
Science is an organic growth, and accurate measurements
give coherence to the scientific organism.
Were it not for the antecedent discovery of the law
of sines, founded as it was on exact measurements,
the rainbow could not have been explained. Again
and again, moreover, the angular distance of the
rainbow from the sun had been determined and
found constant. In this divine remembrancer there
was no variableness. A line drawn from the sun
to the rainbow, and another drawn from the rainbow
to the observer’s eye, always inclosed an angle of
41°. Whence this steadfastness of position—this
inflexible adherence to a particular angle? Newton
gave to De Dominis[4] the credit of the answer; but
we really owe it to the genius of Descartes. He followed
with his mind’s eye the rays of light impinging
on a raindrop. He saw them in part reflected
from the outside surface of the drop. He saw them
refracted on entering the drop, reflected from its
back, and again refracted on their emergence.
Descartes was acquainted with the law of Snell, and
taking up his pen, he calculated, by means of that
law, the whole course of the rays. He proved that
the vast majority of them escaped from the drop as
divergent rays, and, on this account, soon became
so enfeebled as to produce no sensible effect upon
the eye of an observer. At one particular angle,
however—namely, the angle 41° aforesaid—they
emerged in a practically parallel sheaf. In their
union was strength, for it was this particular sheaf
which carried the light of the “primary” rainbow
to the eye.
There is a certain form of emotion called intellectual
pleasure which may be excited by poetry,
literature, nature, or art. But I doubt whether
among the pleasures of the intellect there is any more
pure and concentrated than that experienced by the
scientific man when a difficulty which has challenged
the human mind for ages melts before his eyes, and
re-crystallizes as an illustration of natural law.
This pleasure was doubtless experienced by Descartes
when he succeeded in placing upon its true
physical basis the most splendid meteor of our atmosphere.
Descartes showed, moreover, that the
“secondary bow” was produced when the rays of
light underwent two reflections within the drop, and
two refractions at the points of incidence and
emergence.
Descartes proved that, according to the principles
of refraction, a circular band of light must appear
in the heavens exactly where the rainbow is seen.
But how are the colors of the bow to be accounted
for? Here his penetrative mind came to the very
verge of the solution, but the limits of knowledge at
the time barred his further progress. He connected
the colors of the rainbow with those produced by a
prism; but then these latter needed explanation just
as much as the colors of the bow itself. The solution,
indeed, was not possible until the composite
nature of white light had been demonstrated by
Newton. Applying the law of Snell to the different
colors of the spectrum, Newton proved that the
primary bow must consist of a series of concentric
circular bands, the largest of which is red and the
smallest violet; while in the secondary bow these
colors must be reversed. The main secret of the
rainbow, if I may use such language, was thus revealed.
I have said that each color of the rainbow is
carried to the eye by a sheaf of approximately parallel
rays. But what determines this parallelism?
Here our real difficulties begin. Let us endeavor
to follow the course of the solar rays before and after
they impinge upon a spherical drop of water. Take,
first of all, the ray that passes through the centre of
the drop. This particular ray strikes the back of
the drop as a perpendicular, its reflected portion returning
along its own course. Take another ray
close to this central one and parallel to it—for the
sun’s rays when they reach the earth are parallel.
When this second ray enters the drop it is refracted;
on reaching the back of the drop it is there reflected,
being a second time refracted on its emergence from
the drop. Here the incident and the emergent
rays inclose a small angle with each other. Take,
again, a third ray a little further from the central
one than the last. The drop will act upon it as it
acted upon its neighbor, the incident and the emergent
rays inclosing in this instance a larger angle
than before. As we retreat further from the central
ray the enlargement of this angle continues up
to a certain point, where it reaches a maximum,
after which further retreat from the central ray
diminishes the angle. Now, a maximum resembles
the ridge of a hill, or a watershed, from which the
land falls in a slope at each side. In the case before
us the divergence of the rays when they quit the
raindrop would be represented by the steepness of
the slope. On the top of the watershed—that is to
say, in the neighborhood of our maximum—is a
kind of summit-level, where the slope for some distance
almost disappears. But the disappearance of
the slope indicates, as in the case of our raindrop,
the absence of divergence. Hence we find that at
our maximum, and close to it, there issues from the
drop a sheaf of rays which are nearly, if not quite,
parallel to each other. They are the so-called
“effective rays” of the rainbow.
But though the step here taken by Descartes and
Newton was a great one, it left the theory of the bow
incomplete. Within the rainbow proper, in certain
conditions of the atmosphere, are seen a series of
richly colored zones, which were not explained by
either Descartes or Newton. They are said to have
been first described by Mariotte, and they long challenged
explanation. At this point our difficulties
thicken, but, as before, they are to be overcome by
attention. It belongs to the very essence of a maximum,
approached continuously on both sides, that on
the two sides of it pairs of equal value may be found.
The maximum density of water, for example, is
39° Fahr. Its density, when 5° colder and when 5°
warmer than this maximum, is the same. So also
with regard to the slopes of a watershed. A series
of pairs of points of the same elevation can be found
upon the two sides of the ridge; and, in the case of
the rainbow, on the two sides of the maximum deviation
we have a succession of pairs of rays having the
same deflection. Such rays travel along the same
line, and add their forces together after they quit the
drop. But light, thus reinforced by the coalescence
of non-divergent rays, ought to reach the eye. It
does so; and were light what it was once supposed to
be—a flight of minute particles sent by luminous
bodies through space—then these pairs of equally
deflected rays would diffuse brightness over a large
portion of the area within the primary bow. But
inasmuch as light consists of waves, and not of particles,
the principle of interference comes into play,
in virtue of which waves alternately reinforce and
destroy each other. Were the distance passed over
by the two corresponding rays within the drop the
same, they would emerge as they entered. But in
no case are the distances the same. The consequence
is that when the rays emerge from the drop they are
in a condition either to support or to destroy each
other. By such alternate reinforcement and destruction,
which occur at different places for different
colors, the colored zones are produced within the
primary bow. They are called “supernumerary
bows,” and are seen, not only within the primary, but
sometimes also outside the secondary bow. The condition
requisite for their production is that the drops
which constitute the shower shall all be of nearly the
same size. When the drops are of different sizes, we
have a confused superposition of the different colors,
an approximation to white light being the consequence.
This second step in the explanation of the
rainbow was taken by a man the quality of whose
genius resembled that of Descartes or Newton, and
who in 1801 was appointed Professor of Natural
Philosophy in the Royal Institution. I refer, of
course, to the illustrious Thomas Young.
But our task is not, even now, complete. The
finishing touch to the explanation of the rainbow was
given by the eminent Astronomer Royal, Sir George
Airy. Bringing the knowledge possessed by the
founders of the undulatory theory, and that gained
by subsequent workers, to bear upon the question, Sir
George Airy showed that, though Young’s general
principles were unassailable, his calculations were
sometimes wide of the mark. It was proved by Airy
that the curve of maximum illumination in the rainbow
does not quite coincide with the geometric curve
of Descartes and Newton. He also extended our
knowledge of the supernumerary bows, and corrected
the positions which Young had assigned to
them. Finally, Professor Miller of Cambridge and
Dr. Galle of Berlin illustrated with careful measurements
with the theodolite the agreement which exists
between the theory of Airy and the facts of observation.
Thus, from Descartes to Airy, the intellectual
force expended in the elucidation of the rainbow,
though broken up into distinct personalities, might
be regarded as that of an individual artist, engaged
throughout this time in lovingly contemplating, revising,
and perfecting his work.
The white rainbow (l’arc-en-ciel blanc) was first
described by the Spanish Don Antonio de Ulloa,
lieutenant of the Company of Gentleman Guards of
the Marine. By order of the King of Spain, Don
Jorge Juan and Ulloa made an expedition to South
America, an account of which is given in two amply
illustrated quarto volumes to be found in the library
of the Royal Institution. The bow was observed
from the summit of the mountain Pambamarca, in
Peru. The angle subtended by its radius was 33° 30′,
which is considerably less than the angle subtended
by the radius of the ordinary bow.
The white rainbow has been explained in various
ways. The genius of Thomas Young throws light
upon this subject, as upon so many others. He
showed that the whiteness of the bow was a direct
consequence of the smallness of the drops which produce
it. The smaller the drops, the broader are the
zones of the supernumerary bows, and Young proved
by calculation that when the drops have a diameter
of 1-3000th or 1-4000th of an inch, the bands overlap
each other, and produce white light by their mixture.
SNOW, HAIL, AND DEW — Alexander Buchan
Snow is the frozen moisture which falls from the
atmosphere when the temperature is 32° or
lower. It is composed of crystals, usually in the
form of six-pointed stars, of which about 1,000 different
kinds have been already observed, and many
of them figured, by Scoresby, Glaisher, and others.
These numerous forms have been reduced to five
principal varieties: Thin plates, the most numerous
class, containing several hundred forms of the rarest
and most exquisite beauty; spherical nucleus or plane
figure studded with needle-shaped crystals; six or
more rarely three-sided prismatic crystals; pyramids
of six sides; prismatic crystals, having at the ends
and middle thin plates perpendicular to their length.
The forms of the crystals in the same fall of snow are
generally similar to each other. The crystals of
hoar-frost being formed on leaves and other bodies
disturbing the temperature are often irregular and
opaque; and it has been observed that each tree or
shrub has its own peculiar crystals.
