Large-size versions of illustrations are
available by clicking on them.
THE GEOLOGY AND
GEOGRAPHY
OF NORTHERN NIGERIA
MACMILLAN AND CO., Limited LONDON BOMBAY CALCUTTA
MELBOURNE
THE MACMILLAN
COMPANY NEW YORK BOSTON
CHICAGO
ATLANTA SAN FRANCISCO
THE MACMILLAN CO. OF CANADA,
Ltd. TORONTO
Plate XVII
Inselberge Landscape around Abuja.
Inselberge Landscape in Northern Yola.
[Frontispiece.
BY J. D. FALCONER, M.A., D.Sc. F.G.S., F.R.S.E., F.R.G.S.
With Notes by the
late ARTHUR LONGBOTTOM, B.A., F.G.S., and an Appendix on the Palæontology of the
Cretaceous Deposits, by HENRY WOODS, M.A., F.G.S. University Lecturer in Palæozoology,
Cambridge
WITH FIVE MAPS AND TWENTY-FOUR
PLATES
MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON 1911
Richard Clay
and Sons, Limited BREAD STREET HILL, E.C., AND
BUNGAY, SUFFOLK.
TO
MY OLD TEACHER AND FRIEND PROFESSOR JAMES GEIKIE
PREFACE
The observations
upon which these outlines of the Geology and Geography of Northern
Nigeria are based, were made in the course of the operations of the
Mineral Survey of the Protectorate of which I had the honour to be
the principal officer. The Survey was instituted by the Colonial
Office in 1904 upon the recommendation of Professor Wyndham R.
Dunstan, under whose general direction the work of investigation
was carried out both in Nigeria and in the laboratories of the
Imperial Institute. From 1904 to 1909 five expeditions in all were
made to Northern Nigeria, in the course of which every province of
the Protectorate was visited in turn. On the first two expeditions
I was accompanied by Mr. Douglas R. Home, F.R.G.S., and on the
remaining three by Mr. Arthur Longbottom, B.A., F.G.S. On an
average seven months of each year were devoted to the actual field
work, but the great extent of territory to be investigated, coupled
with the difficulties of transport, rendered it impossible to do
more in the time available than make a series of cross traverses
through the various provinces with the object of ascertaining in
a general way
their geological structure and mineral resources.
In the following pages I have attempted to correlate the various
observations which were made in the course of our travels and to
weave a more or less consecutive story out of the disconnected
contents of our notebooks. I am myself, however, only too conscious
that many of my deductions may have been drawn from insufficient
data and that further knowledge may overturn many of my hypotheses.
In a new land someone must do the pioneering, and I shall be amply
satisfied if my work affords some foundation, however insecure, for
the investigations of future workers.
It is with the deepest regret that I have to record the death of
my friend and former colleague, Mr. Arthur Longbottom, since this
work went to press. He died of dysentery, after much suffering, in
November, 1910, at Naraguta, on the Bauchi plateau in Northern
Nigeria, whither he had returned on a tour of inspection on behalf
of a private syndicate. We had hoped that he would be able to
continue the work which we had begun together and that he would
return to England with a fresh store of observations which would
increase our scanty knowledge of the geology and geography of the
Protectorate. His extensive experience in various parts of Northern
Nigeria would have rendered those observations of the greatest
value, but his untimely death has put an end not only to a most
promising career but to all hope of any further immediate advance
in our knowledge of
the structure of this outlying portion of the Empire.
Before he left England Mr. Longbottom contributed all the
paragraphs and sections throughout the book marked with his
initials (A. L.). He also supplied the photographs from which have
been prepared Plates I, II, VI, VII, VIII, Fig. 1, and XVI. For
Plate VIII, Fig. 2, I am indebted to the Public Works Department of
Northern Nigeria. Mr. Woods has kindly contributed the Appendix on
the Cretaceous Mollusca and the note upon the Eocene fossils on p.
165.
I gratefully acknowledge a Grant towards the cost of publication
of this work from the Carnegie Trust for the Universities of
Scotland. I am especially indebted to Sir Archibald Geikie for help
and encouragement in the matter of publication. My acknowledgments
are due also to various friends for assistance and advice, and more
particularly to Professor James Geikie, Professor Wyndham R.
Dunstan, Professor Knight, Dr. Horne, and Dr. Flett.
Coloured Geological and
Topographical Map and Sections
At end of Volume
THE GEOLOGY AND
GEOGRAPHY
OF NORTHERN NIGERIA
NORTHERN NIGERIA
CHAPTER I — Hydrographical Systems. ( Map. p. 4 ). — The Niger. — The Benue. — The Niger Provinces.—Kabba, Ilorin, Borgu, Kontagora, Nupe. — The Benue Provinces. Bassa, Nassarawa, Muri, and Yola. — The Central Provinces.—Zaria and Bauchi. — The Northern Provinces.—Bornu, Kano, and Sokoto.
PHYSICAL GEOGRAPHY
Introduction. Hydrographical systems; primary
watersheds. The Niger: Lokoja and Mount Patti; Jebba and the Juju
Rock; the rapids of the Niger. The Benue: the great plain of Muri;
sandbanks. The Niger Provinces: the tableland of central Kabba;
domes and inselberge; the plain of Ilorin; Borgu; Kontagora;
typical scenery of crystalline and sedimentary regions; the plains
of Nupe; the sandstone plateaux of the Niger. The Benue Provinces:
Bassa and Nassarawa; the river belt of sandstones; the Karshi hills
and the Abuja Rock; the central plain; the northern tableland; the
plains of Muri; volcanic hills of the middle Benue; the Wase Rock;
the Murchison hills; the plateau of southern Yola; the lower
Gongola; volcanic craters and granite inselberge. The Central
Provinces: Zungeru and the gorge of the Kaduna; the plains and
hills of Zaria; the Bauchi plateau; the Ningi hills; Angass and
Kanna; the eastern plains; the Gongola valley; the cliffs of
Tongolan; the Kudu valley; the Gateri hills; Tangale and Waja; The
Peak. The Northern Provinces: southern Bornu; the Bima hills; the
Barbur volcanic plateau; the Tagum hills; eastern Bornu; firki land
and blown sand; Lake Chad; the central plains of Bornu; the Kerri
Kerri plateau; intersecting trench-like valleys; the plains of
Katagum; Kano; Mounts Dala and Kogon Dutsi; Kazaure; Katsina;
Sokoto; the Gulbin Kebbi; dallols and dissected sandstone
plateaux.
The Protectorate of
Northern Nigeria comprises an area of about 255,700 square miles,
of which the greater part is enclosed within the obtuse angle
formed by the Niger with its great tributary the Benue.
The north-eastern
corner is occupied by Lake Chad, whence irregular lines forming the
northern and eastern boundaries between the Protectorate, the
French Sudan and the Kameruns run west and south respectively to
the Niger and the Benue. Within the oblique figure thus formed lie
the provinces of Sokoto, Kano, Bornu, Bauchi, Zaria, Kontagora,
Nupe, Nassarawa and portions of Muri and Yola. West of the Niger
lie Borgu, Ilorin and Kabba, which adjoin Dahomey and Southern
Nigeria, while south of the Benue lie Bassa and parts of Muri and
Yola, marching with Southern Nigeria and the Kameruns. The
administrative headquarters of the Protectorate are at present
fixed at Zungeru on the Kaduna in western Zaria, while the
subordinate headquarters of the fourteen provinces enumerated above
are located respectively at Sokoto, Kano, Maidugari, Bauchi, Zaria,
Kontagora, Bida, Keffi, Ibi, Yola, Kiama, Ilorin, Lokoja, and
Dekina. Lokoja also, in addition to being the headquarters of Kabba
province, is from its situation at the confluence of the Niger and
the Benue the natural emporium of European commerce.
Many of the more striking physical features of the Protectorate
are now comparatively well known. The Niger with its flat-topped
sandstone hills, the Benue and the plains of Muri, the peaks of the
Murchison Hills, Lake Chad, and the sands and swamps of Bornu, have
become familiar from the writings of both early and recent
explorers.[1] Equally striking, however, are the broad, flat-bottomed valleys
of the north-west, in which diminutive streams now represent the
mighty rivers of old; the rolling sandy plains of Hausaland[2]
with their sluggish rivers, their isolated granite domes and groups
of rounded hills; and the mysterious Bauchi plateau, the home of
innumerable pagan tribes, whose precipitous walls on the south and
west long marked the limit between the known and the unknown.
The Bauchi plateau, which rises to a height of 4,000 to 4,500
feet above the sea, is the hydrographical centre of the
Protectorate. From it the rivers radiate north, south, east, and
west; but while at first they flow in such diverse directions, they
become ultimately included within one or other of the two great
hydrographical systems, the Niger-Benue system and the Chad or
Inland system. All the rivers leaving the plateau, with the
exception of those flowing directly north, belong to the
Niger-Benue system. The “primary watershed,” or dividing line
between the two great hydrographical systems, runs from the
neighbourhood of Daura on the northern frontier of the Protectorate
south-westward between Katsina and Kano to the twelfth parallel.
Thence it runs south-eastward to the latitude of Zaria, and then
southward to the neighbourhood of Bukuru, on the summit of the
plateau. From Bukuru it strikes approximately north-eastward to
Kerri Kerri, and thence
south-eastward to the Yola border in the neighbourhood of Mubi.
Within the Chad system the only important river is the Yo, of which
the rivers Hadeija, Katagum, Delimi, and Shidya form the
headwaters. The Yedseram, the only other large river of Bornu,
forms for some distance the boundary line between Nigeria and the
northern territories of the Kameruns. In the west the primary
watershed of the Niger system coincides approximately with the
boundary of the Protectorate from the eleventh parallel to the
neighbourhood of Aiedi on the eighth parallel, whence it runs
eastward to Kabba, and thence southward through the Egbira and
Kukuruku hills into Southern Nigeria. South of the Benue, the
primary watershed runs through Southern Nigeria and the Kameruns
entirely beyond the southern limit of the Protectorate.
The Niger enters the Protectorate at Ilo in the north-west, and
flows within the Protectorate for a distance of four hundred and
fifty miles to Idah on the Southern Nigerian frontier. Its breadth
varies in places from a few hundred yards in the dry season to
several miles during the rains. The annual variation in depth may
reach at Lokoja as much as thirty-five feet, and when in flood the
river is navigable for small ocean-going steamers as far as Baro,
the terminus of the Baro-Kano railway, and for light stern-wheelers
as far as Bajibo, twenty miles north of Jebba. Its largest
tributaries within the Protectorate join the Niger on the left, those on the right being
comparatively short and unimportant. On the left are the Kebbi,
Malenda, Kontagora, Kaduna, Bako, Gurara, and Benue; on the right
the Wessa, Oli, Teshi, Awo, Oshin, Oyi, Oro, Kampi, Mimi, and
Meadowbank. Of these the Gurara is navigable in flood for small
stern-wheelers for a distance of thirty miles from the Niger, the
Kaduna as far as Wushishi at the foot of the rapids, and the Kebbi
as far as the confluence of the Jega river.
NORTHERN NIGERIA
Primary Watersheds
and
River Systems.
MAP No. 1.
London. Macmillan & Co., Ltd.
Face p. 4.
The alternation of narrow gorge and open plain is a striking
feature of the Niger valley within the Protectorate. Where the
channel is narrow and bounded by steep cliffs on either side the
seasonal variation in level gives rise to comparatively little
change in the aspect of the river. The blackened rocks in the river
bed and the sandy stretches on either shore may be lost to view,
but the flood is still confined within its walls of rock. It is
where a plain tract intervenes in the course of the river that the
contrast between the Niger in flood and the Niger at low water
becomes particularly striking. The sandy bars and shallows which in
the dry season present such obstacles to navigation, as well as the
grassy banks and islands of alluvium with their terraced slopes of
sand and clay, are buried during the rains under a flood, which may
spread for miles over the plain on either side of the river.
At Idah the Niger narrows and flows swiftly between low cliffs
of white sandstone which pass northward into stretches of wooded
rising ground on either
bank. Ten miles above Idah a group of flat-topped hills approach
the right bank of the river, only to turn off abruptly inland in a
line of buttresses six to seven hundred feet in height. Farther on,
the low, flattened ridges on the left bank run out in a line of
rounded hills: and beyond, the river broadens and a broken plain
stretches inland to distant groups of lofty hills. Nearing Itobe
Mount Purdy stands out, a solitary cone, in advance of the
level-crested plateau beyond. From Itobe to Igbo the Niger flows
through the most picturesque portion of its lower course.[3] The
hills rapidly close in, the channel becomes narrow and rocky, lofty
granite domes and rounded hills separated by narrow gloomy valleys
rise up on either bank, and on the left the level crest of the
plateau looms behind on the sky-line (Plate I, Fig.
1). Below Lokoja the river opens out, the hills die down on
either side, and from the left bank a broken hummocky plain
stretches inland to the base of the Bassa hills, while at Lokoja
itself flat-topped Patti (Plate I, Fig. 2), with
its projecting buttresses and the red roofs of the cantonment at
its base, stands sentinel over the confluence of the Niger and the
Benue (Plate II).
An extensive alluvial plain annually flooded during the rains
stretches away to the Benue from the left bank of the Niger for
long miles above Lokoja. The right bank is occupied more or less
continuously by the worn escarpment of the plateau of which Mount
Patti forms the southernmost extremity. Above Koton Karifi the hills reappear on the left
bank, and for more than twenty miles the Niger flows between the
broken walls of what has once been a magnificent river gorge. In
places for considerable distances the flat-topped hills still rise
up with precipitous slopes on either side in all their ancient
grandeur. More frequently, however, the escarpments are rounded off
and much eroded by lateral valleys, while at times they recede to a
distance, and leave between themselves and the river a more or less
extensive swampy plain, with here and there a flat-topped remnant
of the once continuous plateau. Before Baro is reached, however,
the plateau has disappeared on the right bank, and on the left is
represented only by more or less isolated flat-topped fragments.
The bend of the Niger at Egga and Kacha marks the eastward limit of
an extensive alluvial flood plain, which includes the lower courses
of the Kampi and the Bako, and stretches westward on the north bank
as far as the Kaduna. On the south bank the pinnacle peaks of
Egbom[4] tower high above the river, while a flattened
ridge extends westward as far as Pategi. Beyond the Kaduna the
Niger flows through a broad and shallow valley with extensive
swampy plains on either hand, and only at Shonga on the right bank
and at Rabba on the left does the river approach the rising ground
which elsewhere bounds the valley in the distance.
At Jebba the hills close in on either side, and the Niger issues
by a double channel from a picturesque gorge which extends
northward as a deeply cut and rocky valley as far as Bussa, a distance of
seventy-five miles. For eight hundred yards above Jebba the gorge
is narrow and bounded by vertical walls of quartzite. The Juju
Rock[5] and the other islands which interrupt the
channel mount steeply from the river, the rapid current pours over
sunken rocks which rise abruptly from the river bed, and mighty
whirlpools bear witness to the depth of the water. Beyond Jebba the
quartzite gives place to granite, the vertical walls become
rounded, wooded slopes, the river broadens its rocky bed and flows
in a narrow valley between two belts of broken hilly ground which
lead up to the plains of Kontagora and Borgu, five hundred feet
above the level of the river. In spite of the obstructions in the
channel the river is navigable with caution for small
stern-wheelers as far as Bajibo. Above Bajibo, however, navigation
becomes precarious and dangerous even for native canoes. The
current becomes more violent; rocks, cascades, and whirlpools more
numerous, and small rapids more and more frequent, until above
Leaba the Niger flows in a rocky gorge which leads to the base of
the Great Rapid of Wuru.[6] Here the river is formed of
two branches with a rocky island between. The fall of forty feet
between the summit and foot of the rapid is distributed over some
thirteen hundred yards of river, giving a slope of at least 1 in
100 and a current of from fifteen to eighteen miles an hour. In the
rains the granite boulders are completely submerged
under a seething flood,
and even in the dry season the water is thrown up by the projecting
rocks in sheets of foam, and the roar of the rapid is heard for
long distances on either side.
A stretch of two and a half miles of comparatively smooth water,
broken only by some small rapids and cascades, separates the Great
Rapid of Wuru from the rapid of Patassi, which is composed of two
sets of cascades, each twelve to fifteen feet in height and
separated by a straight reach of some two hundred and eighty yards.
Twenty miles farther on, the third important set of rapids is
reached, the rapids of Garafiri, whose cascades and whirlpools
occupy a stretch of about three miles between the villages of
Garafiri and Malali. At Bussa, the scene of Park’s disaster, the
channel is still much obstructed by projecting rocks and islands,
and the small cascades and the rapid current still render
navigation both difficult and dangerous. The right bank of the
river is here formed of low, wooded rising ground, while on the
left ranges of low hills rise in series one behind the other. Soon,
however, the hills die down and the rocks disappear from the river
bed, and at Warra the river is broad and smooth with an extensive
flood plain on either side. From Warra to Ineku the Niger is formed
of two branches, of which the more westerly preserves the calm
aspect of the river at Warra, while the more easterly becomes again
obstructed by rocks and rapids. Between Ineku and Yelwa the river
is bounded by low, rounded hills covered with scanty forest,
innumerable islands rise from the river bed supporting an abundant vegetation, while
several sets of lesser rapids obstruct the channel. At Yelwa the
banks become low and swampy, the hills begin to recede from the
river, and five miles farther on, near the village of Sakassi, the
last set of minor rapids occurs in the channel. Beyond Sakassi the
whole aspect of the river changes, the channel becomes broad and
sandy and obstructed by nothing more serious than sand bars. On the
north bank a broad, swampy flood plain, with low rising ground in
the distance, bounds the river as far as the mouth of the Kebbi,
and from the Kebbi to the swamps of Ilo an open sandy plain
stretches northward between the river and the base of the plateau.
On the south bank the gently undulating plain which bounds the
river stretches westward to the Dahomey border, broken only by some
scattered groups of flat-topped hills.
The Benue enters the Protectorate to the east of Yola, and after
a course of nearly five hundred miles joins the Niger at Lokoja.
Its principal tributaries are on the right bank the Gongola, Kudu,
Wase, Simanka, Ankwe, Modu and Okwa; on the left bank the Faro,
Ini, Mainarawa, Tarabba, Donga and Katsena. The Benue is navigable
when in flood throughout its whole course for steamers of light
draught. The Katsena is navigable as far as Katsena Allah, the Gongola as far as
Nafada, and the Donga and Tarabba for considerable distances up
stream.
In the matter of scenery the Benue presents a striking contrast
to the Niger. There is on the Benue no picturesque alternation of
narrow gorge and open plain; no rapids trouble its surface from
Lokoja to the flat-topped hills of Yola, and even in the dry season
it is only at long intervals that rocks are exposed in its sandy
bed or in its steep banks of alluvium. The precipitous front of Mt.
Patti and the level crest of the plateau on the right bank of the
Niger are visible from the Benue for twenty miles above Lokoja,
across the great alluvial plain which marks the confluence of the
two mighty rivers. Above Mozum a range of flat-topped hills with
occasional peaks follows the right bank for some distance, but soon
turns inland and leaves a broken plain between the river and the
base of the hills. A similar range, also separated from the river
by a broken plain, runs parallel with the left bank a few miles
inland. The escarpments of these ranges on either side of the river
represent the walls of a broad flat-bottomed valley which is
comparable in origin with the steep-sided gorge of the Niger above
Lokoja. Here on the Benue, however, erosion has been more intense,
and the hills have been cut farther back on either hand.
Above Umaisha a tract of broken country stretches eastward on
the north bank and gives place in the neighbourhood of Loko to
alluvial swamps in the foreground, with undulating sandy plains
beyond. On the left bank, meanwhile, the hills approach the river
at Amara and pass eastward into a hummocky plain, which is bounded
to the south by a distant range of flat-topped hills. Above Loko
the Doma Hills touch the river at Udeni on the right bank, but turn
off abruptly inland with their low, buttressed slopes overlooking
an extensive swampy plain which stretches eastward as far as
Akwaneja. Beyond Akwaneja the right bank assumes a similar aspect
to that of the left. The river is bounded by low, well-wooded
banks, with here and there a projecting mass of sandstone, while a
hummocky plain stretches inland on either side to distant rising
ground. The volcanic tract above Abinsi introduces an agreeable
change into the monotony of the scenery. The worn and weathered
cones and banks of lava stand out picturesquely from the wooded
plains, and on the river-side the black basalt and the yellow sand
offer a pleasing contrast to the eye. Soon, however, the low banks
resume their familiar aspect and a featureless plain spreads out on
either hand. Nearing Ibi the right bank becomes low and swampy, and
the great plain of Muri opens out beyond. From Ibi to Lau the
channel is broad and sandy, and the river winds about amongst
everchanging sand-bars and banks of alluvium. In the rains broad
stretches are flooded on either side, and for long distances on the
right bank the plain beyond becomes an almost impassable swamp,
while on the left long tongues of marshland run up the tributary
valleys. Far to the north the peaks of the Murchison Hills[7]
tower high above the plain, while to the south the rock of Bantaji
guards the swamps of the Donga.
Plate I
On the Niger above Igbo.
Lokoja and Mount Patti.
Farther east, and especially between Numan and
Yola, the river plain is traversed by a large number of
anastomosing channels, which in many cases are only connected with
the main stream during the rains, and in the dry season are cut off
by sand-bars into strings of shallow pools and marshes. Alluvial
mud covers the rocks for some miles inland, and while in the rains
travel on foot is impossible and the villages are isolated one from
another, in the dry season the ground shrinks and is broken
everywhere by wide cracks. This alluvial plain is perfectly level,
so that the small volcanic mounds, dotted here and there, which in
a more varied country would be overlooked, form conspicuous
landmarks. The most striking of these hillocks is Jeme Hill, below
Numan, which stands on the river bank and can be seen for twenty
miles around. (A. L.)
The Benue plain, which is so extensive in Muri province,
contracts near Lau, and thence eastward it is limited on both sides
by plateau areas which rise suddenly and steeply at distances of
fifteen miles from the river. These masses of hills with their
varying outlines introduce a pleasing change into the hitherto
monotonous river journey, and contrast sharply with the river
plain. Here the plain of the Benue is like a broad and shallow
trench, bounded by sharply rising walls, whose height is small
compared with the
distance between. At the same time the height is considerable in
itself, for the rise to the plateau is well over a thousand feet.
(A. L.)
These hills running eastward parallel to the river are breached
by the Gongola plain, about ten miles wide, entering from the
north, and by the plain of the Ma-Ini and Maio Belwa entering from
the south. Farther on the hills come in again. To the north they
are rather scattered and detached, with long tabular outlines, or,
as in the Bagele Hills, with terraced sides and abrupt scarps. On
the other side, though flat-topped hills do occur, the outlines are
usually more varied, while south of Yola rises the typical plateau
of the Vere Hills—a solid mass fronting boldly outwards upon the
surrounding plains with continuous rocky faces indented by a few
deeply-cut and narrow valleys. Meanwhile the plain of the Benue
with its flat and marshy character continues eastward beyond Yola
and the Faro river into the German Kameruns. (A. L.)
Along the whole course of the Benue from Lokoja to Yola
exposures of the underlying rocks are very infrequent, and this
remark applies equally to the lower courses of the tributaries. The
main trunks of the Benue system have long passed the stage at which
they effected any appreciable erosion, and now serve only as the
channels along which detrital material brought down by the
head-waters is transported to the Niger. The finer portion of the
material is carried in suspension, and is responsible for the
covering of alluvial mud found over those parts of the river plain which are inundated
during the annual rise. The coarser portion is rolled along the
bottom and gives rise to the sandbanks. These are covered in flood,
but for the other part of the year they are a constant feature of
the river scenery. The stream, much lessened in volume, swings
first to one side and then to the other of the wide river bed, with
the broad sandbanks occupying the bends. (A. L.)
The sand is rather coarse, principally of clean grains of quartz
and felspar, but at times with pebbles of vein quartz or
occasionally of polished limonite. Typically, a spread of sand,
many acres in extent and with its level surface only a few feet
above the water, ends on the down-stream side in a slope which
plunges into deep water at perhaps 40°. While the river is falling,
the formation of these sandbanks can be well studied. The water,
covering the sandbank by only a few inches, is seen to sweep grains
of sand or occasional pebbles along over the level surface till
they reach the end of the bank and roll down into deep water, thus
helping to form the next lamina. In this way the sandbank is built
up of successive laminæ inclined down stream at a steep angle to
the horizontal, and wherever a bank is cut into by a change in the
direction of the current the inclined lamination is beautifully
displayed. The lamination is of course only a particular instance
of the “current bedding” produced whenever detritus in process of
deposition is subjected to currents of air or water, but it is
important to understand that the oblique lamination of the
sandbanks is brought about by the river currents, and not by the wind as has been
supposed. Any slight rearrangement of the surface layers by wind is
obliterated by the next rise of the river. (A. L.)
The orographical structure of Kabba and Ilorin is somewhat
complex. The primary watershed of the Niger basin which runs from
Kabba to Aiedi and thence westward a few miles south of the Ilorin
frontier, is not defined by any range of hills or mountains, but by
the crest line of a lofty plain which, like a low, extended arch,
slopes gently northward towards the Niger and southward towards the
sea. In central Kabba the high plain assumes, to some extent, the
character of a tableland, which is bounded to the north-east by the
Lukke hills and the broken ground beyond the Mimi valley. To the
east and south-east the tableland is abruptly limited by the worn
escarpments of the Shokko Shokko, Agwede and Egbira hills, at whose
base, 500 feet below, are spread out the broad, well-watered and
well-wooded plains which stretch away eastward to the Niger, and of
which passing glimpses are caught from the river below Itobe. To
the south these undulating plains slope gently upward and inward
from the Niger and lose themselves in the narrow valleys of the
Kukuruku Hills.
Although such in brief is the orographical character of the major portion of Kabba
and Ilorin, the actual conditions of the surface and the peculiar
character of the scenery to some extent obscure the issue. In
central Kabba the undulating surface of the tableland is in places
densely wooded, in places covered with thin bush, in places open,
grassy, and park-like with cultivation near the towns and villages,
while above the plain at frequent intervals rise isolated rounded
hills or groups of hills, or even short rocky ranges, which form
secondary watersheds between the minor streams. Similarly, on the
lower plains adjoining the Niger, the attention is arrested by the
dome-shaped island mountains and picturesque clusters of rounded
hills which rise abruptly into view. The full significance of this
peculiar type of scenery can only be grasped when gazing from the
summit of one of the isolated hills themselves, or from some
favourable spot on the crest of the escarpment which separates the
higher from the lower plains. Then all the minor irregularities
disappear, and the level, wooded surface of the plain can be seen
stretching up to and around and between the bases of the hills
which rise like rocky islands from a verdant sea. The hills may be
covered to the summit with angular débris and scanty
forest, or the lower slopes may be littered with exfoliation
fragments, amongst which a few trees find a precarious footing,
while the summit may be smooth and bare, with a few large boulders
perched picturesquely in apparently impossible positions. The
contours of the bare and treeless summits are frequently
particularly graceful with beautiful domes and cupolas rising from gently-rounded slopes
of granite or gneiss. In many cases a precipitous path leads to the
village which caps the summit, while for long acres round the base
the plain is cleared and cultivated. (Plate
XVII.)[8]
The northern part of Kabba is more rugged, the population is
more scanty and forest more abundant. The rocky valley of the Mimi
is bounded to the north by the broken, hilly ground between Jakura
and Akpara, which passes to the east and north under the
flat-topped sandstone hills which fringe the Niger. The Jakpara
Hills with their conical peaks upon rounded bases are landmarks for
miles around. The white quartzite hills in the neighbourhood of
Lukke alternate with belts of soft schists which support a
luxuriant forest growth. Farther west by Moppa, Sanlu and Takete
detached groups of granite hills rise upon an undulating plain. The
thicker vegetation, however, obstructs the view, and it is only
when the landscape is surveyed from the summit of one of the higher
hills that the structure is seen to be very similar to that of
southern Kabba. To the west, Ogga and Eri are set within a cluster
of rocky hills, while to the north there is a comparatively rapid
fall to the plains of the Kampi. Between the Kampi and the Niger a
steep-sided range of sandstone hills capped by the pinnacle peaks
of Egbom (p. 7) runs out westward in a
succession of rounded ridges two hundred feet above the level of the river, and not
until near Share is the river belt of sandstones again bounded by a
low escarpment. The Shappa and Orissa Hills form the only prominent
range in central Ilorin. The Shappa Hills are of granite, and many
of the peaks are gracefully rounded and most picturesque, while the
quartzites and gneisses of the Orissa hills give more rugged and
irregular contours. West of the Orissa hills lies the great plain
of Ilorin, thickly populated and well cultivated, in broad
undulations trending generally north and south, with scattered
inselberge and many kopjes of granite boulders, but with no
conspicuous ranges or groups of hills. The Jebba hills, which
extend southward for some distance towards Ilorin, pass westward
into an extensive tract of hilly country, high lying, little known
and largely uninhabited, which forms a natural boundary between
Ilorin and Borgu.
The primary watershed of the Niger basin follows the frontier of
the Protectorate more or less closely from the eighth parallel to
the south of Ilorin as far as Okuta in the west of Borgu, whence it
runs northward to the latitude of Nikki before turning off westward
into Dahomey. With the watershed the high plains, along whose crest
it runs, also swing round to the west of Borgu and fall off gently
towards the Niger on the right, while along the river itself from
Bussa to Jebba a narrow belt of broken country marks the transition
from the high plains of Borgu to the gorge-like valley of the
Niger.
Southern Borgu is one of the least inviting
districts in the
Protectorate. The rocks are concealed almost everywhere by recent
alluvium or by surface ironstone,[9] which in places forms
extensive platforms entirely bare of vegetation. Where rocks do
emerge from the superficial accumulations they seldom form hills.
The granite hills around Kiama are an exception to this rule, and
along the western frontier a number of granite domes rise from two
to three hundred feet high. These domes, however, are separated by
wide stretches of country over which all geological processes seem
to have come to a standstill. The drainage system is very poorly
defined, and for the most part the water is carried off, not by
stream courses with sand and boulders, but along shallow,
soil-covered depressions in the plain. Such streams as there are
dry up soon after the end of the rains, and the natives then resort
largely to wells dug in the surface soil. The crests of the gentle
undulations follow one another on the sky-line in endless
succession, and the journey becomes monotonous in the extreme. The
northern part of the province is diversified by a number of
sandstone hills, and some swift, clear streams, fed from springs at
the base of the hills, afford a welcome change to the jaded
traveller. (A. L.)
In the provinces which lie to the east of the Niger the intimate
relation which exists between the character of the scenery and the
nature of the rocks becomes increasingly evident. In Kabba, Ilorin,
and Borgu, where crystalline rocks of a granitic or gneissic
character cover by far the larger part of the surface, the prevailing and most
characteristic type of scenery is that of rolling sandy plains with
isolated hills or groups of hills rising abruptly above the general
level. On the other hand, where sedimentary rocks, such as
sandstones and ironstones, cover the surface, as in the river belt
of sandstones in Kabba and Ilorin, the characteristic type of
scenery is that of steep-sided and flat-topped ranges and plateaux,
with detached tabular and conical hills rising from the lower
plains. Over the greater part of Kontagora province, to the north
of the parallel of Kontagora itself, granites and gneisses form the
surface rock. In the west, near N’gashki, a group of granite hills
rises abruptly from the plain which stretches away to the east,
with its surface diversified in typical fashion by occasional
inselberge. A belt of slaty rocks much pierced by quartz veins runs
along the left bank of the Niger from N’gashki to Yelwa, and gives
rise to a stretch of broken country adjoining the river. The
granites and quartzites in the lower valley of the Malenda river
also give rise to much rough and stony country, which culminates in
the quartzite hills of Maibirro. Beyond Jindani and Yelwa the plain
reappears in a more typical character, and stretches northward as a
gently undulating surface covered with deep, sandy drift, from
which emerge at long intervals low, rounded, or rocky hummocks of
granite or gneiss.
Plate II
The Confluence of the Niger and the Benue.
It is in the east of the province, however,
between Banaga and Kotonkoro, that the landscape assumes the
typical aspect of the high plains of Hausaland. In front, behind,
and for long miles on either hand, the open, sandy plain stretches away to the
horizon, with here and there a low turtleback or heap of granite
boulders, and at long intervals a loftier dome or a group of
rounded granite hills. The rivers flow in almost imperceptible
valleys in broad, sandy channels, or over smooth and bare surfaces
of rock, level with the plain, while the low watersheds between the
rivers are crossed almost unconsciously by the traveller.
The country around Kotonkoro and to the south-west is extremely
picturesque. Numerous domes of bare rock rise from a grassy
park-like country. The plain around one of these domes passes
imperceptibly into a gentle slope of sandy wash, which leads the
eye up to the spring of the dome. As the path winds along, first
one and then another of these domes engages the attention. On
approaching Mahorro two of the highest hills have flattened summits
and are found to be capped by about twenty feet of stratified
ironstone. To the north-east of Kontagora these outliers are more
numerous, though seldom of a sufficient size to mark on the map.
The country is in consequence very rough and broken and the track
becomes difficult and stony. The poor agricultural prospects,
however, are compensated for by good iron ore, and the crude native
furnaces soon come to be a familiar sight. When these outliers are
finally left behind at Libo, a belt of very rough broken country
has to be crossed before descending to the Kara river. (A. L.)
The town of Kontagora is set in an open undulating sandy plain,
deeply covered with surface drift, which stretches eastward to Moriga and the Kara
river, with its surface diversified by a few isolated sandstone
hills near Kontagora and a group of granite hills at Beri. To the
south of Kontagora begins the sedimentary series which stretches
southward into Nupe province and westward to within a few miles of
the Niger. The change in the nature of the surface rock is
accompanied by corresponding changes in the character of the
scenery. Here at first are the same rolling sandy plains, if
possible more deeply covered with drift than before; but here are
no turtlebacks or lofty domes of granite forming conspicuous
features in the landscape, no rivers flowing over smooth surfaces
of rock in valleys so shallow that they are almost
indistinguishable from the plain. In place of these are low,
flat-topped sandstone hills rising singly or in groups above the
general level, and deep and narrow canyon-like valleys cut down, it
may be, fifty or a hundred feet below the surface of the plain. A
typical group of hills with their level summits of hard ferruginous
grit and their slopes covered with rounded boulders of the same
occurs at Madara, a few miles south of Kontagora, while at Kagara,
Kaboji, and Sonko the streams run in typical canyon valleys, whose
presence is unsuspected upon the plain until the chasm opens
abruptly at one’s feet. The larger rivers flow in broad,
trench-like valleys, with their steep and wooded slopes covered
with rounded pebbles and boulders of ironstone and grit. To the
west the plains pass into the belt of hilly ground which leads to
the Niger valley, and which in its eastern portion shows the characteristic
tabular outlines of the sedimentary series, and in the west,
adjoining the river, the rugged irregular contours of broken
crystalline hills.
The province of Nupe is somewhat unequally divided into two
parts by the Kaduna river. With the exception of the quartzite and
granite hills of Jebba and Kailema and the crystalline rocks of the
eastern corner of the province between Badegi and the Gurara river,
the entire surface is covered by sandstones, which are continuous
with the sedimentary series of southern Kontagora. West of the
Kaduna, however, the high plains become much dissected and broken
up into extended tabular ridges and irregular plateau-like masses,
with the tracks leading over a succession of low, well-wooded hills
and shallow, flat-bottomed valleys bounded by rough and stony
slopes. On either side of the Kaduna rise isolated tabular hills
and short ranges separated by long stretches of open plain, while
to the east of Bida there open out the great rolling sandy plains
of Nupe, which extend from the Bako river to Badegi and southward
to the bend of the Niger. Beyond Kacha the character of the
landscape again changes: scattered hills and short ranges diversify
the surface and coalesce southward into the extended flat-topped
ranges and plateaux, whose wooded slopes dip steeply down to the
narrow ledge of crystalline rock which fringes the Niger. In the
north-eastern angle of the province beyond Badegi the sandstones
give place imperceptibly to granites and gneiss. The sandy plains
of Nupe are continued eastward, but their drift-covered surface presents now the
characteristic features of a crystalline floor. Knobs and hummocks
and low ridges of gneiss project here and there above the general
level, while lofty groups of rounded granite hills, like those of
Gaung, rise abruptly and picturesquely from the plain.
The sandstone plateaux which bound the Niger to the north of the
Gurara river are continued southward as far as Koton Karifi, and
pass eastward into Nassarawa as a dissected system of flat-topped
and conical hills. To the south of the confluence of the Niger and
the Benue, the scenery on the left bank of the Niger in Bassa
province, as already described (p. 6) is that
of a hummocky plain with groups of rounded hills in the foreground,
backed and capped by lofty plateaux and conical hills. Such a type
of scenery is readily explicable as that of a dissected sedimentary
series resting upon an irregular crystalline floor of granite and
gneiss.
With the exception of the narrow strip of crystalline rocks
exposed along the Niger, the province of Bassa, so far as known, is
entirely covered by the sedimentary series whose western and
northern escarpments are visible respectively from the Niger and
the Benue. These escarpments, however, which from a distance appear
continuous, are found on nearer view to be much incised by river
valleys which have been cut back for long distances into the high ground
behind. A system of lofty, flat-topped and conical hills, extended
ranges and plateau-like masses, separated by narrow, steep-sided
valleys, has thus been dissected in the north and west out of the
once continuous tableland. In the interior the relief lessens, and
the surface of the tableland becomes broken up into a succession of
tabular ranges and rounded ridges, separated by shallow valleys
well cleared for cultivation. The hills are usually thickly wooded
and the paths frequently rough and stony and covered with pebbles
and broken fragments of ironstone. In the south the Akpoto country
becomes in places particularly hilly and difficult, while the
little known Munchi country in the east is said to open out into
undulating park-like plains beyond the belt of forest which fringes
the river.
For purposes of description the province of Nassarawa may be
conveniently divided into three portions, the river belt of
sandstones, the central crystalline plains and hills, and the
northern tableland. The Gurara river forms the greater part of the
western and northern boundary and drains all but the eastern
portion of the elevated plains in the north. The river belt of
sandstones between the Gurara and the Okwa in the west presents the
characteristic and familiar scenery of a dissected sandstone
tableland. Beyond the Okwa river the flat-topped hills and ranges
die down into undulating sandy plains which stretch eastward as far
as the Modu river, while the detached group of hills, which strikes
inland from Udeni, is the last prominent tabular range on the lower
Benue. Farther east,
the low-lying plains stretch into the unexplored land of the
Munchis, while to the north the trade route from Loko to Ibi leads
over well wooded undulating plains deeply covered with sandy drift.
The solid rock is frequently exposed in the beds of the larger
rivers and streams which have cut their channels through the
superficial accumulations; and here and there, as at Doma and
Lafia, groups of low rounded sandstone hills and projecting
hummocks of grit diversify the surface of the plain.