Snowflakes vary from an inch to 7-100ths of an
inch in diameter, the largest occurring when the temperature
is near 32°, and the smallest at very low
temperatures. As air has a smaller capacity for retaining
its vapor as the temperature sinks, it follows
that the aqueous precipitation, snow or rain, is much
less in polar than in temperate regions. The white
color of snow is the result of the combination of the
different prismatic rays issuing from the minute
snow-crystals. Pounded glass and foam are analogous
cases of the prismatic colors blending together
and forming the white light out of which they had
been originally formed. It may be added that the
air contained in the crystals intensifies the whiteness
of the snow. The limit of the fall of snow coincides
nearly with 30° N. lat., which includes nearly the
whole of Europe; on traversing the Atlantic, it rises
to 45°, but on nearing America descends to near
Charleston; rises on the west of America to 47°, and
again falls to 40° in the Pacific. It corresponds
nearly with the winter isothermal of 52° Fahr. Snow
is unknown at Gibraltar; at Paris, it falls 12 days on
an average annually, and at St. Petersburg 170 days.
It is from 10 to 12 times lighter than an equal bulk
of water. From its loose texture, and its containing
about 10 times its bulk of air, it is a very bad conductor
of heat, and thus forms an admirable covering
for the earth from the effects of radiation—it not
infrequently happening, in times of great cold, that
the soil is 40° warmer than the surface of the overlying
snow. The flooding of rivers from the melting
of the snow on mountains in summer carries
fertility into regions which would otherwise remain
barren wastes.
The word hail in English is unfortunately used to
denote two phenomena of apparently different origin.
In French, we have the terms grèle and grésil—the
former of which is hail proper; the latter denotes
the fine grains, like small shot, which often
fall in winter, much more rarely in summer, and
generally precede snow. The cause of the latter
seems to be simply the freezing of raindrops as they
pass in their fall through a colder region of air than
that where they originated. We know by balloon
ascents and various other methods of observation that
even in calm weather different strata of the atmosphere
have extremely different temperatures, a stratum
far under the freezing point being often observed
between two others comparatively warm.
But that true hail, though the process of its formation
is not yet perfectly understood, depends
mainly upon the meeting of two nearly opposite currents
of air—one hot and saturated with vapor,
the other very cold—is rendered pretty certain by
such facts as the following. A hailstorm is generally
a merely local phenomenon, or at most, ravages a
belt of land of no great breadth, though it may be
of considerable length. Hailstorms occur in the
greatest perfection in the warmest season, and at the
warmest period of the day, and generally are most
severe in the most tropical climates. A fall of hail
generally precedes, sometimes accompanies, and
rarely, if ever, follows a thunder-shower.
When a mass of air, saturated with vapor, rising
to a higher level, meets a cold one, there is, of course,
instant condensation of vapor into ice by the cold due
to expansion; at the same time, there is generally a
rapid production of electricity, the effect of which
upon such light masses as small hailstones is to give
them in general rapid motion in various directions
successively. These motions are in addition to the
vortex motions or eddies, caused in the air by the
meeting of the rising and descending currents. The
small ice-masses then moving in all directions impinge
upon each other, sometimes with great force,
producing that peculiar rattling sound which almost
invariably precedes a hail-shower. At the same time,
by a well-known property of ice, the impinging
masses are frozen together; and this process continues
until the weight of the accumulated mass
enables it to overcome the vortices and the electrical
attractions, when it falls as a larger or smaller hailstone.
On examining such hailstones, which may
have any size from that of a pea to that of a walnut,
or even an orange, we at once recognize the composite
character which might be expected from such
a mode of aggregation.
A curious instance of the fall of large hail, or
rather ice-masses, occurred on one of her Majesty’s
ships off the Cape in January, 1860. Here the stones
were the size of half-bricks, and beat several of the
crew off the rigging, doing serious injury. We may
conclude by a description (taken from Mem. de
l’Acad. des Sciences, 1790) of one of the most disastrous
hailstorms that has occurred in Europe for
many years back. This storm passed over Holland
and France in July, 1788. It traveled simultaneously
along two lines nearly parallel—the eastern one
had a breadth of from half a league to five leagues,
the western of from three to five leagues. The space
between was visited only by heavy rain; its breadth
varied from three to five and a half leagues. At the
outer border of each, there was also heavy rain, but
we are not told how far it extended. The length was
at least a hundred leagues; but from other reports
it may be gathered that it really extended to nearly
two hundred. It seems to have originated near the
Pyrenees, and to have traveled at a mean rate of
about sixteen and a half leagues per hour toward the
Baltic, where it was lost sight of. The hail only fell
for about seven and a half minutes at any one place.
The hailstones were generally of irregular form, the
heaviest weighing about eight French ounces. This
storm devastated 1,039 parishes in France alone, and
the damage was officially placed at 24,690,000 francs.
For any assigned temperature of the atmosphere,
there is a certain quantity of aqueous vapor which it
is capable of holding in suspension at a given pressure.
Conversely, for any assigned quantity of
aqueous vapor held in suspension in the atmosphere,
there is a minimum temperature at which it can remain
so suspended. This minimum temperature is
called the dew-point. During the daytime, especially
if there has been sunshine, a good deal of aqueous
vapor is taken into suspension in the atmosphere. If
the temperature in the evening now falls below the
dew-point, which after a hot and calm day generally
takes place about sunset, the vapor which can be no
longer held in suspension is deposited on the surface
of the earth, sometimes to be seen visibly falling in
a fine mist. This is one form of the phenomenon of
dew, but there is another. The surface of the earth,
and all things on it, and especially the smooth surfaces
of vegetable productions, are constantly parting
with their heat by radiation. If the sky is covered
with clouds, the radiation sent back from the clouds
nearly supplies an equivalent for the heat thus parted
with; but if the sky be clear, no equivalent is supplied,
and the surface of the earth and things growing
on it become colder than the atmosphere.
If the night also be calm, the small portion of air
contiguous to any of these surfaces will become
cooled below the dew-point, and its moisture deposited
on the surface in the form of dew. If this
chilled temperature be below 32° Fahr., the dew becomes
frozen and is called hoar-frost. The above
two phenomena, though both expressed in our language
by the word dew, which perhaps helps to give
rise to a confusion of ideas on the subject, are not
necessarily expressed by the same word. For instance,
in French, the first phenomenon—the falling
evening-dew—is expressed by the word serein; while
the latter—the dew seen in the morning gathered
in drops by the leaves of plants, or other cool surfaces—is
expressed by the word rosée.
The merit of the discovery of the “Theory of
Dew” has been commonly ascribed to Dr. William
Charles Wells, who published in 1814 his Essay on
Dew, which obtained great popularity. The merit
should, however, be divided between him and several
others. M. Le Roi of Montpellier, M. Pictet
of Geneva, and especially Professor Alexander Wilson
of Glasgow, largely contributed by experiment
and inducement to its formation.
THE AURORA BOREALIS — Richard A. Proctor
The aurora is one of those phenomena of nature
which are characterized by exceeding beauty,
and sometimes by an imposing grandeur, but are unaccompanied
by any danger, and indeed, so far as
can be determined, by any influence whatever upon
the conditions which affect our well-being. Comparing
the aurora with a phenomenon akin to it in
origin—lightning—we find in this respect the most
marked contrast. Both phenomena are caused by
electrical discharges; both are exceedingly beautiful.
It is doubtful which is the more imposing so
far as visible effects are concerned. When the auroral
crown is fully formed, and the vault of heaven
is covered with the auroral banners, waving hither
and thither silently, now fading from view, anon
glowing with more intense splendor, the mind is not
less impressed with a sense of the wondrous powers
which surround us than when, as the forked lightnings
leap from the thundercloud, the whole heavens
glow with violet light, and then sink suddenly into
darkness. The solemn stillness of the auroral display
is as impressive in its kind as the crashing peal
of the thunderbolt.
The reader is no doubt aware that auroras or polar
streamers, as they are sometimes called, are appearances
seen not around the true poles of the earth, but
around the magnetic poles which lie very far away
from those geographical poles which our Arctic and
Antarctic seamen have in vain attempted to reach.
The formation of auroral streamers around the magnetic
poles of the earth shows that these lights are
due to electrical discharges of electricity, which,
though only visible at night, take place in reality in
the daytime also.
Remembering that the aurora is due to electrical
discharges in the upper regions of the air, it is interesting
to learn what are the appearances presented
by the aurora at places where the auroral arch is
high above the horizon—these being, in fact, places
nearly under the auroral arch. M. Ch. Martins, who
observed a great number of auroras in Spitzbergen in
1839, thus writes: “At times they are simple diffused
gleams or luminous patches; at others, quivering rays
of pure white which run across the sky, starting from
the horizon as if an invisible pencil were being drawn
over the celestial vault; at times it stops in its course,
the incomplete rays do not reach the zenith, but the
aurora continues at some other point; a bouquet of
rays darts forth, spreads into a fan, then becomes
pale, and dies out. At other times long golden
draperies float above the head of the spectator, and
take a thousand folds and undulations as if agitated
by the wind. They appear to be but at a slight elevation
in the atmosphere, and it seems strange that
the rustling of the folds as they double back on each
other is not audible. Generally, a luminous bow
is seen in the north; a black segment separates it
from the horizon, the dark color forming a contrast
with the pure white or bright red of the bow, which
darts forth rays, extends, becomes divided, and soon
presents the appearance of a luminous fan, which
fills the northern sky, and mounts nearly to the
zenith, where the rays, uniting, form a crown, which
in its turn darts forth luminous jets in all directions.