Central Nassarawa is, in the west, particularly broken and
rugged. Around Gwombe and Gwagwada and as far east as Buga a mass
of low rounded hills has been carved out of a series of soft
micaceous schists. The rocks are much decomposed on the surface
into red gritty earth which supports an abundant vegetation and in
which the rivers and streams have cut deep and narrow channels. To
the north these rounded hills pass into a tract of exceedingly
difficult and broken country where precipitous hills of granite and
gneiss, separated by narrow rocky valleys, rise to a height of two
thousand feet above the sea. This mountainous tract, of which the
Karshi hills are the northern prolongation, forms the secondary
watershed between the river systems of the Gurara and the Okwa.
Beyond the Karshi hills a lofty and rapidly undulating plain, in
places rough and stony, and much diversified by inselberge (Plate
IV, Fig. 1) and short ranges of granite hills,
stretches north-westward towards Abuja. The Abuja Rock, a typical
granite mount, streaked and channelled by the summer rains, is a
far-famed local
landmark and an object of superstitious veneration to the
inhabitants of the surrounding country. (Plate III, Figs. 1 and 2.)
Eastward from Buga to Nassarawa extends a broken plain with
hummocks of gneiss and detached groups of granite hills. At Kurudu,
the Okwa river flows through a narrow picturesque gorge between two
rounded granite hills, with the bare rock in the river bed
honeycombed with potholes. Nassarawa itself is set in the midst of
gently undulating park-like plains, well wooded and well
cultivated, which extend northward as far as Keffi and are
continuous southward with the sandy plains of the river belt. To
the east they are limited by the rocky mass of the Anagoda hills
and the broken hilly ground between Keffi and the Modu river. To
the north of Keffi there opens out the great central plain of
Nassarawa which extends from the neighbourhood of Abuja in the west
to the Mada country in the east, and is limited northward by the
worn escarpment of the northern tableland. Turtlebacks and kopjes
of granite and broken hummocks of gneiss rise at intervals from the
undulating surface of the plain, while the minor irregularities are
obscured, as usual, by deep sandy drift. The larger rivers flow in
broad sandy beds with long stretches of swampland on either hand,
or, where the drift is thinner, in narrow rocky channels with belts
of broken plain on either side from which the surface drift has
been in large part removed.
Plate III
Abuja Rock.
Abuja Rock from the Walls of Abuja.
Viewed from the central plain, the margin of the northern
tableland is in places particularly picturesque. A well-marked, though much worn
and rounded escarpment (Plate IV, Fig. 2),
extends continuously from the neighbourhood of Kari in the west by
way of Kukwi, Tatara, and Jaginde, to Darroro and Assab in the
east, and the skyline is variously ornamented by domes, cupolas,
conical peaks and perched blocks, with here and there, as beyond
Darroro, a jagged wall of granite rising perpendicularly from the
plain. The escarpment is steeper in the east than in the west, and
beyond Kari it is lost in the broken hilly ground around Abuja,
which falls in a succession of rocky steps to the plains of the
Gurara. The tableland itself slopes gently towards the Gurara, and
in the west, in the picturesque Gwari country, the numerous domes
and turtlebacks of granite afford ideal sites for pagan villages.
Parallel with the escarpment runs a narrow belt of broken country
which represents the worn edge of the tableland, while to the north
the undulating park-like plains, diversified by inselberge and
groups of rounded hills, wrapped closely round with sandy drift,
recall in every detail of scenery the central plain of Nassarawa
below. A belt of broken rising ground, stretching northward from
Jaginde, separates the Gurara system from the Kogin Kogom which
drains the grassy plains of Morrua and issues behind Jaginde from a
narrow rocky valley cut backward for a considerable distance into
the plateau. The flat-topped granite massif of the Kagoro hills,
whose precipitous wall forms the continuation of the escarpment
from Darroro to Assab, projects also on the north above the plateau
and overlooks the lofty undulating plains of Morrua and Akul which
stretch away northward towards the Kaduna. Far away to the south of
Darroro, the peaks of the Mada hills can be seen across a broken
wooded plain: but this eastern portion of the province is as yet
unknown and unexplored.
The great plain, which forms the major part of Muri province,
extends from the base of the Murchison hills on the north to the
frontier of the Protectorate on the south, and from the Ankwe river
in the west to the Wurkum and Shebshi hills in the east. With the
exception of the narrow margin of crystalline rock in the
north-west which runs parallel with the Murchison hills, and the
broader belt in the south between Takum and Bakundi, the plain
possesses throughout a floor of sedimentary rock. The flat-topped
hills, which are such a conspicuous feature of the scenery of the
lower Benue and the Niger, are here, however, entirely wanting. The
surface of the plain slopes gently towards the Benue from either
side, and into it the tributary rivers have cut a succession of
shallow swampy valleys separated by tracts of gentle rising ground,
which run approximately at right angles to the Benue. The
undulating plain thus produced is deeply covered with surface drift
and relieved only by an occasional hummock of sandstone or grit.
The thin forest, which usually covers the rising ground, gives
place in the neighbourhood of the rivers to extensive stretches of
open grass land, which, in the rains, become almost impassable
swamps.
West of the Ankwe river the surface of the plain is more frequently broken by ridges
of rock, and around Awe and on either side of the Benue to the
south a number of worn volcanic cones are scattered over the plain.
(Plate V, Fig. 1.) Similar volcanic hills
project above the surface on the northern margin of the plain
between Yelua and Wase. The Wase Rock[10]
itself, a pillar of trachyte, is a landmark for miles around.
(Plate V, Fig. 2.) At Namu, also, in the
north-west of the province, and at Bantaji on the Donga to the
south of the Benue, solitary hills of similar origin form striking
features in the landscape. Turtlebacks and kopjes of granite
boulders are found only on the margins of the plain where the floor
is crystalline. In the north-west the narrow belt of hummocky
crystalline ground, with its inselberge and groups of granite
hills, is limited by the domes and pinnacles of the little known
Murchison hills,[11] while in the south the undulating plains
which stretch along the frontier of the Protectorate from the
Katsena river to the Tarabba present all the characteristic
features of a crystalline floor. In the north-east of the province,
between Bashar and Jeb-jeb, the plains stretch northward up the
Kudu valley but are limited to the east by the Ligri, Wurkum, and
Jenoe hills, whose steep and rocky sandstone slopes become in
places quite rugged and picturesque. The Kwana and Mumuye hills,
which bound the plain to the south of the Benue, are largely
granitic and present the usual rounded contours, while the
foothills of the Shebshi range are of quartzite and gneiss and give
rise to more rugged and irregular outlines.
The Mumuye and Shebshi hills are portions of an extensive tract
of hilly country which stretches into German territory and which
occupies the southern part of Yola province.[12]
The western boundary of this tract runs almost northward, past
Jalingu and Kwana, as if to approach the Benue at Lau, but instead
turns sharply at right angles. Running eastward from near Kwana
there is a practically continuous wall of rock, and at Bamga a
steep and almost precipitous climb brings the traveller from Lau to
Yola on to the rim of the plateau area. From this position he looks
out over the Benue plain several hundred feet below. The river is
far away to the north, while beyond that again, just discernible on
a clear day, is the further wall of hills stretching continuously
from the Wurkum country to the Gongola. Turning south, he sees the
tumbled Shebshi hills as a faint silhouette in the distance, while
in the foreground, slightly below him, stretch sandy uplands with a
gentle eastward slope. The track drops down upon the uplands and
then proceeds just within the rim of the plateau towards Yola. For
the first day this rim is made up of a chain of rounded hills. Then
a number of sandstone outliers arrests the attention. (Plate VI,
Fig 1.) The sandstone is massive and breaks down
along vertical faces. The fallen blocks, in assuming their position
of rest, smooth off the irregularities of the granite base, and
from the truncated
cones thus produced, the cliff-sided outliers rise and dominate the
landscape, like great fortresses. From this point the plateau
descends gently to the plain of the Maio Belwa and Ma-ini rivers.
(A. L.)
Opposite Lau the hilly ground to the north has similar outlines
to those of the crystalline plateau to the south. Farther east,
behind Wuduku, its appearance changes and the hills are composed of
sandstone. Here the ridge seems to be a bold escarpment limiting
the unknown Yam-yam country in precipitous cliffs, but with a dip
slope to the south which plunges down below the marshy levels of
the Benue. Further eastward again there is another change and the
outlines of the hills become regular and flat. Where the plain of
the Gongola enters the Benue valley, they turn at right angles and
run northward parallel to the river. At the angle which separates
the two plains the mass is seen from a distance to be made up of
horizontal beds, with seven or eight terraces on its sides. These
hills, which run up to the west of Kombo, are still, however,
unentered and unexplored. (A. L.)
The plain of the lower Gongola is a repetition of the Benue
plain on a somewhat smaller scale. The floor of the trench is about
ten miles wide and the height of the walls perhaps a thousand feet.
On travelling up the Gongola, Kwaba hill is conspicuous two or
three days before it is actually reached. It is a massive hill of
granite with numerous satellites, of which some are of granite
while others are outliers of horizontal sandstones, bounded by
vertical cliffs. The successive beds are exposed in their sides
like string-courses
of masonry, the Gongola outliers differing in this respect from
those of southern Yola, which have been carved out of unbedded
sandstones. (A. L.)
The plateau of the Vere hills has been already mentioned
(p. 14). It is completely separated from the
Karin mountains, Alantika mountains, and other hilly ground to the
south by broad and level plains. The essential feature of the
scenery is the contrast between the plain and the plateau masses.
They rise up from one to two thousand feet higher than the plain,
with no transitional foothills or slopes. The base is concealed by
sandy wash, and at intervals streams, issuing from the plateau by
narrow ravines, trench down through the compacted wash to form
steep-sided nullahs. (A. L.)
Opposite to the Vere hills and across the Benue stand the Bagele
hills. A little further north is the Hossere Furo. Both groups are
of sandstone, rising up from the smooth grassy plain. The rise,
however, is not immediate, as it is in the Vere hills, but through
basal slopes and foothills toward the higher summits. North-eastern
Yola consists of a high-level plain, elevated considerably above
the plain of the Benue, but passing into it more gradually than do
the plateaux to the south. There is, however, one step of about 200
feet running eastward from Song to the frontier, while the belt of
country twenty miles broad to the north of it forms rather a
curious contrast to the remainder of north-eastern Yola. Possibly
there has been a slight local uplift. Granite is seen over almost
the whole surface breaking down into coarse sand and giving
rise to a hummocky
country difficult to travel. The variations in level are only
small, and taken as a whole it is still a plain, but the breaking
down of the granite is going on actively over the whole surface.
Parallel to this country and almost in alignment with it, is an
interesting string of craters and flows of basalt. (Plate VI,
Fig. 2, VII, Figs. 1,2.) The eastern extremity of this line appears
to be at Song, but westward it extends across the Mboi hills into
the Yungeru country, to what distance is not yet known. The
remainder of north-eastern Yola, with its rocky islets set in
extensive thinly wooded plains, is similar in character to that of
so many other parts of the Protectorate. Some of the inselberge,
especially in the Kilba hills (Plate VIII, Fig.
1.), afford excellent examples of exfoliation. (A. L.)
The province of Zaria lies almost entirely within the drainage
basin of the Kaduna river, only a narrow belt of country in the
south and south-west being drained by the rivers Gurara and Bako.
With the exception of the uppermost springs of the Kaduna and the
lower navigable portion of its valley below Wushishi, the whole
course of the river is included within the province. At Wushishi
the river narrows and its channel becomes much obstructed by rocks
and rapids, as it winds through the belt of hills which separate
the rocky plains of western Zaria from the lower plains of Nupe. At
Zungeru the Kaduna flows in the dry season in a narrow picturesque gorge
between white walls of quartzite, which in the rains are buried
under a rushing foaming torrent. (Plate VIII, Fig.
2.) From the bend of the river above Girku in the east of the
province, the Kaduna flows south-westward over stretches of
drift-covered plain and through scattered groups of hills with its
channel marked by frequent rocks and rapids, while beyond Girku the
river meanders peacefully over the lofty plains which stretch
eastward to the boundary of the province. The larger tributaries of
the Kaduna on the north bank, the Kara, Koriga, Tubo, and Shika,
take their rise upon the high plains of Hausaland, along whose
crest runs the primary watershed between the systems of the Niger
and Lake Chad, and, like the Kaduna itself, they flow in their
middle and lower courses over broken plains and between groups of
hills which separate the minor streams. The Leri river in the east
rises on the high plains of the Bauchi border, while the Dunia and
Serakin Pawa, the principal tributaries on the left bank, as well
as the headwaters of the Gurara, take their rise amidst the
scattered hills upon the lofty plains of the south-east. The Bako
river rises in the hilly ground around Fuka and, with the Gurara,
drains the extreme southern portion of the province.
The hills in Zaria province are of two types; (1) ranges or
belts of low rounded and somewhat broken hills, composed largely of
the more resistant schists, or of a mixture of granite and schist;
(2) kopjes, inselberge and small detached groups of rounded granite
hills, with here and there an extensive elongated mass of granite with a more or less
flattened tabular summit and steep though worn and rounded slopes
rising abruptly from the plains which encircle its base. To the
first type belong the tract of hilly country in the west of the
province which stretches from Mikonkeli to the south of the Kaduna
as far as Kagara to the north, the belt of hills between Gwari and
Koriga in the north of the province, the broken stony country
around Fuka in the south, and the line of hills in the centre of
the province from Dorka to Kaserimi. To the second type belong the
solitary turtlebacks and detached masses of granite hills which so
abundantly diversify the plains, while the Kudaru, Ruruma, and
Kerko hills are somewhat worn examples of the steep-sided granite
massifs, whose flattened though hummocky summits form the retreat
of numerous pagan tribes.
Over the greater part of the province the configuration of the
surface presents a very close resemblance to the characteristic
scenery of the plains of Kabba (p. 17).
Between and around the island mountains and groups of hills and
tracts of hilly country, spread undulating thinly-wooded
drift-covered plains. The towns and villages are set picturesquely
on the slopes and summits of the hills or in patches of thick
forest in the midst of the plain. For long distances round the
bases of the hills the plains are cleared and cultivated, and from
the summit of any of the isolated mounts the peculiar character of
the landscape impresses itself, as before, upon the observer. Here
and there the solitary hills raise their rounded crests in lonely
grandeur, while
around and between them the rivers pick their sluggish course over
the monotonous plain. The lofty drift-covered levels of the
south-east are largely uninhabited and cultivated only by the
pagans who daily descend from their rocky strongholds in the Ruruma
Hills or on the margin of the Bauchi plateau. Only in the
north-east between Girku, Leri, and Zaria do the plains assume the
typical aspect of Hausaland (p. 21). The
gently undulating surface is deeply buried in sandy drift, and in
places floored with surface ironstone: extensive stretches of the
grassy plains become swampland in the rains: the landscape is
diversified only at long intervals by occasional turtlebacks and
groups of granite hills, while well-walled towns and villages are
set here and there upon the open plains or built round the bases of
the solitary hills themselves, which serve as watch-towers in times
of danger.
The major part of the province of Zaria may be conceived as a
shallow, elongated basin, draining westward, and shut in on the
north and north-east by the high plains of Hausaland and on the
south-east by the Amo, Piti, and Chawai hills, the lofty plains of
Katab, and the tableland of Nassarawa. The greater part of Bauchi
province, on the other hand, may be looked upon as an undulating
plain sloping gently towards the north-east. The south-western
portion of the province is occupied by the great plateau of Bauchi,
which is limited on the north and south by well-defined walls of
rock. (Map p. 40). The plateau, however, is not
a true tableland in the sense of an elevated plain marked off on
all sides by precipitous walls from the lower plains around. Like the
tablelands of Kabba, Nassarawa, and Southern Yola, which are at
most inclined plains bounded by escarpments on one side only, the
Bauchi plateau from its summit at Bukuru slopes gently eastward and
merges gradually into the plains of the Gongola, while on the west
between the northern and southern escarpments it is continuous for
a short distance between Kwoll and Assab with the highest portion
of the tableland of Nassarawa. The intimate relation which exists
between the Bauchi plateau and the tableland of Nassarawa is
further demonstrated by the continuation of the escarpment of the
tableland beyond Assab to form the southern wall of the plateau.
From Assab the escarpment, here 1,500 to 2,000 feet in height, runs
south-eastward to the neighbourhood of the ninth parallel before
swinging round to the north-east and losing itself amongst the
rocks of Angass, where the single escarpment appears to be replaced
by a succession of rocky steps leading down to the tract of broken
country between Angass and Kanna, which is drained by the
headwaters of the Wase river.
The northern margin of the plateau is more irregular than the
southern and marked by projecting spurs which have now assumed the
character of mountain ranges. In the west the escarpment begins in
the neighbourhood of Kwoll and Miango and stretches westward as a
somewhat worn and rounded cliff, 1,000 feet in height, separating
the lofty plains of Rukuba from the lower plains of Zaria. Beyond
Chawai and Piti the escarpment is continued as the western
slope of the Amo,
Jengre, and Gura hills, which run out upon the broken plains to the
north, while the eastern slope of the hills again assumes the
character of an escarpment and bounds the valley of the Delimi as
far as the gorge of the river below Naraguta. To the east of the
Delimi, however, the single escarpment is replaced by at least two
steps which interpose the broken plains of Toro and the summits of
the Kwandokaya hills between the plateau and the northern plains,
while to the south-east of Polchi the escarpments run out upon the
plains of the upper Gongola in groups of picturesquely rounded
hills. There is some reason also to believe that the plateau is
stepped or broken in the neighbourhood of Basak in the Rukuba
country, but this point must await further investigation.
The surface of the plateau reaches its highest elevation of
about 4,100 feet in the neighbourhood of Ngell and Bukuru, whence
the headwaters of the Delimi, Sungo, Gongola, and Kaduna radiate
approximately north, south, east, and west. Of these rivers the
Delimi ultimately joins the Yo and falls into Lake Chad, the
Gongola and Sungo join the Benue, and the Kaduna the Niger. Thus
the few square miles in the neighbourhood of Ngell and Bukuru may
rightly be claimed as the hydrographical centre of the Protectorate
(p. 3), and even the most casual observer
cannot fail to be impressed here with that peculiar character of
the primary watershed which has already been remarked in the case
of the western tributaries of the Niger and the northern
tributaries of the
Kaduna. There is no group of rocky mountains, no rugged peak at the
hydrographical centre of the Protectorate, no lofty ranges forming
the watersheds between the radiating rivers: only an undulating,
swampy, alluvial plain, diversified by a few short, broken ridges
and low kopjes of granite boulders. Into these plains, however, the
rivers which flow north, west, and south are rapidly carving narrow
rocky valleys, and their descent to the lower plains is marked in
every case by picturesque waterfalls and narrow gorges cut back for
considerable distances into the plateau itself. The wooded gorge of
the Delimi below Naraguta, the steep waterfalls of the Kaduna
between Kwoll and Miango, and the deep rocky channel of the Kogin
Rim above Assab afford striking evidence of the activity of the
rivers. Only in the east where the plateau inclines gently towards
the Gongola do the streams flow in sandy beds between deep banks of
alluvium, with broad stretches of swamp on either bank wherever the
slope is insufficient to drain the summer floods.
THE BAUCHI PLATEAU
and the
NASSARAWA TABLELAND.
MAP No. 2.
London. Macmillan & Co., Ltd.
Face p. 40.
The Bauchi plateau, in the more restricted sense, may be taken
to include all those portions of the elevated plains which rise
above 3,500 feet. Upon this definition the plateau is bounded on
the south by the continuous escarpment which stretches from Sura to
Assab, and on the west by a line drawn northward from Assab to
Kwoll and Miango. Beyond Miango the plateau is again limited by an
escarpment as far as the Jengre Hills, whence the boundary runs
southward to Naraguta and thence eastward along the higher escarpment behind Toro.
The eastern boundary of the plateau is formed by a line drawn
approximately from south to north from Kereng to Toro. The most
striking feature of the greater part of the plateau as thus
defined, with the exception of the Rukuba country to the north, is
the bare and treeless aspect of the surface, which assumes the
character of undulating grassy downs extensively cultivated in the
neighbourhood of the towns and villages (Plate IX, Figs. 1,2). Only amidst the clusters
of rocky hills which diversify the surface do a few wind-worn trees
bear witness to the former extent of the forest, while round the
bases of the hills and among the boulders on their slopes are set
the rectangular compounds of the pagan villages, whose dark green
hedges of prickly cactus add a pleasing touch of colour to the
mellow landscape. The low hills which rise from the plateau have
also a peculiarity all their own, which is to be directly
correlated with the greater diurnal variation in temperature. The
turtlebacks and domes and rounded hills of the lower plains are
here conspicuously absent, and in their place appear heaps of
boulders and jagged ridges of granite, with picturesque peaks and
pinnacles and perched blocks adorning their crests. Here and there,
also, in the Rukuba and Kibyen country in the west and in Sura in
the south-east, more or less worn and rounded volcanic cones rise
above the general level of the plateau (Plate X, Fig. 1). The craters of Kereng (Plate X, Fig. 2) in the south-east, however, are remarkably well
preserved. The surface of the plateau itself is, moreover, deeply
covered with drifted alluvium and provided in places with a crust of surface
ironstone through which the rivers and streams have in many cases
cut to the rock below. Above Assab the drift has been largely
removed from the surface, and between Rim and Woran numerous
grassy, flat-topped hills of red, gritty earth, capped by a hard
ferruginous crust, rise in places to a height of at least 300 feet
above the level of the plateau margin. These hillocks recall in
appearance the tabular sandstone hills of the Niger valley, and
bear striking witness to the former extent and thickness of the
superficial accumulations (Plate XI, Fig. 1).
The broken, rocky character of the marginal belt of plateau on the
north in the neighbourhood of Zammagan, Basak and Naraguta is
probably also largely due to the removal of the covering of drift
which formerly obscured many of the irregularities of the surface.
Isolated hillocks of drift indeed are common throughout Rukuba to
the west of Jos and Naraguta (Plate XI, Fig. 2),
while at Basak they may be seen capping the very summit of the
cliffs.
Beyond the limits of the plateau proper, a thinly wooded
drift-covered plain, in places somewhat broken and hummocky and
diversified in the usual manner by inselberge and scattered groups
of granite hills and drained by the headwaters of the Gongola,
stretches eastward to the meridian of Bauchi. Similarly, beyond the
worn and rounded escarpments and projecting spurs which form the
northern margin of the plateau, an undulating wooded plain, drained
by the Delimi and its tributaries, extends north-eastward towards
the Kano border. The western watershed of the Delimi valley runs along the hummocky
drift-covered plain which stretches northward between the Gura
hills and the detached mass of the Liruei-Kano hills, and thence
through the rocky hilly country round Tirkunya into the Ningi
hills, which form the most striking feature of the north-western
portion of the province. This elongated mass of hills, which
stretches from Bura in the west to Ari in the east, is a typical
instance of those steep-sided and more or less tabular granite
massifs, of which examples have already been noted in the Ruruma
hills of Zaria, the Kagoro hills of Nassarawa and the Vere hills of
Yola. The Ningi massif reaches a higher elevation in the west than
in the east, its slopes are much rounded and incised by narrow
valleys, its summit is broken and hummocky with a scanty soil of
disintegrated granite, while wooded drift-covered plains stretch
outward from its base towards Zaria on the west, towards Kano and
Katagum on the north, and towards the Delimi valley on the east and
south.
The surface of these wooded plains rises gently in the north to
form the low watershed between the valleys of the Delimi and the
Shidya. The headwaters of the Shidya rise in the tract of broken
hilly country which lies to the north and north-east of Bauchi, and
forms in part the northern watershed of the Gongola basin. Around
Bauchi itself domes and turtlebacks and groups of rounded hills,
rising from the stony and drift-covered plains, are particularly
numerous and often remarkably picturesque in their outlines, the
lion couchant being one of the commonest forms which the bare
granite hills assume. (Plate XII, Figs. 1 and
2.) South of the Gongola the plains slope gently upward to the
base of the Jarawa and Kanna hills, becoming more and more broken
and stony as they approach the watershed of the Wase river. The
granite massif of the Wadai hills between Angass and Sura is only
known along the northern margin, while Angass[13]
itself is a particularly broken and rugged district separated from
the Murchison Range by a series of rocky valleys still largely
unexplored. Between Angass and Kanna the high drift-covered plains
of Bauchi pass into a tract of broken country which is drained by
the headwaters of the Wase river and falls rapidly to the plains of
Yergum.[14] The line of fracture to the west which forms
the margin of the plateau appears to be replaced here by a simple
flexure or perhaps by a succession of worn and broken steps.
Farther east rise the Kanna and Kantana hills, with their
projecting spur towards Kwankyam, whose steep though worn
escarpments interpose an ascent of 700 to 1,000 feet between the
lower and higher plains. The western hills, which are prolonged
northward into the Jarawa country, are composed of crystalline
rocks, and the more prominent peaks possess the usual rounded
outlines of granite hills. The Kantana hills to the east, however,
are sedimentary in origin, and from a distance show the familiar
tabular outlines of horizontal sandstone hills, the narrow plateau
on top being, however, much eroded and incised. Eastward the
escarpment appears to be much stepped, and in the neighbourhood of
Tukkur wheels northward to form the eastern slope of
the Jarawa hills
which run out upon the plain of the Gongola.
The eastern portion of the province is almost entirely covered
with sedimentary rocks. In the centre, a little to the south of the
parallel of Bauchi, broad plains stretch eastward from Goram and
Lafiangale to Deba Habe and the Gongola, bounded on the north by
the broken sandstone plateaux which are enclosed within the bend of
the Gongola, and on the south by the slopes of the Kudu valley, the
rounded hills of Gateri and the picturesque peaks of Tangale. In
the west the plains are deeply covered with sandy drift, through
which the headwaters of the Kudu have cut narrow canyon valleys
into the sandstones below. Farther east the summer rains render the
plains for long distances almost impassable swamps which drain
imperceptibly east or west, and leave in the dry season a gently
undulating surface of black, gritty clay, baked and fissured by the
summer sun. To the north of the central plain and within the bend
of the Gongola lies a region of detached tabular masses of
elongated or irregular outline, together with groups of worn and
rounded sandstone hills, separated by stretches of sandy plain,
presenting all the familiar scenic features of the dissected
sandstone plateaux of the Niger. The narrow valley of the Gongola,
between Deu and Gombe, bounded by flat-topped ranges on either
side, presents a marked contrast in scenery to the upper course of
the river above Baddera, where it threads its way over the high
plains of Bauchi between turtlebacks and domes of
granite. Below Gombe,
however, the valley opens out and the river flows as far as Nafada
through an open sandy plain, diversified by occasional flat-topped
sandstone hills. Between Nafada and Ashaka the channel winds
between detached hills and short tabular ranges which rise from an
undulating, thinly-wooded plain, while below Ashaka the river
enters the deeply drift-covered plain which stretches from the base
of the Tilde and Dukul hills on the west to the foot of the
escarpment of the Barbur plateau in Bornu on the east. As far as
Jurara and Deba Filani the plain is thickly populated, well-cleared
and cultivated; southward, however, within the Habe country, the
scattered villages are separated by longer stretches of bush, while
the plain is diversified by hummocks of sandstone and worn hillocks
of basalt with the intervening irregularities of the surface deeply
buried in drift. Beyond Deba Habe, the plain of the Gongola forms
the eastward continuation of the central plain of Bauchi, whose
sandy and swampy surface stretches southward to the base of the
Tangale hills.
The south-eastern portion of the province is occupied in part by
the upper valley of the Kudu river and in part by the extensive
mass of hills which, with the exception of the outer margin around
Gateri, Tangale, and Waja, is still largely unentered and
unexplored. A series of narrow valleys, terminated by canyon-like
gorges, have been cut by the headwaters of the Kudu into the
elevated plains of Pali and Lafiangale. The difference in level
between the upper plains and the river bed is greatest
in Dugari in the
west, where the streams flow in narrow flat-bottomed valleys,
bounded by perpendicular walls of white sandstone three to five
hundred feet in height. From the cliffs above Tongolan the scenery
recalls in miniature the chasms, amphitheatres, and canyons of
Colorado, with their promontories and projecting buttresses, steep
slopes and vertical ledges, while south-eastward towards Yuli the
remnants of a lower plateau mark a former pause in the activity of
the river. Farther east, by Panguru and Putu, the relative height
of the upper plains decreases, the sandstones become broken and
inclined, and the tributary valleys are bounded, not by cliffs, but
by rounded, drift-covered slopes leading upward to the higher
undulating plains. The folded sandstones of Gateri and Ligri give
rise to a group of rounded hills deeply incised by narrow valleys
and backed by the ridges and peaks of the unexplored Wurkum and
Pero hills, which stretch north-eastward towards the Gongola. From
Gateri northward to Putu the track leads over somewhat broken
country, while eastward from Putu to Tangalto stretch open,
undulating, swampy plains, diversified only by occasional conical
stumps of basalt.
Plate IV
A Granite Mount near Abuja.
The Margin of the Nassarawa Tableland from
Jaginde.
The tract of country which lies between Tangale and the Gongola
is one of the most picturesque within the Protectorate. The hills
rise in detached groups from an irregular hummocky plain, and form
the advanced guard of the belt of lofty highlands which intervene
between the northern plains and the Benue. While the whole district
may be spoken of generally as Tangale, the Tangale tribe itself
occupies a very
restricted area and is lodged within only the westernmost group of
hills, the successive groups to the east and south being inhabited
by the kindred but mutually hostile tribes of Chongwom, Ture, Awok,
Tula, and Waja. Tangalto and Tall, the strongholds of the Tangale
tribe, are set on the slopes of rounded granite hills,
picturesquely pierced by worn pillars of trachyte and basalt, and
separated by a narrow undulating plain from the neighbouring group
of granite hills inhabited by the pagans of Chongwom and Kaltungo.
Beyond Chongwom the character of the scenery entirely changes. In
place of the rounded crystalline hills there rise precipitous and
more or less tabular masses of well-bedded sedimentary rock,
forming natural fortresses in the midst of an undulating park-like
plain, diversified by knobs and hummocks and standing rocks of
sandstone (Plate XIII, Fig. 1). At times the
summit ridges are gently curved and the hills are bounded on one
side by a steep escarpment and on the other by a dip slope sinking
gradually to the plains below, while the whole landscape is
dominated by the Ture or Tangale Peak, a picturesque pillar of
trachyte rising from a worn and flattened basal cone of sandstone
(Plate XIII, Fig. 2). Eastward, around Jelengu
and Reme, somewhat similar pillars of volcanic rock project here
and there above the sandstone hills, while the worn escarpments of
Waja appear to be continuous with those which form the western
boundary of the lower Gongola valley. The country which lies to the
south of Tangale and Waja is quite unknown. From the upper slopes
of the Ture Peak, a
second distant range of hills can be seen running parallel with the
jagged ridge in the foreground, and it is probable that, instead of
a mass of hills or a continuous plateau, there is within this
unexplored land a succession of broken valleys and rocky ridges,
forming one of the last strongholds of paganism within the
Protectorate.
The southern portion of Bornu, which lies immediately east of
the Gongola, presents, so far as the character of its physical
features is concerned, a greater affinity with the central
provinces of Bauchi and Zaria than with the northern Hausa States
or with the northern portion of Bornu itself. The valley of the
Gongola to the east of the river is more broken and hummocky than
it is to the west. The sandstones are much faulted and variously
inclined and form belts of rocky plain between the swampy valleys
of the tributary streams. Here and there, as at Wuyo and Gulani, a
circle of jagged ridges forms a natural battlement round a pagan
town, or, as at Balbea, a flattened sandstone hill forms a natural
stronghold in the midst of the plain. More striking, however, are
the level crests, the dip slopes and the steep escarpments of the
Bima and Wadi hills, which form the landmarks of the Gongola plain
from Ashaka to Wuyo. From Gulani north-westward to Ashaka stretches
an extensive undulating plain, broken only by an occasional ridge
of sandstone, with black swamp earth covering all the minor irregularities of the
surface and creeping up the slopes of the sandstone hills
themselves. Eastward the broken plain of the Gongola is bounded by
the worn escarpment of the Barbur volcanic plateau which becomes
higher towards the north, and between Bilaraba and Dukshi is
fringed with a series of flat-topped and conical outliers of
basalt. To the east of Gulani and Balbea the ascent to the plateau
is over basal sandstones capped by black volcanic rock. Farther
north, however, in the neighbourhood of Dukshi, the escarpment is
entirely composed of basalt and rises from an irregular crystalline
floor. The summit of the plateau is broken and rocky in the west,
deeply trenched by the tributary streams of the Gongola, and
diversified by numerous worn and rounded cones of ash and lava.
Eastward towards Buratai the plateau descends by a series of low
rocky steps separated by drift-covered levels to the plains of Hong
and Ajigin, whose deep covering of dark gritty clay, hard and
fissured in the dry season, forms an almost impassable swamp in the
rains. In the south-east, towards Biu and the little-known Burra
country, the volcanic cones become higher and more conspicuous,
while the surface of the plateau, according to native information,
becomes much dissected and broken up into narrow plains and rocky
valleys.
Two tracts of hilly country, separated by the swamps of Hong and
Ajigin, lie respectively to the north-west and north-east of the
volcanic plateau. The worn and flattened hills of sandstone and
ironstone, which stretch westward from Buni and Ligdar towards
Gabai, pass northward
into the high, drift-covered plains of Mutwe and Gujba, which form
the watershed between the Gongola and Lake Chad. In the east the
Tagum and other groups of flat-topped hills around Lua, Ndufa, and
Korode represent the remains of a sandstone plateau once continuous
with the hills of Buni and Ligdar across the swamps of Ajigin. To
the north and north-east of the Tagum hills stretch the great
wooded plains of Bornu, with their gently undulating surface
sloping imperceptibly towards Lake Chad, and with not a single hill
nor even a hummock of boulders to relieve the monotony of the
scenery. To the south-east of the Tagum hills the sandstones run
out upon a crystalline floor which stretches southward into Yola
province, with its surface diversified only at long intervals by
island mountains and groups of granite hills.
The plains of Bornu, which stretch northward from the rocks of
Chibuk and the Tagum hills to Lake Chad and the river Yo, are in
the east so imperfectly drained that extensive tracts of their
surface become marshy and swampy during the rainy season. The
swamps reach their greatest development in the valley of the
Yedseram and in the immediate neighbourhood of Chad, where the
whole country is flooded after the rains. Barth[15]
has described in a sufficiently vivid manner the featureless and
cheerless aspect of the swampy plains to the south and south-east
of Chad. Along the caravan road from Maidugari to Kukawa transverse
stretches of swamp land (firki) alternate with belts of slightly higher ground, hard and
smooth on the surface, or thinly coated with loose yellow sand or
dark grey loam. The character of the vegetation reflects
immediately the changes in the nature of the surface soil. In the
dry season, after the grass is burnt, the cracked and fissured
surface of the firki land is absolutely bare and treeless, the hard
and smooth surfaces of sandy clay are covered with thin mimosa
bush, while the stretches of loose sandy loam support a more
abundant growth of trees and scrub. The ancient dunes and the
undulating sandy plains which bound the northern basin of Chad
support an open forest growth, while recent accumulations of blown
sand are now found only within the towns themselves, or in places
which have been cleared of forest for purposes of cultivation.
The Maidugari river, which rises on the Yola border and loses
itself in swamp a few miles to the north of Maidugari, has cut
deeply into the superficial sandy drift in the neighbourhood of
Maidugari and Maifoni and formed a comparatively narrow valley with
steeply-rounded slopes. From Maifoni a gently rolling plain
stretches westward to Gujba, with occasional thick-stemmed kukas
rising prominently above the low growth of scrubby trees and
prickly mimosa which covers the surface, and with here and there a
dry “tubka” or shallow swampy hollow forming an open space within
the forest. At Murguba, and again at Gujba, the plain is broken by
shallow water-courses, bounded by low and ill-defined ridges of
rising ground. These extended swampy hollows are flooded during
the rains, and in the
dry season marked by a line of wells and occasional pools separated
by sandy bars. Beyond Gujba the rolling plains become again marked
by a rapid alternation of sandy rising ground and clayey shallows
draining southward in the rains. Between Chumga and Abakri,
however, rock begins to appear in places on the surface of the
drift-covered undulations which separate the swampy valleys, while
a few isolated flat-topped or rounded hills are scattered over the
plains. From Abakri can be seen to the west the long flattened and
rounded ridges and tabular hills, half buried in sandy drift, which
introduce the Kerri Kerri plateau. Daura and Durua are set within
this broken country, each at the base of a low flat-topped range of
ironstone hills, while the track leads over a succession of stony
ridges and shallow cultivated valleys to the continuous plateau
surface which is reached about half-way between Durua and Momdo.
The plateau itself is no higher than the summits of the marginal
hills, which with their drift-filled valleys represent merely the
dissected eastern front of the tableland. The surface of the
plateau, at first thinly wooded and in places somewhat stony,
passes westward into the open sandy undulating plains around
Potiskum, which fall away gently to the north and west into the
valley of the Kogin Shidya. Here and there a low hill or a rounded
ridge of sandstone or ironstone rises above the general level, with
the sandy drift which obscures all the minor irregularities of the
surface deeply banked against its slopes.
The Kerri Kerri plateau, like the tableland of Nassarawa and the elevated
plains of Bauchi, is not a true plateau in the strict sense of the
term, but rather an inclined plane, bounded by more or less worn
escarpments on the south and passing gradually northward into the
plains of Ngizem. The escarpments which stretch from the
neighbourhood of Daura south-westward to the provincial boundary
reach their greatest elevation behind Dozi, Lewe, and Gamari to the
north-west of Nafada, where precipitous cliffs of white sandstone
500 to 600 feet high rise abruptly from the sandy plains which
stretch southward to the Gongola. The escarpment, however, is in no
sense a continuous one, for, as already noted in the neighbourhood
of Daura, the margin of the plateau has been much cut up and
dissected, especially in the east, into a system of more or less
detached flat-topped ranges and tabular masses separated by narrow
valleys or by short stretches of open plain.
Westward where the escarpment is higher, as in the neighbourhood
of Dozi and Kadi, the cliffs are furnished with the characteristic
buttresses and indentations of horizontal sandstone rocks, while
the margin of the plateau is fringed with a series of detached
tabular and conical outliers of varying size and elevation (Plate
XIV, Fig. 1), the larger of which form natural
strongholds for pagan villages.[16] The most remarkable
feature, however, about the margin of the plateau is that, behind
the fringing belt of detached ranges and outlying groups of hills,
the main mass of the plateau has been grooved or
incised by a series
of narrow flat-bottomed trench-like valleys, bounded by
perpendicular walls of white sandstone and notched and indented by
lateral gullies (Plate XIV, Fig. 2). For long
distances the narrow valleys preserve a very constant breadth,
while frequently they turn abruptly at right angles or are joined
by lateral valleys of similar character which unite the whole
series into an interlacing system through which it is possible to
travel from Daura to Zai without once ascending to the level of the
plateau above. At their upper ends the valleys rapidly contract and
run out upon the plateau as steep and rocky gullies. Their floors,
like the surface of the plateau above, are well cleared for
cultivation and studded with scattered trees, while the villages
are set aloft on the margin of the cliffs overlooking the wells in
the valley below.