The sky then looks like a cupola of fire; the blue, the
green, the yellow, the red, and the white vibrate in
the palpitating rays of the aurora. But this brilliant
spectacle lasts only a few minutes; the crown first
ceases to emit luminous jets, and then gradually dies
out; a diffused light fills the sky; here and there a
few luminous patches, resembling light clouds, open
and close with incredible rapidity, like a heart that
is beating fast. They soon get pale in their turn,
everything fades away and becomes confused, the
aurora seems to be in its death-throes; the stars, which
its light had obscured, shine with a renewed brightness;
and the long polar night, sombre and profound,
again assumes its sway over the icy solitudes of earth
and ocean.”
The association between auroral phenomena and
those of terrestrial magnetism has long been placed
beyond a doubt. Wargentin in 1750 first established
the fact, which had been previously noted, however,
by Halley and Celsius. But the extension of the relation
to phenomena occurring outside the earth—very
far away from the earth—belongs to recent
times. The first point to be noticed, as showing that
the aurora depends partly on extra-terrestrial circumstance,
is the fact that the frequency of its appearance
varies greatly from time to time. It is
said that the aurora was hardly ever seen in England
during the Seventeenth Century, although the northern
magnetic pole was then much nearer to England
than it is at present Halley states that before the
great aurora of 1716 none had been seen (or at least
recorded) in England for more than eighty years,
and no remarkable aurora since 1574. In the records
of the Paris Academy of Sciences no aurora is mentioned
between 1666 and 1716. At Berlin one was
recorded in 1707 as a very unusual phenomenon; and
the one seen at Bologna in 1723 was described as the
first which had ever been seen there. Celsius, who
described in 1733 no less than three hundred and sixteen
observations of the aurora in Sweden between
1706 and 1732, states that the oldest inhabitants of
Upsala considered the phenomenon as a great rarity
before 1716. Anderson of Hamburg states that in
Iceland the frequent occurrence of auroras between
1716 and 1732 was regarded with great astonishment.
In the Sixteenth Century, however, they had been
frequent.
Here then we seem to find the evidence of some
cause external to the earth as producing auroras, or
at least as tending to make their occurrence more or
less frequent. The earth has remained to all appearance
unchanged in general respects during the
last three centuries, yet in the Sixteenth her magnetic
poles have been frequently surrounded by auroral
streamers; during the Seventeenth these streamers
have been seldom seen; during the last two-thirds of
the Seventeenth Century auroras have again been frequent;
and during the Eighteenth Century they have
occurred sometimes frequently during several years
in succession, at others very seldom.
Connected as auroras are with the phenomena of
terrestrial magnetism, we may expect to find some
help in our inquiry from the study of these phenomena.
Now it appears certain that magnetic phenomena
are partly influenced by changes in the sun’s condition.
We may well believe that they are in the main
due to the sun’s ordinary action, but the peculiarities
which affect them seem to depend on changes in the
sun’s action.
Many of my readers will doubtless remember the
auroras of May 13, 1869, and October 24, 1870, both
of which occurred when the sun’s surface was marked
by many spots, and both of which were accompanied
by remarkable disturbance of the earth’s magnetism.
It may, then, fairly be assumed that the occurrence
of auroras depends in some way, directly or indirectly,
on the condition of the sun. But what the real
nature of that connection may be is not easily determined.
Angström was the first to observe the spectrum of
the aurora borealis. He found that the greater part
of the auroral light, as observed in 1867, was of one
color, yellow, but three faint bands of green and
greenish blue color were also seen. The aurora of
April 15, 1869, was seen under very favorable conditions
in America. Professor Winlock, observing it
at New York, found its spectrum to consist of five
bright lines, of which the brightest was the yellow
line just mentioned. One of the others seems to
agree very nearly, if not exactly, in position with a
green line, which is the most conspicuous feature of
the spectrum of the solar corona. During the aurora
of October 6, 1869, Flögel noticed the strong yellow
line and a faint green band. Schmidt, on April 5,
1870, made a similar observation. He saw the strong
yellow line, and from it there extended toward the
violet end of the spectrum a faint greenish band,
which, however, at times showed three defined lines,
fainter than the yellow line.
It was not till the magnificent aurora of October
24-25, 1870, that any red lines were seen in the spectrum
of an aurora. On that occasion the background
of the auroral light was ruddy, and on the ruddy
background there were seen three deep red streamers
very well defined. The ruddy streamers, on the night
of October 25, converged toward the auroral crown,
which was on that occasion singularly well seen.
Förster of Berlin failed to see any red line or band
despite the marked ruddiness of the auroral light.
But Capron at Guildford saw a faint line in the red
part of the spectrum; and Elger at Bedford observed
a red band in the light of the red streamers, the band
disappearing, however, when the spectroscope was
directed on the white rays of the aurora.
As yet the auroral spectrum has not been interpreted.
The reason probably is, that the conditions
under which the light of the aurora as of the corona
is formed are not such as have been or perhaps can
be attained or even approached in laboratory experiments.
CLOUDS — D. Wilson Barker
Those who are professionally engaged in the
scientific work of weather bureaus recognize
the importance of accurate observations of cloud
forms and nature, and much good work has been
done in this connection in recent years by scientific
observers in England, Australia, and the United
States; but as a popular study, nephology is almost
entirely overlooked, and this notwithstanding the
fact that, perhaps, no branch of knowledge offers
greater facility and ease of acquisition. Each cloud
has its history fraught with meaning; its open secret
is writ on its face, and may be read by any one who
will, give himself a little trouble, nor need he go
deeply into the study in order to make observations
interesting to himself, and perhaps of great use in
the furthering and perfecting of weather lore. To
the ancients, the sky was doubtless an object of constant
remark and interest, and possibly their intuitive
knowledge of weather forecasting was much more
accurate than ours. The dwellers in our modern
cities see little of the sky, clouds have no interest for
them beyond the personal consideration as to the advisability
of taking out an umbrella or not. But
farmers, fishermen, sailors, and others following
open-air avocations are dependent on the weather,
and to be wise in its forecast is of importance to
them. To these, especially, cloud study should appeal;
it can not fail to be profitable to them in their
personal work, and they have all the opportunity, if
the will be there, to forward the general knowledge
of the subject by careful painstaking observations,
which they may transmit to those scientifically engaged
in dealing with weather laws, and thus assist
in the elucidation of questions on which we are at
present but very imperfectly informed.
In this article the broad distinctions of clouds
will be dealt with. There are two well-defined
types—Stratus and Cumulus—so distinct in actual
appearance and in physical formation that they may
be taken as the basis of classification. Sometimes
both types appear to merge into each other, in which
case no variety of classification suffices to describe
them satisfactorily, as any one who has studied cloud-forms
must allow. “Stratus” is a sheet-like formation
of cloud. “Cumulus” is recognizable by its
heaped-up appearance and vertical thickness. Numerous
varieties of cloud-forms may be observed
graduating from one of these types to the other, but
when an observer can clearly distinguish Stratus
from Cumulus he has already acquired valuable
knowledge.
The presence of either type of cloud alone indicates
a more or less set condition of the atmosphere,
and generally foretells a continuance of the existing
weather. The simultaneous presence of both types
indicates a coming change, the gradation of Stratus
into Cumulus foreboding worse weather, and of
Cumulus into Stratus heralding good. Again, as we
shall show later on, the vertical thickening of the
stratiform clouds is a distinctly bad indication.
Up to quite recently, Luke Howard’s division of
clouds, formulated in 1802, held first place; even
now it is in constant use, for though attempts have
been made at a more scientific classification, all of
them, with the single exception of that proposed by
the late Rev. Clement Ley, can only be termed make-shifts.
Mr. Ley’s classification, unfortunately, is
long, and not well adapted to the use of any but professional
investigators, or enthusiasts with ample
time on their hands. There exists a so-called “international”
system of cloud nomenclature, but, for all
that, each country has its own especial system, with
the result that vast collections of cloud statistics are of
little value as helps to a classification, and are useful
only as records of clouds present at certain times.
Clouds owe their existence to two causes:
1. Through the passing of warm, moist air into
colder, when, owing to condensation, a certain proportion
of the moisture becomes visible in the form
of a cloud.
2. Through changes occurring in the atmosphere
as it rises into higher regions of atmosphere, where
decrease in pressure and expansion and consequent
loss of heat take place and cause condensation of
moisture.
The first process may be described as the condensation
formation of clouds, and the second as the
adiabatic formation of clouds. As a matter of fact,
no hard and fast line separates these two operations;
they act in unison, and the combination of vertical
and horizontal currents goes to make up the diversity
of forms which clouds assume.
In settled states of the atmosphere, Stratus clouds
are common, or the sky may be clear. In unsettled
conditions, Cumulus or Heap clouds are formed.
We shall now describe a few familiar forms of
cloud, giving them simple names and endeavoring to
compare them with other nomenclatures.
Of Cumulus clouds there are five well-defined varieties.
Rain Cumulus, of which there are two sub-varieties:
(a) Shower-cumulus, when rain falls from the
cloud without increment of wind. The edges of this
cloud are not cirrus-topped.
(b) Squall-cumulus, when the rain is accompanied
by wind, or by wind with hail and snow falling from
this cloud.
In these cases the Cumulus cloud is generally
much serrated, having a cirriform edging. In some
cases this cirriform edging extends far over the sky
and forms halos, particularly at the rear.
Two rarer varieties of Cumulus are:
Pillar-cumulus, generally noticed over the calm
belts of the ocean, and distinguishable by its slender
forms, which rise to great altitudes.