Beyond the western boundary of Bornu the escarpments die down,
and the drift-covered surface of the plateau falls gently to the
swampy valley of the Kogin Shidya. The last trace of the sandstones
of Kerri Kerri is found in the low hills of conglomerate and grit
around Hardawa, while twenty miles to the north according to native
information the sandstones disappear in the neighbourhood of Lise
under the sands and swamps of Katagum. Beyond Hardawa begin the
undulating plains of Hausaland (pp. 21,38), which stretch westward, to the north of
the Ningi massif, through the heart of Kano and the north-eastern
portion of Zaria into Kontagora and Sokoto. Here and there a heap
of granite boulders or an isolated group of rounded hills, as at
Shira, Fagam and
Kila, rise out of the red sandy drift which is deeply banked
against their slopes, and which elsewhere effectually buries the
irregularities of the crystalline floor. Occasionally a smooth and
rounded surface of gneiss or granite outcrops level with the plain,
while the rivers which flow in broad, shallow and ill-defined
valleys, indistinguishable at a distance from the rest of the
plain, have here and there succeeded in cutting their channels
through the sandy drift to the solid rock below. Where uncultivated
the plains are covered with prickly mimosa bush, or with trees well
spaced or in bushy clumps with open grass around. The farm-land,
however, is well cleared and comparatively bare with a few large
trees scattered here and there amongst the furrows, while the
fallow is thickly covered with tall aromatic grass and low scrubby
trees of a few seasons’ growth.
From Jemaari north-eastward to Katagum, and thence westward to
the immediate neighbourhood of Kano, not a single rock is anywhere
exposed, and not a single block or boulder or even a pebble of
quartz is to be found in the sandy loam which covers the surface.
North of Jemaari the track leads through gently undulating country
presenting the usual alternations of farm-land and thinly wooded
plain. Between Itas and Murmur the surface becomes in places quite
hummocky with ancient dunes in the form of low rounded ridges of
loose sandy drift, extended approximately east and west, rising in
succession upon the level plain and bearing witness to the former
prevalence of desert conditions. Around Katagum the drainage is so
imperfect that long
stretches of the plain are under water in the rains, while westward
beyond Kwotolu the surface resumes its undulating aspect with its
alternation of sandy rising ground and clayey shallows. Here and
there, as at Koya, a rounded ridge of loose sand, now covered with
grass and scattered trees, affords evidence of the former activity
of the wind, while in places in the hollows are small sheets of
fresh water forming lakes which are more or less permanent
throughout the year.
Around Kano and north-westward towards Katsina the open
undulating plain presents all the characteristic features of a
crystalline floor. For long distances round the towns the red sandy
loam which covers the surface has been cleared of all but the
larger trees and subjected for generations to intense cultivation.
Here and there around Kano smooth surfaces of rock appear level
with the plain, or rise into low rounded turtlebacks or heaps of
granite boulders. The two prominent tabular hills of weathered
diorite, Dala and Kogon Dutsi,[17] within the walls of Kano,
have a peculiar interest of their own which will be related in the
sequel (p. 201). Northward between Kano and
Kazaure the undulating sandy plain is diversified only by the low
flat-topped hillocks of earthy grit and conglomerate in the
neighbourhood of Sai and Ingogo and by the broken ridges of
amethystine quartz which rise from the plain immediately south of
Kunchi. Kazaure is set in the midst of rocky ridges of blue
quartzite with red sandy drift filling up the valleys and
banked against the
slopes of the hills. Around Rauni rise low extended flattened
ridges, which appear to be composed throughout of dark red
vesicular ironstone. At Tummas, half way to Ingawa, low granite
knobs and heaps of boulders project above the surface, but from
Ingawa north-westward to Katsina the undulating plains are
featureless, with no prominent granite hills or kopjes to break the
monotony of the landscape. Smooth surfaces of rock are here and
there exposed level with the plain or in the channels of the
streams, but otherwise the deep sandy drift which covers the
surface effectually obscures all the minor irregularities.
Around Katsina both sandstones and granites are exposed upon the
surface of the plain, but it is not until the neighbourhood of
Kurefi is reached that a decided change takes place in the
character of the scenery. From Kurefi to Runka the track leads
through rocky granite country which assumes its wildest aspect in
the neighbourhood of Tummo, where the landscape, as far as one can
see, is made up of an assemblage of low rounded hills, domes, tors,
heaps of boulders, perched blocks and smooth surfaces of granite,
with no particularly high or prominent peaks. On the margin of the
mass near Kurefi and Runka the aspect of the country is smoother;
the turtlebacks and rounded hills are separated by narrow stretches
of plain, and often half-buried under the sandy drift which is
banked up against their slopes. From Runka south-westward to
Kotorkoshi, and thence north-westward to Karakai, the landscape
assumes its familiar aspect of a gently undulating thinly-wooded
surface, diversified by isolated groups of granite hills which, at Duru and
Kotorkoshi, rise abruptly and picturesquely from the drift-covered
plain. A belt of broken hilly country stretches N.N.E. and S.S.W.
between Karakai and Maradu, parallel with the strike of the
quartzites, phyllites, and gneisses out of which the hills are
carved. Immediately beyond Maradu knobs and hummocks of volcanic
agglomerate project from the surface of the plain, while northward
the track leaves the hummocky crystalline floor in the
neighbourhood of Gora and follows the level swampy plain of the
Gulbin Gandi as far as Dankaiwa. At Dankaiwa, in the cliff which
faces the river, is the first exposure of the red earthy sandstones
of Sokoto, while at Dampo, a little farther on, the first
flat-topped sandstone hill rises above the sandy plain. From Dampo
the track again runs along the swampy valley of the river as far as
Rara, whence it leads across undulating park-like plains to the
broken sandstone hills of Sokoto.
The northern and western portions of Sokoto province, which
consist of a dissected plateau of horizontal sandstones, possess in
general all the familiar scenic features of such a formation as
already described from other parts of the Protectorate. The
apparent differences are differences more in degree than in kind.
Like the Niger above Lokoja, the Benue in its lower and upper
course and the Gongola above Gombe, the Gulbin Kebbi or Sokoto
river flows in a broad flat-bottomed trench-like valley between two
steep and worn escarpments, which lead up to the plateau on either
side. The breadth of the valley of the Gulbin Kebbi is, however, out of all proportion
both to the height of the bounding walls and to the volume of the
river itself. The floor of the valley is usually several miles in
breadth, while the cliffs on either side rarely reach 200 feet in
height. In the rainy season a sluggish flood covers the bottom of
the valley, but in the dry season the river is represented by one
or more diminutive streams, which disappear from time to time in
stretches of swamp. The continuity of the flat-topped escarpments
is much broken by lateral valleys, and here and there the vertical
walls are replaced for long distances by steeply-rounded rising
ground. The tributaries of the Gulbin Kebbi, where they flow
through sandstone country, are likewise bounded by worn and broken
escarpments. Similar broad flat-bottomed valleys with diminutive
streams are common also within the French Sudan to the north of
Sokoto, and are there generally known as “dallols.” Of them all,
however, the Gulbin Kebbi possesses the greatest volume of water,
which is contributed practically entirely by its numerous southern
affluents which rise on the high and well-watered plains on the
borders of Kano, Zaria, Kontagora and Sokoto.
Plate V
Worn Volcanic Cones near Awe.
The Wase Rock.
The plateau areas between the larger valleys are themselves much
dissected, especially in the east and north, by the smaller
tributary streams. In the west there are some fairly extensive
stretches of continuous tableland, whose gently undulating and
thinly-wooded surface is diversified by occasional flat-topped and
conical hills, the relics of a former plain which has now been
almost entirely removed in the course of denudation. To the north and north-east of
Sokoto the irregularities of the dissected plateau are softened in
contour by accumulations of blown sand which covers the floors of
the smaller valleys and creeps up their lateral slopes, rounding
off the steeper cliffs of the flattened ridges on either side.
[1]Mungo Park; Denham and Clapperton; R. and J.
Lander; Laird and Oldfield; Allen and Thomson; Baikie; Crowther;
Barth; Vogel; Passarge; Lenfant; Lugard; Wallace;
Boyd-Alexander.
[2]“Hausaland,” in the general sense of the
term, may be taken to include the provinces of Sokoto, Kano,
Kontagora, Zaria and Bauchi.
[3]Cf. Laird and Oldfield,
Expedition to Africa, 1837, p. 136. The hills between
Itobe and Igbo are here erroneously described as the eastward
prolongation of the Kong Mountains.
[4]Cf. Vandeleur, Geographical
Journal, X, 1897, p. 349.
[5]Vide Lenfant, Le Niger,
1905, illus., pp. 72, 73.
[7]Boyd-Alexander, From the Niger to the
Nile, 1908, p. 47.
[8]An excellent illustration of this peculiar
type of scenery (Inselberglandschaft) is given by Bornhardt,
Deutsch-Ost-Afrika—Géologie, Bd. VII, 1900, Taf. 3, p.
30.
[9]Erroneously designated lava by the earlier
explorers.
[10]Cf. Boyd-Alexander, From the
Niger to the Nile, 1908, p. 86.
CHAPTER II — Typical Traverses. — Borgu and the Niger Valley between Jebba and Yelwa. — Kabba and Ilorin. — 4. The Central Crystalline Area. — Southern Muri and Yola. (A. L.)
CRYSTALLINE ROCKS
Distribution of the crystalline rocks. Typical
traverses. The harder and softer gneisses. The Jebba quartzites:
the phyllites and slaty schists of the Niger valley; the sheared
trachytes of Bussa; the Ngashki phyllites; replacement of the
softer gneisses by the harder banded types. The gneisses and
schists of Ilorin; the quartzites of the Orissa hills; the
stanniferous pegmatites of Eri. The muscovite schists of Northern
Kabba; the quartzites of Lukke; the crystalline limestone between
Jakura and Wa; the mica schists of Sequarre; the magnetite and
hæmatite schists of Eastern Kabba; the Igbo limestone. The
phyllites and schists of Sokoto province; the volcanic agglomerate
of Maradu; the blue quartzite of Kazaure; the diorites of Kano;
Mounts Dala and Kogon Dutsi; the gneisses and schists of
north-eastern Kontagora; the Zungeru belt of phyllites and schists;
auriferous quartz reefs; the quartzites and schists of Eastern
Zaria; the gneisses and granites of Abuja; the tourmaline rocks of
Darroro; the mica schists of Gwombe; the banded gneisses of Bauchi
province; the granites of the Murchison Hills; the later
intrusions. The inliers of Arofu; the granites and gneisses of
southern Muri; the Vere Hills; exfoliation in the Kilba Hills; the
crystalline rocks of Bornu.
While crystalline rocks
undoubtedly everywhere form the foundation of the Protectorate,
there are extensive tracts over which the later sedimentary rocks
obscure the underlying granites, gneisses, and schists. The largest
area over which these crystalline rocks are exposed is that which
occupies the centre of the Protectorate and extends from the Niger
in the west to beyond
Bauchi in the east, and from Nassarawa in the south to the
neighbourhood of Katsina in the north. The major portion of the
provinces of Kabba, Ilorin and Borgu to the west of the Niger also
possess a crystalline floor, while similar rocks cover a narrow
strip on the southern border of Muri and Yola, as well as a
considerable tract of country to the east of the Gongola, lying
partly in Yola and partly in Bornu.
The following typical traverses show the character of the
exposures along the tracks.
I. Filelin (near Lokoja) to Ohoto:
9.20
A.M.—
Left Filelin.
9.34
„
Blocks of ferruginous sandstone and
grit.
9.38
„
Masses of biotite pegmatite; large
felspars often graphic.
9.40
„
Fine-grained, non-foliated reddish
biotite granite with streaks and veins of graphic biotite
pegmatite. The granite at places on the margin follows the
foliation of the adjoining biotite gneisses, in places crosses the
foliation, or is in part injected into the gneiss. The biotite
gneiss is often folded and distorted; in places there is an
alternation of dark bands of biotite gneiss with lighter coloured
felspathic layers. The foliation is much disturbed, but in a
general way E.N.E. and vertical. The gneiss is cut by granite and
pegmatite veins and dykes.
9.45
„
Reddish granite continued. In places the
granite shows distinct banding and alternation of coarse and fine
layers; sometimes banded with pegmatites.
9.50
„
Same pink granite; but here an irregular
mixture of coarse and fine material with in places knots of coarse
granite in a fine grained matrix.
10.00
„
Small granite knobs on right.
10.30
„
Sandstones.
10.10
„
Biotite gneiss, strike N.E.–S.W.,
foliation dip 40° N., cut by coarse pegmatites as before; in places
interfoliated bands of segregation pegmatite.
10.15
„
Soft biotite gneisses.
10.30
„
Large blocks of pink granite and
pegmatite.
10.40
„
Granite piercing hard white biotite
gneiss with knots of amphibolite. Strike of gneisses N. 35° E.,
foliation dip N.W. There is a sharp junction between the granite
and gneiss, but no chilled margin.
10.50
„
Granite.
11.00
„
No exposures.
11.30
„
Extensive mass of coarse-grained biotite
gneiss, crossed by pegmatite veins and dykes of coarse red granite
with large felspars.
11.35
„
Biotite augen-gneiss with felspars
sometimes two inches long; crossed by veins of fine red granite or
aplite.
11.40
„
No exposures.
12.30
P.M.—
Bare surface of augen-gneiss level with
the plain.
12.40
„
Dyke or sheet of fine grained reddish
granite showing distinct foliation.
12.45
„
Fine grained and finely banded biotite
gneisses crossed and injected by granite veins.
12.50
„
Sandstones.
1.15
„
Ohoto.
II. Jakura to Wa (north side of Mimi
Valley).
6.30
A.M.—
Left Jakura hill; on brow of hill quartz
schist, strike N. 10° W., foliation vertical; at foot of hill fine
grained pink granite with pegmatite, passing into friable pink
granite with large crystals of graphic quartz and felspar in a
finer matrix.
6.55
„
Soft biotite gneisses, strike N.N.E., dip
W.N.W.
6.58
„
Band of amphibolite, dip unascertainable,
followed by soft biotite gneisses. Pebbles of hornblende epidote
schist.
7.12
„
Bare exposure of red pegmatite dyke
cutting fine grained felspathic gneiss, with in places a sharp
junction and in others not.
7.20
„
Quartz schist.
7.35
„
Magnetite hornblende schist, strike N.
10° W., foliation dip practically horizontal.
7.40
„
Quartz hæmatite schist, dip
unascertainable.
8.00
„
Massive and granular quartzite and quartz
schist.
8.15
„
Biotite gneiss.
8.30
„
Quartz-muscovite schist, strike N.N.E.,
dip low to W.N.W.
8.35
„
Pebbly muscovite schist, strike N.–S.,
foliation vertical or highly inclined.
8.45
„
Pegmatites.
8.50
„
White marble forming rounded knobby hills
cut by pegmatite veins and enclosing streaks and veins of
quartz-epidote-calcite-idocrase rock on right hand; on left the
marble is foliated, with folia of pyrites and knots of quartz;
strike of foliation N. 10° E., dip 10° W.
9.50
„
Marble ends.
9.80
„
No exposures; grass land.
9.15
„
Quartzites.
9-35
„
Soft biotite gneiss; foliation generally
vertical; strike N.N.E., dip vertical or high towards W.
9.50
„
Garnetiferous biotite gneiss with quartz
segregations; folia crumpled and contorted; strike generally
N.–S.
10.10
„
Garnetiferous biotite gneiss as before;
folia practically horizontal.
10.25
„
Soft micaceous schists pierced by
granite; strike N. 35° E., dip S.E.
10.35
„
Pink granite in places foliated; graphic
pegmatites on slope of Wa hill.
10.45
„
Wa. On top of hill white mica schists
with magnetite knots, strike N.–S., dip 20° W.; cut by
quartz-muscovite-magnetite pegmatites.
III. Olle to Sura (Central Kabba).
6.20
A.M.—
Left Olle.
6.25
„
Quartz schists and quartz-muscovite
schists, strike N. 40° E., dip 55° W.
6.27
„
Soft fine-grained quartz-muscovite
schists, strike N. 10° E., dip 70° W.
6.35
„
Muscovite schists, strike N. 30° W.,
foliation vertical.
6.40
„
Thick dyke of graphic quartz and felspar,
with crystals of tourmaline.
7.50
„
Fine grained white granite.
7.25
„
Coarse white granite in river bed.
7.30
„
No exposures.
8.10
„
Micaceous gneiss, strike N. 35° E.; much
folded and contorted with streaks and knots of pegmatite, probably
proceeding from a thick intrusive mass of quartz-felspar pegmatite
which adjoins the gneiss.
Muscovite schists, strike N. 10° W.,
foliation vertical.
9.50
„
Broken fragments of quartzite,
quartz-muscovite pegmatite, and quartz-tourmaline pegmatite on
road; intrusive masses of graphic felspar and quartz with crystals
of muscovite, associated with exposures of coarse muscovite
granite.
10.10
„
Biotite gneisses much crumpled and
folded, exposed in bed of stream, strike N.W.
10.15
„
Quartzites and muscovite pegmatites.
10.20
„
Intrusive mass of muscovite granite with
large crystals of graphic quartz and felspar in a fine grained
granitic matrix; banded in places with coarse and fine layers.
10.40
„
Exposures of quartz-muscovite
pegmatite.
11.00
„
Soft micaceous schist, strike N. 10° W.,
foliation vertical.
11.50
„
In bed of river and for 500 yards beyond,
garnetiferous mica schists with quartz lenticles, strike N. 10° W.,
dip 15° W.
11.30
„
Schistose biotite granite, with schistose
pegmatite veins forming Sura hill.
11.40
„
Sura.
IV. Agwede to Kabba.
6.30
A.M.—
Left Agwede hill. On top, white granite
usually showing faint foliation and frequently enclosing portions
of dark mica schist; on slope, whitish ribboned gneisses, often
folded and contorted, strike N.E. to E., foliation vertical.
6.40
„
Dyke of graphic pegmatite in gneiss;
sharp junction. Frequent bands of schistose pegmatite in the pale
coloured banded gneisses; in places the gneisses are hornblendic
with knots of fine-grained hornblende gneiss or schist.
7.00
„
Ascent to plateau over contorted
gneisses.
7.20
„
Same contorted gneisses passing into and
invaded by white schistose granite; wooded plain, few
exposures.
8.20
„
Occasional exposures of similar contorted
and granitised gneiss; strike E. 10° N. to E. 10° S.
9.50
„
Faintly banded and indistinctly foliated
biotite gneiss.
9.30
„
Odouape.
10.30
„
Level surface of composite gneiss.
11.30
„
Much crumpled and folded biotite gneiss;
strike N.–S. Followed by garnetiferous biotite gneiss.
11.45
„
Composite gneiss; foliation vertical;
strike N. 10° W.
12.00
„
Kabba.
V. Izon to Abuja (Nassarawa province).
6.25
A.M.—
Left Izon. Open grassy or thinly-wooded
plains for two hours; no exposures.
8.20
„
Hills; ascent of 300 feet; biotite and
hornblende gneiss and amphibolite; epidote-hornblende gneisses; in
places twisted and contorted and extensively invaded and injected
by masses of biotite granite; strike varies between N. 10° E. and
N. 10° W.; foliation mostly vertical.
Hills; soft micaceous gneisses, strike N.
10° W., foliation vertical or high to E.
10.35
„
Hard biotite gneisses, strike N.–S.,
foliation linear and vertical.
10.45
„
Ribboned and banded biotite gneisses with
quartz veins parallel to or crossing the foliation; strike N.–S.;
foliation vertical.
11.00
„
Abuja granite with foliated margin;
vertical crush lines in granite striking N.–S.
11.15
„
Abuja Residency.
VI. Boi to Ronn (Southern Bauchi).
6.30
A.M.—
Left Boi; hornblende epidote gneisses
broken and invaded by granite; in places much folded and
crumpled.
6.40
„
Felspathic gneiss, strike E.–W., dip high
to S.
7.20
„
Complex of quartz epidote rock,
amphibolite, epidote hornblende schist, and gneiss, in places much
granitised; general strike E. 10° N.
7.45
„
Hummocks of banded gneiss; open country;
few exposures.
8.40
„
Hornblende epidote gneisses.
8.45
„
Goshin Duchi. Hills of biotite granite
behind town.
9.30
„
Quartz schists, fine grained gneisses,
epidote hornblende and quartz hornblende rocks, much granitised in
places veins and dykes of pegmatite and basalt.
9.20
„
Blocks of basalt and porphyry.
9.35
„
Hills of fine grained biotite
granite.
10.15
„
Granite rocks, in places schistose.
10.45
„
Rocky, hilly country; banded and
contorted gneisses micaceous and hornblendic; general strike E. 10°
S.
11.10
„
Ronn.
It is unnecessary, however, to quote the
details of particular traverses at greater length, especially as
such descriptions serve only to show the character of the phenomena
to be correlated. It is sufficiently evident that there are within
the Protectorate two fairly well-defined series of crystalline
rocks, a less metamorphosed series of quartzites, schists and
gneisses representing an original sedimentary succession, and a
more highly metamorphosed series of banded and contorted gneisses,
whose relation to the less metamorphosed group will be discussed in
the sequel. The less metamorphosed gneisses present in the field
somewhat granular and incoherent outcrops and may be conveniently
termed the “softer” gneisses, while the “harder” or more highly
metamorphosed gneisses are readily distinguishable by the smooth
and compact character of their exposed surfaces. The softer
gneisses appear to disintegrate readily into a gritty sand under
present conditions, while the harder gneisses exfoliate and pass
into angular débris. Both series are extensively invaded by
granitic and other igneous intrusions.
In the province of Borgu the superficial accumulations are so
extensive and the exposures so few and so widely scattered that the
general character of the underlying rocks is somewhat difficult to
ascertain. It is probable, however, that, like the neighbouring
region of Nikki,[18] there is a floor of well foliated
and granitoid
gneisses of varied origin, much pierced and invaded by veins of
granite, pegmatite, quartz porphyry and quartz. Tremolite schists
are known from Mori, and sheared porphyries from Kanikoko. The
occasional inselberge are composed invariably of grey porphyritic
granite. At Jebba the gorge of the Niger is carved out of a series
of sheared quartzites and pebbly grits with the shear planes at
times well spaced and the deformed pebbles still distinctly
visible. Secondary mica has been sparingly formed along the planes
of movement, together with knots and streaks of tourmaline and
tourmaline quartz. The predominant strike is N.–S., and the
prevailing dip of the foliation 65° W. The well-jointed Juju Rock
is composed of a laminated quartzite schist, which splits readily
into thin curving slabs, whose surfaces are coated continuously
with white mica and show a well-defined strain-slip corrugation.
Westward the Jebba quartzites pass into highly sheared quartz
schists, quartz mica schists and granulitic gneisses, which are
followed by crumpled and contorted muscovite biotite schists and
gneisses and by granite at the river Awo. The quartzites and
granulitic gneisses are produced along the strike to form the range
of low wooded hills which stretches southward from Jebba towards
Ilorin. To the east of Jebba the quartzites are abruptly limited by
hard banded and composite gneisses pierced by masses of porphyritic
biotite granite.
To the north of Jebba the Kailema granite, over which the track
leads from Jebba to Bajibo, is a massive hornblende granite,
porphyritic with white and pink felspars and somewhat finer-grained on the
margin. The granite has evidently pierced a series of hornblende,
epidote and talc schists and micaceous gneisses which outcrop
imperfectly to the east. Still farther north, at Wuru, the channel
of the Niger is occupied by a hard and coarsely foliated epidote
hornblende gneiss, much pierced by irregular veins of schistose
granite and pegmatite. The foliation planes of the gneiss are
vertical and strike approximately N. 20° E. On the left bank the
gneiss is succeeded by thinly bedded quartzites and quartz schists
with scattered knots, lenticles and veins of quartz and finely
fibrous tourmaline. Intercalations of much corrugated and wrinkled
mica schist bear witness to differential movements along certain
lines. The whole series, which strikes N.–S. and dips vertically or
at high angles to the east or west, has apparently suffered a rapid
and repeated folding. These rocks are followed by a series of
fine-grained green and yellow quartzites and well-cleaved slaty
schists with numerous quartz reefs, which pass eastward into a
complex of hornblende gneisses, epidote hornblende gneisses,
amphibolites and fine grained schistose granites which disappear at
Kumboji under the sandstones of southern Kontagora.
At Bussa on the right bank of the Niger, near the Residency,
there are exposures of much decomposed shaly phyllites, vertical or
highly inclined to the east and striking N. 10° E. These are
followed on the left bank by a thick series of little altered grey
and greenish granular quartzites, which in places become pebbly and
conglomeratic and enclose bands of greenish trachyte, also showing little or no sign
of shearing. Further exposures of black and coarsely-sheared
trachytes are found in the swamps between Bussa and Libata.
Eastward the quartzites are much invaded by masses of dark green
diorite and by thick veins of tourmaline quartz, the latter showing
every variation between a banded tourmaline quartz rock and a
felted mass of finely fibrous tourmaline. Nearer Auna these rocks
are succeeded by well-foliated augen gneisses which are overlaid by
the sandstones of southern Kontagora.
To the north of Bussa phyllites are again exposed in the low
rounded hills to the north-west of Ngashki, where they strike
N.–S., and possess a high dip of 65° to the east. Locally, however,
the strike may be directed as much as 35° to the east of north,
indicating that the rocks do not pursue a constantly rectilineal
direction from south to north, but rather possess a zig-zag
character with short and sharp deflections towards the east. The
phyllites, which are mostly grey in colour, but in places mottled
with large irregular whitish spots on a grey ground, pass eastward
into soft grey sericitic schists with crystals of magnetite and
pyrites. The whole series is pierced by quartz tourmaline veins, in
which all proportions are to be found, from almost pure quartz
through banded quartz tourmaline rocks to masses of pure tourmaline
of the finely fibrous variety. Sills of much decomposed vesicular
dolerite are common also amongst the phyllites. Farther east
intercalations of micaceous gneiss become abundant within the
schists, and a red fine-grained granulitic gneiss intervenes
between them and the massive biotite granite of Abara, with its large crystals of
felspar. To the north of Ngashki, phyllites and green slaty schists
with lenticular streaks and veins of quartz and tourmaline quartz
occur near Lokofe. A good exposure in the river Malenda shows
pebbly lustrous schists striking N.–S., and dipping steeply towards
the east. At Yelwa a narrow strip of phyllites and slaty schists on
the left bank of the Niger, passes westward into knotted mica
schists and biotite gneisses in the river bed and eastward into the
granitic gneiss of Bin Yauri. In the country which lies to the east
of Lokofe and Yelwa, and especially in the neighbourhood of Tabachi
and Maibirro, green slaty schists of varying hardness are
frequently exposed, usually associated with harder gritty bands and
much invaded by veins of quartz and tourmaline quartz, and
irregular intrusions of granite. In places also there may be found
intercalations of hard, gritty, carbonaceous and
little-metamorphosed slaty beds. At Chuggu, moreover, between
Tabachi and Maibirro, there is a parallel belt of biotite gneisses,
hornblende gneisses, and epidote hornblende gneisses, while at
Maibirro the slaty schists, here associated with thinly-bedded
quartzites, quartz schists and quartz muscovite schists, striking
N. 35° E., with a dip to the north-west of 65°, are succeeded in
the river Malenda by well-banded biotite gneisses much veined with
felspathic and granitic material. The actual junction of the banded
gneisses with the quartz schists is, however, not exposed.
The rocks of the Niger valley, between Jebba and Yelwa, are thus of many and
varied types. The quartzites and phyllites, the slaty schists and
micaceous gneisses, and the green hornblendic and epidotic beds
represent the sandstones, shales, grits and arkoses of the
sedimentary series, while the sheared trachytes and porphyries, the
schistose granites and the coarse-grained amphibolites represent
the original igneous intrusions within the sedimentary succession.
The whole series strikes fairly constantly N.–S., and forms a
practically continuous belt between Jebba and Yelwa. In most places
there are indications that the rocks have suffered a repeated
folding into fairly sharp anticlines and synclines with vertical
axial planes, the folding being apparently more rapid in the
phyllites and schists than in the quartzites and gneisses. There
has unfortunately been no opportunity of tracing the belt in detail
along the strike, and little is known of the lateral variation or
repetition of the individual members or of the original sequence of
the sedimentary succession. There is no reason to believe that
there has been any increase or decrease in the intensity of the
metamorphism throughout the belt from south to north, and no
indication has been found of the occurrence of any thermal
metamorphism in the neighbourhood of the later granitic
intrusions.
The most remarkable feature, however, about this belt of
schistose rock within the Niger valley is the manner in which the
less metamorphosed series of quartzites, phyllites and schists is
bounded abruptly and locally replaced by a more highly
metamorphosed and much granitised series of well-foliated and
often banded
gneisses. At Maibirro, for example, the schists are abruptly
succeeded by banded gneisses of a type which is nowhere found
between Maibirro and the Niger. Similarly at Wuru and Jebba, the
quartzites give place unexpectedly to composite granitised
gneisses, which must have originated under very different
conditions from those which brought about the metamorphism of the
phyllites and schists. This is a peculiarity of the fundamental
architecture which is repeated in many other parts of the
Protectorate.
Over the greater part of the province of Ilorin the surface
accumulations are so extensive that the nature of the underlying
rocks can be judged only from small and scattered exposures. So far
as known there are no belts of phyllites and quartzites upon the
western or central plains comparable with that of the Niger valley
above Jebba. The predominant rock in central Ilorin is a fairly
coarse-grained, granular and felspathic muscovite biotite gneiss,
sometimes an augen-gneiss with large eyes of felspar, which
disintegrates readily into a felspathic sand. With it are
associated in places thin bands of quartzite, quartz schist, quartz
muscovite schist, mica schist and gneiss, often garnetiferous,
amphibolite and schistose granite, which, however, are neither
sufficiently extensive nor sufficiently resistant to give rise to
any prominent feature upon the surface. The whole series which
strikes approximately N.–S. is much veined by muscovite and tourmaline
pegmatites and invaded by masses of granite usually of a more or
less foliated character. The larger masses rise into turtlebacks or
heaps of boulders, and in their neighbourhood the gneisses have
often been so intimately veined and traversed by the granite as to
produce a composite or banded rock. At Share hard biotite gneisses
with linear foliation are extensively developed and associated with
hornblendic gneisses and amphibolites and cut by tourmaline
pegmatites.
In the Orissa hills, which lie to the east of the great plain of
Ilorin, the softer gneisses enclose for the first time a prominent
series of quartzites, quartz schists, quartz mica schists and
granulitic gneisses which, with their general meridional strike and
high dip to the west, are very similar to those which form the low
hilly ground to the south and west of Jebba. The Shappa hills which
succeed the Orissa hills to the south are composed in great part of
biotite and hornblende granites, coarse-grained, fine-grained and
porphyritic, which appear to be intrusive into a series of hard
biotite and hornblende gneisses, in places well-banded and much
veined with pegmatites, which occupies the south-eastern corner of
the province. Around Eri the softer gneisses are again well
developed and extensively exposed, the series here consisting of
thin quartzites, felspathic gneisses and much contorted and
occasionally garnetiferous micaceous gneisses and schists, with
interbedded hornblende schists and irregular masses of
coarse-grained amphibolite, probably of intrusive origin. The whole
series is much
pierced by veins of quartz, tourmaline quartz and muscovite
pegmatite, the latter in places carrying tinstone.
Plate VI
Yendam Hill, Yola Province.
Craters at Mbol.
The softer gneisses and schists are found
everywhere in northern Kabba between Eri and the Ilorin border and
the sandstone plateaux which fringe the Niger. The usual succession
of thinly bedded micaceous and felspathic gneisses and hornblende
schists is varied by the intercalation of several belts of platy
muscovite schists and quartz muscovite schists, whose deeply
decomposed surface usually supports a luxuriant forest growth.
Within the area mentioned there are at least four of these belts,
more or less well-defined and continuous along their strike from
north to south. The first runs southward from the neighbourhood of
Idofin by way of Kponyon, Onore, Ote, and Omwo into southern Kabba:
the second strikes southward from Okoro and Iye, skirting Sanlu to
the east and Moppa to the west: the third runs from Shale and
Oferre by way of Lukke, Olle, and Appe to the neighbourhood of
Odouape, and the fourth is exposed along the track leading from Wa
to Jakura on the north side of the Mimi valley. While the fissile
quartz muscovite schist may be taken as the index rock of the
various belts, each belt is in itself a complex of muscovite
schists, quartz muscovite schists, quartz schists, quartzites,
biotitic and hornblendic schists and other rocks and the
proportions of these rocks vary not only in the successive belts
but at different points within the same belt. In the third belt for
example there is along with the muscovite schist a great
development of
quartzites and quartz schists around Lukke, where they form
prominent rocky hills (v. Traverse No. III. p. 66). Farther north several bands of chlorite and
talc schist are found in the same belt in the neighbourhood of Iye,
while in the fourth belt between Wa and Jakura there are
intercalations of such other rocks as quartz magnetite schist,
magnetite hornblende schist, and crystalline limestone (v.
Traverse No. II. p. 65). The limestone is a
white coarse-grained saccharoid marble which may be traced for at
least 1,000 feet across the strike without affording any indication
of repetition by folding. On the margin the marble is distinctly
schistose with the foliation planes striking N. 10° E. and dipping
at a low angle towards the west. The rocks in the neighbourhood are
somewhat irregularly folded with the foliation planes as a rule
parallel with the bedding. Their junction with the marble is not
exposed, but if the marginal foliation of the latter be also
parallel with the bedding, the actual thickness of the original
limestone must have been very much less than 1000 feet. The marble
is very pure, consisting almost entirely of calcite with occasional
streaks and segregation bands of quartz-epidote-calcite-idocrase
rock representing the original impurities within the limestone.
Minute spots of graphite are abundant throughout and on the margin
streaks and knots of quartz and pyrites are common along the
foliation planes. In places the marble is irregularly crossed by
later pegmatite veins.
The tracts which intervene between the various belts are
occupied by a variety of soft micaceous and hornblendic schists and differ from the
belts already described only in the comparative rarity of muscovite
schists and quartzites. The absence of sufficiently detailed
investigation, however, makes it impossible to say to what extent
the various belts described above represent repetitions of one and
the same belt, or to what extent they represent the original
sequence of the sedimentary succession. While the average strike of
the whole series is meridional, its undulating or zigzag character
is evident from the occurrence of individual exposures showing
every angle between north and east. The different members of the
series, while frequently vertical, may dip at various angles to the
east or west or even in places approach the horizontal. Individual
bands are frequently puckered and contorted and the whole series is
invaded in places by extensive masses of biotite granite which rise
into turtlebacks and groups of rounded hills. The granites are
usually foliated on the margin and sometimes throughout and
frequently show at their junction with the gneisses and schists a
narrow zone of injected rock. Small detached exposures of the
harder banded gneisses, elongated upon a parallel strike, are
distributed irregularly throughout the softer series. The gneisses
and schists are abundantly pierced by muscovite and tourmaline
pegmatites and veins of quartz and tourmaline quartz, the latter in
the neighbourhood of Iye being minutely banded as in the case of
some of the tourmaline quartz veins at Bussa and Ngashki. With the
pegmatites appear to be connected some small intrusive masses of
non-foliated granite which become more numerous in the eastern portion of the
area under consideration (v. Traverse No. I. p. 64).
The striking feature of the architecture of the high plains of
central Kabba is the manner in which the softer gneisses of the
north with their intercalated bands of schists and quartzes are
invaded, and in part replaced, by masses of granite and of hard
banded and composite gneisses. The first belt of schists can be
traced southward with intervals from Omwo to the neighbourhood of
Arigidi and Sequarre, where the soft micaceous schists carry much
staurolite. Harder gneisses however appear to bound this softer
belt irregularly to the west and east. The prolongation of the
second belt of schists in northern Kabba is probably represented by
two narrow strips of quartzite enclosed in the harder gneisses
between Kabba and Lupa. The third belt appears to be somewhat
abruptly replaced in the neighbourhood of Odouape by a complex mass
of granites and hard composite and banded gneisses which extends
westward across the high plains of Kabba from Agwede and Shokko
Shokko to Ikinre and Omwo (v. Traverse No. IV., p. 67), and is apparently prolonged southward into
the Egbira hills. The fourth belt appears to suffer much lateral
variation, and is represented on the main road between Lokoja and
Kabba only by some thin bands of muscovite schist enclosed within a
series of micaceous and felspathic gneisses.
In southern Kabba the few facts at our disposal seem to indicate
the almost complete absence of quartzites and muscovite schists and
a repeated alternation of the softer and harder gneisses, the whole
series being pierced by pegmatite veins and intrusive masses of
granite. Quartzites and quartz schists reappear, however, in the
Kukuruku hills on the borders of Northern and Southern
Nigeria.[19]
The lower plains which stretch eastward to the Niger from the
escarpments of the Egbira and Agwede hills are occupied very
largely by gneisses of the softer series. Quartzites, quartz
schists and quartz-muscovite schists occur within the series, but
their distribution over the plains is so irregular, and their
prolongation along the strike so inconstant, that they cannot in
any way be correlated with the quartzites and schists of either the
third or fourth belts in northern Kabba. It would seem, indeed,
that instead of the regular alternation to be found in the north,
there is over these eastern plains a greater mixing of types and a
greater variation in character of the individual members when
traced along the strike. Quartz-magnetite and quartz-hæmatite
schists are fairly common, typical occurrences being found in the
neighbourhood of Bangede and around Okuruku and Anamu near Okeli,
and on the plains to the west of the Againyi hills, as well as at
the foot of their western slope. Amongst the granites and granitoid
gneisses which form the escarpment of the Egbira hills, bands of
felspathic gneiss and micaceous schist are frequently enclosed, and
on the plains the gneisses and schists are crossed in the usual
manner by pegmatite veins and pierced by masses of more or
less foliated
granite, which form the prominent isolated hills which diversify
the surface of the plain. Here and there also may be found a small
exposure of composite or banded gneiss, but it is only in the
immediate neighbourhood of the Niger that the harder gneisses
attain any extensive development. There they form a narrow belt,
marked by prominent intrusions of granite, which stretches
southward from Lokoja along the right bank of the river, and adds
picturesqueness to the Niger valley between Igbo and Itobe
(p. 6). At Lokoja the harder gneisses are
exposed in the river bed, and the softer gneisses and schists at
the foot of Mt. Patti. The average strike is N.E. to S.W., and the
whole series is much pierced by veins of granite and graphic
pegmatite, the latter containing frequent knots of magnetite.