Roll-cumulus generally accompanies strong winds,
particularly polar west winds, which succeed cyclonic
disturbances. Here we have the ordinary
Cumulus cloud so blown along by the wind as to assume
the roll formation from which it is named.
A still rarer form of Cumulus appears in scattered
patches over the sky, and is indicative of an electrical
state of the atmosphere.
Cumulus clouds form at a low altitude, but they
frequently tower upward to great heights.
It should be noticed that in these clouds the fine
weather form is of soft, smooth outline, and has a
quiet appearance.
Stratus Clouds may be divided into four varieties
as follows:
1. Fog, so well known as not to need description.
It is, in fact, a Stratus cloud resting on the earth’s
surface.
2. Stratus, a cloud sheet which covers the whole
sky at a moderate elevation. Here and there the
cloud is thin, and under surfaces appear as parallel
lines all round the horizon. This is the characteristic
cloud of anti-cyclonic, or dry, fine weather conditions.
It may continue to cover the sky for several
days in succession.
3. High Stratus, including all the varying forms
of Cirro-cumulus from the mackerel skies to the
Cirro-macula of Clement Ley. Many beautiful varieties
of this cloud of minute cumuliform appearance
are caused by the changes taking place in the
atmosphere. We notice waves, wavelets, stipplings,
and flecks. To it are due the coronas sometimes seen
round the sun, as also iridescent clouds occasionally
noticed in the same vicinity. The wave-like appearance
of the clouds is due to the passage of a more
rapidly moving air current over a slower one, or of
a wave current crossing a motionless portion of the
air. When two air currents pass over one another at
an angle, the particles of clouds tend to fall into
different shapes, hence our mackerel skies. But this
cloud, although beautiful, is essentially one of warning,
more especially when the flecks are of a thin,
scaly appearance (resembling the scales of certain
fishes so closely that I have called it the scale cloud).
Sometimes these detached flecks appear in lines, and
very striking is the effect produced.
4. Cirrus.—The highest form of cloud and the
most important as a factor in the science of weather
forecasting. Cirrus, ordinarily, appears as wisps
and feather pieces scattered over the sky, and its
significance is then of no import.
When, however, this cloud takes the form of lines
parallel to the horizon, or of lines appearing to
radiate as wheel-spokes from any one part of the horizon,
it should be carefully noted as indicative of approaching
weather. Its movement and propagating
transition should be observed. This cloud is composed
of ice-dust or crystals.
When a cyclonic disturbance is about to pass over
an observer, Cirrus generally appears first in parallel
lines, or at a radiant point; the threads gradually
increase and interlace until a complete sheet of
Cirro-stratus covers the sky, causing a halo. The
cloud further thickens, the halo disappears, all becomes
overcast, and rain comes on. The cloud is now
known as Nimbus, and after it has endured some
time, the wind shifts, the Nimbus clears off, and it is
succeeded by a polar west wind.
In addition to these forms of clouds, we may often
notice, particularly during high winds, fragments of
clouds hurrying across the sky. These are known as
“scud”; they are generally pieces carried off by the
winds from the main bodies of clouds.
Occasionally two forms of cloud are present at the
same time. This is ordinarily taken as a case of
Cumulus and Stratus, and has become known as
Cumulo-stratus; but, if observed in the zenith, it may
readily be noted that the two forms of cloud are distinct,
and they had better be dealt with separately.
The appearance of Cumulo-stratus is an effect of
perspective.
Clouds float at varying altitudes, according to the
latitude and elevation of the ground; the vertical
temperature and adiabatic gradients determining the
level at which the vapor becomes visible as cloud. It
is desirable in all cloud observations, that note should
be made of the approximate relative altitudes of
clouds and of their velocity of motion. This is particularly
desirable when dealing with the stratiform
clouds, whether as ordinary Cirro-cumulus or as
very high Cirro-macula.
The beautiful coloring of clouds results from the
breaking up of light beams in passing through them
or along their edges. This phenomenon is caused by
diffraction, and to it is due our lovely sunrises and
sunsets. When the sun is high in the heavens, the
light is white, but as the orb nears the horizon, and
its rays pass through thicker layers of atmosphere, the
smaller light waves get gradually cut off, until the
sun sinks as a red ball below the horizon. The largest
waves of light produce the red rays and the after
glow which are so beautiful. Sunrise and sunset
effects are matters of much interest, but are of too
complicated a nature to be fully gone into here; we
must, however, notice them briefly, because of their
importance in weather forecasts. Soft sunset colors
indicate fine settled weather; fiery brilliant hues denote
change to stormy or wet weather.
Other color effects in clouds are due to phenomena,
known as halos and coronas. Halos appear as rings
round the sun and moon; they are caused by the
shining of the orb through very high Stratus or
Cirrus clouds, and have a diameter of 42°. Sometimes
shades of color, resembling those of a rainbow,
are visible—red appears on the inside and blue
on the outside. These rings of color are due to the
reflection and refraction of light passing through the
fine ice crystals of which high Stratus or Cirrus
clouds are composed. Occasionally a complicated
series of beautifully colored rings is noticeable.
Generally speaking, these rings are due to the thinness
of the high cloud through which the light is
passing. Still more curious arrangements of halos
sometimes occur.
Coronæ are broader rings seen quite close to the
sun or moon, and are due to the shining of light
through the edges of loose Cumulus or Stratus
clouds. They have red on the outside and blue on
the inside of the ring; the colors are, generally, easily
distinguishable. The more brilliant hues occasionally
seen, as has been said, in the vicinity of the
sun and moon, would appear to be incomplete sections
of circles intermediate in size between coronæ
and halos. An interested observer will be well repaid
if he chooses to study more closely the many
curious optical phenomena connected with clouds,
but it would be beyond the scope and object of this
paper to go into them more fully here.
Whoever wishes to be weatherwise, and who has
time to study the weather charts published daily,
may easily acquire such knowledge of local characteristics
as will enable him to forecast fairly accurately.
Cirrus clouds, as a rule, are reliable
guides; they form, as we have said, in parallel
threads, from the position and movements of which
forecasts may be made. Should the threads appear
on, and parallel to, the west horizon, and moving
from a northerly point, a depression is approaching
from the west, but, although causing some bad
weather, it will probably pass to the north of the
observer. Should the lines appear parallel to the
southwest or south-southwest horizon, and be moving
from a northwesterly point, the depression will very
likely pass over the observer and occasion very bad
weather. These are two of many possible prognostics.
Weather forecasting is much helped by a study
of the daily weather charts. Again, weather is often
very local, and to predict with fair accuracy a
knowledge of local conditions is necessary.
WINDS — William Hughes
Among the secondary causes affecting climate,
probably none is of greater importance than
the direction of prevailing winds. The currents of
air are warm or cold, wet or dry, according as they
have had their origin in warm or cold latitudes, and
have traversed inland tracts, or the expanse of ocean,
in their advancing course. With us, and in the
northern half of the globe in general, north and east
winds are cold and dry, while south and west winds
are warm, and often accompanied by moisture.
Within the Southern Hemisphere these conditions
are reversed, and the hottest currents of air come
from a northwardly direction. The prevailing
winds of western Europe are from the west and
southwest; and it is to this fact that we must mainly
ascribe the high winter temperature, as well as the
comparative freedom from extremes of heat and cold
which distinguishes the countries of western Europe.
The same cause explains the abundant moisture
which belongs to those regions in general, and which
distinguishes the western shores of our own islands
in a remarkable degree. Such winds have traversed
the immense expanse of the Atlantic, and come to the
western seaboard of Europe laden with the moist
vapors gathered on their course. These vapors, condensed
upon the high grounds which line the western
side of the British Islands, or, further to the
northward, upon the long chain of the Scandinavian
Mountains, fall to the earth in copious torrents of
rain. In the process of condensation, a vast quantity
of latent heat is disengaged, and the temperature is
correspondingly raised. Warmth and moisture are,
indeed, speaking generally, concomitant conditions
of European climate, and are especially so in the case
of western Europe.
Even in the case of lands which nearly approach
the tropic, the influence of prevailing winds in raising
or lowering the temperature is strikingly seen.
At New Orleans, bordering on the Mexican Gulf,
and throughout the adjacent portions of the United
States, the winters are often of excessive severity.
Cold winds, generated in the higher latitudes of the
New World, and blowing for weeks in succession
from the northern quarter of the sky, are the cause of
this. The generally level interior of the North
American Continent—a vast lowland plain, bounded
only to the east and west by the Alleghanies and the
Rocky Mountains—presents no obstacle to the advance
of these cold northerly blasts. The middle
and eastwardly parts of North America are subject
to like influences, in this regard, to the plains of
eastern Europe. To the westward of the Rocky
Mountains, on the other hand, the conditions affecting
climate present greater analogy to those that belong
to western Europe.
In the case of many countries, some local wind, of
occasional prevalence, forms a marked characteristic
of climate. The most remarkable of these local
winds are the simoon, the sirocco, the föhn, the harmattan,
and the mistral.
The often-described simoon of the desert is an intensely
heated and dry wind, which raises the temperature
like the blast of a furnace, and fills the air
with particles of sand, of suffocating quality. The
same wind is known in the deserts of Turkestan as the
tebbad (fever-wind), the terrible conditions of which
are thus described by the pen of a traveler. “The
kervanbashi (leader of the caravan) and his people
drew our attention to a cloud of dust that was approaching,
and told us to lose no time in dismounting
from the camels. These poor brutes knew well
enough that it was the tebbad that was hurrying on;
uttering a loud cry they fell on their knees, stretched
their long necks along the ground, and strove to
bury their heads in the sand. We intrenched ourselves
behind them, lying there as behind a wall; and
scarcely had we, in our turn, knelt under their cover,
than the wind rushed over us with a dull, clattering
sound, leaving us, in its passage, covered with a crust
of sand two fingers thick. The first particles that
touched me seemed to burn like a rain of flakes of
fire. Had we encountered it when we were deeper
in the desert we should all have perished. I had not
time to make observations upon the disposition to
fever and vomiting caused by the wind itself, but
the air became heavier and more oppressive than
before.”
The sirocco of the Mediterranean coasts is the hot
wind of the African desert, tempered, before reaching
the coasts of southern Europe, by its passage
across the great expanse of inland waters. The enervating
influences of this wind are well known to the
resident on the shores of Sicily, the Italian mainland,
or the islands of the Archipelago. The same wind,
when it reaches the high mountain regions of the
Apennines and the Alps, is known as the föhn.
The föhn, or warm south wind, is an important
agent in modifying the climate of the higher Alpine
region, where its prevalence for a few days in succession
causes the snow-line to recede, and is often
accompanied by inundations occasioned by the suddenly
melted snows. Its absence during a longer
period than usual is attended, on the other hand, by
a prolongation of the glaciers into a lower region of
the mountain valleys. The Swiss peasants have a
saying, when they talk of the melting of the snow,
that the sun could do nothing without the föhn.
The harmattan of Senegambia and Guinea is a
cold and intensely dry wind, which blows from the
northeast during the months of December and
January.
The mistral of southern France possesses similar
qualities to the last-named wind, and blows, for days
together, down the valley of the Rhone.
Winds transport particles of dust, and, with them,
the minuter forms of vegetable and animal life, to
vast distances. The phenomena known to sailors as
red fogs and sea-dust are evidence of this. In the
Mediterranean, and also in the neighborhood of the
Cape Verde Islands, showers of dust, of brick-red
or cinnamon color, are sometimes experienced in
such quantity as to cover the sails and rigging hundreds
of miles away from land. Among this sea-dust,
examination with the microscope has detected
infusoria and other organisms native to the tropical
regions of South America.
The prevailing currents of the atmosphere, or
winds, constitute an important feature in the climate
of any country, and it belongs to Physical Geography
to explain the prevalent winds which distinguish
great regions of the globe. Such explanation is more
easily made in regard to the warmer latitudes of the
earth, where alone the direction of the wind is constant,
than might be at first supposed by those whose
personal experience is limited to such countries as
Britain, and other temperate lands, where the variable
condition of the atmosphere is the well-known
subject of common observation and remark. But
within those parts of the globe which experience a
vertical sun, and for a few degrees beyond the exact
line which marks the limit of the sun’s vertical influence
on either side of the equator, the conditions
either of perennial calm, or of currents of air that
constantly blow in one given direction, are the uniform
characteristics of climate.
Throughout a zone of a few degrees in breadth,
which extends round the globe in the neighborhood
of the equator, and the limits of which undergo a certain
amount of variation, dependent on the sun’s
passage of the equinox, the variation of temperature
throughout the year is confined within very narrow
limits, and the result is a general prevalence of
calms—that is, of undisturbed atmosphere. Wind
is air set in motion, mainly by the existence of different
conditions of temperature between adjacent
bodies of air—of colder and denser air pressing
against warmer and lighter air, and taking the place
which is left vacant by the latter, as it rises into the
higher regions of the entire aerial sea. Between
the heated air of the tropics in general, and the comparatively
cooler air of the regions lying some distance
north and south of the tropics, for example,
there is a very manifest difference as to temperature,
as well as in regard to other conditions; but for a few
degrees in the immediate neighborhood of the
equator there is no such obvious difference, and,
consequently, nothing to occasion disturbance (temperature
alone being considered) in the general
equilibrium of the atmosphere. Hence the prevalence
of calms in that region. Within the parallels
of 8° or 10° on either side of the line, the angle at
which the solar rays reach the earth is at no time
more than a few degrees from the perpendicular, for
the equator divides the total amount of angular
difference which is involved in the entire yearly path
of the sun.
The average breadth of the calm latitudes—or the
Zone of Calms, as it is the custom, in books and maps,
to term it—may be stated at about six or seven degrees.
The mid-line of this zone does not coincide
with the equator, for the reason that the equator
does not represent the line of the earth’s highest
temperature, owing to the preponderance of land in
the Northern Hemisphere. Hence the Zone of
Calms is, for the most part, to the northward of the
equator—extending, with varying seasonal limits,
from about the first to the seventh or eighth parallel
of north latitude. But the limits oscillate with the
sun’s passage of the equinox and consequent place
in the heavens vertically over either side of the
equator.
The calm latitudes are the dread of the mariner,
whose ship is often delayed for weeks together within
their limits. The wearisome and tantalizing nature
of this delay can, perhaps, only be adequately appreciated
by those who have experienced the monotony
attendant on a calm in mid-ocean, when, with a still
and glassy sea around, a glittering atmosphere, and
a burning sun overhead, the sails hang idly by the
yards, and the vessel makes no appreciable progress.
Between the oscillating limit of the Zone of Calms
and the parallel of 28° in the Northern Hemisphere,
on one side of the globe, and between the correspondent
limit and the parallel of 25° south latitude, on
the opposite hemisphere, there prevail through above
two-thirds of the earth’s circumference, steady winds,
blowing with almost undeviating uniformity from
the eastward. These are the trade-winds. More precisely,
the trade-wind of the Northern Hemisphere
is a wind blowing from the northeastward—that is,
a northeast wind. The trade-wind of the Southern
Hemisphere blows from the southeastward, and is
a southeast wind.
The trade-wind belts stretch round more than two-thirds
of the earth’s surface. They comprehend
(within the latitudinal limits already defined) the
Atlantic and Pacific Oceans, with the countries that
lie adjacent to those vast areas of water. In the
Pacific, however, their limits are less distinctly
marked, and their influence less powerful, to the
southward of the equator than to the north of that
line. Over the Indian Ocean and its shores, the atmospheric
currents follow, during portions of the
year, an opposite course.
The trade-winds of the Atlantic and Pacific—blowing
constantly, and with almost undeviating
steadiness, from the eastward—regulate the course
of the mariner across those oceans. They, of course,
facilitate the passage of either ocean in a westerly
direction—that is, from the shores of the Old World
to the eastern seaboard of America, or from the
western coast of the New World to the eastern shores
of the Asiatic and Australian Continents. It was the
trade-wind of the Northern Atlantic that carried
Columbus to the westward, on the adventurous voyage
which resulted in the discovery of the New
World, inspiring terror in the breasts of his companions,
while in the mind of the great navigator
himself it strengthened the assurance of reaching
land by pursuing the direction in which his vessels’
prows were turned. On a like great occasion, the
trade-wind of the Pacific carried Magellan’s ship
steadily forward through the ocean which he was the
first to cross, and facilitated the earliest circumnavigation
of the globe. On the other hand, the same
winds compel the return voyage across either ocean
to be made in higher latitudes, where westerly winds
prevail.
The explanation of the trade-winds is found in the
different measure in which the sun’s heat is experienced
by regions within or nearly adjacent to the
tropics, and by those of higher latitudes. They are
currents of air set in motion by the differences of
density consequent upon such various conditions
of temperature—conditions which are of uniform
prevalence, and the result of which is also constant.
To sum up, we may say that the trade-winds, like
the currents of the ocean, are due, first, to the sun,—that
is to the different measure in which the solar
heat is distributed on the globe’s surface; and,
secondly, to the earth’s axial rotation, which affects
the direction of currents in the aerial ocean in manner
precisely analogous to that in which it affects
the like currents in the aqueous ocean. In truth,
the ocean of water, and the ocean of air—in contact
with one another, and possessing many properties in
common—act and react upon one another, mutually
imparting their respective temperatures, movements,
and other conditions. This is only one among the
instances of mutual harmony—one of the many mute
sympathies—which abound in the natural world.
The monsoons are winds which blow over the
Indian Ocean, and the countries adjacent to its
waters. In general terms, it may be said that they
prevail within the same latitudes as those over which
the trade-winds of the Atlantic and Pacific blow.
But the monsoons differ from the trade-winds of the
two greater oceans in the fact that they are periodical
winds, not perennial. The monsoon blows for half
the year from one quarter of the heavens, and for
the other half from an opposite quarter.
Charts Showing the General Directions of Wind and Tide Currents
Over the northerly portion of the Indian Ocean—from
the neighborhood of the equator to the shores
of the Asiatic Continent, including the Malay
Archipelago and the adjacent China Sea—a northeast
monsoon blows during the winter months of the
Northern Hemisphere; that is, from October to
March, inclusive. During the summer months—April
to September—and within the same limits,
the southwest monsoon blows. Southward from the
equator to the neighborhood of the tropic of Capricorn,
the southeast monsoon blows during the winter
of those latitudes (April to September): this is exchanged,
during the other half of the year, for a
northwest monsoon in the neighborhood of the Australian
coasts, and for a northeast monsoon along the
line of the African shores. The term monsoon—derived
from a Malay word which signifies “season”—expresses
the periodical nature of these winds, and
indicates to how large an extent the climate of Indian
seas and lands is dependent upon their periodical
recurrence.