To the east of the Niger the softer gneisses reappear in the
narrow strip of crystalline rocks in Bassa province which fringes
the left bank of the river. Typical exposures are found in the
neighbourhood of Igbo and Itobe, where the gneisses are associated
as usual with early foliated or partially foliated granites and
with later intrusions of muscovite and tourmaline granites and
pegmatites, the tourmaline occurring as a rule in large and
well-formed crystals. At Igbo the micaceous gneisses enclose a
number of narrow bands of fine-grained, crystalline limestone, each
3 to 4 feet in thickness. The limestone varies much in colour and
hardness, the purest varieties being blue, while weathering and
impurities produce grey and brown tints. Much of the
limestone is gritty,
but the commonest accessory is a pale-coloured tremolite in
rosettes and felted veins of fibrous crystals and in scattered
well-formed prisms.
The crystalline rocks of Kabba and Ilorin thus present
considerable resemblances in individual types to those of the Niger
valley above Jebba. The most striking differences are the
occurrence of such new types as crystalline limestone and magnetite
and hæmatite schist in eastern Kabba and Bassa, and the apparent
absence in the southern provinces of the characteristic phyllites
and slaty schists of the Niger valley to the north. The place of
the phyllites, however, is probably taken by the muscovite and
quartz-muscovite schists, which form such prominent members of the
metamorphic series in northern Kabba. If so, there would seem to
have been a slight increase in the intensity of the regional
metamorphism in these southern provinces as compared with the Niger
valley above Jebba. Of thermal metamorphism, on the other hand,
there is little trace. The schists and gneisses in the
neighbourhood of Sequarre in western Kabba and in several
localities in northern Kabba and Ilorin contain a considerable
quantity of staurolite and other metamorphic minerals. Their
distribution, however, appears to possess a linear character which
is somewhat inconsistent with a contact origin, while sufficiently
extensive igneous intrusions, to whose influence their local
development might be ascribed, have nowhere been definitely
recognised in the vicinity. The numerous intrusive masses of both
foliated and unfoliated granite throughout these provinces appear to have induced little
or no contact metamorphism in the surrounding gneisses and schists.
An exception to this rule may perhaps be made in the case of the
tourmaline granite behind Igbo on the Niger, to whose influence may
possibly be ascribed the extensive development of tremolite in the
neighbouring crystalline limestones. At the same time it should be
remembered that the occurrence of tremolite in metamorphic
limestones cannot be taken as an invariable indication of contact
action.
The irregular association of a less metamorphosed series of
quartzites, schists, and gneisses with a more highly metamorphosed
series of well-foliated, banded and contorted gneisses, exhibited
by the crystalline rocks of the Niger valley above Jebba, forms
likewise a striking feature of the crystalline rocks of Kabba and
Ilorin. The banded gneisses may be either biotitic or hornblendic
and the lighter and darker bands may alternate so regularly and
rapidly as to give rise to a striped or ribboned appearance even in
the hand specimen. They are for the most part of a medium and even
grain and never porphyritic, although sometimes they enclose
lenticular masses of finer grained biotite gneiss or of
amphibolite. As a rule they are much invaded by granite and
frequently rendered composite by thin granitic intrusions. The
foliation, which may be parallel or oblique to the striation, is
their least conspicuous feature. Pegmatite veins are usually
abundant and frequently foliated. As already described (p. 74) these hard banded and contorted gneisses are
found irregularly replacing the softer gneisses both along and across the
strike.
So far as known there are no schistose rocks in Kabba and Ilorin
comparable to the sheared trachytes of the Niger valley and the
sheared porphyries of Borgu. Rare occurrences of thin basaltic
veins have been observed, but as a rule the later intrusions in
Kabba and Ilorin take the form of small masses of red or white
aplitic granite or of dykes and veins of muscovite and tourmaline
pegmatite or of graphic granite. These later granites have usually
a peculiar granular and incoherent character and never possess
chilled margins. Frequent occurrences have been noted of the
granular granite acting as a kind of ground mass to large scattered
porphyritic masses of graphic quartz and felspar (v.
Traverses Nos. I, II, III, pp. 64–66).
Frequently also the granites show an ill-defined banding caused by
an alternation of coarser and finer grained layers. It is possible
that the muscovite and tourmaline pegmatites, the veins of graphic
granite, the quartz reefs, the banded tourmaline quartz rocks, and
the small intrusive masses of acid granite may all possess a
certain genetic connection. Only at Igbo, however, does there occur
an extensive mass of reddish muscovite tourmaline granite which to
some extent unites the characters of all the dyke rocks and
represents in a general way the original magma from which the
various dykes and veins and intrusive masses have been
differentiated. The abundance of tourmaline throughout Kabba and
Ilorin led to the hope that tinstone would in places be associated
with it, but only at
Eri in eastern Ilorin have the pegmatites proved stanniferous. The
quartz reefs also have nowhere been found to yield more than the
minutest trace of gold.
The peculiarities of structure which have been remarked in the
western provinces are continued throughout the larger crystalline
area in the centre of the Protectorate. To the east as to the west
of the Niger there has been an irregular and local replacement of a
series of softer gneisses, quartzites, and schists by a complex of
harder composite and often well-banded gneisses. There is also in
the western part of the central area within the softer gneissic
series a well-defined grouping of the quartzites and phyllites or
micaceous schists into regular and definite belts which preserve
their identity for considerable distances along the strike. In the
east, however, this regular grouping appears to be absent, the
quartzites and mica schists being distributed irregularly and in
short and narrow bands amongst the felspathic and micaceous
gneisses.
In the east of Sokoto province there are at least two broad and
well-defined belts of phyllites and indications of a third. The
first is found between Bukwium and Anka near the Kontagora border,
and is well exposed in the Anka river. The strike of the phyllites
varies from N.–S. to N. 20° E. Surface creep here and there makes
the dips uncertain, but the inclination is usually high, and is
sometimes to the
west, sometimes to the east. This fact probably accounts for the
great width of the belt. It is not known whether the phyllites
extend to the east of Anka, but after crossing them at right angles
to the strike for thirty miles to Bukwium they are still in full
force, and the western limit is unknown. There are numerous
quartzite bands, especially about Anka and to the south of it. The
phyllites are soft, fine-grained and dark-coloured, and are
traversed by abundant quartz veins, some of which carry pyrites. It
is interesting to note that the phyllites are only exposed in the
neighbourhood of the river. Away from the river, on the level
plain, there is no indication of them, and this fact is of
significance when considering the possible extension of the
phyllite belts. There is a numerous and interesting series of
igneous rocks within the phyllite area, including such types as
hornblende granite and syenite, pegmatite and quartz porphyry.
These rocks, as a rule, show no sign of crushing, and are probably
later intrusions. Some of the porphyries, however, show an
imperfect foliation, and appear to have been exposed to some of the
later crustal movements. Bands of amphibolite and veins of
tourmaline quartz also occur within the phyllitic series. (A.
L.)
The second group of phyllites and quartzites forms a belt of low
hilly country striking N.N.E.–S.S.W. across the Sokoto-Kano caravan
route between the villages of Maradu and Karakai, and continues
northward at least as far as Bazai. The series attains across the
strike a breadth of about thirteen miles, and consists of a succession of slaty schists
and knotted phyllites and quartzites, much pierced by quartz veins
and thin bands of schistose granite, and dipping fairly constantly
in the west at 70° E.S.E., but gradually heeling over in the east
to the W.N.W. Towards Karakai the quartzites predominate and
contain much tourmaline, and many samples may be found showing
quartz and tourmaline or quartz, slate and tourmaline in
alternating ribbon-like bands. The third belt is very imperfectly
exposed on the borders of the province between Woneka and Duru, and
is represented only by some scattered outcrops of slaty phyllite
and mica schist.
Between Maradu and the margin of the Sokoto sandstones to the
west, the softer gneisses are exposed upon the sandy plains in a
very fragmentary manner, and appear to consist of micaceous and
hornblendic gneisses and schists, and of hornblende epidote
gneisses and amphibolites with numerous small granitic intrusions.
The rocky knobs and kopjes two miles west of Maradu are formed of a
coarsely banded volcanic agglomerate, dipping eastward at angles of
30°–40°. The agglomerate may be traced to within a mile of the
village, where it gives place to a thinly bedded series of red ashy
sandstones and ash beds. East of the rest camp, these beds strike
N.N.E. and dip at 40° E.S.E. while west of the camp they strike
E.N.E. and dip at 30° S.S.E. The ash is exposed in, and to the west
of, the village, but to the east its place is taken by a much
broken and shattered granitic rock. The actual junction, however,
was not observed.
Such evidence as there is points to the former existence of a
volcanic neck or vent in this locality.
Between Karakai and Woneka the surface accumulations obscure the
rock for long distances, especially to the west and east of the
granite hills of Kotorkoshi. Around Bungudu, however, there are
numerous exposures of a coarse-grained composite gneiss which
probably extends eastward to the boundary of the province. Beyond
Duru a grey biotite granite with a distinctly foliated margin forms
the broken hilly country between Runka and Kurefi, but between
Kurefi and Sani the granite gives place to a group of dark-coloured
biotite gneisses before the crystalline rocks disappear under the
sandstones of Katsina. Knobs and hummocks of granite, however,
protrude through the sedimentary rocks round the walls of the town,
and Barth[20] has reported the occurrence of similar mounds
and ridges immediately to the north. To the south-east of Katsina
the surface rock appears to be everywhere a reddish granite as far
as Duru, but beyond Duru the superficial accumulations so
effectually cover up all but the occasional kopjes and boulders of
granite that the character of the underlying rocks is difficult to
ascertain. From the scattered fragments of quartzite, gneiss and
amphibolite, and occasional granite boulders in the surface drift,
it is probable, however, that for a considerable distance to the
west of Kazaure the floor is formed of the sedimentary gneisses
with the usual granitic intrusions. The hills around Kazaure are
composed of hard blue
quartzite much pierced by later veins of white quartz, and striking
N.N.E.–S.S.W. with a dip of 30° to the west. This belt can be
traced southward to the neighbourhood of Kasuan Kuka, where the
quartzites become more micaceous and where some rocky exposures of
granular amethystine quartz probably represent the intrusive quartz
veins of Kazaure. From Kasuan Kuka to Kano, granites and an
occasional quartz vein are the only rocks exposed, while the deep
drift effectually prevents the discovery of the relationship
between the surrounding rocks and the decomposed diorites of the
two flat-topped hills within the walls of Kano. A few miles east of
Kano the crystalline rocks disappear under the sands and clays of
Katagum and Bornu. There is every reason, however, to believe that
they are continuous to the north-east with the granites, gneisses
and quartzites of Machina, Zinder and Mounio.
The two detached hills, Dala and Kogon Dutsi, the landmarks of
Kano, with their precipitous slopes and tabular summits, 150 feet
above the level of the plain, present from a distance a decidedly
sedimentary appearance. Clapperton,[21] indeed, described them as
consisting of “argillaceous ironstone mixed with pebbles and a
rather soft kind of marl,” while Dala, the eastern mount, “is of
clay-ironstone and conglomerate lying upon a bed of soft light clay
apparently mixed with vegetable remains.” Closer investigation has
shown, however, that both hills are of crystalline origin, Dala
being composed entirely of earthy decomposition products with a gritty
ferruginous cap, while Kogon Dutsi still retains cores or kernels
of undecomposed diorite in the midst of the friable earthy material
which forms the greater part of the hill. The felspar and
hornblende of the original diorite have decomposed in the lower
part of Dala into a whitish clay streaked and spotted with dark
green chloritic material. In the upper part the colouration is more
uniformly white or yellow, the bulk of the iron having been leached
out to form the red ferruginous cap. That the latter is itself
composite, however, is evident from the occurrence of gritty
grains, subangular fragments of quartz and rolled pebbles of
ironstone in the vacuolar and concretionary crust. Kogon Dutsi
presents a similar composition, with the exception that the
material becomes coarser and more granular in the neighbourhood of
the kernels of undecomposed rock. The extensive decomposition of
the crystalline rocks to which the earthy hills of Dala and Kogon
Dutsi bear witness will be found in the sequel (p. 201) to be an important factor in the evolution
of the characteristic scenery of the high plains of Hausaland.
Plate VII
Craters in the Mboi Hills.
A Crater in the Mboi Hills.
Around Kontagora and beyond the northern limit of
the sandstones which occupy the southern portion of the province of
the same name, there are found hard biotite gneisses in places well
striated with pink felspathic and dark biotitic bands and crossed
by numerous pegmatite veins. To the east the crystalline floor is
deeply covered with drift, and outcrops are comparatively rare. In
the Beri river, however, there is a good exposure of soft
felspathic biotite gneisses, with knots and bands of pegmatite, while at
Beri a pink granite forms a compact group of rocky hills. The
granite possesses a distinctly foliated margin, and is followed to
the east by hard hornblende and biotite gneisses much pierced by
granite veins, which may be traced to within six miles of Bobi
before they are replaced by the softer micaceous series. On account
of the extent of the superficial accumulations, however, these
softer gneisses are well exposed for the first time only in the
rocky bed of the Kara river which forms the boundary between the
provinces of Kontagora and Zaria.
Though the structure of the north-eastern part of Kontagora
province is complicated in detail, it may be described in general
terms as a succession of gneiss and quartzite bands alternating
with one another. They strike a few degrees east of north, and
probably pass to the east of the Bukwium-Anka phyllites. The
quartzites are frequently tourmaline-bearing, and one band west of
Kotonkoro carries scales of graphite. This fact is interesting in
connection with the graphitic schists reported from a neighbouring
part of Zaria.[22] The gneisses are usually biotite gneisses,
sometimes markedly banded, at other times indistinctly so.
Schistose rocks are seen in places, quartz schists being prominent
to the south of Mahorro, where talc schists are also seen. The
gneiss and quartzite country is traversed by pegmatite and quartz
veins, but intrusions of unfoliated granite are rather uncommon.
Most of the turtle-backs are foliated throughout, differing in this respect from
those of many other parts of the Protectorate. Where the Sokoto
road leaves the province, the boundary is marked by a high sharp
ridge of amphibolite. (A. L.)
A tract of broken hilly country, almost entirely carved out of
the softer gneissic series, occupies the south-western corner of
Zaria province, and a diagonal traverse across the hills from
Moriga on the Kara river by way of Garun Gabbas and Zungeru to Kato
and Mikonkeli presents a particularly instructive section. The
hills which begin about ten miles south-east of Moriga consist
between Inga and Garun Gabbas of a succession of quartz schists,
quartz muscovite schists, micaceous and hornblendic schists,
felspathic gneisses and amphibolites, pierced by numerous quartz
reefs and veins and by a few dykes of basalt and porphyry, the
whole series possessing an average strike of N. 10° E. Between
Garun Gabbas and the Kaduna river ridges of quartz and quartzite
rise conspicuously at intervals out of the pebbly drift which
covers the surface. In the railway cutting on the right bank of the
Kaduna near Zungeru, quartzites and quartz schists are associated
with bands of a hard and minutely foliated granulitic gneiss
containing knots and lenticles of quartzo-felspathic and chloritic
material. In the river bed below the railway bridge the most
conspicuous rock is a well-bedded white quartzite striking N. 30°
E. and dipping at an angle of 64° to the south-east (Plate VIII,
Fig. 2). The quartzite varies much from a
massive variety with knots of pyrites and scattered flakes of white
mica through a streaky lustrous micaceous quartzite with linear
foliation planes, into a well-foliated quartz muscovite biotite
schist. The different varieties are often arranged in alternating
parallel bands and the whole series is pierced by occasional narrow
veins of quartz. The trench which at the Kaduna bridge forms the
low water channel of the river has been carved out of a band of
hornblende schist intercalated along the strike of the quartzites.
The schist, which in places is composed almost entirely of
hornblende, but in others interfoliated and streaked with felspar,
has been more easily worn away than the quartzite, and grooved and
pot-holed by the action of the river. The quartzites and quartz
schists, with intercalated sills of hornblende schist, continue
eastward to the adjoining channel of the Dago, where they are
replaced by finely striated biotite gneisses. At Zungeru micaceous
and felspathic gneisses are mingled with soft, thinly bedded and
repeatedly folded hornblende mica epidote gneisses or schists. The
whole series is pierced in the usual manner by veins of quartz,
pegmatite and granite and strikes in a general way N. 10° E. with a
dip of 60° to the east. Immediately east of the railway station,
however, these rocks give place to well-foliated granulitic
gneisses with knots of felspar, striking N. 20° E. and dipping at
45° E. These gneisses, which probably represent more completely
altered portions of the sedimentary series, are in places well
striated, with the bands wavy and contorted and crossed irregularly
by veins of quartz. They are followed in the vicinity of
Kwarra-town by coarsely schistose grits which are among the
least altered rocks
within the Protectorate. Between Zungeru and Gidan Motu there are
numerous exposures of green hornblendic beds similar to those at
Zungeru. They become, however, more fissile and contorted towards
the east. The strike oscillates between N.N.E. and N.N.W., and the
dip varies between vertical and high angles to the east or west,
while at Gidan Motu there is an intercalated belt of platy biotite
gneisses which strike N. 60° W. and dip at 60° S.S.W. Between Gidan
Motu and Kato green slaty schists and phyllites are extensively
developed and steeply folded at Kato into sharp anticlines and
synclines which possess an average strike of N. 20° W. The
phyllites enclose much quartz in lenticles and veins and contain
abundant knots and spots of pyrites. Beyond Kato the phyllites are
succeeded by soft mica schists, which in turn give place to
micaceous and felspathic gneisses. The latter around Mikonkeli,
Minna and Paiko are replaced by hard composite and in places banded
rocks and invaded by extensive masses of granite which usually show
a distinct marginal foliation.
Southward the Zungeru belt of quartzites and phyllites
disappears under the sandstones of Nupe, but any correlation with
the quartzites and schists of Kabba cannot yet be attempted.
Northward the belt has been followed as far as Birnin Gwari and
from its pronounced development in the neighbourhood of that place,
it is probable that it extends considerably farther north again.
For twenty or thirty miles north of Tegina there are a number of
granite hills strung
together on a roughly north and south line, the granite having
apparently been intruded along the strike of the schistose rocks.
The granites become foliated locally. Between Bugai and Koriga
there is a twenty mile belt of phyllites and subordinate mica
schists which runs northward to Birnin Gwari, where the phyllites
form a number of sharp north and south ridges and are excellently
exposed, both in the sides of the ridges and in the intervening
streams. The phyllites are traversed by many quartz veins and gold
is widely disseminated in the streams. At Koriga there is an
extensive development of hornblende gneiss. To the east are mica
schists, and graphitic schists are reported from the
neighbourhood.[23] The rocks which rise above the plain in the
vicinity of Zaria are all of granite. (A. L.)
So far as known, the Zungeru belt is the last prominent group of
phyllites which can be traced within the Protectorate. To the east
their place is taken by muscovite and biotite schists, which are to
be regarded as more completely reconstructed representatives of the
original shales, the metamorphic action having apparently been
somewhat more intense in eastern and southern Zaria and Nassarawa.
Several belts of quartzites and schists are enclosed in the usual
manner within a complex of softer and harder gneisses to the east
of Minna and Mikonkeli. At Kuta, for example, there is a belt of
quartz muscovite schist passing on the one hand into knotted
micaceous schists and on the other into quartzites and
carrying much
sillimanite and magnetite and large lenticles and knots of quartz
and tourmaline quartz. The whole series strikes approximately N.
30° E. and dips at 45° N.W. Again between Chiri and Guni there is
much soft micaceous schist rising into low rounded and well-wooded
hills, striking generally N.–S., well frilled in places, with a
high content in ilmenite and numerous knots and veins of tourmaline
quartz. Farther east the hills which run from the neighbourhood of
Dorka to Kaderri and Kaserimi are composed very largely of
quartzites, quartz schists, quartz muscovite schists and micaceous
gneisses, striking N. 10° W., dipping at an angle of 60° W., and
pierced by quartz veins, muscovite pegmatites and granites. Between
these more prominent belts occur numerous smaller exposures of
schists and quartzites and of both the softer and the harder
gneisses. To the east of Kaserimi and Duchi and in the valley of
the Kaduna the surface is so deeply and extensively covered with
drift that little rock beyond the marginally foliated biotite
granites of the isolated hills is available for investigation.
There are, however, indications from occasional fragments in the
drift and rare exposures in the stream courses that a considerable
extent of the buried surface is composed of a softer gneissic
series. To the east of the Kaduna, the Ruruma and Kerko hills which
rise so prominently from the south-eastern plains are composed
throughout of a coarse-grained biotite granite, locally carrying
hornblende in addition and in places distinctly foliated on the
margin. The junction with the gneisses is only rarely seen
on account of the
thick masses of drift which envelop the base of the hills. At Guri
on the eastern slope of the Ruruma hills there are exposures of
finely laminated hornblende epidote gneisses, while the granites
and the marginal rocks are pierced by numerous dykes of basalt,
dolerite and quartz porphyry.
The high plains of south-eastern Zaria are occupied by mica
schists and gneisses, with granite hills in one or two localities.
Along the eastern border of the province the Rukuba plateau ends in
a ragged edge of biotite granite which veins the gneisses at its
foot. The country around Jengre and Limoro is interesting on
account of the occurrence of masses of soda granite and especially
on account of numerous intrusions of dyke rocks, felsites often
with flow structure, porphyries and quartz porphyries. Tinstone is
frequently met with in the streams of the district. Similar dyke
rocks are exposed around the base of the Kudaru hills to the
north-west of Leri and there, in addition, dykes and veinlets of
basalt penetrate the granite. These hills are made up of hornblende
granites with syenitic modifications, and locally, towards the
boundary, the granite takes on a distinctly foliated structure,
with rounding of the porphyritic felspars as in augen-gneiss. (A.
L.)
In Nupe, between the Gurara river and the sandstones of the
Niger valley, the crystalline series is introduced by muscovite
biotite gneisses, quartzites, hornblende and talc schists and
amphibolites, pierced by extensive masses of granite around Gaung,
Lefu and Paiko. In the bed of the Gurara, between Gaung and Izon, soft gneisses and
hornblende epidote rocks alternate with hard and well-foliated
gneisses striking a little west of north, and varying in dip from
vertical to 40° or 50° E. Five miles beyond Izon, the Abuja road
ascends over a crumpled and contorted series of biotite,
hornblende, and epidote hornblende gneisses much injected by masses
of biotite granite. Beyond Abuchi a second ascent over soft
micaceous gneisses, striking N. 10° W. and vertically foliated,
leads up to the striated, ribboned and banded gneisses which have
been invaded by the biotite granite of Abuja. The banded gneisses
strike N.–S., and are much granitised in the immediate
neighbourhood of the intrusive rock, while the granite itself is
marginally foliated and crossed by crush lines at the Izon gate
(v. Traverse No. V, p. 68).
Partially foliated granites and banded and composite gneisses
continue to within five miles of Kao, whence a belt of soft
micaceous gneisses and quartz muscovite schists, striking N. 20°
E., stretches eastward to the neighbourhood of Kari. At Kari the
escarpment of the northern tableland is formed of soft biotite
gneisses, much pierced by veins of quartz and tourmaline quartz.
Eastward by way of Kukwi, Anta, Tatara and Jaginde the escarpment
shows much granite, associated both with banded gneisses and with
softer micaceous and hornblendic rocks. The gneisses and granites
are extensively invaded by muscovite and tourmaline pegmatites, and
by irregular intrusions of basalt and dolerite. Between Jaginde and
Darroro quartzites, quartz schists and mica schists outcrop along
the escarpment, and
at Darroro a similar series of thin quartzites and gneisses,
striking generally N.–S., is pierced by extensive masses of
quartz-muscovite-tourmaline pegmatite, and capped by the basalts of
the Morrua plateau. The pegmatite is rudely banded parallel with
the strike of the gneisses, the various bands being distinguished
by their richness in tourmaline. All proportions may be found
between quartz muscovite and quartz tourmaline rock, the tourmaline
in the latter being either in large well-formed prisms, or in thin
alternating bands of felted fibrous crystals. North-east of Darroro
the vertical granite cliffs of the Kagoro massif form the margin of
the Bauchi plateau, while in the neighbourhood of Assab the later
basalts much obscure the crystalline rocks of the lower plains. At
Assab the escarpment is largely composed of biotite granite, in
places porphyritic, and apparently intrusive into a series of
micaceous gneisses which strike N. 10° E. and dip at an angle of
40° to the west.
The northern tableland and the central plains of Nassarawa are
composed very largely of the softer gneisses with scattered
outcrops of harder composite rocks. Quartz muscovite schists occur
in a broad belt on the tableland in the vicinity of Aribi, and
calc-silicate rocks between Kwai and Jaginde. The composite and
banded gneisses reach their greatest development in the Karshi
hills between Abuja and Keffi. The Mada hills and the greater part
of eastern Nassarawa are still unexplored. In southern Nassarawa a
broad belt of quartz muscovite schists, mica schists, hornblende
schists and chlorite schists extends between Gwombe and Buga, the mica schists in the
neighbourhood of Zagabutu being particularly rich in staurolite,
garnet and imperfectly crystallised knots of andalusite, presumably
of contact origin. The whole series has been steeply folded on a
meridional axis and then coarsely corrugated transversely, the
result being that individual exposures may strike at any angle
between E.–W. and N.–S. The average strike is inclined around
Gwombe to the north-west, and around Buga to the north-east. To the
east of Buga the schists give place to the usual varied succession
of softer gneisses, pierced in the usual manner by veins of quartz
and tourmaline quartz, and associated from time to time with banded
and composite rocks and detached masses of granite. Numerous dykes
of basalt and quartz porphyry appear to be associated with the
granites of the Kurudu and Anagoda hills, which lie respectively
west and south-east of the town of Nassarawa.
The crystalline rocks of Bauchi province present similar
characters to those of other parts of the Protectorate. The broad
belts of schists and quartzites found in other provinces are here,
however, conspicuously absent, such types being represented only by
thin bands intercalated within the softer gneisses. Banded and
composite rocks, on the other hand, appear to be present in
somewhat greater proportion than elsewhere, while domes and
turtlebacks of foliated or partially foliated granite (Plate XII,
Figs. 1,2) are in places
particularly abundant. The softer and harder rocks are, however, so
intimately mingled
that a detailed description of the succession is impossible.
Excellent exposures of the harder series are to be found in the
Kogin Delimi below Naraguta in the west of the province, and of the
softer gneisses in the Jarawa hills to the south of Bauchi, and of
both in the broken rocky country between Kanna and Angass in the
southern portion of the province (v. Traverse No. VI,
p. 68). The strike of the gneisses, while
very variable, appears to have a greater tendency to assume an
E.–W. direction in Bauchi than in other provinces, a tendency which
is probably due to the greater intensity of the transverse forces
which have elsewhere corrugated the prevailing meridional strike of
the gneisses and schists. This feature is particularly marked in
the neighbourhood of the Ningi massif, which is elongated
approximately along the strike of the surrounding softer and harder
gneisses.
Another notable feature of the province of Bauchi is the number
and variety of the later igneous intrusions. Pegmatites, quartz
reefs and veins of tourmaline quartz are as abundant in Bauchi as
elsewhere, but the attention is somewhat diverted from these
commoner intrusive types by the presence of an abundant series of
dyke rocks, and of extensive masses of non-foliated granite and
syenite. The pegmatites are in places particularly rich in
ilmenite, rutile and chalcedonic quartz; the dyke rocks present
almost every variety between basalt and quartz porphyry; and many
of the granites are especially rich in soda, and in places show
chilled margins at
their junction with the gneisses. The summit of the plateau around
Bukuru is occupied by one of these extensive granitic intrusions,
which is of peculiar interest from the fact that in places it is
veined and impregnated with tinstone and other minerals. A similar
granite, also carrying tinstone, forms the greater part of the
Kwandokaya hills to the north of Toro. Many intrusions of syenite
are found around and to the east of the town of Bauchi. The Ningi
massif is largely composed of a soda granite, with a well-chilled
margin, accompanied by numerous dykes of granite and porphyry, and
in places carrying tinstone. Similar granites are found at Kila and
Fagam on the borders of Bauchi and Kano, at Shira in Katagum to the
north, and at Gurkawa on the Muri border to the south. The
Murchison Hills are believed to be largely granitic, but they and
the southern margin of the plateau are still practically
unexplored. A small detached area of granite and gneiss, pierced by
numerous pipes of basalt and trachyte, is found around Tangalto and
Chongwom in the south-east of Bauchi province.
The strip of crystalline rocks which runs along the southern
boundary of Muri and Yola has an average width of forty miles and
is separated from the central area by the sedimentary rocks of the
Benue plain. The crystalline floor appears at the surface, however,
in a number of small inliers around Arofu in western Muri, and thus quite near to
the river. Five miles east of Arofu a small area of fresh
biotite-granite about fifty yards in diameter peeps from beneath
the limestone. Immediately north of the town itself there is a
knoll of granitoid gneisses striking north and south. On proceeding
southward along this line of strike limestone is passed over for
three miles, but then the gneiss appears at the surface again, and
this time forms a ridge running southward for at least some miles.
It may well be that other inliers of crystalline rocks occur in the
unexplored Munchi country.
Farther south the transition from sedimentary to crystalline
rocks is not accompanied by any feature in the drift-covered plain.
Though the exposures are not good, it appears that biotite-gneisses
and schists with the customary northerly strike are penetrated by
granite veins. The rocky masses which lie south and south-west of
Takum are probably largely or entirely of granite. The bulk of the
material which forms the scattered hills around and to the north of
Takum is also granite, but here it is inextricably mixed up with
gneiss which is met with in patchy fashion from top to bottom of
the hills. The foothills of Shebshi are schistose grits and
gneisses invaded by granites in the usual manner.[24]
Plate VIII
Exfoliation in the Kilba Hills.
The Gorge of the Kaduna at Zungeru.
Only the northern part of the Mumuye-Manna plateau has been
traversed. The rock there is almost always biotite-granite, but it
is by no means always of the same variety. Occasional quartz veins
are seen on the plateau and also on the lower ground between Numnai
and Panteti, where the quartz carries pyrites. Intrusions of
quartz-porphyry are often seen cutting through the granites,
e.g., east of Numnai, around Shega Mundi hill, around
Panti Baturi and at Yendam Hill. The granites only occasionally
carry included blocks and fragments of gneiss, but on descending to
the plain biotite-gneiss is met with striking N. 10° E., and the
plain from Chukol eastward to the Vere Hills is occupied by
biotite-gneiss, with some intrusions of granite. The Vere Hills
themselves are made up almost entirely of granite, though to a
small extent of gneiss also. Biotite-gneiss is frequently seen
round the base of the plateau, generally much penetrated by granite
or quartz-porphyry. Thus, speaking generally, the plains of
southern Yola are mainly of gneiss and the plateau areas mainly of
granite.
To the north of the Benue plain again, the country around Song,
Goila and Korubi is a biotite-granite area with some conspicuous
dykes of quartz-porphyry running for miles across the country. Near
Zummu porphyry rises just above the soil of the plain and exhibits
flow-structure, while between Zummu and Holma a felsite dyke is
seen cutting through the granite. The hills around Holma are built
of a well-jointed biotite-granite, and the sides of the hills are
concealed by the loose blocks split off along the joints. The
granite of the Kilba Hills, near Pella, is a very fresh,
coarsely-crystalline biotite-granite, and locally, where the Kilba
Hills rise out of the plain to the west, the granite takes on an
augenstructure. In consequence of exfoliation (Plate VIII, Fig. 1) most of these hills rise up as smooth bare
rocks, but some are made up of heaps of blocks, and it is not
possible to explain the contrast as due to a difference in
material. Between the hills around Holma and those around Pella
there is a plain of gneiss about fifteen miles broad. The gneiss
has been formed by the injection of a micaceous gneiss or schist
with innumerable strings and veins of granite. The exposures are
excellent throughout, but the gneiss never forms hills.
The country north of Pella is comparatively uninteresting. It
consists of an extensive plain with isolated hills and rocks of
biotite-granite. The nature of the rocks underlying the plain is
practically unknown. The small area of the Kwaba Hills is composed
of biotite-granite with some quartz-porphyry. (A. L.)
Crystalline rocks, largely of a gneissic character, are exposed
also in southern Bornu around Kwoia to the east of the Gongola and
along the base of the escarpment on the north-western margin of the
Burra volcanic plateau. In eastern Bornu the granite rocks of
Chibuk and Kobshi are the first prominent outcrops of crystalline
rocks to the south of Maidugari. Barth,[25]
however, mentions an isolated granitic cone, Mt. Deladeba, to the
N.N.E. of Isge, while Courtet,[26] Gentil and
Freydenberg[27] note the occurrence of knobs of
crystalline rock
between Lake Chad and Lake Fittri. It is highly probable indeed
that the thick alluvial accumulations to the east and south-east of
Chad and in the lower Shari valley rest for the most part upon a
crystalline floor, provided with scattered relics of Tertiary
sandstone. (J. D. F.)
[27]Gentil and Freydenberg, Bull. Soc. Geol.
Fr. (4), VIII, 1908, p. 44.
CHAPTER III — The Gneisses and Schists . — The Intrusive Rocks . — ( a. ) The older Granites. — ( b ) The Younger Intrusions.
PETROGRAPHY OF THE GNEISSES AND SCHISTS AND
ASSOCIATED IGNEOUS INTRUSIONS.
Subdivision of the gneisses and schists;
predominant meridional strike. Gneisses and schists of sedimentary
origin; variation in degree of metamorphism; phyllites and slaty
schists; green schists; quartzites; quartz schists and granulitic
gneisses; crystalline limestones; micaceous and felspathic
gneisses; calc silicate rocks; epidote hornblende rocks; magnetite
and hæmatite schists; sheared trachytes of Bussa; altered dolerites
of Ngashki; amphibolites and talc schists; granulites. Petrography
of the banded Archæan gneisses: relationship of the two groups of
gneisses. The gneisses and schists of the Sahara and of West
Africa. Age of the sedimentary gneisses. Intrusive rocks: the older
granites; variation in degree of reconstruction; brecciation and
injection structures; the younger granites; aplitic and
tourmaline-bearing granites and pegmatites; tourmaline quartz
veins; stanniferous pegmatites; schorl rocks; soda granites;
biotite and riebeckite granites; tinstone associated with soda
granites; biotite and augite syenites; augite diorites; riebeckite
quartz porphyries; ægirine granophyre; augite syenite porphyries;
alkali syenite porphyries; diorite porphyrites; mica porphyrites;
hornblende porphyrites; diabases and basalts. Granites and dyke
rocks of the Sahara. Age of the soda granites; successive periods
of intrusion.
For descriptive purposes it
has been found convenient to separate the crystalline rocks of the
Protectorate into two groups, (1) a group of quartzites, phyllites,
schists, and soft felspathic and micaceous gneisses, and (2) a
group of hard well-foliated, much granitised and
frequently banded
and contorted micaceous and hornblendic gneisses. The first group
occupies by far the greater area within the Protectorate, and
presents in the west a well-marked alternation of belts in which
quartzites, phyllites and schists predominate, with belts of which
the prevailing members are felspathic and micaceous gneisses. The
second group has no definite distribution within the Protectorate,
but appears to replace locally and irregularly the members of the
first group throughout the whole crystalline area. The members of
both groups appear to be irregularly folded along axial planes,
which are usually vertical or highly inclined, but it is probable
that detailed investigation would reveal a regular system of major
folds, accompanied by much local and minor crumpling. The folding,
which is typically meridional, has been affected by equatorial
stresses in such a way as to produce a transverse twisting or
corrugation of the predominant strike. The result is that
individual exposures may vary in direction of strike from N.–S. to
E.–W., while in places, as in Bauchi province, the strike may
become regionally equatorial. A similar equatorial divergence of
the prevalent meridional strike has been noted by M. Hubert[28]
in eastern Dahomey, but his suggestion that the equatorial
direction becomes predominant in Borgu and the Niger valley has
been found to be unsupported by local evidence. It is impossible to
believe, moreover, that there can be any connection, as suggested
by M. Chudeau,[29] between this locally divergent strike of the
gneisses and the distribution of the overlying Cretaceous and
Eocene rocks.
The sedimentary origin of the first group is fairly certain. The
pebbly quartzites, quartz schists, schistose grits, mica schists,
slaty schists and phyllites represent corresponding varieties of
sandstones, grits and shales, and frequently exhibit traces of
their sedimentary origin in the deformed pebbles and clastic grains
which they contain. The granular felspathic and micaceous gneisses
on the other hand are more highly reconstructed, and afford in
themselves little or no evidence of a clastic origin. Their
constant association and alternation with the quartzites and
schists make it probable, however, that they represent the arkoses
and greywackes of the original sedimentary series. Many of the
green hornblendic and epidotic beds probably also represent
original gritty accumulations of chloritic and epidotic character,
while the quartz magnetite schists of eastern Kabba correspond to
ferruginous grits or gritty ironstones of sedimentary origin. The
fine-grained crystalline limestones of Igbo, and the course
granular marble between Jakura and Wa, similarly represent original
clastic calcareous rocks within the sedimentary succession. To what
extent the recurrence of belts of quartzites, phyllites and schists
in the western part of the Protectorate represents the repetition
of one or more belts is quite unknown. The phyllites and schists
are moreover everywhere unfossiliferous and the age of the original
sedimentary rocks is consequently undetermined. The foliation
of the
reconstructed rocks is, as a rule, parallel with the bedding, but
the more micaceous schists frequently show a more or less
well-defined and oblique strain-slip corrugation.
The alternation of belts of less and more highly metamorphosed
rocks in the west of the Protectorate coincides to some extent with
an alternation of belts in which shear structures predominate, with
belts in which reconstruction is prevalent. The various belts
appear to pass insensibly into each other, but it has been found
impossible in the course of a rapid survey to decide either to what
extent shearing may have everywhere preceded reconstruction, or to
what extent the varying degree of reconstruction in alternate belts
may represent a corresponding variation in the intensity of the
accompanying folding. All that can be said with certainty is that
the sedimentary series as a whole has been subjected to a highly
complex folding accompanied by much shearing and reconstruction,
and that while over the greater part of the Protectorate the
stresses have been for the most part relieved by, or translated
into, a molecular and mineralogical rearrangement, there are
certain belts in which relief has been primarily obtained by
mechanical readjustment or shearing. Moreover, as in the
reconstructed areas there are certain beds which appear to be more
coarsely crystalline and less distinctly foliated than others, so
in the shear belts there are certain beds which have assumed the
character of granulitic gneisses, while others in the immediate
neighbourhood have retained to a very large extent their original clastic and
gritty character. Most of the reconstructed gneisses exhibit under
the microscope faint strain phenomena which, with the incipient
strain-slip foliation of the more micaceous schists, are probably
to be ascribed to regional stresses affecting the whole series
after the period of maximum reconstruction.
Grey and purple sericitic phyllites with knots of magnetite and
pyrites are extensively developed in the neighbourhood of Ngashki
in the Niger valley, around Anka and Bukwium on the borders of
Sokoto and Kontagora, and in the Zungeru belt in eastern Zaria from
Kato to Birnin Gwari and Koriga. In Kabba and Nassarawa and
southern Zaria their place is taken by fine-grained quartz
muscovite schists with abundant magnetite. With the phyllites are
associated in the Niger valley and Sokoto province green quartzose
slaty schists (1195, 1325),[30] whose colouration is due
to recrystallised chloritic and hornblendic material. Mr.
Parkinson[31] has described similar slaty schists from
Southern Nigeria. In the neighbourhood of Zungeru these green
schists become softer and show under the microscope (1563–4) ragged
porous or fibrous crystals and crystal aggregates of reconstructed
hornblende in a matrix of granulitic quartz, felspar and magnetite,
which encloses also scattered grains of hornblende, biotite,
chlorite and epidote, as well as relics of original clastic grains
of quartz and
felspar. These rocks present a considerable resemblance to the
“green beds” of the Scottish Highlands.[32]
True quartzites are comparatively rare and only at Kazaure do pure
siliceous rocks attain any extensive development. Elsewhere the
rocks which present the characteristic appearance of quartzite in
the field are usually more or less felspathic and micaceous, and
include schistose grits, micaceous quartzites, quartz muscovite
schists and granulitic gneisses which show all the familiar stages
in the deformation of felspathic grits and sandstones. Some of the
white and pink granulitic gneisses, such as those of Zungeru, may,
however, be ultimately proved to have been originally of an
eruptive character. A characteristic feature of these rocks is the
development of tourmaline and sometimes of pyrites along the shear
and foliation planes as the result of later pneumatolytic action.
At Kuta a belt of coarse quartz muscovite schist contains abundant
sillimanite in sheaf-like aggregates.
Mica schists in which biotite predominates are common throughout
and frequently carry garnet, magnetite and staurolite in
conspicuous crystals. In the Igbo limestones the original
impurities have recrystallised as tremolite, diopside, scapolite
(122), sphene, biotite, felspar, quartz and pyrites, while the
streaks of silicates in the limestone between Jakura and Wa contain
diopside, epidote, idocrase, felspar, and quartz. In the latter
limestone also, scales of graphite are common throughout and
pyrites on the
margin. Amongst the reconstructed gneisses, which show a remarkable
and rapid variation in texture, the paler-coloured felspathic rocks
are usually rich in orthoclase and microcline, and occasionally
muscovite, while the darker-coloured varieties, in which biotite
predominates, frequently carry garnet, sillimanite, staurolite and
in places also sphene, tremolite and carbonates. Andalusite-bearing
rocks are known from certain localities, but kyanite gneisses and
schists have not been definitely recognised in place. The local
occurrence of kyanite on the river sands, however, points to its
presence within the neighbouring rocks. Calc silicate types are
represented by the tremolite schists of the Nassarawa tableland
(668, 689), by a diopside tremolite schist from Borgu (1221), a
diopside and wollastonite-bearing rock with sphene from Maibirro in
the Niger valley (1192), a biotite tremolite schist with calcite
from Olle in northern Kabba (879), and a diopside and
scapolite-bearing rock with sphene and green spinel from Atuka,
near Okuruku, in eastern Kabba (843). The epidote hornblende
gneisses of Boi in southern Bauchi (976) contain much diopside and
sphene, and are probably of sedimentary origin, while relics of
clastic felspar are common in an epidote rock from Goshin Duchi a
few miles east of Boi (983). A similar clastic origin is presumed
for many of the epidosites and epidote hornblende rocks associated
with the sedimentary gneisses in other parts of the Protectorate.
The quartz magnetite and quartz haematite schists include coarse
granular and fine-grained granulitic types (844, 1370), the latter presenting
considerable resemblance to the itabirites of the United States.
Their sedimentary origin is presumed throughout and only in the
case of the massive quartz magnetite rock of Okuruku (148), which
is associated with a magnetite-bearing epidiorite, is there any
probability of an igneous origin.
Two stages of alteration can be recognised among the sheared
trachytes of Bussa (p. 72). The least
altered types (1164, 1167) show fracturing of the phenocrysts of
felspar and the replacement of the original ferro-magnesian
minerals by fibrous uralite, chlorite, epidote and calcite. The
more altered types (1166) are characterised by the production of
abundant white mica in the ground-mass and the development of
biotite, magnetite and carbonates from the original ferro-magnesian
material. Original vesicles of quartz and calcite in these rocks
have been deformed and the quartz replaced by a mosaic of angular
grains. A sheared quartz-porphyry from Kanikoko (1206) exhibits
deformation and granulation of the phenocrysts of quartz and
felspar and the abundant development of white mica in the
ground-mass. In the altered dolerites of Ngashki (1175, 1176) the
original structures are almost entirely obscured by the extensive
production of granular epidote, calcite, uralite, chlorite,
magnetite, felspar and quartz. Knots of tourmaline are common in
these altered rocks. The types grouped as amphibolites range from
epidiorites to hornblende schists and amongst them may be
recognised all the familiar stages of saussuritisation
and
recrystallisation of original tonalitic, dioritic and diabasic
rocks.
It is probable, however, that amongst the amphibolites are
included rocks which were intruded at many different periods during
the metamorphism of the gneisses and that to difference of age as
well as to local intensification of the metamorphism is to be
ascribed the variable degree of construction which the rocks have
severally undergone. The chlorite and talc schists are to be
regarded as basic intrusions which had suffered considerable
decomposition before being subjected to metamorphism.
Amongst highly reconstructed rocks of doubtful derivation,
associated with the sedimentary gneisses, are a garnetiferous
quartz enstatite biotite granulite from Atuka near Okuruku in
southern Kabba (841) and a garnet granulite from the adjoining
village of Kuturu (168) composed of labradorite, quartz and garnet
with a distinct centric structure. The garnet is in spongy masses
associated with a small quantity of green pyroxene, sphene, apatite
and iron ore. A pyroxene granulite, from Againyi (823), consists of
a porous intergrowth of pale green pyroxene and green hornblende,
the former predominating, in a matrix of clear labradorite. Sphene
and ilmenite are also present in irregular intergrowths as well as
a little accessory epidote and apatite. Another variety (832)
contains no amphibole and consists of pale green pyroxene and
garnet in a matrix of labradorite and quartz. A third type from the
same locality (821) is an epidote granulite consisting of
predominant epidote and garnet with subordinate green pyroxene, sphene and
ilmenite in a matrix of labradorite. A pyroxene granulite from
Nassarawa province (854) shows ragged crystals of diallage with
quartz inclusions and rounded garnets with inclusions of both
quartz and diallage in a matrix of coarsely granular quartz. A
hornblende granulite from the same province (859) consists of
rounded spongy garnets and radiate groups of actinolite fibres with
a little accessory ilmenite in a typical fine-grained
quartzo-felspathic mosaic. A remarkable biotite granulite from Eri
in eastern Ilorin (1056) contains in addition to biotite and
granular garnet as the predominate ingredients much sillimanite,
staurolite and andalusite in a matrix of untwinned felspar and
quartz.
The harder gneisses of the second group are subordinate in
amount and irregularly distributed throughout the micaceous and
felspathic gneisses of the first group. They consist of a series of
much granitised medium and even-grained biotitic and hornblendic
gneisses which usually show a lenticular or linear foliation but
frequently also a ribboned or banded and contorted character. In
some cases the banding is broad and only to be distinguished in the
field, while hand specimens present quite a homogeneous character.
All variations may be found between acid aplitic and basic
hornblendic bands, and knots and lenticular masses of more basic
material are common within the more acid types. As a rule the
banding is much interrupted by pale coloured granitic material into
which the lighter coloured bands seem to pass imperceptibly. A
striking feature is the presence of a series of early pegmatite veins folded
and foliated with the gneisses. There is in fact every indication
that these banded rocks are of a primarily composite character and
represent either an originally sedimentary or a differentiated
igneous succession invaded and injected by acid granitic material
and pegmatite veins in the deeper portions of the crust and
subsequently subjected to a later folding, foliation and
recrystallisation. The foliation is in general vertical and
parallel with the banding. Under the microscope the striated and
banded gneisses exhibit all the characteristic features of typical
archæan rocks (801, 846). They are composed of a granular aggregate
of quartz, felspar, hornblende or biotite with sphene, apatite and
iron ores as accessories. The felspars are usually allotriomorphic
and vary from labradorite in the more basic to orthoclase and
microcline in the more acid rocks. The biotite occurs in tabular
flakes singly or in aggregates with the basal plane well-developed
but with ragged margins in the prism zone, while the hornblende
forms spongy growths or irregular plates with embayed margins and
inclusions of quartz. Strain shadows are common in the quartz and
felspar but only traces of cataclastic granulisation or of
secondary micropegmatite are as a rule recognisable. Van Hise[33]
is of opinion that the strain phenomena and the vermicular quartz
to be observed in these thoroughly reconstituted gneisses are due
to the partial lagging of reconstruction behind deformation. It is
equally probable, however, that the strain effects and micropegmatitic
structures have been induced after the period of maximum
reconstruction by later strains of lesser magnitude.
The mutual relations of the two groups of gneisses are difficult
to determine. The actual contact between the two has nowhere been
found clearly exposed and its character must therefore be to a
certain extent conjectural. The apparent irregular replacement of
the softer gneisses by members of the harder group when traversed
across the strike and the apparent extension into the softer
gneisses of tongue-like masses from the larger areas of harder rock
is certainly suggestive of an intrusive character as advocated by
Mr. Parkinson[34] in Southern Nigeria and Liberia. Upon this
hypothesis the harder gneisses would appear to correspond to an
intimate mixture of granitic and dioritic material, whose intrusion
took place during the folding and reconstruction of the sedimentary
gneisses. It is difficult, however, to understand why these rocks
should present in their even-grained and closely foliated and
banded character such an entirely different aspect in the field
from that of even the earliest recognisable granitic intrusions
within the sedimentary gneisses and why they alone should possess
an early set of folded and foliated pegmatite veins in addition to
the later pegmatites which are common to both groups of gneisses.
All the phenomena indeed are more readily explicable on the
assumption that the two groups differ in age, that the sedimentary gneisses were
originally deposited unconformably upon an earlier and probably
archæan gneissic or igneous complex and that the latter series was
refoliated and recrystallised during the reconstruction of the
former. The peculiar distribution of the harder gneisses and their
intimate alternation with the softer series would then be due in
part to the original unconformability between the two groups and in
part to the irregular character and the varying amplitude of the
later folding to which both were subjected. The abrupt replacement
of the sheared but little altered quartzites of the Niger valley by
the harder gneisses at Jebba, Wuru and Maibirro probably indicates
also a certain amount of faulting and thrusting accompanying the
folding. On account of the subordinate and ill-defined character of
the distribution of the harder rocks no attempt has been made to
distinguish upon the map between the archæan and the later
sedimentary gneisses.
Plate IX
On the Summit of the Bauchi Plateau.
The Parade Ground at Bukuru.
On the Plateau looking Westward from Bukuru.
In the Sahara to the north, M. Chudeau[35]
has described the occurrence of archæan and sedimentary gneisses
whose mutual relations are similar to those which prevail in
Northern Nigeria. The archæan areas are of comparatively small
extent, and are primarily distinguished, according to M. Chudeau,
by the presence of numerous domes and turtlebacks of granite. This
method of distinguishing between the two groups, however, cannot be
applied in Nigeria, where intrusive granites are equally common
amongst both the earlier and the later gneisses. The
metamorphic rocks
of sedimentary origin attain a great development in the
Sahara[36] and include quartzites, phyllites, schists,
gneisses and crystalline limestone similar in type to those of
Nigeria, and folded in a similar complex manner along axes whose
direction is predominantly meridional. The decrease northward of
the intensity of the metamorphism, as advocated by M. Chudeau,
requires much further investigation in view of the local occurrence
of little altered rocks within the Protectorate. M. Chudeau
believes in the original unconformability of the later gneisses
upon the archæan and points out, with M. Haug,[37]
the analogy between the system of folding of the gneisses of the
Sahara and the Sudan, and the Caledonian folding of the crystalline
rocks of Scotland and Scandinavia. He demonstrates the pre-Devonian
age of the sedimentary gneisses, and assigns them provisionally to
the Silurian on the strength of their supposed continuity with the
graptolite-bearing shales of Tindesset[38]
and Hassi el Kheneg.[39] The stratigraphical evidence available,
however, seems rather to point to an unconformability between the
shales and the schists, and M. Flamand[40]
prefers to assign a pre-Cambrian age to the whole series of
crystalline schists and gneisses.
In Dahomey, M. Hubert[41] has described a
similar complex of
archæan and sedimentary gneisses in which the latter clearly
predominate. His separation as Silurian of the quartzites and
schists of Atacora from the other metamorphic rocks appears,
however, to be unjustifiable. Quartzites, gneisses and schists of
sedimentary origin associated with granites and archæan rocks, are
found also in West Africa, in French Guinea and the Fouta
Djalon,[42] in Liberia,[43] the Ivory Coast,[44]
the Gold Coast,[45] Togoland,[46] and Southern
Nigeria,[47] and in Central Africa to the south and
south-east of Chad,[48] in the Kameruns,[49]
and in the French Congo.[50] Similar rocks attain also
a remarkable development to the south of the equator.[51]
The comparison which M. Chudeau has drawn between the strike of
the gneisses and schists of the Sudan on the one hand, and of the
Scottish Highlands on the other, may be further extended not only
to the petrographical character of the constituent groups of the
gneissic complex in West Africa, but also to the respective
relations of the various groups to each other, and to the intrusive
rocks which they enclose.[52] Dr. Voit[53] has, moreover, arrived at
similar conclusions in the course of his investigation into the
structure of the crystalline complex of the northern Transvaal, the
constitution and mutual relationships of the Swaziland system and
the older granites and gneisses being closely paralleled in the
rocks of Nigeria.
Many of the thin bands of well-foliated augen and granitoid
gneiss enclosed within the metamorphic series undoubtedly represent
original granitic intrusions, but these are so intimately bound up
with the adjoining gneisses and schists, that no discrimination can
yet be attempted. Attention has been more particularly directed in
the field to those granitoid rocks which form the prominent
inselberge and isolated groups of hills, and amongst them it has
been found possible to distinguish an older from a younger series
of intrusions. The older intrusive rocks consist very largely of
granite, while the younger intrusions include granitic, syenitic
and diabasic types with numerous associated dyke rocks. To what
extent the epidiorites, amphibolites, and granulites of the
gneissic series represent more basic intrusions associated with the
older granites is quite unknown.
The older granites are never sharply marked off from the
adjoining gneisses, and invariably show a certain amount of foliation in the
field, and of cataclastic deformation and reconstruction under the
microscope. They appear to have been intruded at different times
within the period of the evolution of the gneisses, and the amount
of reconstruction which they themselves have undergone may be taken
as a general indication of their age. Vermicular micropegmatite is
most extensively developed in those schistose granites which have
stopped short of complete reconstruction and which still afford
abundant evidence of cataclastic deformation. It may also occur,
however, in small amount in thoroughly foliated and reconstructed
granites, in which it is assumed to have been developed, as in the
case of the banded and striated gneisses (p.
118), by minor crustal strains, after the period of maximum
reconstruction.
The most highly altered types form extensive lenticular or
phacolitic masses of well-foliated and fairly coarse-grained
biotitic and hornblendic gneisses (849, 481), containing streaks
and patches of darker-coloured and finer-grained basic material.
Under the microscope they possess a coarsely granular and
thoroughly reconstructed character, and in the mutual relations of
their constituents present a considerable resemblance to the banded
gneisses of similar composition. The quartz and felspar show much
later straining with incipient granulisation and traces of
micropegmatite. These rocks have participated in all the changes
which the adjoining gneisses have undergone, and were apparently
intruded before the period of maximum reconstruction of the
latter.
The least
altered of the older granites (806, 1560) are typically
distinguished by the irregular batholitic character of the
intrusions and the partial and frequently only marginal character
of the foliation. They sometimes have a tendency to become
finer-grained towards their junction with the gneisses, and they
usually enclose portions of the adjoining rocks and vein and inject
with granitic material the marginal gneisses and schists. The
inclusions may be of either the harder or softer gneisses according
to the character of the rocks which the granite has invaded. In
places a parallel arrangement of the felspars appears to have been
induced through streaming during crystallisation. The partial
foliation is parallel in a general way with that of the surrounding
gneisses and schists. The granites vary much in character from
biotitic and hornblendic rocks with syenitic modifications to
muscovite and microcline granites and aplites. The quartz and
felspar are strained throughout and cataclastic granulitisation is
common on the margins of the crystals. Vermicular quartz is most
abundant in the marginal and more distinctly foliated portions.
These rocks would appear to have been intruded after the period of
maximum reconstruction of the gneisses, but before the final
cessation of stress and strain. In places, as at Abuja, crush lines
cross the granites, and in their immediate neighbourhood the rocks
show a higher degree of reconstruction and a greater development of
micropegmatite in the matrix.
These later members of the older granites appear to have
produced little or no contact metamorphism in the surrounding rocks. Brecciation and
injection structures on the other hand are very frequent. The hard
banded gneisses on the margin of the Abuja granite, for example,
are abundantly injected with quartz and quartzo-felspathic
material. Similarly at Zwoll, in Bauchi province, a porphyritic
biotite granite has brecciated a series of banded and contorted
biotite and hornblende gneisses, and many of the enclosed fragments
are partially absorbed or minutely injected with granitic material.
The Okuruku granite in southern Kabba, on the other hand, has
invaded and brecciated members of both the harder gneisses and the
softer series of biotite schists, magnetite schists and
amphibolites. In the neighbourhood of the inclusions the granite
becomes knotted and richer in biotite, hornblende, sphene and other
minerals, while the inclusions themselves are irregularly veined
and minutely penetrated by quartzo-felspathic material. In every
case it would appear that the rocks, which the granites have
brecciated and invaded, possessed their gneissic or schistose
character before the actual intrusion of the granites, and that
they, as well as the larger inclusions, suffered little further
change as the result of the stresses and strains which brought
about the partial foliation of the granites. The gneisses as a
whole show strain phenomena, but only traces of granulitisation and
of secondary formation of micropegmatite, while the granites show
extensive marginal formation of micropegmatite, together with
intense cataclastic deformation. It would seem indeed that these
granites were intruded after the period of maximum
reconstruction of
the gneisses, but before the final cessation of stress and strain,
and that while the gneisses which had originated under conditions
of maximum strain were comparatively stable under similar strains
of diminishing intensity, the granites which had crystallised from
igneous fusion were in a condition of unstable equilibrium with
respect to the stresses and strains to which they were subjected
after crystallisation. The gneisses, as it were, possessed a
certain amount of elasticity and transmitted the later strains with
little resultant deformation, while the rigid masses of granite
arrested those strains and suffered in consequence a partial
deformation. The production of a strain-slip foliation in some of
the more micaceous beds is probably to be referred to the same
period of differential deformation of the granites. These later
strains became locally intensified along crush lines, and in the
case of the Abuja granite produced granulitisation both of the
granite and of the felspathised gneisses in the neighbourhood of
the crush line.
In addition to the two extreme types of the older granites there
are many elliptical masses of similar material, elongated in a
direction parallel with the strike of the gneisses, which show a
decided foliation throughout, sometimes more and sometimes less
intense (48, 1223). As a rule they are indistinctly marked off from
the adjoining rocks and preserve their normal texture up to the
margin of the intrusion. The granites are predominantly biotite
granites, more rarely muscovite biotite granites, and the felspars
are frequently porphyritic. Under the microscope they show a higher degree of
reconstruction than the least altered granites, but a less complete
reconstitution than the most highly altered types. The felspars
vary from oligoclase to microcline, and the plagioclases often show
basic interiors and zoned margins. Both the felspar and the quartz
show abundant strain phenomena, and the felspars are usually
partially surrounded by a secondary fringe of vermiculate
micropegmatite in optically discontinuous grains. Frequently also
the felspars are corroded or irregularly penetrated by quartz or
micropegmatite, while the matrix consists largely of granular and
partially recrystallised felspar, muscovite, microcline and quartz.
The darker patches within these granites may be taken to represent
portions of the adjoining rocks caught up during intrusion and
partially absorbed, recrystallised and foliated with the granites.
These rocks may possibly be correlated in the period of their
intrusion with the least altered granites and regarded as the
smaller and therefore more readily reconstructed representatives of
the larger partially foliated masses. On the other hand, in the
absence of the inclusions of adjoining gneisses which form the
characteristic feature of the least altered granites, it is
possible to regard them as of somewhat earlier origin and as having
participated to a considerable extent in the general reconstruction
of the gneisses themselves.
It seems probable indeed that the crustal movements which
brought about the metamorphism of the gneisses were of a somewhat
spasmodic character and spread over a very considerable interval of
time and that the
intrusion of granitic and other igneous material took place at
intervals throughout the whole period. No hard and fast line can
therefore be drawn between the highly reconstructed and gneissose
granites and the imperfectly or only partially foliated types, a
complete series of intermediate varieties being apparently possible
between the two extreme members of the series.
1. Granites and pegmatites.
Amongst the later granites, which present for the most part a
batholitic habit, two types may be distinguished, differing in
mineralogical composition and probably also to some extent in age.
The rocks of the first type never show chilled margins, and were
evidently intruded while the adjoining gneisses still possessed a
comparatively high temperature. They include non-foliated aplitic
granites and pegmatites usually rich in muscovite and tourmaline.
The second type includes riebeckite and biotite granites which are
sharply marked off from the adjoining rocks by felsitic and
fine-grained margins. The rocks of the first type usually exhibit
under the microscope a faint straining of the quartz and felspar
and were probably intruded before the final cessation of the
crustal movements to which the reconstruction of the gneisses and
the foliation of the earlier granites were due. The members of the
second type, on the other hand, are practically free from
cataclastic phenomena and evidently assumed their present position
after the cessation
of movement within the crust. There is no reason, however, to
assume any great difference in age between the two types or any
marked discontinuity in magmatic conditions. The same magma, of a
somewhat acid and alkaline character, was probably present in the
depths throughout the whole period of intrusion, and the varying
character of the products may be taken as affording some clue to
the nature of the differentiation within the magma itself.
The alkaline granites of the first type are pale-coloured
granular rocks composed almost entirely of quartz, microcline,
orthoclase, perthite, and acid plagioclase. Biotite and other
ferro-magnesian minerals are comparatively rare, while muscovite
and tourmaline are frequently abundant and in large primary
crystals. The quartz and felspar have a tendency to pegmatitic
intergrowth, and sometimes masses of graphic pegmatite occur
porphyritically in a normal quartzo-felspathic matrix. The
tourmaline and muscovite may occur also as at Igbo in two
generations, in large crystals and in the ground mass. Frequently
also these granites possess a tendency to a coarsely banded
structure, the banding being due either to variation in size of
grain of the quartz and felspar or to variation in the abundance of
tourmaline and muscovite in successive bands. These granites rarely
form intrusions of any size, the largest known being the tourmaline
granites of Igbo and Darroro, but even they are insignificant in
comparison with the alkaline granites of the second type.
The pegmatite dykes and veins which are so abundant throughout
the gneissic series evidently belong to the same period of intrusion. Like the
aplitic and tourmaline-bearing granites they never exhibit
fine-grained margins at their junction with the gneisses. They vary
much in composition, and include graphic granites, quartz-felspar
pegmatites with biotite, magnetite or ilmenite, quartz-felspar
pegmatites with muscovite or tourmaline or both, and
quartz-muscovite pegmatites with or without tourmaline. At Eri
there are stanniferous pegmatites carrying quartz, muscovite and
cassiterite, but no tourmaline. The massive quartz-reefs which
frequently enclose portions of the surrounding schists and carry
tourmaline, pyrites and sometimes gold, are also to be referred to
the same period of intrusion. Inseparable from them and from the
pegmatites are the smaller veins of quartz and tourmaline quartz
and the schorl rocks of a foliated or striated character which
occur as a rule in association with the veins of tourmaline
quartz.[54] The schorl rocks with a gneissic habit in which
the quartz is minutely granulitic and the tourmaline is in small
crystals evidently recrystallised and full of quartz inclusions,
are probably altered gneisses and schistose granites in which the
felspars have been replaced by quartz and tourmaline. Traces of
decomposed felspathic material are to be found in every specimen,
but no pseudomorphs in tourmaline of felspar or of other minerals
have been definitely recognised. The striated schorl rocks in which
the darker bands are formed of finely fibrous tourmaline, appear to
arise also as modifications of the adjoining rocks in the vicinity of the tourmaline
quartz veins. Sometimes a foliated rock may be traced through a
striated type into a massive tourmaline rock on the margin of the
vein in which the original structures have entirely disappeared,
and in which the original minerals, including the quartz, have been
entirely replaced by finely fibrous tourmaline. In the quartz
veins, pegmatites and granites, on the other hand, the tourmaline
is in well-formed crystals of primary origin. It would seem,
therefore, that the pneumatolytic or metasomatic formation of the
schorl rocks was brought about by emanations from the quartz veins
and pegmatites before the final consolidation of the latter. The
extensive development of tourmaline and pyrites in the quartzites
and schists of certain areas is also to be ascribed to the
circulation of mineralising vapours and solutions throughout the
adjoining rocks during the same period. The remarkable absence of
tinstone in the tourmaline pegmatites, together with its appearance
in pegmatites at Eri which do not carry tourmaline, seems to point
to the stanniferous pegmatites having originated at a somewhat
later date than the normal tourmaline-bearing pegmatites. This
suggestion is supported by the occurrence of tinstone in
association with the soda granites of Bauchi province.
The alkaline granites of the second type are typically
distinguished by possessing fine-grained porphyritic and felsitic
margins which indicate a considerable difference in temperature
between the granites and the surrounding gneisses at the period of
intrusion. To this group are referred the biotite granite of
the Anagoda hills
in S. Nassarawa, the riebeckite granite of the Gurkawa Hills in
northern Muri, the biotite granite of Bukuru and Ngell, and the
riebeckite granites of the Kwandokaya Hills and of the Ningi-Burra
massif in Bauchi province, the riebeckite granite of Fagam, Gadama
and Galambi on the borders of Bauchi and Kano, the riebeckite
granite of Shira in the sub-province of Katagum, and probably also
the Kailema granite near Jebba and the biotite granite of Abara
near Ngashki in western Kontagora. The alkaline character of the
original magma is reflected in the nature both of the felspars and
of the ferro-magnesians. The felspars are much kaolinised and
consist chiefly of tabular crystals of untwinned orthoclase
probably rich in soda, and of streaky perthitic intergrowths of
orthoclase and albite. Where enclosed in quartz, the terminations
of the crystals are frequently idiomorphic. Microlites of
riebeckite are common inclusions and small plagioclases of an
earlier growth are frequently enclosed in the later felspars and in
the quartz. The quartz forms aggregates of irregular grains showing
faint strain shadows and has a great tendency to form coarsely
pegmatitic intergrowths with the felspar. Where biotite alone is
present, it is of a normal greenish-brown colour. Riebeckite,
however, is the most characteristic ferro-magnesian, and occurs for
the most part in irregular plates moulded on the felspars and
sometimes enclosing small crystals of felspar in poikilitic
fashion. Occasionally, as in the Shira granite, the riebeckite
builds elongated crystals more or less idiomorphic in the prism
zone and frequently
intergrown with ægirine. Crystals of riebeckite over an inch in
length are common in knots in the Kila granite, while on the other
hand the margin of the Gurkawa granite is stippled with minute
porous crystals of the same mineral. Associated with the riebeckite
in places are other blue and brown soda amphiboles[55]
as well as green ægirine and a golden-coloured mica, while zircon,
apatite and iron ores are constant but scanty accessories. Pyrites
is sometimes abundant in the porphyritic and felsitic marginal
rocks.
A remarkable feature of some of these later granites is the
occurrence in them of tinstone and numerous sulphide ores. It would
appear that in places these later granites have been broken and
fissured, and that along these fissures vapours and solutions have
risen which have chloritised and mineralised the adjoining rock and
in part refilled the fissures with ore-bearing pegmatite and vein
stuff. In the Bukuru granite the cassiterite is associated in the
neighbourhood of Ngell with pyrites, chalcopyrite, tetrahedrite,
blende and galena. Topaz and brown zircon are found accompanying
the cassiterite in the tin-bearing alluvium at Naraguta and
elsewhere, and are probably also of pneumatolytic origin. The
granites of the Kwandokaya hills, of the Jengre and Limoro hills,
of the Ningi-Burra massif, and of the Gadama and Fagam hills also
yield in places a tin-bearing drift, and have probably suffered,
like the granite of Bukuru, a local fissuring and mineralisation. Tourmaline has not
been definitely recognised as a product of these later
emanations.
Similar soda granites with microgranitic and felsitic margins
have been described by MM. Foureau, Gentil and Lacroix[56]
from Zinder, by M. Chudeau[57] from Mounio and Goure, by
M. Garde[58] from Machina and by M. Hubert[59]
from Dahomey, while Captain Freydenberg[60]
has noted the occurrence of a soda syenite with granitic
modifications at Melfi to the south-east of Chad.
Plate X
A Worn Volcanic Hill on the Bauchi Plateau near
Zamagan.
The Twin Cones of Kereng from Panyam
2. Syenites and Diorites
These rocks, which are by no means abundant, occur as small
bosses or intrusive masses within the gneisses and schists. As a
rule their junction with the surrounding rocks is not exposed, and
their relationship to the later granites has not been definitely
ascertained. They should probably be regarded, however, as somewhat
basic modifications or differentiation products of the later
granitic magma. Slight strain effects are usually apparent in the
quartz and felspar.
The low hills around the town of Bauchi are composed of a
coarse-grained augite syenite, in which orthoclase felspar
predominates in large twinned crystals with a peculiar brownish tint and a resinous
lustre. Under the microscope (49) subordinate oligoclase is present
in smaller crystals, while the knots of darker minerals are
composed of intergrowths of violet-coloured augite, green
hornblende, brown biotite and iron ores. Quartz is interstitial and
occurs also in secondary vermicular growths on the margins of the
felspars. The accessories are zircon and apatite, and the whole
rock is much stained with secondary limonite. A rock of similar
appearance from Kende to the east of Bauchi (69) is somewhat more
basic and may be termed a quartz augite-diorite. The predominant
felspar is a basic andesine, while the ferro-magnesians are
pale-coloured augite and green hornblende in irregular plates and
intergrowths. The quartz is interstitial or intergrown with the
felspar in pegmatitic fashion. A similar rock from Kanna (1677)
contains much biotite in addition to the augite and hornblende. The
rock of Kogon Dutsi (1338) within the walls of Kano is a basic
augite diorite without quartz, consisting predominantly of acid
labradorite and pale-coloured augite. Both the augite and felspar
have a tendency to schillerisation and the augite is accompanied
by, and in part intergrown with, green hornblende, brown biotite
and iron ores, while hypersthene is a scanty accessory. An
augite-diorite from eastern Kontagora (1302) contains a little
interstitial quartz and felspars varying from oligoclase to acid
labradorite. The ferro-magnesian minerals are much intergrown and
include pale-coloured augite, green hornblende and brown
biotite.
3.
Porphyries, Porphyrites, Diabases and Basalts.
The later sills and dyke rocks are most abundant in the central
provinces and especially in Bauchi, eastern Zaria and northern
Nassarawa, but are somewhat sparingly distributed throughout the
remainder of the Protectorate. They include quartz porphyries,
orthoclase porphyries, mica and hornblende porphyrites, diabases
and basalts. In the central area the high content in soda which
characterises the more acid types undoubtedly indicates a genetic
connection between the later soda granites and the dyke rocks,
although investigation has not yet proceeded far enough to allow of
the recognition of the major foci to which the dyke rocks may be
referred. Amongst the quartz porphyries micro-granitic, granophyric
and felsophyric types are equally common. In the central provinces
riebeckite is frequent both among the phenocrysts and in the ground
mass, while elsewhere biotite is the commonest coloured
constituent. An ægirine granophyre occurs at Chikobo (1417) on the
borders of Bauchi and Zaria. Orthoclase porphyries are commonest in
the central area and possess typically a trachytic ground mass, of
which riebeckite and ægirine are frequent constituents. Augite
syenite porphyries are known from the Kudaru Hills in eastern Zaria
(1397) and from the vicinity of Limoro on the borders of Bauchi and
Zaria (1413). The latter occurrence contains hornblende and biotite
in addition to the augite, as well as a small amount of quartz
in the ground mass.
An extensive mass of granophyric diorite porphyrite occurs in
western Kontagora between Auna and Bussa (1162). The felspars are
turbid and much epidotised and surrounded by a fringe of more acid
material which passes outwards into the granophyric matrix, while
smaller irregular crystals of green augite and ilmenite and
abundant minute plates of green mica are also enclosed within the
pegmatitic material. A peculiar rock from Toro in Bauchi province
(958) may be termed a quartz diorite porphyrite, and presents a
certain affinity with the basic members of the charnockitic series
of the Ivory Coast.[61] The phenocrysts have a tendency to
aggregate in groups and include rounded quartz grains, tabular
crystals of andesine and labradorite with inclusions of granular
augite, elongated crystals of hypersthene and irregular plates of
biotite much corroded by the ground mass and enclosing grains of
augite and apatite. The ground mass is composed of granular green
augite, felspar and quartz, with much magnetite and apatite. The
felspar of the ground mass is frequently untwinned and more acid
than the phenocrysts, while the quartz forms in places
micropoikilitic plates enclosing the other ingredients of the
ground mass. As alkali-syenite porphyries may be grouped the
bluish-grey trachytoid rocks of Kanikoko (1217–1219). The
phenocrysts include tabular plates of sanidine, microcline,
microperthite and microcline-microperthite and fairly well-formed
crystals of diopside and enstatite, much intergrown with brown mica
and bluish
amphibole. The ground mass consists of microlites of alkali felspar
and granules of the ferro-magnesian minerals. Typical mica
porphyrites with phenocrysts of biotite and zoned andesines occur
at Barazara (1274), and between Dan Gerumfa and Gwashi (1276) in
eastern Kontagora. A hornblende porphyrite between Naraguta and
Tilde (956) is inconspicuously porphyritic and composed of
allotriomorphic green hornblende with subordinate biotite and
augite in a matrix of acid plagioclase, orthoclase and quartz. A
hornblende porphyrite from Borgu (1220) contains large poikilitic
crystals of augite and hornblende in a granular matrix of
labradorite, augite, hypersthene, hornblende and iron ore. A
somewhat similar rock from Bauchi province (1602) contains large
poikilitic hornblende crystals in a matrix of granular augite,
magnetite and lath-shaped crystals of labradorite. A labradorite
porphyrite from Toro in Bauchi province contains large tabular
phenocrysts of acid labradorite in a ground mass of green augite,
brown biotite, secondary green hornblende and lath-shaped crystals
of acid plagioclase. The diabases and basalts of the central area
are particularly numerous round the margin of the Bauchi plateau
and the Nassarawa tableland, and probably include intrusions of
many different ages. Some of the coarser-grained diabases present
affinities with gabbros in the idomorphism of their augite crystals
(47, 1695). Others exhibit a typical ophitic structure (962, 985),
while others are predominantly granulitic (690). A hornblende
diabase with pale green augite, greenish-brown hornblende and deep
red biotite occurs
in the Ruruma hills (1466) in eastern Zaria. Biotite also
accompanies the augite in an ophitic diabase from the Kudaru hills
(1390) in north-east Zaria. Uralite diabases are fairly common, but
olivine diabases (747) are conspicuously rare. The basalts present
few features of interest and are typically compact, non-porphyritic
rocks with a holocrystalline, ophitic or granulitic structure and
free from olivine.
MM. Gentil[62] and Chudeau[63] have described the
occurrence of intrusive granites, pegmatites, porphyries,
lamprophyres, diabases and gabbros, throughout the crystalline
areas of the Central Sahara, and Dr. Passarge[64]
has recorded similar intrusive rocks from the northern gneissic
region of Adamawa. Ægirine rhyolites and alkaline trachytes of an
intrusive character are known also from Zinder, Gabana, Hadj el
Hamis,[65] and M’Burao.[66] M. Chudeau believes that
the alkaline granites of Mounio with their associated hypabyssal
types pierce the sandstones of the Tegama, and would therefore
refer all the known occurrences of soda granite in Central Africa
to a late Cretaceous or Tertiary age. In Nigeria, however, there is
little or no evidence to support this view. The soda granites are
nowhere found piercing sedimentary rocks within the Protectorate,
and while it is possible that they may have been originally
intruded into rocks of Cretaceous age which have now been removed, it is
remarkable that, with the exception of rare sills of dolerite and
the Tertiary pipes of basalt, no intrusive dykes or masses of
hypabyssal or plutonic material are to be found within what remains
of either Cretaceous or Tertiary rocks within the Protectorate.
Such intrusive rocks, however, are particularly numerous within the
crystalline areas, and especially in the neighbourhood of the
larger masses of soda granite, which are themselves frequently
pierced by marginal dykes of basalt and porphyry. In view of the
abundance of soda types among the dyke rocks, moreover, it is hard
to escape the conclusion that the later non-foliated alkaline
granites of the second type (p. 132) with
the associated syenites, diorites and dyke rocks of acid,
intermediate and basic composition, all possess a certain genetic
relationship, and should be properly ascribed to one and the same
period of igneous activity. All the facts, indeed, are more readily
explicable on the assumption of the pre-Cretaceous age of this
period of intrusion within the Protectorate. The stratigraphy of
Mounio and Koutous is somewhat doubtful, and M. Chudeau’s
section[67] of the faulted margin of the Goure granite does
not indicate an actual transition from the mica schists and
quartzites into the sandstones and clays of the Tegama. M.
Garde,[68] moreover, while indicating the occurrence of
crystalline gneisses to the north of Goure does not appear to
confirm M. Chudeau’s interpretation of this important section. It should be
remembered, however, that even although further investigation may
prove the truth of M. Chudeau’s correlation, there is no reason to
believe that the intrusion and effusion of igneous rocks rich in
alkalis has been confined, as M. Chudeau would suggest, to one
particular period in the history of the Sudan.
It is interesting to trace in a general way, and subject to the
limitations indicated above, the successive changes in the
character of the igneous intrusions from the earlier to the later
granites and dyke rocks on the hypothesis of a continuity in
magmatic conditions, or at least of a periodic renewal of activity
in the same igneous focus. The successive intrusions may be
tabulated as follows:—
1. Earliest well-foliated granites of
intermediate composition with numerous associated amphibolites and
epidiorites.