The change from the one monsoon to that from an
opposite quarter is not accomplished at once. The
breaking-up of the monsoon, as it is termed, is attended
by thunderstorms and other meteorological
phenomena, which prevail during some weeks,
until the setting-in of the coming monsoon is
fairly accomplished. The nature of these changes,
and the general characteristics of the monsoon itself,
are admirably depicted in the following passage, by
a master hand:
“Meanwhile the air becomes loaded to saturation
with aqueous vapor drawn up by the augmented
force of evaporation acting vigorously over land and
sea; the sky, instead of its brilliant blue, assumes the
sullen tint of lead, and not a breath disturbs the motionless
rest of the clouds that hang on the lower
range of hills. At length, generally about the middle
of the month, but frequently earlier, the sultry
suspense is broken by the arrival of the wished-for
change. The sun has by this time nearly attained
his greatest northern declination, and created a
torrid heat throughout the lands of southern Asia
and the peninsula of India. The air, lightened by
its high temperature and such watery vapor as it may
contain, rises into loftier regions, and is replaced by
indraughts from the neighboring sea, and thus a
tendency is gradually given to the formation of a
current bringing up from the south the warm humid
air of the equator. The wind, therefore, which
reaches Ceylon comes laden with moisture, taken
up in its passage across the great Indian Ocean. As
the monsoon draws near, the days become more overcast
and hot, banks of clouds rise over the ocean to
the west, and in the peculiar twilight the eye is attracted
by the unusual whiteness of the sea-birds that
sweep along the strand to seize the objects flung on
shore by the rising surf.
“At last sudden lightnings flash among the hills
and shoot through the clouds that overhang the sea,
and with a crash of thunder the monsoon bursts over
the thirsty land, not in showers or partial torrents, but
in a wide deluge, that in the course of a few hours
overtops the river banks and spreads in inundations
over every level plain.
“All the phenomena of this explosion are stupendous:
thunder, as we are accustomed to be
awed by it, affords but the faintest idea of its overpowering
grandeur in Ceylon, and its sublimity is
infinitely increased as it is faintly heard from the
shore, resounding through night and darkness over
the gloomy sea. The lightning, when it touches the
earth where it is covered with the descending torrent,
flashes into it and disappears instantaneously; but
when it strikes a drier surface, in seeking better conductors,
it often opens a hollow like that formed by
the explosion of a shell, and frequently leaves behind
it traces of vitrification. In Ceylon, however,
occurrences of this kind are rare, and accidents are
seldom recorded from lightning, probably owing to
the profusion of trees, and especially of cocoanut
palms, which, when drenched with rain, intercept
the discharge, and conduct the electric matter to the
earth. The rain at these periods excites the astonishment
of a European; it descends in almost continuous
streams, so close and so dense that the level
ground, unable to absorb it sufficiently fast, is covered
with one uniform sheet of water, and down the sides
of acclivities it rushes in a volume that wears channels
in the surface. For hours together, the noises of
the torrent as it beats upon the trees and bursts upon
the roofs, flowing thence in rivulets along the ground,
occasions an uproar that drowns the ordinary voice
and renders sleep impossible.”
The monsoons of the Indian Ocean are not divided
by any such distinctly defined belt of calms as separates
the opposite trade-winds of the northern and
southern Pacific and Atlantic. The southeast monsoon
of the southern Indian Ocean passes gradually
into the southwest monsoon, which prevails at the
same time in the northern half of that ocean. Nor
is the season of change from the one monsoon to the
other precisely the same over all parts of that ocean.
Indeed, the Indian Ocean, from the geographical
conditions already adverted to, is exposed in much
higher measure than either of the other oceans to
the disturbing influences consequent upon proximity
to land, and its winds are hence affected in a vastly
greater degree by local conditions. Thus the Indian
monsoon, the Arabian and East African monsoon,
and the monsoon of northwestern Australia, assume
in each case a direction which is dependent upon
the geographical position and contour of the lands
whence they derive their distinguishing names. In
the Red Sea, the monsoons follow the direction of its
shores, and blow, for six months of the year, alternately,
up and down its long and trough-like valley,
confined and guided in their passage by the mountain-chains
which bound it upon either side.
We have hitherto spoken of the monsoons only in
connection with the Indian Ocean. But, in truth,
a monsoon, or season-wind—which is what the word
monsoon means—is experienced upon a large portion
of the West African coasts, and thence far out into
the mid-Atlantic, within the proper region of the
Atlantic trades. The evidence of this is one among
the many valuable results due to the Wind and Current
Charts of Maury, and the cause of it is precisely
the same as that which occasions the monsoon of the
Indian coasts. Between the equator and the parallel
of 13° north, the intense heat of a vertical sun, acting
upon the western coasts and adjacent interior of the
African Continent, occasions a reversal of the ordinary
wind of that region. The intensely heated atmosphere
of the land, owing to superior rarity,
ascends, and the cooler air of the neighboring sea
sets in to fill its place. The monsoon thus generated
lasts as long as the sun remains to the northward of
the equator. Further to the south a like phenomenon
accompanies, in those localities, the passage of the
sun into south declination. The influence of these
monsoons extends to a distance of a thousand miles
or more from land, the entire space within which
they prevail forming a cuneiform (or wedge-shaped)
region in the midst of the Atlantic, the base
of which rests upon the African Continent, while its
apex is within ten or fifteen degrees of the mouth of
the Amazon.
A similar reversal of the trade-winds of the North
Pacific occurs off the western shores of Central
America, capable of explanation in precisely like
manner—due, that is, to the excess of heat which the
summer sun brings to the adjacent lands, and the consequent
rarefaction and rising of the currents of air
over those lands. This, and the like instance of the
West African monsoons, show in the most striking
manner how powerfully the land is affected by the
sun’s heat, and to how wide a distance the atmospheric
movements which are generated by such influences
extend over the adjacent seas. Even such
limited tracts of land as the Society and Sandwich
Islands have a marked influence upon the winds experienced
over the surrounding waters. They interfere,
says Maury, with the trade-winds of the Pacific
very often, and even turn them back, for westerly and
equatorial winds are common at both groups, in their
winter time.
Upon the coasts of most countries that are within
the warmer latitudes of the globe, there occur daily,
at or shortly before the hour of early dawn, and
toward the approach of sunset, breezes that blow
respectively off the shore or from off the adjacent
waters. The former is known as the land-breeze;
the latter as the sea-breeze.
These refreshing movements of the air are not confined
to countries within, or even very near to, the
tropics, though they are more powerful in the case
of countries that are within the torrid zone than in
the case of other lands. But they are felt upon the
coasts of the Mediterranean, and in even much
higher latitudes than those of the Mediterranean,
during the warmer portions of the year. The hour
at which they begin to be perceptible is not the same
in all localities; but, speaking generally, the land-breeze
begins to be felt about an hour before sunrise,
and the sea-breeze toward the early evening, as the
time of sunset approaches. During the midday
hours the intense heat of the atmosphere, accompanied
by general calm and almost perfect repose
of the animal world, is painfully felt by all residents
in warm countries, and the cooling sea-breeze which
sets in as the sun approaches the horizon is welcomed
with intense delight. To the sojourner in Indian
lands, it is the signal for outdoor exercise, and is accompanied
by a general reawakening of the outer
world of nature. The dweller on the African or
Australian coasts equally rejoices in its refreshing
power. The mariner within Indian seas, frequently
becalmed during the stillness of the night-watch,
finds like relief in the breeze which blows off the
land with the approach of early morning.
The land and sea-breezes are due to a cause strictly
analogous to that which produces the monsoon of
eastern seas—that is, the influence of the sun heating
in various measures the lands and seas, and with
them the superincumbent air. Successive movements
are generated in the atmosphere according as different
portions of the whole acquire, with difference of
temperature, various degrees of density. During
the hours of midday heat, the air over the land becomes
relatively hotter, by many degrees, than the
air which is above the adjacent water, for it is the
well-known attribute of land to experience much
greater extremes of temperature than water does.
As afternoon, with its sultry temperature, advances,
this continued heat occasions the land-air to form
an ascending current, while the cooler (and relatively
denser) air from the neighboring waters flows
in to take its place. This cooling breeze is an effort
of nature to restore equilibrium in the atmosphere,
the heavier portions of the whole body of air assuming
the place of lower strata, and the higher
portions spreading over the superior regions. This
effort continues until the desired balance is attained,
and, with the approach of midnight, the air is again
calm and settled. But during the night, while the
water retains a nearly uniform temperature, the land
rapidly parts with the heat, so that the air over the
land becomes at length colder than that over the
water. This latter, therefore, relatively the warmer
of the two, tends to rise, while the cooler air of the
land fills its place. A wind blowing from off the land
is thus generated. In some localities this blows during
great part of the night. But the period of its commencement
varies in different places, and the intervals
of calm between both land and sea-breezes are
often of uncertain duration.
The land and sea-breezes repeat, on a scale of
diurnal variation, the phenomena shown by the monsoons
on a scale of yearly change. They show how
readily the atmosphere yields to the slightest pressure,
and how powerful an influence on the laws of
climate, and, with them, on the condition of mankind,
is exercised by every change, of temperature, or
otherwise, to which it is subject. Similar winds—alternating
from opposite quarters of the heavens—are
experienced in inland districts, as on the banks
of the Tapajos River, in South America.
The rotary storms which occur, at uncertain intervals,
in particular latitudes, are to be included
among the exceptional phenomena of atmospheric
change. They prevail, however, over larger areas
than was formerly supposed, and perhaps belong to
a general system of atmospheric movements in which
electric and magnetic influences fill an important
place. The hurricanes of the West Indies, the tornadoes
and cyclones of the Indian Ocean, and the
typhoons of the China Sea, are winds of this description.
Within the Southern Hemisphere, the direction
of the rotating circle is always found to correspond
to the movement of the hands of a watch
(i. e., from west to north, east, and south): to the
north of the equator, the circle of wind follows an
opposite direction (or west to south, east, and north).
By a knowledge of this law, combined with careful
observation of the track usually taken by such
storms, mariners are enabled to avoid some of the
dangers incident to their occurrence. The destruction
which they occasion, however, within maritime
tracts exposed to their influence, as well as upon the
high seas, is at times fearfully great.
Waterspouts are another form in which the rotary
movements of the air are manifested. In the case
of these phenomena, a taper column of cloud, descending
from above, is joined by a spiral column of
water which winds upward from the agitated surface
of the sea, the two together forming, by their
union, a continuous column which moves over the sea.
Waterspouts seldom last longer than half an hour.
They are more frequent near the coast than on the
high seas, and more commonly seen in warm climates.
SQUALLS, WHIRLWINDS, AND TORNADOES — Sir Ralph Abercromby
If we watch the stages of gradually increasing
wind, we find that as the strength rises the tendency
is more and more to blow in gusts. Gradually
these gusts get still more violent, and in their highest
development come with a boom like the discharge
of a piece of heavy ordnance. This is what sailors
call “blowing in great guns,” and these are the gusts
which blow sails into ribbons, and dismast ships
more than any amount of steady wind. These gusts
only last a few minutes, but they seem to be very
closely allied to the simplest form of squalls. In a
true, simple squall the wind generally need not be
of the exceptional violence which causes “guns”;
but after it has rather fallen a little, the blast comes
on suddenly with a burst, and rain or hail, according
to intensity, or other circumstances, while the
whole rarely lasts more than five or ten minutes.
At sea one often sees two or three squalls flying
about at a time. Then we readily observe that over
the squall there is firm, hard, cumulus cloud; that the
disturbance only reaches a short distance above the
earth’s surface; that the squall moves nearly in the
same direction as the wind; and that there is little
or no shift of the wind before or during the squall.
We also see that the shape of the squall is merely that
of an irregular patch, with a tendency rather to be
longer in the direction of the wind than in any other
quarter; and that the motion of the squall as a whole
is much slower than that of the wind which accompanies
the first blasts. If, at the same time, we watch
our barometer closely, we find that if the squall is
sufficiently strong, the mercury invariably rises—sometimes
as much as one-tenth of an inch—and returns
to its former level after the squall is over. No
difference is observed in this sudden rise, whether
the squall is accompanied with rain, hail, or thunder
and lightning; and though we are unable exactly to
explain why the wind sometimes takes this irregular
method of blowing, we have still to do with a comparatively
simple phenomenon.
The simplest kind of thunderstorm may more
properly be described as a squall accompanied by
thunder and lightning, instead of only with wind and
rain. On a wild, stormy day, with common squalls,
one or two of these, which are exceptionally violent,
will be accompanied by one or two claps of thunder
with lightning. The principal interest which attaches
to this type of thunderstorm consists in the
proof which is afforded that there is no essential difference
between a common squall and another which
may be associated with electrical discharge, except
intensity. The look and motion of the clouds, and the
sudden rise of the barometer, are identical in both
cases. In western Europe this class of thunderstorm
is much more common in winter than in summer,
which is the reverse of what takes place with all other
kinds of thunderstorm. So much is this the case that
in Iceland there are no summer thunderstorms, but
only winter ones, of this simple squall type. In Norway
both types occur; and the winter ones are there
found to be the most destructive, because they are
lower down, and therefore the lightning is the more
likely to strike buildings. In that country, however,
the summer thunderstorms are not nearly so violent
as in more southern latitudes.
We must now just mention a class of thunderstorms
which are more complicated than a simple squall,
and yet differ in many ways from line-thunderstorms.
They are associated with secondary cyclones, and are
much commoner in England than line-thunderstorms,
but none have been tracked over a sufficiently
long area to allow us to say anything about their
shape or motion. All we know is, that as surely as
we see a secondary on the charts in summer, so certainly
will thunderstorms occur during the day,
though we can not say in what portion of the small
depression.
The special features of this class of thunderstorm
are the calm sultry weather with which they are associated,
so different from the squall of a line-thunderstorm,
and the limited rotation of the surface-wind
during the progress of the storm. Another very
remarkable feature is that this surface circling of the
wind extends only a very short distance upward, and
whenever a glimpse can be caught of the drift of the
upper clouds, they are found to move in the same direction
throughout the whole period of the disturbance.
This is the familiar class of thunderstorm
which we associate with sultry weather, and with the
thunder coming against the wind.
One of the first things which must strike everybody
is, that even in the temperate zone some countries are
far more ravaged by thunderstorms than others. For
instance, France suffers more than any other part of
Europe, and England the least. We may probably
find at least two causes which modify the development
of thunderstorms. In the first place, the geographical
position of the country relative to the great
seasonal areas of high and low pressure. From this
point of view we can readily see that France is far
more exposed to the influence of small secondaries,
which come in from the Atlantic, and which die out
before they reach central Europe, than any other
portion of that continent.
In Great Britain, though the bulk of winter rain
is cyclonic, a great deal of summer rainfall is non-isobaric;
in Continental Europe a still larger proportion
is of the latter character; so are most tropical
rains, except the downpour of hurricanes; while
the whole of the heavy rain on the equator, and all
that falls in the doldrums, is also absolutely non-isobaric.
A moderate whirlwind may be two hundred feet
high, and not above ten feet in diameter. The dimensions,
however, are very variable, for a whirlwind
may vary in intensity from a harmless eddy in a
dusty road to the destructive tornado of the United
States.
But by far the most striking non-isobaric rain in
the world is the burst of the southwest monsoon in
the Indian Ocean. The quality of the rain, if nothing
else, distinguishes the monsoon from cyclonic
precipitation. The rain in front of a Bengal cyclone
seems to grow out of the air, while that of the monsoon
falls in thunderstorms and from heavy cumulo-form
clouds. The only rational suggestion which
has been made to account for this burst of rain would
look to a sudden inrush of damp air from the region
of the doldrums as the source of the change in
weather, but not of the direction of the wind, or of
the shape of the isobars; for the burst is apparently
almost coincident with the disappearance of the
belt of high pressure to the south of the Bay of
Bengal.
The word “pampero” is, unfortunately, used in a
very vague manner in the Argentine Republic and
neighboring states. Every southwest wind which
blows from off the pampas is sometimes called a
pampero; and there is a still further confusion caused
by calling certain dry dust-storms pamperos sucios,
or dry pamperos. The true pampero may be described
as a southwest wind, ushered in by a sudden
short squall, usually accompanied by rain and thunder,
with a very peculiar form of cloud-wreath.
The barometer always falls pretty steadily for
from two to four days before the pampero, and always
rises for some days after the squall. Temperature
is always very high before the squall, and then
the sudden change of wind sends the thermometer
rapidly down, sometimes as much as 33° in six hours.
Thunder accompanies about three out of four pamperos;
but more or less rain always falls, except in
the rarest cases. The wind before this class of pampero
almost invariably blows moderately or gently
for some days from easterly points, and then with
a sudden burst the southwest wind comes down with
its full strength, and, after blowing thus from ten to
thirty minutes, either ceases entirely or continues with
diminished force for a certain number of hours. In
all cases but one the upper wind-currents have been
seen to come from the northwest before, during and
after the pampero.
The general appearance of a pampero will be best
understood by a description of an actual squall. “In
the early morning of a day in November, the wind
blew rather strongly from the northeast. The sky
was cloudy, but not overcast, save in the southwest
horizon. The clouds were moving very slowly from
the west, or a little south of it, throwing out long
streamers eastward. About 8 A. M. the threatening
masses in the southwest had advanced near enough
to show that at their head marched two dense and
perfectly regular battalions of cloud, one behind the
other, in close contact, yet not intermingling, and
completely distinguished by their striking difference
of color, the first being of a uniform leaden gray,
while the second was as black as the smoke of a
steamer. On arriving overhead, it was seen that
the front, although slightly sinuous, was perfectly
straight in its general direction, and that the bands
were of uniform breadth. As they rushed at a great
speed under the other clouds without uniting with
them, preserving their own formation unbroken, their
force seemed irresistible, as if they were formed of
some solid material rather than vapor. The length
of these wonderful clouds could not be conjectured,
as they disappeared beneath the horizon at both ends,
but probably at least fifty miles of them must have
been visible, as the ‘Cerro’ commands a view of
twenty miles of country. Their breadth was not great,
as they only took a few minutes to pass overhead,
and appeared to diminish from the effects of perspective
to mere lines on the horizon. At the instant
when the first band arrived, the wind—which was still
blowing, and something more than gently, from the
northeast—went round by north to southwest; at the
same time a strong, cold blast fell from the leaden
cloud, and continued to blow till both bands had
passed.”
A whirlwind may be described as a mass of air
whose height is enormously greater than its width,
rotating rapidly round a more or less vertical axis.
A tornado is simply a whirlwind of exceptional
violence; if it were to encounter a lake or the sea, it
would be called a waterspout. Its most characteristic
feature is a funnel, or spout, which is the visible
manifestation of a cylinder of air that is revolving
rapidly round a nearly vertical axis. This spout is
propagated throughout the northern temperate zone
in a northeasterly direction at a rate of about thirty
miles an hour, and tears everything to pieces along
its narrow path.