2. Partially foliated granites of both
acid-alkaline and intermediate character, with partially sheared
dyke rocks of acid, intermediate and basic composition.
3. Aplitic granites and pegmatites, rich in
muscovite and tourmaline with associated reefs and veins of quartz
and tourmaline quartz.
4. Alkaline granites rich in soda with associated
syenites, diorites, and dyke rocks.
If the normal sequence of events
within the period of activity of any igneous focus be taken to be
the successive manifestation of volcanic, plutonic and
hypabyssal
activity, there may perhaps be recognised in the successive
intrusions indicated above four periods of igneous activity, each
of which is now represented by the products of the plutonic and
hypabyssal phases only, while the products of the accompanying
volcanic phases, if originally present, have been removed in the
course of denudation. The tendency to the production of rocks rich
in alkalis should be remarked as a notable feature of the various
periods of intrusion; and in this connection it is interesting to
note that Messrs. Horwood and Wade[69] have likewise emphasized
the alkaline character of many of the earlier granitic intrusions
to the south of the equator.
[28]Hubert, Mission Scientifique au
Dahomey, 1908, p. 448.
[65]Garde, C. Rd. Ac. Sc., 5 July, 1909
(149), p. 43; cf. Gentil and Freydenberg, Bull. Soc.
Geol. Fr. (4), VIII, 1908, p. 44; cf. also Suess,
The Face of the Earth, Vol. IV, Oxford Trans.,
1909, p. 283.
[68]Garde, C. Rd. Ac. Sc., 149, 5 July,
1909, p. 43.
[69]Horwood and Wade, Geol. Mag. V, 6,
1909, p. 455.
CHAPTER IV. — Muri Province. — Yola Province. (A. L.) — Bauchi and Bornu.
CRETACEOUS ROCKS.
Distribution: relation to crystalline rocks. The
Cretaceous rocks of the Benue valley: the succession around Awe;
the position of the salt-bearing beds; the Arofu and Akwana
limestones and galena veins; the limestone-shales and brine springs
of eastern Muri; the Muri sandstones; the Gateri grits; the
limestones and galena veins of the Kudu valley. The Cretaceous
rocks of Yola and of the Gongola valley: the Bima sandstones; the
Gongila limestone; the rocks of Gulani and Wuyo; the upper
sandstones of Ture and Waja. Classification of the Cretaceous
rocks. The Cretaceous rocks of the French Sudan, Southern Nigeria
and the Kameruns.
No rocks of Devonian or
Carboniferous age or of any period preceding the Cretaceous[70]
have been discovered within the Protectorate. The Cretaceous rocks,
moreover, are themselves represented only by the upper members of
the series, no deposits of earlier age than the Turonian having
been anywhere definitely recognised. The Upper Cretaceous rocks are
confined within the Protectorate to the Benue valley between long.
8°·30′ E. and long. 12°·15′ E., and to the valley of the Gongola
between its confluence with the Benue and lat. 11°·15′ N. They form
the greater part of the plains of Muri and Yola, and of the narrow plain of the
Gongola in Bauchi and Bornu, but only around Ligri and Gateri and
in Wurkum, Tangale and Waja do they rise into ranges of rounded or
tabular hills. The rocks consist of a well-bedded series of red and
white sandstones and grits, carbonaceous and calcareous shales,
ironstones and limestones, usually somewhat faulted and gently
folded into low anticlines and shallow synclines, and in places
pierced by intrusive masses of dolerite or basalt. The original
thickness is unascertainable, partly on account of the folding and
faulting to which the rocks have been subjected and partly on
account of the thick covering of drift which obscures the rocks for
long distances and renders difficult or impossible the correlation
of the various exposures. In the west the Cretaceous rocks are
believed to pass unconformably under the sandstones of the Lokoja
series in the neighbourhood of Udeni, but the actual junction has
nowhere been observed. How far westward they extend under the
Lokoja sandstones is also undetermined. An exposure of white
sandstones and carbonaceous shales in the bed of a stream near
Akwacha in Bassa province seems to indicate that they extend for a
considerable distance west of Loko, but that they do not reach the
Niger is abundantly evident from the absence of any trace of them
in the neighbourhood of the river below Lokoja.
To the north and to the south of the plains of Muri, the
Cretaceous sandstones give place imperceptibly to a crystalline
floor. The actual contact, however, has been found wherever
examined to be so
much obscured by drift that it is impossible to say whether any
part of the margin is bounded by faults or not. In the east the
Cretaceous rocks pass under the upper Benue sandstones and, so far
as known, do not reappear anywhere east of Yola. Similarly in the
Kudu valley they disappear under the sandstones of Duguri and in
the Gongola valley under the Gombe sandstones to the west and
north. To the east of the Gongola between Bilaraba and Dusua they
rest upon a hummocky crystalline surface, while farther south they
pass behind Balbea under the Barbur basalt plateau. To the west of
Kwoia, however, their junction with the crystalline rocks, although
somewhat badly exposed, has every appearance of being fractured,
while on the lower Gongola the relations between the Cretaceous and
the crystalline rocks to the east are such as to suggest most
forcibly an extensive faulting along the contact. Within the area
covered by the Cretaceous rocks inliers of granite and gneiss are
exposed around Arofu on the middle Benue, around Tangalto and
Chongwom in southern Bauchi and in various places upon the narrow
plain of the Gongola and, while in some cases the crystalline rocks
exposed upon the plain of the Gongola have undoubtedly been brought
to the surface by faulting, it is at the same time fairly certain
that the earlier Cretaceous rocks were deposited upon a highly
irregular crystalline floor, the more elevated portions of which
have been exposed at the surface in the course of denudation. For
the same reason also the apparent base of the Cretaceous in the
neighbourhood of these inliers of crystalline rock cannot be regarded as the
actual base of the series, which is at most very imperfectly
known.
In Muri province a series of limestones and shales definitely
known to be of Upper Cretaceous and Turonian age[71]
is exposed in western Muri around Awe to the north and around Arofu
and Akwana to the south of the Benue. In eastern Muri, the same
series is found at Kumberi on the right bank of the Benue a few
miles above Amar. At Awe a sharp little anticline runs E.N.E. along
the north side of the town and brings up coarse white felspathic
grits and sandstones with two bands of carbonaceous shales (Plate
XV, Fig. 1). It begins to the north-east of the
town as a gentle arch with its axis dipping gently westward. The
intensity of the folding increases rapidly in that direction until
the anticline passes into a fault with the downthrow towards the
north, but farther west the fold reappears and speedily flattens
out. The northern limb of the fold is much more steeply inclined
than the southern and the beds are even in some places overturned.
At the same time the sandstone outcrop takes the form of a bow with
the concave side to the north, the whole structure being evidently
produced by a local intensification of the folding. (A. L.)
To the north and south of Awe, there are synclinal areas in
which rocks higher in the series have been preserved. From the
various exposures in the neighbourhood the following approximate sequence can be
made out:
Soft white quartzose sandstones, often with calcareous
cement.
Thinly-bedded light-coloured calcareous shales and sandstones
with bands of impure shelly limestone.
Greenish shales with ironstone bands and nodules.
White grits and sandstones and carbonaceous shales with brine
springs.
Base not exposed.
(A. L.)
Grits and sandstones similar to the lowest beds seen at Awe,
with brine springs but without carbonaceous shales, come up to the
surface again along a line running from Akiri to Azara and Ribi,
but north of that again comes another synclinal area, for between
Azara and the Ankwe river exposures of shales and limestones are
met with. Over the whole of this part of Muri indeed the beds
appear to be thrown into gentle anticlines and synclines whose axes
run approximately E.N.E. A lower and essentially sandy division,
often yielding brine, appears along the cores of the anticlines as
at Azara, Akiri and Awe, while an upper, essentially shaly
division, is preserved in the intervening synclinal areas. (A.
L.)
Limestones are known to occur in the volcanic tract on the
Benue, south-west of Awe, while at Tunga wharf, immediately south
of Awe, a river section exposes the lower sandstones striking
E.N.E., and dipping northwards at 27°. At Arofu a small inlier of
granite to the east of the town is seen to be five-sixths
surrounded by silicified limestone with the remainder of its
perimeter concealed by marshy ground. An excavation proved the
limestone to rest directly on the granite and we are therefore
compelled to conclude that this granite forms a knoll surrounded
and at one time overlaid by the limestone. Whether similar relations hold good between
the other inliers of crystalline rocks in the neighbourhood and the
limestone is not certain. The outcrop of the limestones is a broad
one, about half a mile across in the vicinity of Arofu and still
more to the south of Akwana. The broad outcrop is accounted for
partly by the gentle dip, partly perhaps by gentle undulations of
the strata, but partly also by the thickness of the limestone. What
this is it is impossible to say in the present state of our
knowledge, but it is certainly much greater than that of the
limestone bands known in the neighbouring parts of the province.
(A. L.)
The saltings of Arofu and Akwana bear witness to a salt-bearing
series underneath the limestones although that series itself is
nowhere exposed. Above the limestones comes a series of shales,
shaly sandstones and ironstones with a few thin intercalated
limestone bands. This shaly series outcrops between Arofu and the
river and runs a little south of west to beyond Akwana. The dip is
about 10° and almost north. At Shaii however, twelve miles east of
Arofu, similar shales dip at 25° towards N. 40° E., so that the
folding is somewhat variable. Around Arofu the limestones are
seldom seen in an unaltered condition, for almost all the rock
exposed is silicified. Thus the country is crossed by innumerable
dyke-like masses of silicified limestone standing up several feet
above the soil. In the rising ground south of Akwana the silicified
bands give rise to several small cascades. Knots of galena are
common in the silicified limestone and in places as at Arofu there
are veins of galena and fluorspar. The original structures of the
limestone which was partly foraminiferal but very largely oolitic and
pisolitic have been excellently preserved during the change. At
Arofu, however, a compact unaltered variety contains imperfect
remains of ammonites. It is possible that these limestones which
lie to the south of the Benue may be on a somewhat different
horizon from those of Awe, but the presence of the underlying
salt-bearing series indicates that the general succession is the
same. (A. L.)
At Kumberi some miles above Amar there occurs the following
excellent riverside section of the limestone-shale series, in which
the subordinate character of the limestone bands is very
evident:
Occasional sandstone seen in the
river bank:—
About 100
ft.—
Fine-grained sandstone, mudstone and
shales alternating as below.
12
„
Sandstone.
About 50
„
Shales.
2
„
Good white limestone with numerous teeth,
bones, and phosphatic casts of lamellibranch and gasteropod shells.
Ammonites.
6
„
Shales.
1½
„
Shelly limestone.
8
„
Shales.
2
„
Friable sandstone.
10
„
Shales with mudstone bands.
15
„
Blue-gray shales.
20
„
Shales with mudstone bands.
2
„
Mudstone with large calcareous
concretions round ammonite shells.
5
„
Shaly sandstone.
10
„
Friable sandstone.
2
„
Sandy shales.
10
„
Soft, friable sandstones with shaly
partings.
About 200
„
Fine-grained and soft sandstones,
mudstones, and shales alternating as above.
Occasional sandstones in river bank.
Total: 450 to 500 ft. Limestone :
3½ ft.
Strike uniformly N. 50° E.; dip,
30°–35° S.E.
(A. L.)
Farther east
again scattered fragments of limestone indicate the presence of the
limestone-shale series in a synclinal between Bomanda and Muri. An
anticlinal axis, running a few hundred yards south of Bomanda,
brings up a lower series of soft thinly-bedded calcareous grits
with partings of green calcareous marl along the crest of the arch.
Brine springs like those of Awe rise through the northern limb. At
Muri the lower reddish grits are seen in full force and dip
southwards at about 60°. A careful estimate of the thickness
exposed worked out at over 1,500 ft. though neither the base nor
the upper limit was seen. To the north fragments of limestone are
found in the swamps of the Kudu valley to the west of Jebjeb and
Gateri but the associated rocks have nowhere been observed. (A.
L.)
The white grits and sandstones and carbonaceous shales of Awe
and the calcareous grits and marls of Bomanda represent the passage
beds between the upper limestone-shale series and the lower
sandstones exposed at Akiri, Ribi and Tunga wharf in the west and
at Muri town in the east of the province. The weak brine which
rises in springs through the passage beds at Awe and Bomanda and
even through the limestones at Akwana appears to take its origin in
the upper part of the lower sandstone series. Brine springs and
surface incrustations are found also where the lower sandstones
outcrop at Jebjeb, Jebero, Jenoe, and Langa in eastern Muri, at
Akiri, Azara, Ribi, Kanje and Abune in western Muri, and at Doya
and Keana on the borders of Nassarawa. These localities occur immediately round the
margins of the more marked synclinal areas containing the remains
of the limestone-shale series, a fact which warrants the assumption
that the upper part of the sandstones underlying the limestones is
impregnated with salt and was originally accumulated under arid
conditions. Indications of the salt-bearing strata are also found
at Lafin Gisheri between Tunga and Ibi and at a place of the same
name between Shishi and Wurio to the east of Ibi. In these
localities we are justified in assuming that the rocks immediately
below the surface belong also to the upper part of the lower
sandstone series.
To the north of the salt area in western Muri well bedded
sandstones are exposed in many places between Lafia in Nassarawa
province, and Wase and Bashar to the north of Ibi. The prevailing
type of rock is a reddish grit or sandstone, frequently brick red,
with knots and spots of white felspathic material, everywhere
unfossiliferous, and nowhere yielding any indication of salt
impregnations. At Bashar thin beds of white and red clay are
interstratified with the sandstones. The rocks are usually inclined
at a low angle to the horizontal and appear to be thrown into a
series of gentle undulations. From their position with respect to
the salt-bearing beds and the limestone-shale series, it may be
assumed, although continuous exposures are wanting, that these
rocks lie conformably at or near the base of the sedimentary
succession in western Muri. Their thickness is unknown, but if, as
seems probable, they are to be correlated with the sandstones of Muri town
which underlie the limestones in the eastern part of the province,
they must be, according to Mr. Longbottom’s determination (p. 151), at least 1,500 ft. thick, and they may
well be more. The whole series underlying the passage beds of Awe
and Bomanda and impregnated with salt in the upper part may
conveniently be termed the “Muri sandstones.”
Plate XI
Hillocks of Drift on the Bauchi Plateau near
Vom.
Hillocks of Drift to the West of Naraguta.
Upon the drift-covered plains of southern Muri are found
occasional detached exposures of sandstones and grits, the majority
of which probably belong to the lower sandstone series. The
presence of the “Muri sandstones” underneath the limestones of
Arofu has not been demonstrated but, as already stated, the
occurrence of brine springs at Akwana affords a considerable
presumption in favour of their occurrence. Whether they outcrop in
the unexplored Munchi country to the south and south-west of Akwana
is, however, quite unknown.
In the north-eastern part of the province the “Muri sandstones”
become more steeply folded, and in the Jenoe, Ligri, and Gateri
hills appear to be underlaid by a series of very coarse-grained
white felspathic grits or arkoses. The axes of the folds strike in
a general way a few degrees north of east, and the folding appears
to be most intense in the immediate neighbourhood of Gateri where
the grits are much hardened, and in places set on end. On the
western slopes of the Gateri hills are white and red grits and
sandstones, the representatives of the “Muri sandstones,” dipping
generally westward and apparently underlying the fragmentary limestones of the
Kudu valley. In the Gateri hills, moreover, there are indications,
as at Awe, that the axes of the folds are not horizontal, but
tilted gently to the west, and the distribution of the upper
limestones in Muri province in elliptical areas or irregular
centroclinals surrounded by the lower sandstones clearly suggests a
later transverse corrugation of the whole series of gently folded
rocks. It is probable, therefore, that the limestones of the Kudu
valley, although originally occupying a much higher place in the
series than the Gateri grits, have reached their present relatively
low position, not so much as the result of faulting, but rather as
the result of the irregular folding to which the whole series has
been subjected. At the same time the occurrence of galena veins in
the swamps to the west of Jebjeb indicates that the rocks of the
Kudu valley, like those of Awe and Arofu, have undergone a certain
amount of shattering and fissuring contemporaneously with the
folding. Amongst the fissure-veins may also be included the broad
dykes of limonitic ironstone which form conspicuous hillocks around
Akiri in western Muri.
The limestone-shale series runs up the plain of the Benue from
Lau almost to Yola, and may even be found eventually farther east
than Yola. This series occupies the lower ground and nowhere forms
hills. In addition to it there are sandstones of various kinds,
sometimes occupying the plain and sometimes building up hills above the plain. The
sandstones of the hills are approximately horizontal and rest upon
the lime-stone-shale series. They will be referred to as the “Upper
Benue sandstones.” The question of their conformability or
otherwise to the limestone-shales will be discussed later (p. 188).
The limestone-shale series, being soft and easily denuded, is
very poorly exposed, and is generally hidden by the river alluvium.
Calcareous sandstone and greenish yellow shale are seen near Lau,
and south-west of Kunini large blocks and slabs of shelly limestone
a few inches thick lie scattered about over the silt. An excavation
at Kunini proved that the limestone is in thin bands, 12 or 18
inches thick, intercalated between shales. The series is thrown
into small anticlines and synclines. Of one small anticline noticed
the strike of the axis was N. 10° E., one limb being inclined at
20°, the other at 60°. The shales are seldom seen but they give
rise to a characteristic soil which is easily recognised and which
leads to the discovery of limestone lumps and fragments on further
search. Low-lying country of this character, with numerous slabs
and lumps of limestone lying about on the surface stretches up the
Benue to the vicinity of Namtari.
Similar areas of limestone-shales are found on the Gongola
between Kiri and Shillen and between Mada and Kombo. The latter
area runs up the plain of the Maio Hawal, but for what distance is
unknown. In both areas the limestone is shelly and very
fossiliferous and there are no sections showing the shales or
giving the dips.
The only natural section of this series which was met with in Yola
province was at Numan. Just to the east of the village, white
fine-grained sandstone crops out and is followed by still finer
sandstones and mudstones on which the village itself is built. No
limestone appears in the section and only a little shale. The dip
is 10° towards N. 55° W. The mudstone is usually reddish, but the
fresher parts are greenish and glauconitic-looking. Phosphatic
nodules are found in this material and also in lumps of shelly
limestone set up by the natives in Dong. A small inlier of
Cretaceous limestone is exposed also between Jibaru and Murki in
eastern Yola to the north of the Bagele hills.
The thick grits and sandstones which emerge from beneath the
limestone-shale series in Muri province are probably responsible
for the bold escarpment behind Wuduku, but in the absence of
detailed examination it is impossible to say if these
infra-limestone grits were ever deposited in Yola province, and if,
being so deposited, they crop out between the limestone-shale
series and the crystallines or are faulted out. The problem, a
simple one in itself, is enormously complicated by the fact that
the “Upper Benue Sandstones” rest upon the limestone-shales and
conceal their boundary with the crystallines. Consequently, the
relation of the Cretaceous rocks to the crystallines is at present
unknown.
The Cretaceous rocks floor the trench of the upper Benue valley,
while the plateau areas to the south and, east of the Gongola, to
the north also, are of crystallines. It is improbable from the
characters of the
limestone-shale series that these beds were formed in a narrow
inlet running up into a land mass. On the contrary they are
fine-grained marine sediments which were probably deposited in open
water. Their present position, on the floor of the valley, at a
much lower altitude than the crystalline plateaux north and south
must be explained as the result of tectonic movements. The wrinkled
character of the series is consistent with this explanation, and in
this connection sections in the neighbourhood of Kombo are
suggestive. There the broad channel of the Gongola, usually half a
mile wide, becomes suddenly constricted to less than a hundred
yards by a ridge of grits dipping at 50° towards S. 30° E. These
grits pass under limestone-shales to the south-east, and must
therefore be correlated with the Awe and Bomanda grits. When
followed eastward, however, for only a few miles, this prominent
ridge of grits disappears, and the limestone-shales are seen to
approach to within at least a few yards of the granite. This
strongly suggests that the limestone-shale belt running up the Maio
Hawal is faulted down to abut against the granite. In a similar
way, it is believed that the Upper Benue valley, to the east of its
confluence with the Gongola, was initiated as a tectonic valley.
(A. L.)
To the north as to the west of Gateri, a series of white
sandstones intervenes between the Gateri grits and the limestones
which outcrop in the swamps between Putu and Tangalto. These limestones
are exposed in a very fragmentary manner, but the presence of a
narrow belt of the lower white salt-bearing sandstones in the midst
of the limestones half-way between Putu and Tangalto indicates here
as elsewhere the occurrence of the usual succession and an
irregular folding of the whole series. Approaching Tangalto the
Cretaceous rocks run out upon the crystallines as white sandstones
and quartzose conglomerates whose horizon probably coincides with
those of the Ture grits to the east. In the hilly country to the
east of Tangalto and on the plains to the north, the inclination of
the Cretaceous rocks is in part due to a gentle folding, and in
part to an extensive faulting which has in places on the Gongola
plain brought up even the base of the series and the underlying
crystalline rocks to the same level as the limestones. Around Ture
(Plate XIII, Fig. 1) and Awok, the varied dip of
the well-bedded sandstones indicates a gentle curving into low
domes and shallow basins, while a similar structure is very evident
at Gulani, to the east of the Gongola, where the town is set within
a circle of low hills which possess a well-marked centroclinal dip.
On the other hand, the Wadi, Shinga, Bima and Gunna hills, are
separated by faults, and owe their greater northerly inclination to
the effect of a larger fault which is well seen at Gunna on the
left bank of the Gongola, and on the track between Wadi and Wuyo,
where the basal sandstones of the Bima hills with the underlying
crystalline rocks are brought up to the surface of the plain. The faulting, like the
folding, appears to be in two well-marked directions at right
angles to each other and approximately N.N.W., and E.N.E., and it
is probable that there is here, as in the neighbourhood of Awe,
some intimate relation between the faults and folds, for whose
complete demonstration much more detailed investigation would be
required.
If the Gunna fault be taken as showing the base of the
Cretaceous, the succession in the Gongola valley begins with a
series of sandstones and grits at least 1,000 ft. thick. The basal
beds are green and purple earthy grits, composed of the debris of
crystalline rocks and containing large rounded fragments of granite
and pegmatite. These are followed by alternating bands of red,
white and purple grits, which pass into the white and red
sandstones and grits of the Bima and Wadi hills. The marginal
rocks, which overlie the crystallines between Bilaraba and Dusua
are white, red, and purple grits, much broken and current bedded
and probably belonging to the same horizon as the lower sandstones
of the Bima hills. These “Bima sandstones” are succeeded after an
unknown interval by the Cretaceous limestones which outcrop in many
places upon the plains to the east and to the west of the Gongola,
and are particularly frequent over the tract of country which lies
between Dukul and Deba Habe to the west, and Wuyo and Gulani to the
east of the river. In one locality, a mile to the south of the
village of Gongila, between Nafada and Ashaka, the limestone is
clearly shown resting upon the lower series of white and red sandstones and grits.
As a rule, however, the rocks associated with the limestone, are
badly exposed and obscured by drift, while the shales which
presumably accompany the limestones have nowhere been observed. At
Gulani, the limestones pass conformably under a series of coarse
white and red grits and white, yellow and brick red sandstones with
intercalated bands of clay and flaggy ironstones. With these upper
rocks the red and white sandstones, clays, and ironstones of Wuyo
present much lithological resemblance and probably belong to the
same horizon. Similarly at Reme in the extreme south-east of Bauchi
province the limestones are overlaid by a well-stratified
succession of white and red sandstones, grits, and flaggy
ironstones, which are at least 800 ft. thick, and form the
escarpments of the flat-topped hills of Waja. The whole series
becomes markedly ferruginous towards the summit, and while the
contact with the limestones below is not actually exposed, there is
no reason to believe that it is other than conformable. These rocks
in Waja represent the highest known members of the Cretaceous
series, and it is they which are continued westward by Tula, Awok,
and Ture, and run out upon the crystallines to the east of
Chongwom. Around Awok and Ture these upper sandstones are
distinctly and variously inclined, but in Tula and Waja the dips
are so gentle as to be almost imperceptible. The white grits and
sandstones are usually unfossiliferous, but in the neighbourhood of
Ture and the Peak (Plate XIII, Fig. 2), they
contain much silicified wood from the presence of which a subsidiary argument may
perhaps be drawn in favour of their Cretaceous age. The Gongola
valley to the east and north-east of Waja retains the faulted
character which it exhibits to the north, while the country which
lies between the escarpments of the Waja hills on the north, and
the Yam Yam hills fronting the Benue on the south, is quite unknown
and may contain representatives of both granitic and sedimentary
rocks.
From the various exposures described, the Cretaceous series
within the Protectorate may be tabulated as follows:
Muri and Yola.
Bauchi and Bornu.
Upper grits and sandstones
Sandstones, grits, clays, and ironstones
of Waja, Ture, Gulani and Wuyo.
Limestones
Turonian limestones and shales of the
Benue valley
Turonian limestones of the Gongola
valley.
Lower grits and sandstones
Passage beds (white sandstone and
carbonaceous shales at Awe: calcareous grits and green calcareous
marls at Bomanda).
Muri sandstones (upper part
salt-bearing).
Bima sandstones and grits.
Gateri grits.
No evidence of contemporaneous volcanic activity has been found
within the Cretaceous rocks of the Protectorate. Intrusive sills of
dolerite, however, have been observed within the Cretaceous
sandstones in the
neighbourhood of Awe, Bashar and Serakin Kudu in Muri province, but
they, together with the numerous pipes and necks of basalt and
trachyte which pierce the Cretaceous rocks in the valleys of the
Benue and Gongola, are probably to be referred to some phase of
Tertiary volcanic activity (see Chapter
VIII).
In the French Sudan limestones of Turonian age have been
described from the Damergou[72] to the north of Zinder,
from Bilma to the north of Lake Chad,[73]
and from Tabankort, Tabrichet and Mabrouka[74]
to the south and west of the Adr’ar’ des Ifor’as. The occurrence in
the Damergou of gypsiferous clays in association with Cretaceous
limestones has not been paralleled in Northern Nigeria. On the
other hand the presence of a salt-bearing series within the
Cretaceous has not hitherto been recognised in the Sahara. Any
correlation of the lower grits and sandstones of Northern Nigeria
with the Lower Cretaceous of the Tegama, as established by M.
Chudeau,[75] would, however, be premature, in view of the
absence of determinative fossils in both and the marked
lithological differences between the two. It seems better for the
present to avoid any definite subdivision other than that indicated
above and to refer the whole series of grits, sandstones, clays,
ironstones and limestones provisionally to the Upper Cretaceous. To the south of
the Protectorate rocks of a similar age have been described by Mr.
Parkinson[76] and Dr. Esch[77] from Southern Nigeria and
the Kameruns respectively.
[70]See Chudeau, Sah. Soud., 1909, pp.
47, 11, for pre-Cretaceous rocks of the Sahara.
CHAPTER V — 1. The Sokoto and Lokoja series. — (2). The Duguri, Gombe, and Kerri Kerri sandstones. — 3. The Upper Benue sandstones. (A. L.)
EOCENE ROCKS
The Sokoto and Lokoja series: the nummulitic
limestone of Sokoto; the upper ferruginous beds; the pisolitic clay
of Giro; the white and pink clays of Kurukuru; the partial
colouration of the lower sandstones and clays; the Lokoja
sandstones and ironstones; the pisolitic ironstones of Mount Patti;
outliers of the Lokoja series. The Duguri sandstones and grits: the
clays of Pali, Kirifi and Deu; the Gombe sandstones; the ironstones
of the Tagum hills; the grits and ironstones of Kerri Kerri; the
age of the Duguri, Gombe and Kerri Kerri sandstones. The Upper
Benue sandstones: their relation to the Cretaceous and crystalline
rocks of Yola; age of the Upper Benue sandstones; outliers of
Eocene sandstones in Muri province; Tertiary history of the Upper
Benue valley. The Eocene rocks of the Upper Niger and the Kameruns.
The stratigraphy of the Chad area.
The dawn of the Eocene was
marked by regional upheaval, by the folding and fracturing of the
Cretaceous rocks and by subsequent intense erosion, as the result
of which the underlying crystalline floor was again laid bare over
the greater part of the Protectorate. A period of depression,
however, ensued, during which the sandstones, clays, limestones and
ironstones of the middle Eocene were extensively and unconformably
accumulated both upon the crystalline rocks and upon the worn and
denuded edges of the Cretaceous. The existence of this hitherto
unsuspected unconformity between the Eocene and the Cretaceous introduces a new
element of complication into the history of the Sudan and the
southern Sahara.[78] In the opinion of M. Chudeau[79]
deposition was continuous at least from the lower Turonian to the
close of the Eocene, while the early Eocene was marked by a
widespread transgression of the waters over the Niger valley. The
presence of the unconformity, however, makes it evident that the
known distribution of the Cretaceous cannot in any way be taken to
indicate the former limits of the Cretaceous sea.
The Eocene rocks within the Protectorate may be conveniently
classified into three groups:
1. The Sokoto and Lokoja series.
2. The Duguri, Gombe and Kerri Kerri
sandstones.
3. The Upper Benue sandstones.
The occurrence of fossiliferous limestones of Eocene age at
Kalfu, Tamaske, Garadimi and Toronkawa to the north of Sokoto has
been known since 1903.[80] The limestones have been found, however, to
extend much farther south and to be associated with sandstones,
clays and ironstones which are continuous with those in the Niger
valley from Ilo to Lokoja. One of the finest sections of the
limestone series is found in the cliffs facing the river beyond
Wurnu to the north-east of Sokoto where the following succession
may be observed:
10
ft.—
Dark red ferruginous grit on top.
10
„
Ferruginous oolite.
30
„
Soft, chalky, fossiliferous limestone
with phosphatic nodules and rounded lumps of hard and somewhat
phosphatised limestone.
10
„
Calcareous clay.
80
„
Blue shale, much obscured by
landslips.
20
„
Compact yellow calcareous clay, with
efflorescence of gypsum.
40
„
Red, yellow, green and black gypseous
shales and clays.
50
„
Black carbonaceous shales with radiating
crystals of gypsum alternating in thin bands with yellow
ferruginous clays with knots of ironstone; efflorescences of alum
and gypsum.
At Wurnu the same succession is represented by red sandstones
above and white, yellow and blue clays and shales below, with
fragments of limestone at the foot of the slope indicating a thin
band of limestone intercalated probably near the summit of the
shales. Around Sokoto there is much earthy ferruginous oolite on
top passing downwards as a rule into well-bedded red sandstones.
Below the Wurnu gate, however, there is an exposure of about thirty
feet of blue and white calcareous shales and clays with thin bands
of hard limestone 3–4 inches thick, numerous fossils and occasional fragments of
gypsum. At Alkamu, near Sokoto, a chalky limestone with nodules and
echinoids, associated with calcareous shales and overlaid by earthy
oolite, encloses rounded lumps of harder nummulitic and crinoidal
limestone which look as if they had been originally washed into
place. This appears to be the first record of the occurrence of
nummulitic limestone in West Africa. A similar hard nummulitic
limestone is found as far south as the neighbourhood of Jega and
more detailed investigation would probably reveal its presence in
many localities between Sokoto and Jega. The limestones, however,
nowhere form thick and massive beds, but appear to occur always in
thin bands or as nodules in calcareous shales or clays. The
occurrence of gypseous shales below the limestones at Wurnu and
Sokoto indicates the earlier presence of arid conditions, but
whether or not the various exposures of limestone occur on one and
the same horizon has not been determined.
Neither limestones nor calcareous shales are known from the
immediate neighbourhood of the Gulbin Kebbi below Sokoto.[81]
There are, however, numerous indications in the cliffs which bound
the valley of a lower lighter-coloured earthy series overlaid by a
succession of gritty and ferruginous beds. At Tozo white clays and
earthy sandstones with thin bands of limonite are covered by
oolitic ironstone which is overlaid in turn by hard red sandstone
and pebbly ferruginous grit between Tozo and Kantami. The
escarpments of
Gulma are composed of soft white earthy grit below, becoming redder
and more sandy towards the top and capped by a layer of hard
ferruginous sandstone. At Birnin Kebbi the lower massive white and
yellow earthy sandstones become mottled red and pass into dark red
ferruginous sandstones on top. At Giro the lowest rock exposed is a
yellowish pebbly grit with intersecting silicified ribs which
passes in places into a friable white pebbly marl. This is followed
by a thick (100 ft.) bed of white clay, frequently stained red
along the joints and irregular cracks, almost every specimen of
which shows a pisolitic or concretionary structure. Traced
southward this bed becomes much silicified in its upper part, with
retention of the original structure, and all stages may be traced
between the normal pisolitic clay and a pure silicified and
somewhat opalescent pisolite. Northward and eastward red and yellow
sandy beds and friable pebbly grits become intercalated in the
clay, while fragments of a fine-grained and very ferruginous
sandstone afford indications of the former presence of ferruginous
beds on top. Fragments of a similar silicified clay are found as
far south as Marake where the prevailing rock is a brick red earthy
sandstone, in places spotted with felspathic knots. At Kurukuru,
near Ilo on the Niger, soft white and pink clays are again
prominently exposed under a cap of ferruginous oolite. A pisolitic
clay very similar to that of Giro has been described by M.
Hubert[82] from Gaya on the Niger.
The sedimentary rocks run out upon the crystallines as friable pebbly yellow
sandstones at Shunga to the east of the Niger, as a hard yellow
silicified sandstone at Bakinrua in Borgu to the west of the Niger,
and farther south as ferruginous sandstones and conglomerates at
Konkwesso and Kabe, and soft white earthy sandstones on the Niger
to the west of Warra. On the plateaux throughout Sokoto province
the prevailing rock belongs to the upper ferruginous series of hard
red sandstones, grits and ironstones, the softer and
lighter-coloured rocks below being exposed for the most part only
in the cliffs which bound the river valleys. The occurrence of
outliers of still higher beds upon the surface of the plateaux
indicates that the actual summit of the series has probably nowhere
been observed.
There can be little doubt that the partial colouration of the
lower beds, and to some extent also the hardening of the upper
sandstones, is due to some considerable extent to the percolation
of iron-bearing solutions from above downwards. There is equally
little doubt that the oolitic ironstones are of primary and not of
secondary or metasomatic origin and that the deposition of the
ferruginous material took place during the latest stage in the
history of Eocene sedimentation. All gradations indeed can be
traced between a pure oolitic ironstone and a ferruginous sandstone
with scattered oolitic grains of limonite. M. Chudeau[83]
insists that even where enclosed between thick beds of clay, the
oolitic ironstones are in every case the product of the
decalcification of limestone. He has himself, however, supplied a sufficient
answer to his own argument in calling attention to the fact that
the limestones where now exposed never show any trace of even a
superficial alteration into ironstones and in admitting, as he
does, the truth of M. Hubert’s[84] contention that the
oolitic ironstones of the Niger valley are nowhere accompanied by
or traceable into limestones of a similar character. The flints of
Ansongo,[85] moreover, like the nodules of Sokoto, indicate
nothing more than the local occurrence of limestone within the
Eocene accumulations.
Plate XII
A Granite Mount near Bauchi.
A Granite Mount near Bauchi.
The sandstones of southern Kontagora and of the Niger valley
below Jebba, which may be conveniently grouped as the “Lokoja
series” from the locality of their greatest development, represent
the southward continuation of the Sokoto sedimentary series and
more particularly of the sandstones, grits, conglomerates and
ironstones of the southern portion of the series, lying below the
confluence of the Gulbin Kebbi and the Niger. The break in
continuity at Warra is more apparent than real and is due merely to
the fact that the Niger has entirely removed the sedimentary rocks
from its channel and thus exposed the underlying crystallines. A
well-marked difference in character may be traced between the
Eocene rocks of Sokoto province and their southern representatives,
a difference, however, which is entirely due to a dissimilarity in
the earlier conditions of sedimentation. In the Sokoto series there
is a well-marked succession of marine and deeper water accumulations by shallow water
and lacustrine accumulations. Amongst the southern sandstones,
however, there is no trace of a marine series, no trace of
limestones or of carbonaceous or calcareous shales and even clays
are conspicuous by the rarity of their occurrence. The series
throughout, which attains at Lokoja a thickness of 1,000 feet, is a
shallow water accumulation of sandstones, grits, conglomerates and
ironstones, comparable to some extent with the upper shallow water
portion alone of the Sokoto series. The resemblance indeed between
the uppermost members of both series is heightened by the
prevalence in both of oolitic and pisolitic ironstone, and it seems
probable that, in spite of their earlier dissimilarity, the
conditions towards the close of sedimentation in Eocene times were
identical over the whole area from Sokoto to Lokoja. A further
resemblance between the Lokoja series and the rocks of the lower
valley of the Gulbin Kebbi is the frequent staining of the lower
members of the series by descending ferruginous solutions. Wherever
the lowest rocks of the Lokoja series are exposed they are almost
invariably light-coloured sandstones and conglomerates. The
colourisation begins along the joints and fissures and spreads
inward as an irregular pink or reddish mottling. Traced upward the
rocks gradually become uniformly coloured and the tints gradually
deepen until the rocks assume the characteristic dull dark red or
chocolate colour of the uppermost members of the series. The
staining in places, owing probably to local differences in texture,
has reached a greater depth than in others with the result that at certain points
within the series the lower lighter-coloured rocks appear to attain
a less development than in others. There is, however, to be
observed in places towards the upper portion of the series an
alternation of more and less ferruginous beds underneath the richly
ferruginous cap of chocolate-coloured sandstones and ironstones,
while here and there intercalations of ferruginous beds are to be
found even within the lower lighter-coloured members of the series.
In northern Borgu, moreover, it is common to find the basal
conglomeratic beds richly ferruginous, a fact which is probably to
be explained as due to the gradual extension of the marginal swamps
during the advent of shallow water conditions.
The outliers of the Lokoja series which lie to the north and
north-east of Kontagora belong to the uppermost portion of the
series and consist for the most part of oolitic and pisolitic
ironstones. Immediately south of Kontagora, soft white sandstones
and earthy grits are exposed in the gorge of a stream at Kagara.
Again at Sonko in a narrow canyon valley a fine-grained hard
ferruginous sandstone is intercalated between beds of yellow gritty
clay. Between Sonko and Jebba, there is a great development of
ferruginous sandstones, ironstones and grits which extend also over
a large portion of western Nupe. In western Kontagora, yellow grits
and sandstones occur round Auna, while at Shaffini yellow and
reddish pebbly sandstones form the base of the hills of ferruginous
grit which rise above the plain. At Kumboji the rocks are red
pebbly sandstones and ferruginous grits, while at Chika a lower bed of yellow
sandstone is succeeded by red grit and ferruginous
conglomerate.