The diameter of the actual spout often does not
exceed a few yards, and the total area of destructive
wind is rarely more than three or four hundred yards
across. The height of the spout is that of the lowest
layer of clouds, which are then never high; and, as in
thunderstorms, the upper currents are unaffected by
the violent commotion below.
The spout as a whole has four distinct motions:
1. A motion of translation generally toward the
northeast at a variable rate, but which may be taken
to average thirty miles an hour.
2. A complex gyration. The horizontal portion of
this rotation is always in a direction opposite to that
of the hands of a watch—that is to say, in the same
manner as an ordinary cyclone. But in addition to
this there is a violent upward current in the centre
of the cylinder of vapor or dust which constitutes the
spout, and sometimes small clouds seem to dart down
the outer sides of the funnel whenever these float
in close proximity. There are, however, no authentic
instances of any object being thrown to the ground
by the individual effort of a downward current. The
slight downward motion of a few small clouds is
probably only a slight eddying of a violent uprush.
3. A swaying motion to and fro like a dangling
whip, or an elephant’s trunk, though the general direction
of the spout is always vertical.
4. A rising and falling motion, that is to say, that
sometimes the end of the funnel rises from the surface
of the ground and then descends again, and so on.
Owing to this rise and fall, the general appearance of
the tornado changes a good deal. When the bottom
of the spout is some distance above the ground, the
whole is somewhat pointed, and does comparatively
little harm as it passes over any place. As the spout
descends, a commotion commences on the surface of
the ground. This latter gradually rises so as to meet
the descending part of the spout, and then the whole
takes the shape of an hour-glass. This is the most
dangerous and destructive form, because the ground
gets the whole force of the tornado.
The general appearance of the cloud over a tornado
or whirlwind is always described as peculiarly
smoky, or like the fumes of a burning haystack.
The tornado is also never an isolated phenomenon;
it is always associated with rain and electrical disturbance.
The destructive effects of the tornado are very curious,
from the sharp and narrow belt to which the injury
is confined. It appears that in the passage of
some tornadoes wind-pressures of various amounts,
from eighteen to a hundred and twelve pounds per
square foot, have been demonstrated by destruction
of bridges, brick buildings, etc. The upward pressures
are sometimes as great as the horizontal, and
even greater. Downward pressures or movements of
wind have not been clearly proved. Upward velocities
of 135 miles per hour seem not to be unusual,
and horizontal velocities of eighty miles have been
recorded with the anemometer. The destructive
wind-velocities are confined to very small areas. A
destruction of fences, trees, etc., is often visible over a
path many miles long and a few hundred yards wide,
but the path of greatest violence is very much narrower.
The excessive cases above referred to are
observed only in small isolated spots, less than a hundred
feet square, unequally distributed along the
middle of the track. Thus, in very large buildings,
only a small part is subject to destructive winds. In
different parts of this area of maximum severity, the
winds are simultaneously blowing in different, perhaps
opposite, directions, the resultant tending not
to overturn or carry off or crush in, but rather to twist
round a vertical axis. Buildings are generally lifted
and turned round before being torn to pieces. As the
chances are very small that a building will be exposed
to the violent twisting action, it is evidently the average
velocity of rectilinear winds within the path of
moderate destruction that it is most necessary to provide
against in ordinary structures. These winds
may attain a velocity of eighty miles an hour over
an area of a thousand feet broad, and generally blow
from the southwest; the next in frequency blow from
the northwest. The time during which an object is
exposed to the more destructive winds varies from
six to sixty seconds. An exposed building experiences
but one stroke, like the blow of a hammer, and
the destruction is done. Hence, in a suspension-bridge,
chimney, or other structure liable to be set
into destructive rhythmic vibrations, the maximum
winds do not produce such vibrations. The duration
of the heavy southwest or northwest winds over the
area of moderate destruction is rarely over two minutes.
The motion of translation of the central spout
of a tornado, in which there is a strong vertical current,
is, on ah average, at the rate of thirty miles an
hour.
Tornadoes mostly occur on sultry days and either
in the southeast or right front of cyclones, or in front
of the trough of V-depressions.
The general character of all tornadoes is so similar
that the description of one will do for all. We
shall therefore give some of the description furnished
by an eye-witness to the United States Signal
Office, which is described in the reports as the “Delphos
tornado”:
“On Friday morning, May 30, 1879, the weather
was very pleasant, but warm, with the wind from
the southeast, from which direction it had blown for
several days. The ground was very dry, and no rain
had fallen for a number of weeks. About 2 P. M.
threatening clouds appeared very suddenly in the
west (against the wind), attended in a few minutes
by light rain, the wind still in the southeast It
stopped in about five minutes, and then commenced
again, wind still the same, accompanied by hail,
which was thick and small at first, but rapidly grew
less in quantity and larger in size, some stones measuring
three and a half inches in diameter, and one
was found weighing one-fourth of a pound. This
last precipitation continued for about thirty minutes,
after which a cloud in the shape of a waterspout
was seen forming in the southwest, and moving
rapidly forward to the northeast. The cloud from
which the funnel depended, seen at a distance of
eight miles, appeared to be in terrible commotion;
in fact, while the hail was falling, a sort of tumbling
in the clouds was noticed as they came up from the
northwest and southwest, and about where they appeared
to meet was the point from which the funnel
was seen to descend. There was but one funnel at
first, which was soon accompanied by several smaller
ones, dangling down from the overhanging clouds
like whiplashes, and for some minutes they were appearing
and disappearing like fairies at a play.
Finally one of them seemed to expand and extend
downward more steadily than the others, resulting at
length in what appeared to be their complete absorption.
This funnel-shaped cloud now moved
onward, growing in power and size, whirling rapidly
from right to left, rising and descending, and
swaying from side to side. When within a distance
of three or four miles, its terrible roar could be
heard, striking terror into the hearts of the bravest.”
The eye-witness judged that the funnel itself would
reach a height of about five hundred feet from the
ground. As the storm crossed a river, a cone-shaped
mass came up from the earth to meet it, carrying
mud, débris, and a large volume of water. The
cloud then passed the observer’s house very near
to 4 P. M. The progressive velocity at the time was
considered to be about thirty miles per hour, although
at Delphos, three and a half miles distant,
it had slackened down to near twenty miles. A
few minutes previous to and during the passage of
the funnel, the air was very oppressive; but ten minutes
after the wind was so cold from the northwest
that it became necessary to wear an overcoat when
outside.
The actual diameter of this storm appears to have
been only forty-three yards. On the right of the
track, destructive winds extended to a further distance
of from one to two miles, sensibly deflected
winds for another mile and a half, beyond which
only the usual wind of the day was experienced. On
the left or northern side of the tornado path, the
damage did not extend quite so far, for the width
of the belt of destructive winds was not more than
twenty-eight yards across and that of sensibly deflected
winds one mile and a quarter.
As a specimen of the damage done a large two-horse
sulky plow, weighing about seven hundred
pounds, was carried a distance of twenty yards,
breaking off one of the iron wheels attached to an
iron axle one and three-quarter inches in diameter.
A woman was carried to the northwest two hundred
yards, lodged against a barbed-wire fence, and instantly
killed. Her clothing was entirely stripped
from her body, which was found covered with black
mud, and her hair matted with it. A cat was found
half a mile to the northwest of the house, in which
she had been seen just before the storm, with every
bone broken. Chickens were stripped of their
feathers, and one was found three miles to the
northwest.
A few miles further on, another eye-witness says,
“the dark, inky, funnel-shaped cloud rapidly descended
to the earth, which reaching, it destroyed
everything within its grasp. Everything was taken
up and carried round and round in the mighty
whirl of the terrible monster. The surrounding
clouds seemed to roll and tumble toward the vortex.
“The funnel, now extending from the earth upward
to a great height, was black as ink, excepting
the cloud near the top, which resembled smoke of
a light color. Immediately after passing the town,
there came a wave of hot air, like the wind blowing
from a burning building. It lasted but a short time.
Following this peculiar feature, there came a stiff
gale from the northwest, cold and bleak, so much so
that during the night frost occurred, and water in
some low places was frozen.”
END OF VOLUME TWO
FOOTNOTES:
[1] The lakes of Sweden, which cover one-twelfth of the surface
of the country, exercise an important influence on climate
according as they are frozen or open.
[2] Another variety or species of seal inhabits Lake Baikal.
[3] Count von Helmersen, however, has stated his belief that
for this extreme northern prolongation of the Aralo-Caspian
Sea there is no evidence. The shells, on the presence of which
over the Tundras the opinion was chiefly based, are, according
to him, all fresh-water species, and there are no marine shells
of living species to be met with in the plains at the foot of the
Ural Mountains.
[4] Archbishop of Spalato and Primate of Dalmatia.
TRANSCRIBER’S NOTE
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg 470: ‘chiefly Brachipods of’ replaced by ‘chiefly Brachiopods of’. Pg 472: ‘these same familes’ replaced by ‘these same families’. Pg 483: ‘constituing links’ replaced by ‘constituting links’. Pg 563: ‘Camaroons Mountains’ replaced by ‘Cameroon Mountains’. Pg 563: ‘with Teneriffe in’ replaced by ‘with Tenerife in’. Pg 569: ‘existing, denundation’ replaced by ‘existing, denudation’. Pg 650: ‘their relativ size’ replaced by ‘their relative size’. Pg 718: ‘incalulable ages’ replaced by ‘incalculable ages’. Pg 722: ‘greatly diminshed’ replaced by ‘greatly diminished’.