On the Niger below Jebba, ferruginous sandstones and gritty
ironstones are exposed at Rabba on the left bank, and red
sandstones and pebbly grits at Shonga on the right. At Share white
pebbly sandstones and conglomerates with thin bands of clay occur
below the ferruginous grits which form the summit of the tabular
hills. At Padda, to the east of Pategi, the basal beds are white
conglomerates and sandstones, knotted with fragments of felspar and
passing upward into the alternating series of white and reddish
sandstones which rise into the pinnacle peaks of Egbom. On the
banks of the Kaduna above Wuyo, white earthy sandstones are
frequently exposed passing upward in the isolated hills through
alternations of white and red sandstones into hard ferruginous grit
and ironstone. Throughout southern and western Nupe, the rocks,
where exposed, are invariably red sandstones or ferruginous grits.
Between Kacha and Baro, however, the lower beds are white or
mottled red and pass upward into a succession of red and white
sandstones and grits. A similar alternation occurs in the base of
the outlying Jakpara hills in northern Kabba which are capped by
red ferruginous grit.
From Baro southward along the Niger and eastward into Nassarawa
and Bassa, oolitic ironstones and earthy limonites associated with
ferruginous sandstones and grits attain a greater development while
the colouration of the lower beds is in general much more complete.
At Baro a white or reddish sandy clay with pebbles of quartz and kaolinised felspar
is exposed at the base of Mt. Gidda. This is followed by a thick
mass of oolitic ironstone covering the greater part of the slope,
in places much decayed and ribbed, banded and knotted with
concretionary limonite. The upper portion of the hill is composed
of red sandstone and ferruginous grit, the uppermost beds being
well banded with coarser material, and enclosing pebbles of quartz
and felspar. At Lokoja the lowest rock exposed at the base of Mt.
Patti is a soft white felspathic sandstone with pebbles of quartz,
kaolinised felspar and pegmatite which occasionally accumulate into
conglomeratic bands. The greater part of the hill, however, is
composed of white sandstones and grits indistinctly bedded and
stained irregularly, and in places to a considerable depth, with
iron ore. In the upper part of the hill the white sandstones give
place to ferruginous sandstones with concretionary knots of earthy
limonite and intercalated bands and irregular lenticular patches of
oolitic ironstone. The uppermost 150 feet are made up of oolitic,
pisolitic and earthy ironstones enclosing irregular masses of hard
lustrous concretionary limonite, while the summit is capped by a
pebbly ironstone composed of small rounded and elliptical pebbles
of compact ironstone cemented by earthy limonite containing a few
grains of quartz.[86]
The prolongation of the Lokoja series into Kabba, Nassarawa and
Bassa is accompanied by much variation in the extent and thickness
of the lower lighter-coloured rocks, and in the proportions
of ferruginous
sandstone and oolitic ironstone in the upper part of the series.
There are also indications to the north and east of Lokoja of the
ironstones occupying in places a much lower horizon than they do in
Mt. Patti, as for example in Mt. Gidda where the ironstones are
found only a little way above the river level and at Amara on the
Benue where a ferruginous conglomerate very similar to that on the
summit of Mt. Patti is exposed in the bank of the river. On the
Benue, the Lokoja series runs out unconformably and presumably upon
Cretaceous rocks in the Doma hills behind Udeni which present the
usual succession of white sandstones below and red sandstones
above. In Bassa the Lokoja series, although not actually traced, is
believed to be continuous with the white and red sandstones and
clays with plant remains exposed in the neighbourhood of Idah and
Igore on the borders of Southern Nigeria. A small outlier of red
and white sandstones and earthy limonite, near Ojerami and Akwi in
southern Kabba, bears witness moreover to the former extent of the
Lokoja series to the west of the Niger.
The post-Cretaceous rocks of eastern Bauchi and Bornu, while
more or less continuous from south to north, may be conveniently
divided into three groups, the Duguri sandstones and grits, the
Gombe grits and clays, and the Kerri Kerri grits and ironstones.
The series
throughout is a well-bedded succession of grits, sandstones, clays
and ironstones, usually lighter-coloured below, and dark red and
ferruginous above. Although everywhere unfossiliferous the series
presents many analogies with the Eocene rocks of the Niger valley,
and it is largely upon the strength of those analogies that these
eastern rocks are assumed to be also of Eocene age. Their
stratification throughout is practically horizontal, though in a
few localities a gentle dip may be observed. Their junction with
the underlying Cretaceous is believed to be unconformable, but the
actual contact has nowhere been observed. Their total thickness,
from the level of the Gongola at Nafada to the summit of the Kerri
Kerri plateau, is about 1,000 ft., and there is no reason to
believe that they elsewhere exceed this figure to any great
extent.
In Duguri, the basal beds are not exposed, but in the cliffs
behind Tongolan there is an excellent section of 400 feet of white
and purple grits capped by hard red ferruginous sandstone, and
banked against the crystalline rocks to the west. Between Tongolan
and Yuli there is an alternation of white and red sandstones with
occasional beds of gritty clay, while at Samia a series of white
sandstones and clays overlaid by red sandstones and grits is
exposed in the cliffs which bound the valley of the Yuli river. The
boundary between the Cretaceous and Eocene is very approximate, and
has been drawn provisionally a little east of Samia, where the
horizontal sandstones appear to give place to sandstones possessing
a distinct inclination to the west and probably belonging to the
inclined Cretaceous
series. Southward, the white grits of Duguri appear to be largely
replaced by red ferruginous sandstones, which are well exposed
around Tukkur to the east of Kantana. On the plateau at Kantana, a
thin series of red sandstones with bands and streaks of flaggy
limonite, underlaid by white sandstones, pebbly grits, and
conglomerates, rests directly upon the crystalline rocks, while the
presence of outliers of hard red sandstone on the summits of the
hills around Kanna affords some indication of the originally
greater westward extension of the series. The correlation of the
various rocks in Duguri and the Kanna district is rendered
difficult both by the rapid lateral variation in character of
individual beds and by the occurrence of much faulting along the
eastern margin of the plateau. The distinction of a lower and
lighter-coloured series from an upper ferruginous series is less
marked in Duguri than elsewhere, the lower series being apparently
in great part an irregular succession of white and red sandstones
and grits.
Over the central plains of eastern Bauchi the prevailing rock is
a soft reddish grit in places quite pebbly, in others passing into
white and purple sandstones. Exposures are found mostly in the deep
and narrow channels which the streams have cut below the surface of
the plain. At Pali bands of white gritty clay in every respect
similar to the clays of Kurukuru on the Niger (p. 168) are intercalated amongst the sandstones,
some of them containing knots and lenticular patches of purer
kaolin. At Panguru rounded masses of earthy and oolitic
ironstone are found
in the surface alluvium, but it is doubtful whether they are
concretions in situ or rolled fragments of an earlier deposit. A
thin series of soft white and red sandstones and earthy limonites
capped by hard red grit and possessing a dip of 10° S. and
belonging probably to the upper part of the series is exposed
immediately south of Putu. At Kirifi and Deu on the Gongola, thick
beds of white clay similar to the clays of Pali are exposed below
harder beds of red sandstone and red ferruginous grit. Similarly at
Keffin Serakin Yaki, to the west of the Gongola, banks of white
clay occur as large lenticular patches in reddish grit. Between Deu
and Gombe white sandstones and clays are frequently exposed in the
lower part of the cliffs which bound the river valley, the upper
part being invariably red sandstone or hard ferruginous grit. At
Gombe a characteristic section is exposed in the hill behind the
Residency. A coarse white clay with gritty and sandy bands in its
upper part and somewhat stained with iron forms the lower slopes of
the hill, while on top are dark red sandstones and pebbly grits
passing in places into flaggy or oolitic ironstones. The Gombe
grits and clays are continuous within the bend of the Gongola with
the grits and ironstones of the escarpments behind Tilde and Dukul
which bound the narrow plain of the Gongola in the extreme east of
Bauchi province. These eastern escarpments, however, are much
higher than the escarpments of the west and the upper ferruginous
series attains in them a much greater development. At Dukul the
thinly bedded soft
white sandstones which outcrop at the base of the escarpment are
covered by a thick series of hard red grits with bands of flaggy
limonite. Around Tilde and Ribadu the ferruginous grits alone are
exposed, but at Birri to the north the lower series is again
exposed as thinly bedded white sandstones under the usual
ferruginous cap. To the east of the Gongola, the Gombe series is
continued in the low hills of ferruginous grit, conglomerate, and
ironstone around Ligdar and Buni, and farther east in the
sandstones and ironstones of the broken hills which stretch from
Ndufa to Lua in southern Bornu. Around Ndufa yellow and brick-red
sandstones are exposed below with ferruginous grits above,
containing in places scattered knots of hæmatite and streaks of
lustrous pisolite. At Lua the Tagum hills are entirely composed of
ferruginous grit enclosing in the upper layers large knots,
lenticles, and irregular masses of hard pisolitic limonite similar
in every respect to the lustrous pisolite of Mt. Patti at Lokoja.
Immediately south of Koje the lower rocks run out upon the
crystallines in alternating bands of yellow sandstone, coarse grit,
and fine white clay somewhat stained with iron ore.
In the Kerri Kerri country in western Bornu to the north of the
Gongola, the post-Cretaceous rocks are magnificently exposed in the
southern escarpments of the Kerri Kerri plateau. The stepped cliffs
of Kadi consist of two thick and imperfectly stratified beds of
soft white earthy grit, passing in places into a coarser pebbly
grit and separated by a thin bed of red ferruginous sandstone with streaks of
limonite which forms the floor of the step. The summit of the upper
bed is reddened and capped by a thin layer of concretionary
ironstone. The detached tabular hills which fringe the plateau are
either of the same height as the plateau or cut down to the level
of the intercalated bed of hard red sandstone (Plate XIV, Fig. 1). In many cases the detached hills are stepped
like the escarpment of the plateau, and at times the upper grits
are represented merely by a conical peak set upon a platform
floored by the ferruginous band which separates the upper and lower
grits. Farther east the prominent tabular hill behind Fika has a
very similar structure. The base is formed of white and brick-red
grits upon which is set a thick bed of white earthy grit vertically
jointed and bounded by precipitous cliffs. A narrow ledge of hard
red sandstone separates this lower grit from an upper bed of
similar character somewhat stained with iron, whose gentler slopes
are covered with broken fragments of the red ferruginous grit which
caps the summit. Beyond Fika the Potiskum road leads as far as Boza
through the trench-like intersecting valleys so characteristic of
the margin of the plateau with the rocks well exposed in the
vertical cliffs on either side (Plate XIV, Fig.
2). Almost invariably soft reddish grits are found below and
white earthy grits with bands and streaks of white and pink clay
above, the whole being capped at Boza by a series of coarse red
ferruginous grits. The stratification which is indistinct in the
lower part becomes more marked towards the summit of the series and
affords indications
in places of a very gentle inclination towards the north. The upper
ferruginous grits frequently rise into low hills and mounds on the
surface of the plateau between Farsawa and Potiskum and near Gojin
they contain abundant pisolitic knots of limonite, whose presence
renders the grits very similar in appearance to those of the Tagum
hills in southern Bornu.
To the west, the Kerri Kerri grits run out upon the crystalline
rocks in the neighbourhood of Hardawa, as hard white conglomerates
and grits, while to the north and east they are gradually lost
under the recent accumulations of blown sand and earthy drift.
Between Potiskum and Gujba, the surface rocks belong mostly to the
upper ferruginous series, the lower red and white grits being
exposed only in the base of the ironstone hills of Durua. The
ironstones are very variable in character and include richly
ferruginous grits with streaks of limonite, gritty pisolitic
ironstones, pure lustrous pisolitic ores and flaggy and earthy
limonites, the latter containing in places much decayed and
unrecognisable plant remains. East of Abakri outcrops become rare
and the last trace of the Kerri Kerri rocks is found at Girboa in
some scanty exposures and broken fragments of ferruginous grit. In
central Bornu the only known occurrence of rock of any kind is the
outcrop of sandstone recorded by Barth in the banks of the
Maidugari river.[87]
It should be remembered that the assumption of an Eocene age for
the Duguri, Gombe, and Kerri Kerri grits, clays, and ironstones rests entirely
upon analogy. They exhibit frequent resemblances to the Eocene
rocks of the Niger valley and more particularly to the Lokoja
series and to the southern portion of the Sokoto series. They are
throughout shallow water accumulations, for the most part
distinctly bedded, still practically horizontal and presenting,
like the rocks of the Niger valley, a succession of a lower
lighter-coloured series by an upper and more richly ferruginous
series. The lithological resemblances are also particularly
striking. The earthy grits of Kerri Kerri can be exactly paralleled
at many points in the Niger valley and in the valley of the Gulbin
Kebbi, although in Kerri Kerri they attain a much more extensive
development. The clays of Pali, Kirifi and Deu are similar in every
respect to the clays of Kurukuru on the Niger, while the peculiar
pisolitic ironstones and lustrous limonites of Kerri Kerri and the
Tagum hills are exactly similar to those of Mt. Patti at
Lokoja.
The assumption of an unconformable contact of the Duguri, Gombe,
and Kerri Kerri rocks with the underlying Cretaceous rests upon the
practically constant horizontality of the former and the
practically constant folding and faulting of the latter. Certain
broad resemblances, however, can be detected between the uppermost
and least inclined Cretaceous rocks of Ture and Waja and the
sedimentary series of Duguri, Gombe, and Kerri Kerri. Like the
latter the rocks of Waja are of shallow water origin and show a
gradual transition from lighter-coloured sandstones below to hard
red ferruginous sandstones and ironstones above. They are, however, as a rule
more distinctly bedded and inclined and much harder and more
compact than the Gombe and Kerri Kerri series, while in
lithological character they differ in the absence of intercalated
clays in the lower part and in the absence in the upper part of the
oolitic and pisolitic ironstones so characteristic of the
ferruginous series in the Kerri Kerri and the Tagum Hills.
There is little doubt that in structural and lithological
characters the rocks of Duguri, Gombe, and Kerri Kerri present a
greater resemblance to the Eocene rocks of the Niger than to the
Cretaceous rocks of Waja. The unfortunate absence of
palæontological evidence and of any decisive contacts with known
Cretaceous rocks is however to be greatly regretted. On the whole
it seems improbable that rocks so distinctly inclined and so
evidently undulating as the Cretaceous of the Gongola valley should
give place so rapidly in the west and north to such an extensive
series of horizontal sandstones and ironstones as is to be found in
eastern Bauchi and Bornu. The entire absence from the latter
moreover of the pipes and necks of basalt, so abundant in the
Cretaceous of the Gongola valley, is subsidiary evidence in favour
of a difference of age. It would seem indeed that, as in the case
of the Lokoja sandstones and the Cretaceous rocks of the Benue, all
the facts are much more readily explicable on the assumption of an
unconformity between the Cretaceous and the Eocene in Bauchi and
Bornu.
The possibility of contemporaneous volcanic activity during the deposition of the
Eocene sandstones and ironstones is discussed in Chapter VIII.
Upon the Cretaceous rocks which occupy the Benue plain in Yola
province there rests a thick series of sandstones which will be
referred to as the “Upper Benue Sandstones.” The relation between
these sandstones and their basement is extremely interesting and
yields valuable information as to the history of this part of
Nigeria.
The southern boundary of the sandstones is a very irregular one.
Thus on passing from Gidan Jauro to Bamga the path proceeds over
sandstones for a time, then passes on to granite for a short
distance, then on to sandstone (an arkose) again and finally on to
the crystallines. North of Bamga the sandstone partly wraps round a
small hillock of granite. But in the cliff-like sides of the Yendam
Hills the evidence that the sandstones have buried up an old and
hilly land-surface is very clear. Thus in Plate
XVI a hill some hundred feet high is seen to be composite. The
bulk of the hill (off the plate to the left) is granite, but its
base is hidden by conglomerates, which pass upwards into pebbly
sandstones. The latter are banked up against the granite and
overtop it, thus forming a tabular hill. Many other sections in the
district show a similar relation between sandstones and
crystallines. (Plate VI, Fig. 1.) There is here
no question of a faulted margin, but evidence on all sides that an
older and hilly
surface has been buried up in sandstones. If now one stands amongst
these hills, there is the Benue plain to the north, floored by the
Cretaceous limestone-shales, with the “Upper Benue Sandstones”
resting upon them; around are the composite hills with the
sandstone banked up against the granite, while to the south is the
crystalline plateau of southern Yola, with the highest of its hills
capped by sandstone. The accompanying section[88]
is approximately correct, though the maps will not permit of great
accuracy.
Passing on towards Yola, there are crystallines at Bikola and
again beyond Jelima, but in the lower ground between the two are
pebbly sandstones and conglomerates running up from the plain into
the crystalline area as a tongue. Here the present stream course
seems to occupy the same position as an older drainage channel
existing before the deposition of the detrital rocks. The mass of
the Gumba hills is almost all of granite. The southern part,
entirely of granite, rises to 1,886 ft. above sea-level, but the
cone of the northern peak, 1,727 ft. high, is of sandstone which,
as in the case of the Yendam hills, must have been banked against
the granite at the time of deposition.
The observations made may be summarised by saying that if the
“Upper Benue Sandstones” were stripped off, so as to lay bare the
underlying basement, no great change in the physical geography
would result. Some detached hills and outliers would disappear, but
the essential features of the Benue plain with its Cretaceous rocks
and the crystalline plateau of southern Yola would still remain. The sandstones
evidently accumulated upon lower ground to the north, at first
abutting against the granite hills of the south but eventually
rising above them. The physical features were buried up and have
been exhumed by later denudation with great modification in detail
but with the essential characters persisting throughout.[89]
The sandstones are found to run up the Benue plain past Yola to
Garua and they also extend some little way north and north-west of
Garua. The main spread, however, appears to turn southward, and
according to German writers,[90] they are found between
Djerim and Rei-Buba on the uppermost Benue, and also on the hilly
area to the west.
Bearing in mind this distribution of the sandstones and their
field-relations to the underlying basement, there is seen to be an
intimate connection between the sandstones and the river, and it
appears fairly certain that the sandstones were accumulated in the
valley or valley-plain of an “Older Benue.” The outliers of the
Yendam Hills and the Bagele Hills at Yola, which rise a thousand
feet or so above the river, bear witness to the former thickness of
these accumulations, now so largely removed.
The Upper Benue Sandstones cannot yet be subdivided even
locally. No clay or shale bands have been found which would serve as horizons.
The material does not vary much. The bulk is very felspathic and
has evidently been formed out of granitic debris. At Bamga it is
not always easy to distinguish between the arkoses of the sandstone
series and the compacted granite sand at the base of the hills. The
base of the series may become conglomeratic and the sandstone is
liable to be pebbly even in the highest layers. Most of the rocks
are white or slightly stained with iron oxide, but there are also
intercalated layers of very ferruginous sandstone which occur as
definite bands a few inches thick, traceable across the entire
section exposed. The ferruginous character of these bands was
determined at the time of deposition, possibly by local
concentration of the heavier minerals. In addition to such bands
there are in places much thicker masses of highly ferruginous grit
occurring at what is locally the base of the series.
Much of the sandstone is massive, without bedding planes, and it
tends to form caps to the hills with vertical sides, fifty feet or
more in height. Cross bedding is beautifully exhibited in places in
the lower part of the series, especially in the Bagele Hills,
around Tagumbilli, and at Lamurde. The laminæ succeed each other in
a perfectly regular manner through considerable thicknesses of
material, one set of laminæ being truncated and overlaid by another
set with a different inclination. Cross bedding is frequently met
with in the Cretaceous grits of the Protectorate, where the laminæ
are oblique to the major bedding planes. The cross-bedded sandstones
of the Upper Benue, however, are built up of sets of laminæ,
indistinctly marked off from those above and below, and without any
true bedding planes at all. The structure resembles in every way
the cross bedding of the Benue sandbanks, and in size of grain, in
the presence of occasional pebbles and even of pebbly layers, and
in the material itself, a coarse felspathic sand, the sandstones
closely resemble the sandbanks of the present day (p. 15). The oblique lamination has been likened to
that of dunes, but the coarse size of grain and the presence of
pebbles indicate running water rather than wind as the immediate
cause.
The flat tops of the sandstone hills and the terrace-like
features on their sides show that the Upper Benue sandstones have
not been folded. They rest on the Cretaceous limestone-shale
series, which is folded in Yola province as it is in Muri. This
suggests that the Upper Benue sandstones rest unconformably on the
Cretaceous and are therefore post-Cretaceous in age. The actual
boundary between the sandstones and the Cretaceous was only met
with in one place, between Murki and Jibaru, about twenty-five
miles north of Yola. There a coarse, dark red grit rests directly
on Cretaceous limestone without any transition beds between the
two. Unfortunately the dip of the limestone is not known, and
therefore the evidence of the contact is not conclusive. The
immediate change from limestone to unbedded ferruginous grit is
easily understood if the overlying series is unconformable to the
lower one, but it is not easy to account for it on the view of a continuous
sequence. This section is regarded as strongly supporting the
argument drawn from the unfolded character of the sandstones.
Though the reliance to be placed on lithological character is
uncertain, so far as it goes the evidence is all in favour of
correlating the Upper Benue sandstones with the Lokoja series. It
is true that the oolitic ironstones so frequently met with in that
series are not known from the Upper Benue area, but the soft
friable sandstones and the ferruginous character of much of the
material strongly remind one of the Lokoja series.
The above conclusions, and certainly the map of this area, will
require modification in details, for the following reason. Two
groups of sandstone are known in the Benue area, one below the
limestone-shales and therefore Cretaceous, the other resting upon
the limestone-shales, in all probability unconformably, and
therefore post-Cretaceous. In the absence of fossils from either
group of sandstones, the only method of determining the age of any
particular exposure is to prove continuity either with the
sandstones passing below the limestone-shales or with the
sandstones resting upon the limestone-shales. This would require
careful work under the most favourable conditions, and is out of
the question in a preliminary survey. Thus in the Yola province it
may ultimately prove that Cretaceous (infra-limestone) sandstones
have in places been mapped with the Upper Benue sandstones.
In a few places in Muri Province there are
sandstones having
lithological resemblances to the post-Cretaceous series. Near Mutum
Biu, for example, there is a porous, very ferruginous grit,
breaking up into small fragments, each coated with a skin of
limonite—characters exactly paralleled, in many localities, by the
Lokoja series. Again at Loko, sixteen miles north of Katsena Allah,
there is a soft unbedded pebbly sandstone which is much more like
the Upper Benue sandstones than the Cretaceous. It may yet prove
that on the widespread Cretaceous rocks of Muri there are here and
there patches of post-Cretaceous sandstone, serving to link up the
Upper Benue sandstones with the Lokoja series.
In spite of such future modifications, it is believed that the
history of the Upper Benue can be already sketched out as
follows:
1. Crustal movements (folding and perhaps faulting) dropped down
the Cretaceous rocks below the crystalline basement along the
Lau-Yola line.
2. The soft character of the Cretaceous rocks marked them out
for rapid denudation, and an “Older Benue” was established along
this line with crystalline plateaux north and south.
3. For some reason the detritus carried by the river
accumulated, and on consolidation it formed the Upper Benue
sandstones. The unstratified character of the material and the
absence of alluvial mud or clayey bands indicates a rapid
accumulation, probably under somewhat different conditions from
those now prevailing. The sands were banked up against
the granites of the
south and buried up at least the foot hills.[91]
4. Later denudation has largely removed the sandstone and has
exposed the Cretaceous floor again in many places. (A. L.)
Plate XIII
The Sandstone Hills of Ture.
Ture or Tangale Peak.
The great development of rocks of Eocene age in the region of
the Upper Niger has been sufficiently emphasised by M.
Chudeau.[92] The Niger sandstones of Dahomey[93]
are undoubtedly continuous with the western members of the Sokoto
series, and are properly ascribed by M. Chudeau to the Eocene. The
sandstones of the Upper Benue valley in the northern territories of
the Kameruns described by Dr. Passarge,[94]
are likewise continuous with the Upper Benue sandstones of N.
Nigeria, and therefore most probably also of Eocene age. From the
Southern Kameruns Dr. Esch[95] has described Cretaceous
and Eocene rocks whose mutual relations are very obscure. The
locally folded character of the Cretaceous, however, suggests an
unconformity at the base of the Eocene. In Southern Nigeria Mr.
Parkinson[96] has indicated the occurrence of both Cretaceous
and post-Cretaceous rocks whose relations are at present unknown.
The clays and lignites of Asaba[97] may possibly,
however, have been
accumulated contemporaneously with the upper shallow-water portion
of the Eocene of Northern Nigeria.
M. Chudeau’s[98] admirable attempt at the construction of
a map of the central Sahara is somewhat marred by his admittedly
audacious method of deducing the solid geology of certain areas
from the average depth of the wells. A striking example of the
inaccuracy thus introduced is the relegation by M. Chudeau of the
Eocene rocks of Sokoto to the Lower Cretaceous. Another fruitful
source of error is the failure of M. Chudeau to recognise,
doubtless through the want of sufficiently suggestive exposures,
the possibility of an unconformity between the Cretaceous and the
Eocene in the Sudan. There is every reason to believe that the
erosion which followed the post-Cretaceous elevation was active
over the whole of the Sudan, and that the greater part of the
hummocky and irregular surface thus produced was again under water
during the period of the middle Eocene depression. Moreover, the
deposits then accumulated were in turn subjected to erosion during
a subsequent movement of elevation, with the result that the early
Eocene surface of crystalline and Cretaceous rocks was again laid
bare over considerable areas. In view, therefore, of the former
probable extension of the soft sandstones and ironstones of Kerri
Kerri and the Tagum Hills over the plains of Bornu, it is
permissible to doubt the accuracy of much of M. Chudeau’s suggested
stratigraphy of the Chad area. It seems more reasonable to regard the sandstones of
Kanem and Manga as Eocene rather than Lower Cretaceous, and to look
upon them as having been deposited, as in the case of the Eocene
rocks of the Gongola valley, upon an earlier surface of Upper
Cretaceous and crystalline rocks now buried beneath the superficial
accumulations of Bornu. The rocks of Koutous and Alakhos, Ouame and
Tirminy[99] present, moreover, characteristics which ally
them more with the Eocene rocks of the south than with the Turonian
of the Damergou, and it is possible that even the southern
escarpment of the Tegama itself[100] where it overlooks the
Damergou should be referred to the Eocene. The red, white, and
yellow sandstones of Ennedi and of Snoussi on the Upper Shari are
probably also of the same age, and in no way related (as suggested
by M. Courtet[101]) to the Karroo beds of South Africa. (J.
D. F.)
[78]Cf. Suess, The Face of the
Earth, Vol. IV, Oxford Trans., 1909, p. 89.
[80]De Lapparent, C. Rd. Ac. Sc., 136,
1903, pp. 1, 118; Bather, Geol. Mag., [5], 1, 1904, p.
292.
The Eocene fossils from Sokoto and Jega are similar to those
collected by Col. Elliot and Capt. Lelean from the Sokoto frontier,
of which accounts have been published by Dr. F. A. Bather
(Geol. Mag., 1904, p. 292, Plate XI) and by Mr. R. Bullen
Newton (G. J., XXIV, 1904, p. 522), who regard them as of
Middle Eocene age. The fossils consist principally of echinoids and
internal casts of mollusca. The latter are not sufficiently well
preserved for exact determination.
From Sokoto numerous examples of Hemiaster sudanensis
(Bather were obtained, and also specimens of Lucina and
gasteropods. At Jega Hemiaster sudanensis is also abundant,
and a portion of Plesiolampas (probably P. Sahara,
Bather) was obtained. The mollusca belong to the genera
Nautilus, Turritella, Voluta, Cardium,
Lucina, Ostrea, etc.
(H. W.)
[81]Fossiliferous limestone has, however, been
reported from Yelu at the confluence of the Dallul Mauri and the
Niger valley.
[89]The crystalline rocks at the base of the
Bagele hills, mentioned by Barth (Central Africa, Vol. II,
p. 478) may be taken to represent the outcrop of a portion of the
earlier hummocky floor which was buried under the sandstones of
Yola.
[90]Passarge, Adamaua, 1895, p. 363.;
Edlinger, Niger-Tsad Expedition, 1904, p. 154.
[91]Cf. Passarge, Adamaua,
1895, p. 390. Passarge did not recognise the presence of Cretaceous
rocks in the Upper Benue valley.
CHAPTER VI — 1. Hausaland and Borgu. — 2. Bornu and Katagum. — 3. The Gongola Valley. — 4. The Benue Valley.
SUPERFICIAL ACCUMULATIONS
The general absence of a deeply decomposed crust
upon the rocks of the Sahara and the Sudan. The drift of Hausaland
and Borgu; kankar; origin of the plains of Hausaland; the surface
ironstones; origin of the drift; Mounts Dala and Kogon Dutsi; the
surface drift of the Bauchi plateau; conditions of formation of the
surface ironstones; periods of maximum and minimum erosion; periods
of elevation and depression of the crust; the recent arid period.
The drift of Bornu and Katagum; the absence of surface ironstones;
the “firki”-land of eastern Bornu; the shelly marls of Kukawa; the
conditions of formation of the drift of Bornu; the early existence
of swampy and lacustrine conditions in Bornu; regional oscillations
of level in the Chad area; erosion of the floor of the present
lake; the cuvettes of Manga. The drift of the Gongola valley. The
drift of the Benue valley. The ferruginous crusts of the
flat-topped hills of the Lower Niger and Benue.
No fossiliferous deposits
of later age than Eocene have been discovered within the
Protectorate. M. Chudeau[102] quotes the occurrence of
Miocene shales at Boutoutou[103] to the north of Gober
and concludes that the sea did not finally quit Central Africa
until the close of the Miocene. If, however, any deposits of
similar age were ever accumulated within the Protectorate, they
appear to have been subsequently removed and replaced by unconsolidated
accumulations, most probably of late Pliocene age, which now in
places cover Eocene, Cretaceous, and crystalline rocks alike. These
accumulations, which vary considerably in composition in different
parts of the Protectorate, are of continental and in part of
lacustrine origin and represent the products of weathering and
disintegration of the crystalline and sedimentary rocks, sorted and
rearranged by the action of rain, river, and wind. Their
superficial portion now forms the surface soil which except in
swampy localities is characterised by the small quantity of humus
which it contains. M. Chudeau[104] has remarked upon the
absence of any deep decomposition of the rocks in situ in the
Sahara and Sudan to the north of lat. 11°, and his remarks may with
justice be extended to the greater part of Northern Nigeria, where
the original products of decomposition have rarely remained in
place but have travelled outwards for a variable distance from the
parent rocks and in places accumulated to very considerable depths.
This drifted or travelled character of the surface alluvium may be
typically observed round the margins of the larger groups of
granite hills. The disintegrated debris rapidly accumulated during
torrential rains at the mouths of the narrow valleys or at the base
of the weathered slopes is gradually broken up and spread out upon
the plain during succeeding periods of less intense fluviatile
action.
The red sandy loam which covers the greater part of the
crystalline plains of central Hausaland and Borgu, while
predominantly of local derivation, possesses for the most part a
drifted or travelled character, and in composition presents
relatively little affinity with the rocks which anywhere
immediately underlie it. A rude stratification may at times be
observed within the drift, but for the most part it belongs to the
category of ill-defined alluvium and surface wash.[105] Red earth and subangular quartz grains in
varying proportions naturally make up the bulk of the deposit, but
it contains also more or less worn and weathered crystals of all
the common rock-forming minerals together with pebbles and
fragments of quartz, quartzite, granite, gneiss, amphibolite, and
other rocks which are naturally most abundant where the drift is
thinnest. Calcareous concretions similar to the kankar of India are
common in places where the drift is predominantly earthy. They
have, however, nowhere been observed forming any continuous sheet
of sub-surface limestone, while, contrary to M. Hubert’s
suggestion,[106] their distribution appears to have little or
no relation to the occurrence of crystalline limestones within the
schistose series. This composite drift, while evidently resulting
from the rearrangement of weathered and disintegrated material, can
frequently be seen covering up and wrapping round smooth
and little
weathered surfaces of rock in the midst of the open plain. The
smaller inselberge moreover appear to have little influence upon
the composition of the drift, which preserves its heterogeneous
character up to the base of the dome, and even encloses on the
margin sub-angular blocks and boulders of granite detached at an
earlier period from the surface of the dome itself. In places, as
on the high plains of Zaria and Bauchi, a very considerable
thickness of drift has accumulated, and where locally it has been
in part removed the rocky floor below is hummocky and irregular and
as a rule remarkably free from any evident decomposition. It is
highly probable, indeed, that the sandy loam with its heterogeneous
composition and its subangular pebbles of various rocks was slowly
accumulated upon an irregular surface and spread out and levelled
by wind and rain.[107] That it was accumulated, moreover, under
very different conditions from those which now prevail is evident
from the fact that it is nowhere still in process of deposition
upon the open plains of Hausaland. On the contrary it is now
undergoing erosion, rearrangement, and gradual removal by the
rivers and streams which in many parts of their course have not yet
succeeded in exposing the rock below. Its presence also to a
remarkable depth upon the watersheds and the loftiest plains makes
it highly improbable that such regional accumulation could have
taken place at the present elevation of the central provinces. It
would seem rather that there has been within comparatively recent
times a rearrangement of the drainage system of the Protectorate,
brought about by a crustal movement of sufficient intensity to
transform the plains of Hausaland from low-lying areas of
accumulation into high-lying areas of erosion.
Such an hypothesis serves also to explain the present typical
aspect of the plains themselves when it is remembered that their
gently undulating character is due not so much to the smooth and
even surface of the underlying rocks as to the covering of drift
which has obscured all the minor irregularities of the crystalline
floor below. There can be little doubt that the plains of Hausaland
arose primarily as approximately base-levelled plains of erosion.
There is, however, no reason to believe that the actual surface of
the underlying rocks coincides in more than a general way with that
of the open rolling plains of to-day. All the indications indeed
point to quite the opposite conclusion. The base-levelled plains
appear to have suffered some slight regional elevation and erosion,
and the consequent hummocky and irregular surface has been in turn
levelled up and buried in drift. The plains as we know them,
therefore, while primarily plains of erosion, are in their more
obvious characteristics to some considerable extent plains of
accumulation.[108] The processes of erosion and
accumulation moreover, to which they owe their present aspect, are
for the most part the same as those which are now active in these
regions. The isolated hills and the bare surfaces of rock frequently show
evidence of abundant pot-holing and of extensive fluviatile action.
Messrs. Andrews and Bailey[109] have suggested that a
sub-aerial plain of erosion may in time, by means of dry weathering
and run-off, replace a tract of hilly country at an altitude of
even more than 3,000 ft., without the help of river erosion or
planation. Whatever local influence these processes may have,
however, the possibility of such a result upon a regional scale is
exceedingly doubtful. The efficacy of dry weathering indeed, as an
agent of extensive erosion, is, in spite of the able championship
of Professor Walther,[110] becoming more and more
discounted with the progress of research in the Sahara.[111]
A characteristic feature of the red sandy loam on the high
plains of Borgu, Zaria, and Bauchi is the occurrence in places of a
ferruginous cap or crust, which frequently forms a continuous floor
many acres in extent. The ferruginous cap has arisen through the
cementation of the quartz grains and pebbles into a hard resistant
mass by means of hydrated ores of iron originally deposited in the
subsoil above ground water level. The characteristic cellular or
vesicular character and hard lustrous surface of the ironstone are
due to the leaching out of the uncemented material and the
rearrangement and partial dehydration of the iron ores upon
exposure at the surface. Wherever, therefore, the ferruginous cap
is now exposed, it may be assumed that the soil originally
overlying it has
been removed in the course of denudation. In southern Bauchi and
Zaria[112] the vesicular ironstone may at times be found
forming low platforms raised a few feet above the general level of
the plain, which is itself in places provided with a similar floor
of ironstone. At other times where erosion has been more intense it
caps low flat-topped hillocks of drift whose slopes are littered
with fallen blocks and boulders of gritty ironstone. Rolled pebbles
of concretionary limonite and subangular fragments of an earlier
ferruginous crust are frequently also characteristic constituents
of the surface soil. It is evident, therefore, that in addition to
the period of formation of the ferruginous crust which now in
places forms the floor of the plain, there has been at least one
earlier period of formation of gritty ironstone within the drift.
It may therefore be presumed that the drift was originally of
sufficient thickness to cover the highest platform of ironstone of
which traces still remain, and that the ironstone platforms of
lower levels were formed during subsequent intervals in the later
erosion of the drift. In places fragments or masses of pebbly
ironstone of heterogeneous composition can be seen adhering to bare
and unweathered surfaces of granite, sandstone, and other rocks.
Such occurrences are to be regarded as relics of earlier sheets of
ironstone formed, it may have been only locally, when the drift was
thicker and the ground-water at a higher level than it is at
present.
The clue to the
origin of the drift is found within the walls of Kano. The two
flat-topped hills, Dala and Kogon Dutsi, each 150 feet in height,
are composed almost entirely of soft and thoroughly decomposed
rock, stained red towards the summit and capped by a layer of
vesicular ironstone (p. 90). The presence of
rolled fragments of an earlier ironstone in the ferruginous cap
indicates that the flattened summits once formed the floor of the
plain and were themselves probably covered by an earlier sheet of
drifted alluvium. These hills bear witness, however, not only to
the extensive decomposition in situ to which the
crystalline rocks were once exposed, but also to the intensity of
the subsequent erosion which has left them as solitary relics of
the earlier surface of the plain. It is possible, indeed, that the
formation of such a sheet of weathered rock possessed a regional
character, and that in its later erosion is to be found the
explanation of the general absence of any deep decomposition of the
rocks in situ not only within the Protectorate but also
over the whole surface of the central Sahara and Sudan. To the
erosion of this weathered crust is also to be ascribed the
formation of the hummocky and irregular surface upon which the
drift was afterwards spread out, and it is probable that the
greater part of the drift itself was derived from the further
erosion of those portions of the original weathered surface which
still remained above the level of accumulation. No facts have been
ascertained with regard to the occurrence of true lateritic
material either in the original sheet of weathered rock or in
the drifted
alluvium. There is some reason to believe, however, that the
surface ironstones in places contain considerable quantities of
free alumina.
It is possible that before accumulation began the weathered rock
had not been entirely removed from the surface of the plains, and
that portions of the original sheet of decomposed material may
therefore be preserved in places underneath the drift. Moreover
worn surfaces of the original weathered rock may have been left
practically devoid of any covering of drift, and these surfaces, as
around Aribi and Gantam on the Nassarawa tableland, may have been
in course of time provided with a typical crust of gritty and
vesicular ironstone. The floor of the plain of Kano around the two
prominent hills probably represents in this manner a platform cut
in the earlier sheet of weathered rock and covered with a
ferruginous crust. A similar origin should perhaps be ascribed to
the plains of Rauni which are characterised by the great depth of
their surface ironstones and the presence of hillocks of similar
composition which recall the buttes of Djougou in Dahomey.[113] Between Kano and Katsina, moreover, the amount
of drift varies much from place to place, and appears to have been
locally very largely removed and replaced by recent products of
weathering and considerable accumulations of blown sand and clay
which have probably arisen to some extent through the rearrangement
of the earlier drift under the influence of the winds. These recent
accumulations are sometimes found resting upon older surfaces of
cellular ironstone, but are never themselves covered with a
ferruginous crust.
The red earthy drift which forms the detached flat-topped hills
upon the summit of the Bauchi plateau (Plate XI, Figs. 1 and 2), while located for the
most part upon a crystalline floor, may possibly have originated to
a considerable extent through the disintegration of the fragmental
material of the volcanic cones in the neighbourhood, and through
the rearrangement of the original weathering products which may be
presumed to have formed upon the lavas of the Morrua plateau. There
are, however, few remaining traces of any earlier weathering of the
crystalline and igneous rocks preserved underneath the superficial
accumulations. For the most part the latter rest upon smooth and
comparatively unweathered surfaces of rock (Plate XV, Fig. 2), while their travelled origin is indicated not
only by the presence of quartz grains and granitic debris in the
ferruginous cap, but by the occurrence of tinstone in the drift
throughout the whole area. The tinstone is naturally most abundant
around Ngell in the neighbourhood of its centre of dispersion, but
traces of it are to be found even on the hills of Rukuba from which
the drift itself has now been almost entirely removed. There is
good reason to believe, indeed, that the Bauchi plateau up to a
comparatively recent date was continuous with the central plains of
Hausaland, and that it participated in all the oscillations of
conditions to which the latter were subjected. It is probable,
therefore, that the
plateau was at first covered with a sheet of decomposed rock of
variable thickness, by the subsequent erosion of which the
irregular surface was produced upon which the drift was accumulated
and spread out. It is obvious, however, that the drift itself with
its crust of compact or vesicular ironstone is now undergoing
erosion, and that the present tin-bearing alluvium is derived for
the most part from the rearrangement of the drift and only to a
small extent from the denudation of the underlying rocks which is
now again in process. It is probable that, during the earlier
stages in the erosion of the plateau drift, accumulation continued
upon the plains of Zaria and Nassarawa round the base of the
plateau, but any such action is practically negligible under the
present régime.
M. Hubert[114] in Dahomey, and Dr. Passarge[115] in Adamaua, assign only a very subordinate
place to the formation of surface ironstones upon drifted alluvium.
They believe that the ironstones have almost always formed within
material which has arisen as the result of the decomposition in
situ of the rocks immediately underlying.[116] M. Hubert’s insistence upon this point of view
has led him to suggest that the crystalline rocks of the plains of
Borgu were levelled before being decomposed and covered with
surface ironstones, and that the plains themselves have preserved
their present character and elevation since the close of the
Eocene. It may be granted that the crystalline rocks of
Nigeria were at one
time deeply decomposed, but Mounts Dala and Kogon Dutsi bear
unmistakable testimony to the subsequent intense erosion of this
former crust of weathered rock. In southern Bauchi and Zaria,
moreover, extensive sheets of surface ironstone can frequently be
seen capping typical gritty alluvium, and there can be little doubt
that within the Protectorate as a whole, the formation of surface
ironstone upon material actually weathered in situ is the
rarer phenomenon. For the most part the ironstones rest upon and
have been formed within drifted and travelled superficial material
which has been accumulated, near or at base-level, upon an
irregular and hummocky surface, and spread out in such a manner as
to obscure all the minor irregularities and to produce the present
smooth and even character of the plains of Hausaland. M.
Chautard[117] has noted the similar extensive formation of
surface ironstones upon the ancient alluvial plains of Upper Guinea
and the Futa Djalon.
The actual conditions of formation of the surface ironstones are
very problematical. Their association with the older alluvium
affords some reason for believing that their extensive formation in
the past has been accomplished under somewhat different conditions
from those which now prevail. It seems probable also that the
presence of surface ironstone as a ferruginous cap upon drifted
alluvium is an analogous phenomenon to its occurrence upon material
which has arisen, as in Mounts Dala and Kogon Dutsi, through the
decomposition in situ of the rocks below. Drs. Hatch and Corstorphine[118] believe that the surface ironstones of South
Africa have been formed very largely as bog iron ores by the
oxidation of ferruginous solutions or by the deposition of the iron
salts during the evaporation of pools of stagnant water. Captain
Freydenberg[119] has noted the accumulation of ferruginous clay
on the northern shores of Chad, but extensive sheets of ferruginous
material comparable to the platforms of surface ironstone have
nowhere been observed in process of formation upon the surface of
recent alluvium or of æolian accumulations under the present
régime of periodic rains. The surface waters accumulated
during the summer rains are for the most part rapidly and
completely drained, and any probability of the formation at high
altitudes within the Protectorate of pan or bog iron ores upon an
impervious subsoil would appear to be prohibited by the general
open and porous character of the drift. Moreover wherever true
swamps, subject to partial or complete annual desiccation, are to
be found, surface ironstones are typically absent, a waterlogged
condition of the subsoil being apparently unfavourable to the
deposition of the iron ores. M. Lacoin[120] believes that in alluvial deposits such as
those of the Shari and Oubangui an enrichment in iron takes place
in the lower beds at the expense of the upper, and that
the banks of
ferruginous material, formed by sub-surface evaporation and
oxidation after the manner of the “alios” of Landes,[121] are afterwards exposed by the removal of the
overlying accumulations. Such an hypothesis, however, is equally
open to the objection that the surface ironstones of Nigeria are
most typically found where the sub-surface drainage is most
perfect. Professor Walther[122] believes that the
surface ironstones are entirely the product of arid conditions, and
that their origin is similar to that of the protective crust of the
Libyan desert. While, however, there can be little doubt that the
hardening of the surface of the ironstone and the production of the
characteristic varnish is the result of the partial dehydration of
the iron ores upon exposure to conditions of excessive evaporation,
it is difficult to believe that the occasional rains and dews of an
arid climate could suffice to produce any extensive superficial
accumulation of the iron ores. MM. Chautard[123] and Hubert,[124] in view of the sporadic
distribution of the ironstones, would extend to their formation Sir
T. H. Holland’s hypothesis[125] of the bacterial
production of laterite under humid conditions, an hypothesis,
however, which still awaits experimental verification. Dr.
Passarge[126] is of opinion that surface ironstones are
formed under semi-arid conditions where the aridity causes the
rapid evaporation
of the occasional rains, while the high temperature promotes rapid
chemical weathering of the rocks, and the excess of ozone and
oxygen compounds in the rains, together with the absence of
reducing substances in the soil, brings about the rapid oxidation
and precipitation of the iron ores. Mr. Maufe[127] believes that the formation of surface
ironstones (muram) in East Africa is an immediate consequence of
deforestation. There is some reason to believe, however, that even
underneath a covering of vegetation the slow precipitation of iron
ores in the subsoil may give rise to the formation of ferruginous
concretions which may in time coincide to form a continuous layer.
Where the streams have trenched the deeply decomposed surface of
certain belts of schists in Kabba and Nassarawa which support an
abundant vegetation, it may be observed that the iron shows a
decided tendency to accumulate in the upper portion. The solution
of the iron is brought about by the descent of water charged with
carbon dioxide and with organic acids, arising primarily from the
decomposition of the superficial vegetation, while the subsequent
rise and redeposition of the iron ores above ground water level is
accomplished during periods of less relative humidity. It is
conceivable that a similar action may take place within alluvial
deposits when covered with a rich vegetation, and that a subsequent
change in conditions may bring about the more rapid evaporation of
the ferruginous solutions, the removal of the
vegetation and of
the surface soil, and the hardening and polishing of the crust into
a typical surface ironstone. The present irregular distribution of
the ironstones may have been conditioned by the amount of
subsequent erosion as well as by the original composition of the
drift, the content in iron of the circulating surface waters, the
abundance of the vegetation and the character of the drainage, the
free access of oxygen to the subsoil being a necessary condition of
their formation.
Dr. J. M. Maclaren,[128] Mr. F. P. Mennell[129] and Mr. J. M. Campbell[130] have suggested an analogous origin for certain
laterites and ironstones of India, Rhodesia and Upper Guinea
respectively, and postulate for their formation an alternation of
wet and dry seasons similar to the present régime. Messrs.
Mennell and Campbell are of opinion, moreover, that the
concentration of the ores upon definite horizons to form the floors
of successive platforms or terraces may be explained as the result
of a spasmodic lowering of the ground water level under climatic
conditions similar to those which now prevail. While, however, the
erosion of the platforms themselves within the drift may certainly
be ascribed to this cause, it is much more probable that, at least
in the case of Nigeria, the spasmodic activity in the deposition of
the iron ores has been brought about by an alternation of humid and
arid conditions, the formation of ironstone being most marked
during periods of gradual desiccation. It is well known that the
present régime of periodic rains within the Sudan
represents a return to more humid conditions after a period of
aridity, and there is some reason to believe that there have been
many such climatic oscillations in the past. Nothing is more
probable, therefore, than that certain periods of desiccation
should immediately follow the formation of certain platforms of
erosion within the drift, and that to the incidence of these
periods of desiccation is to be ascribed not only the maximum
deposition of the iron ores but also the removal of the overlying
alluvium and the hardening of the surface of the ironstones. It is
probably correct to assume that an annual alternation of rains and
droughts similar to the present régime is favourable to
the deposition of iron ores in the subsoil, and there is every
reason to believe that their accumulation is still going on where
conditions favourable to their formation are to be found. Under
present conditions, however, the amount of iron salts dissolved in
the ground water must be relatively less than it would be at the
close of a period of maximum humidity, and it may therefore be
presumed that in Northern Nigeria the present is a period of
comparatively slow accumulation of iron ores. There is indeed, as
already remarked, little or no evidence of any extensive formation
of surface ironstone, since the beginning of the present
régime.
The ironstones must be exposed actually at the surface before
they can assume their characteristic lustrous carapace, and the removal of the
vegetation and of the surface soil can be accomplished only by a
climatic change. Mr. Campbell[131] in Upper Guinea, and Mr.
Maufe[132] in East Africa, would assign the deforestation
very largely to the influence of man, and the subsequent removal of
the soil to the agency of the tropical rains.[133] It is difficult to believe, however, that the
characteristic ironstone platforms of Hausaland can possess such a
comparatively recent origin, and there can be little doubt that the
highest platforms of which traces still remain should be referred
to at least the later Tertiary period. Once formed, the hard
resistant crust may be covered by later alluvial or æolian
accumulations or by the products of disintegration and
re-arrangement of the ironstone itself, upon the return of more
humid conditions. Wherever covered by later deposits, however, the
ironstone is softer and lacks its characteristic varnish. Moreover,
where a scattered vegetation is now found creeping over a surface
of lustrous ironstone, it is frequently a difficult matter to
decide how much of the scanty soil is a recent accumulation, and
how much of it represents the original covering of alluvium which
had not been entirely removed.
Plate XIV
Dozi Hill, Kerri-Kerri.
A Trench-like Valley near Lele, in Kerri-Kerri.
The formation of the surface ironstones of the plains of
Hausaland is thus by no means readily explicable. Their origin is
probably due, in part, to many different causes, and their
fragmental occurrence at various levels, sometimes upon weathered
rock, sometimes
upon unweathered rock, and sometimes upon drifted alluvium, points
to repeated formation and repeated erosion at several different
periods. It is believed that, if thoroughly investigated, their
distribution and the distribution of the surface drift would afford
some clue to those repeated climatic and crustal changes to which
within comparatively recent times the surface of the central Sudan
has been subjected. The evidence already detailed appears to be
sufficient to establish the following succession of events upon the
plains of Hausaland:—
(1) A period of minimum erosion during which the superficial
rocks were deeply decomposed to a variable depth and covered with a
ferruginous cap.
(2) A period of maximum erosion during which the earlier sheet
of decomposed rock was largely removed, with the exception of such
relics as Mounts Dala and Kogon Dutsi.
(3) A period of minimum erosion and accumulation during which
the surface drift was spread out upon the plains.
(4) The present period of erosion of the drift during which, in
places, successive platforms have been cut in the drift and floored
with concretionary ironstone.
The cause of the alternation of these periods of accumulation
and erosion is probably to be sought in crustal changes of a
regional character. If a condition of low relief be granted to
begin with, a general depression of the surface to the
neighbourhood of base level may be postulated during the periods
of accumulation,
and a corresponding elevation of the surface above base level
during the periods of erosion. The various periods indicated above
may therefore be termed respectively:—
(1) The first period of depression.
(2) The first period of elevation.
(3) The second period of depression.
(4) The second period of elevation.
The history of the earlier periods is somewhat obscure and it is
possible that there may have been many minor oscillations of the
crust during the first period of depression and the first period of
elevation. It is possible also that the first period of depression
may have been represented merely by a general base levelling of the
surface through erosion, and that the original sheet of weathered
rock may have been formed in part underneath sedimentary rocks
which were afterwards removed. The flat-topped hillocks of
consolidated granitic grit and clay between Kano and Kazaure may
perhaps represent contemporaneous accumulations during the first
period of erosion, which were themselves subsequently exposed to
denudation. There is some reason to believe, also, that there has
been within the various periods a minor oscillation in climatic
conditions. These minor oscillations would naturally be most
clearly recognisable within the second period of elevation which is
continuous with and includes the present. There is little doubt
that the formation of the surface ironstones was favoured by the
successive occurrence of periods of maximum evaporation, and it is highly probable
that these periods assumed at times a typically desert character,
and that the partially eroded surface of the drift was covered up
at intervals by æolian accumulations. One such period at least
immediately preceded the present more humid conditions, and is
clearly recognisable in the worn and rounded dunes of the dead
erg of northern Hausaland. The dunes are now grassed over
and covered with scattered trees, but their origin is still
sufficiently indicated in their composition and general outlines as
well as in the characteristic banking of the sand against rocky
exposures. The surface soil and the recent alluvium of the plains
of northern Hausaland probably contain much wind-blown material,
and the swampy character of much of the surface of the plains
during the rainy season is probably to be in part explained as the
result of the interference of the accumulations of this recent arid
period with the earlier system of drainage. At the same time,
however, it should be remembered that the erosion of the drift has
in most places not yet become very intense and that the average
slope of the plains of Hausaland is as a rule so gentle that a
considerable volume of water must accumulate before a perceptible
flow can be established. Much swampland is found on the high plains
around and to the east of Zaria, where Dr. Baikie[134] first recognised the drifted character of the
alluvium.
A striking feature of the plains of Bornu and Katagum and of the
Gongola valley between Nafada and the Yola border is the absence of
any superficial crust of vesicular ironstone. The drift which may
reach in Bornu a depth of more than 300 feet differs considerably
in composition from the characteristic drift of Hausaland. Upon the
open plains of Bornu the drift is composed for the most part of a
mixture of white or yellow sand and light or dark grey clay. A red
ferruginous staining of the drift is comparatively rare to the east
and north-east of Gujba. The clays in places yield irregular
concretions of kankar and small nodules of concretionary ironstone
are dug from the surface sands in the neighbourhood of Kukawa, but
no evidence has been anywhere obtained of the formation or earlier
existence of any continuous sheet of vesicular ironstone. Well
sections exhibit below the surface alternations of sand and clay in
beds of very varying thickness.[135] The sand varies much in
coarseness of grain and is in part angular and of drifted
fluviatile origin and in part well rounded and of æolian origin,
the latter apparently predominating largely towards the north-east.
The clays are finely divided and flocculent in water and mixed with
minute grains of blown sand. In places they become darker coloured
from the inclusion of organic matter and pass into the “firki”
(p. 52) or so-called black cotton
soil of Chad and
the lower Shari. Where exposed at the surface, the clay becomes
plastic and very tenacious in the rains, but hard and brittle and
much cracked and fissured in the dry season. These clays are rich
in azote which contributes much to their fertility.[136] This advantage, however, is counterbalanced by
the almost complete absence of lime, potash, magnesia, and
phosphoric acid, and by the presence of large quantities of
silicate of alumina and of salts of soda. Fragments of broken
shells of land snails and lacustrine molluscs are common in the
clays of eastern Bornu, while thin beds of calcareous marl[137] with abundant remains of recent molluscs are
intercalated among the sands and clays of Mongonu and Kukawa. It is
evident that the conditions of accumulation of the drift in Bornu
were different from those under which the drift of Hausaland was
accumulated. The latter was preeminently a terrestrial or
sub-aerial accumulation while the former in the finer-grained
character of the sand, the abundance of clay and the absence of
pebbles and of any intense ferruginous staining affords indications
of having been deposited to a great extent under water in a shallow
lacustrine or swampy region. It is probable, however, that the
accumulation of the drift of Bornu was, at least in the earlier
stages, contemporaneous with the accumulation of the drift of the
plains of Hausaland and may therefore be referred to a considerable
extent to the second period of depression. No traces are left, however,
upon the plains of Bornu of the earlier periods of depression and
elevation, and it may therefore be presumed that the surface upon
which the drift was deposited was entirely cleared of earlier
accumulations during the first period of erosion and that its minor
irregularities have been completely obscured by the later drift.
The second period of elevation which brought about the erosion of
the drift in Hausaland appears to have resulted in comparatively
little erosive action upon the drift of Bornu. It is highly
probable indeed that Bornu retained its swampy and lacustrine and
therefore receptive character to a comparatively recent period and
that the oscillations of less and more humid conditions which
probably prevailed during the accumulation and erosion of the drift
of Hausaland are reflected in Bornu in the intermingling of æolian
and fluviatile and lacustrine material. The local abundance of
alkaline salts in the drift of Bornu may also be taken as
indicating an occasional and local desiccation.
The presence of swampy and lacustrine conditions in Bornu dates,
therefore, from the second period of depression and there is no
reason to believe that there has been any marked break in
continuity between these later Tertiary swamps and the present Lake
Chad. There is, however, no evidence that the present lake or the
former more extensive swamps in any way represent the shrunken
relics of the early Tertiary seas. The later swamps indeed, with
which the lake is more or less continuous in point of time, appear never to have
reached any great depth and to have been liable to repeated
desiccation. Desert periods may have changed them partially or
wholly into sand fields, but the return of humid conditions
constantly re-established their former character.[138] It may be presumed also that during periods of
exceptional rainfall the swamps became united into a shallow lake
of considerable extent which may even at times have covered the
greater part of central and Eastern Bornu. There is no reason to
believe, however, that this was to any extent a normal condition of
things, or that the drift was primarily accumulated in a large
permanent lake. So far as known, marly beds occur in Bornu only in
the neighbourhood of the present lake, and whatever the extent of
the marginal swamps may have been, the deepest and clearest water
seems always to have been confined to eastern Bornu. The thinness
and local character of the marls would seem to imply, however, that
the requisite conditions for their formation were by no means
common or universal. Captain Freydenberg has described in
sufficient detail the former extension of the swamps to the north,
east, and south of Chad.
The character and thickness of the drift deposits appear to
indicate that Bornu has in the past formed part of an area of
predominant depression. The shelly marl of Kukawa which lies at
least fifty feet above the present level of Chad, undoubtedly
indicates that the floor of the lake was at one time at a higher
level than it is at present. The long-continued accumulation was probably
conditioned at first by the second period of depression and later
by the gradual uprise of the Congo watershed during the latest
period of elevation. In the neighbourhood of Kukawa, however, the
low hillocks of drift show evident signs of erosion, and it may
therefore be presumed that the present floor of the lake has been
excavated out of the earlier lacustrine and drift deposits. This
erosion which M. Garde[139] would presumably ascribe
entirely to æolian activity, was probably in great part conditioned
by the continued relative subsidence of the Bodele depression, the
consequent eastward draining of the lacustrine region and the
establishment of the Bahr el Ghazal as a waterway from west to
east. The lower relative level of Bodele has now been definitely
established, and M. Chudeau’s[140] arguments in favour of
the eastward flow of the Bahr el Ghazal may be provisionally
accepted until fuller information has confirmed or contradicted
them. Desert conditions, however, followed close upon erosion. The
worn and rounded hillocks of drift around Kukawa were partially
buried in blown sand and the Bahr el Ghazal was blocked by the
action of the wind. On the establishment of the present conditions
the lake as we now know it was formed and limited towards the west
by the line of sand dunes, now covered with abundant vegetation,
which stretches from Kukawa to the river Yo along the margin of the
lake. There is no reason, however, to postulate the presence of an
earlier lake
actually on the site of the present Chad at a period immediately
anterior to the coming of the desert.
It may be presumed that during the period of erosion which
preceded the advent of the desert, shallow watercourses were cut in
the drift of Bornu and that they were in part dismembered and
filled up again during the succeeding period of desiccation. One
such watercourse may be clearly traced from Murguba to Maigumeri in
central Bornu. The present channels of the Yo, the Maidugari river,
the Yedseram, and the Shari were probably also established at the
same period. The cuvettes of Manga probably originated at the same
epoch and represent portions of earlier watercourses detached and
isolated by æolian accumulations. The sites of the cuvettes may
possibly have been determined in part by the presence of springs in
the earlier river beds. While, therefore, there is little reason to
believe with Captain Freydenberg[141] that the bottoms of the
cuvettes represent detached portions of the original floor of an
extensive lake almost entirely filled up with æolian accumulations,
there is equally little reason to believe with M. Chudeau[142] that the cuvettes have arisen through the
local subsidence of the underlying rocks brought about by the
dissolution of beds of soluble salts, hypothetically contained
within the sandstones of the Tegama below. M. Chudeau’s objection
that an approximately level surface such as that of the plains of
Bornu cannot be produced under the influence of the wind is clearly
untenable in view
of the known occurrence of extensive level sandfields in modern
deserts. Moreover, in Katagum solitary dunes or isolated hummocks
of blown sand formed during the recent arid period may be found
alone in the midst of an open undulating plain. There is some
reason also to believe that the winds which never attained any
great intensity varied in direction in different parts of the
Protectorate. In Bornu the dunes are elongated in a N.–S. direction
while in Katagum they are more generally E.–W. and it is possible
that a successive variation in the direction of the winds over the
same area may have been responsible for much of the levelling of
the present surface of the plain.
The lacustrine conditions which prevailed over central and
eastern Bornu during the deposition of the drift extended also for
a considerable distance westward through Katagum. The drift of
Katagum is very similar in character to that of Bornu and the
abundance in it of wind-blown material, as in the drift of Bornu,
bears witness to the constant struggle between humid and desert
conditions during the period of its accumulation. The predominance
of the latter at a period immediately preceding the present is
indicated as in Bornu by worn and rounded dunes and banks of sand.
There is every reason to believe that Bornu and Katagum have been
for long on the margin of the desert and that the history of the
repeated advance and retreat of the desert can be clearly read in
the varied character and composition of the drift.
To the west of Gujba, thick accumulations of red sandy loam
effectually obscure the irregularities of the topography of eastern
Kerri Kerri. The drift has evidently been largely derived from the
disintegration of the red sandstones and ironstones of Kerri Kerri,
but the surface of the original deposits has been in great part
rearranged by subsequent æolian action. Upon the plateau itself the
æolian accumulations reach in places a very considerable thickness.
In the valley of the middle Gongola to the south of Kerri Kerri
there is as in Bornu and Katagum, a remarkable absence of surface
ironstone. Around Nafada and southward upon the narrow plain to the
west of the Gongola the drift is predominantly a red sandy loam. To
the east of Ashaka, however, the drift consists very largely of a
stiff tenacious black or dark grey clay, very similar in appearance
to the “firki” of eastern Bornu. This material also covers large
areas upon the central plain of Bauchi to the south and south-east
of Deba Habe. It is clear from the manner in which the red loam and
the dark coloured clay obscure the irregularities of the surface
and creep up the flanks of the sandstone hills that the superficial
accumulations are here also, as on the plains of Hausaland, of
drifted or travelled origin and not primarily of the nature of
material weathered and disintegrated in situ. The local origin of
the drift may however be presumed. The red sandy loam of the
northern and western plains has been derived very largely from the
disintegration of the Kerri Kerri and Gombe sandstones, while the
clays of the east and south have probably arisen in great part from
the decomposition of the lavas of the Burra plateau. The original
depth of the drift is unknown, but must have been very
considerable. The Gongola in places has not yet succeeded in
cutting its way through the drift to the rock below, while the
highest portion of the plain which is still buried in drift lies
about 300 feet above the present river bed. The total thickness,
however, may well have been much greater and there is every reason
to believe that what is now the valley of the middle Gongola
between Nafada and Gasi formed, along with that portion of the
central plain of Bauchi which lies between Deba Habe and Putu, an
area of special accumulation throughout the second period of
depression and that the activity of the river and its tributaries
during the most recent period of elevation has been directed mainly
towards the removal of the drift which had accumulated in the
earlier valley.
The alternation of the major periods of accumulation and of
erosion as established for the plains of Hausaland was probably to
some extent of a regional character, although the relative amount
of respective depression or elevation may have varied much in
different parts of the Protectorate. The irregular movements to
which the Benue valley as a whole was subjected during the latest
period of elevation renders the correlation of the superficial deposits
with those of the northern States a matter of considerable
difficulty. There is good reason to believe, however, that the
lower Benue valley and the greater part of the provinces of Kabba
and Ilorin were exposed during the same period to very intense
erosion, in consequence of which the original drift deposits have
been almost entirely removed and the underlying Eocene sandstones
deeply trenched and worn. The middle Benue, on the other hand,
appears to have been long a region of relative depression and
accumulation, and although platforms of surface ironstone may be
found in places even on the plains of Muri,[143] the requisite conditions for its formation
appear to have been for the most part wanting. It is possible that
its absence may be due, as in Bornu and the Gongola valley, to a
prolonged water-logged condition of the subsoil. Wherever, on the
other hand, the surface of a hillock of red sandstone was exposed
above the drift, there seems to have been a rearrangement of the
ferruginous material and a concentration of the iron ores in the
upper part which has given rise upon exposure to a typical
honeycombed limonitic cap. The ferruginous crusts of the
flat-topped hills and plateaux of the lower Benue and the Niger
have a somewhat similar origin and have arisen in part through the
rearrangement and deposition in the subsoil of the original iron of
the sandstones and oolites and in part, like the ferruginous
conglomerate on the summit of Mount Patti, through the
cementation of
travelled material of a similar composition. Where the summits are
bare and lustrous it may be presumed, as in the case of the
ironstone platforms of Hausaland, that the surface soil, originally
overlying, has been removed in the course of denudation. At Awe, on
the middle Benue, a sheet of sintery calcareous material appears to
have been deposited upon the upturned edges of the Cretaceous
before the accumulation of the drift.
Plate XV
Cretaceous Sandstones and Shales at Awe.
A Flat-topped Hillock of Drift above Naraguta.
Attention has been already directed (pp.
27,77) to the occurrence in Kabba,
Nassarawa, and Southern Zaria of belts of felspathic muscovite
magnetite schists whose deeply decomposed crusts support a
luxuriant vegetation. Where partially eroded, it may be observed
that the iron ores have a tendency to accumulate in the subsoil,
and it is possible that if the overlying soil and vegetation were
removed, the subsoil might harden into a typical vesicular
ironstone. It is difficult to decide, however, to what extent the
decomposed crusts may be considered the result of weathering under
present conditions. It seems probable in view of the absence of any
deep decomposition of the adjoining rocks that they may to a large
extent represent the irregular base of the earlier sheet of
decomposed rock to which the presence of Mounts Dala and Kogon
Dutsi bears witness upon the plains of Hausaland and which may
possibly have extended at the same time over the remainder of the
Protectorate. In some places, moreover, as at Kanna, there is
evidence that the crystalline floor had been locally and deeply
decomposed as the result of differential weathering underneath the Eocene
sandstones. A similar occurrence has been noted by M. Hubert[144] at Youri in the Niger Valley.
Little consolidated accumulations of late Tertiary age,
frequently provided with a ferruginous cap like the drift of
Northern Nigeria, have been described under the designation of
“Benin Sands” by Mr. Parkinson[145] from Southern Nigeria,
of “Ossa Sandstones” by Dr. Esch[146] from the Kameruns, of
“terre de barre” by M. Hubert[147] from Dahomey, and of
reg from the Northern Sahara by numerous French
authorities.[148] It is yet premature, however, to attempt any
correlation of these deposits with the drift of Northern
Nigeria.
[118]Hatch and Corstorphine, The Geology of
South Africa, 2nd Ed., 1909, p. 330. Cf. also Hatch,
Mineral Resources of Natal, 1910, p. 73; Rogers and Du
Toit, Geology of Cape Colony, 1909, p. 390.
Post-Cretaceous erosion in the Sudan; the Middle
Eocene depression; western and eastern basins of sedimentation;
crustal oscillations of the Middle Tertiary period; later Tertiary
oscillations; the first period of depression; the first period of
elevation; early drainage system of the Protectorate; the second
period of depression; lacustrine conditions in Bornu; the second
period of elevation; flexuring of the crust; α- and β-flexures;
differential elevation of the Nassarawa tableland and the Bauchi
plateau; deflection of the Gongola; depression of the Chad area.
Peculiarities of the watersheds and river valleys of the
Protectorate: explanation of the characteristic features of the
valleys of the Niger, Benue, and Gongola; the origin of inselberge;
M. Hubert’s explanation of the hydrography of Dahomey; the period
of formation of the “dallols” of Sokoto; correlation of the
Tertiary crustal movements in the Sudan with those of the
Mediterranean area.
The close of the Cretaceous
was marked by regional upheaval, accompanied by fracturing and
folding of the crust and by subsequent intense erosion. The
Cretaceous sandstones, shales, and limestones, which may be
presumed to have at one time covered the greater part of the
Protectorate, were largely removed from the underlying hummocky and
irregular surface of crystalline rocks. The plateau of southern
Yola, as described by Mr. Longbottom (p.
32), and the eastern cliffs of the central tableland of Kabba
(p. 16), would appear to have originated at
this period. The
final character of the post-Cretaceous drainage system and of the
early Eocene land surface is, however, very imperfectly known. Mr.
Longbottom believes that there were earlier river valleys in the
place of the Upper Benue and Lower Gongola of the present day
(p. 190), and there is some reason to
believe that there was also an earlier river in the present valley
of the Lower Niger. At all events it is in these localities that
the greatest thickness of Eocene sediments is now preserved,
although there is little doubt that the later crustal movements
have also, to some extent, affected their present distribution.
The movement of depression in the Middle Eocene which resulted
in the establishment of open sea in the north-west and in the
deposition of the marine beds of Sokoto province, brought about
also the gradual submergence of the land surface and the
predominance of shallow water conditions over the greater part of
the Protectorate. In the earlier river valleys the greatest
thickness of sediments would naturally be deposited, while only the
ferruginous sandstones and ironstones of the latest lacustrine
phase would be deposited over the higher lying portions of the
early Eocene land surface. The formation of the lava fields of
Bauchi and Burra was probably contemporaneous with the latest phase
of sedimentation. There is some reason to believe, moreover, that
during the greater part of the period occupied by the movements of
depression, there were two major regions or basins of
sedimentation, a western and an eastern, separated by a stretch of
higher ground which extended from Zinder and the northern frontier of Hausaland to
the southern border of the Benue valley, and of which the Murchison
Range formed the only striking physical feature. The thinning out
of the Eocene sandstones to the east of the Niger valley, on the
middle Benue, and to the west of the Gongola, lends some support to
this hypothesis. The accumulations within the western basin, which
was probably continuous westward with the Atlantic, would then
include the lacustrine series of Southern Nigeria, the shallow
water deposits of the middle Niger, the marine limestones of Sokoto
and the sandstones of Dahomey and the Futa Djallon, while the
Duguri, Gombe, and Kerri Kerri sandstones, the Upper Benue
sandstones and their prolongation into the northern Kameruns,
together with the ferruginous sandstones of the Upper Shari and the
Congo, would represent the accumulations within the eastern basin.
The outliers of Eocene upon the Kanna hills and the relics of
ferruginous sandstones upon the plains of Muri indicate, however,
that the western and eastern basins became finally united, and it
seems probable that towards the close of Eocene sedimentation,
shallow water and lacustrine conditions prevailed over the greater
part of the southern Sudan. Free communication may even have been
established at this time over Central Africa,[149] between the western seas of the Upper Niger
and the eastern seas of Lybia and Egypt. There is no reason to
believe, however, that the lower marine facies of Sokoto ever
extended eastward much beyond its present limits.
There can be
little doubt that the close of the Eocene was marked by the
cessation of the predominant movement of depression and the
beginning of those crustal oscillations which culminated in late
Pliocene or early Pleistocene times in the formation of the Bauchi
plateau and the establishment of the present river system. No
deposits of later recognisable age than Eocene have been discovered
within the Protectorate and it may therefore be presumed that until
the close of the Miocene, when the sea is believed to have finally
retired from the Sahara (p. 194), the
surface of the Protectorate remained near or at the level of the
sea. The predominant movement, however, was probably that of
elevation, with the result that the uppermost members of the Eocene
suffered some considerable erosion and were entirely removed from
the greater part of the surface of the central provinces. The later
Tertiary oscillations, which may be conveniently grouped as
Pliocene, began with a negative or downward movement, which has
already been termed the “first period of depression” (p. 213), and during which the crystalline rocks of
the central provinces were deeply decomposed and probably also
covered with a layer of drift. The “first period of elevation”
which followed was marked by a somewhat irregular upheaval of the
surface of the Protectorate. The maximum movement appears to have
been attained in the east, where the Eocene and underlying rocks
were elevated in a low extended arch whose axis formed the primary
watershed and ran approximately from Eastern Yola to Zinder and
Air. As a result of
the erosion which followed the movement of elevation the greater
part of the sheet of decomposed rock formed during the first period
of depression was removed from the surface of the central plain.
The crystalline areas assumed very largely their present physical
features and characteristic scenery. The plains of Hausaland,
however, were continuous with the plains of Nassarawa and Muri
across the site of the Bauchi plateau and the Nassarawa tableland.
The Murchison Range, the Wadai Hills, and the Ningi and Kagoro
massifs remained then as now relics of a former erosion, and beyond
acting as secondary watersheds exerted comparatively little
influence upon the general system of drainage. The relics of higher
beds occasionally found on the sandstone plateaux of the Niger may,
however, be referred to this period of erosion. The larger rivers
probably drained south-westward into Southern Nigeria and Dahomey
across the present valley of the lower Benue and the Niger. The
gorges of the Niger above Lokoja and between Jebba and Yelwa
clearly indicate the much more recent origin of the present river
valley. In the east, however, along the watershed the sedimentary
rocks were deeply trenched and worn on either side by the
headwaters of the rivers. The valleys of the middle Gongola and of
the Upper Benue present when compared with the Niger Valley all the
characteristics of a more ancient topography. It seems probable
indeed that towards the close of erosion the rivers which drained
westward over the plains of Bauchi had already established the
escarpments of
Kerri Kerri, the narrow plains of the middle Gongola, and the
cliffs of Waja and Tangale. An earlier river, moreover, flowed
westward from the hills of Yola with its valley limited as now by
the highlands of Wurkum and Waja, which for a time interposed a
continuous barrier between the rivers of Bauchi and the headwaters
of the Benue. The establishment of the gap at Kombo was probably
accomplished by a tributary of the Maio Haul towards the close of
erosion. To the east of the watershed the plains of eastern and
central Bornu were likewise established at this period by rivers
which probably drained eastward and southward over the site of Chad
and the Shari valley into the Congo basin.
There followed, however, the “second period of depression,”
during which the drift was accumulated upon the irregular surface
of the plain. The maximum movement would appear to have taken place
in the east in the region of the former elevation. It seems
probable that not only was the earlier watershed depressed to such
an extent that the lateral valleys in Bauchi and Bornu were filled
with drift to a depth of 400 to 500 feet, but that lacustrine
conditions prevailed farther east over the plains of Bornu and Chad
and the present valleys of the Logone and the Shari. Another area
of special depression was apparently situated in southern Zaria and
western Bauchi, where the remains of a covering of drift 200 to 300
feet in thickness forms a series of flat-topped hills on the
highest portion of the plateau. The older Benue gravels, as described by
Passarge[150] and Edlinger,[151] were probably also
accumulated at this period. The drainage in the earlier stages may
be presumed to have been, in general, towards the areas of maximum
depression, while the close of the period was marked by the
prevalence of conditions of accumulation and of uncertain drainage
over the whole surface. The inland seas of the Upper Niger, as
described by MM. Chevalier,[152] Gautier and
Chudeau,[153] should probably also be referred to this
period, but it should be remembered that, although they may have
been at times connected with the open sea to the west, there is, as
in the case of Lake Chad (p. 217), no
reason to postulate any kind of continuity between them and the
earlier Miocene or Eocene seas.[154]
Radical changes ensued upon the cessation of the movement of
depression and the return of a positive movement of elevation in
late Pliocene or early Pleistocene times. It should be remembered,
however, that the irregular upheaval of the surface during the
“second period of elevation” and the consequent changes of drainage
were in no sense catastrophic. On the contrary, the movements and
changes were gradual, and possibly to some extent successive and
spread over a considerable interval of time. The regional elevation
of the Protectorate, with the exception of the middle Benue valley, to an average
height of 1,000 feet was accompanied and complicated by a gentle
flexuring of the crust into low arches and shallow troughs along
axes which ran approximately W.S.W.–E.N.E. and N.N.W.–S.S.E. (see
map, p. 236). The two sets of flexures may be
termed respectively the α-flexures and the β-flexures, and it may
be presumed that they were produced for the most part
simultaneously and only to a small extent successively by crustal
movements, which were themselves contemporaneously functional. The
irregular elevation and depression of the surface which gave rise
to the present system of drainage may therefore be looked upon as
the result of the interference of the two sets of flexures. Two
axial arches of the α-series, a northern (α₁) and a southern (α₂),
are readily distinguishable within the Protectorate, the southern
being the more important and of the greater elevation. The northern
(α₁) runs from Borgu over the high plains of Kontagora, Sokoto, and
Katsina towards Machina and Zinder, and forms the watershed between
the Gulbin Kebbi and its tributaries on the one hand, and the
Kaduna and the Yo on the other. These rivers may be presumed to
occupy the shallow troughs on either side of the axial ridge. The
axis of the southern arch (α₂) runs from central Kabba through
Nassarawa and southern Bauchi into northern Yola, and the Benue may
be presumed to occupy the trough to the south. The two α-arches are
not strictly parallel, but diverge somewhat towards the east. Of
the β-flexures two axial arches are also
distinguishable
within the Protectorate, a central (β₁) and a western (β₂), the
latter being the more important of the two. The central arch (β₁)
is of a very partial character, and is recognisable only in the
sinuous watershed which runs northward between the two α-arches
from the summit of the plateau towards the Sokoto-Kano frontier and
separates the headwaters of the Kaduna from those of the Yo. The
western arch (β₂) forms, in part, the western watershed of the
Niger, and runs northward from the high plains to the south of
Ilorin approximately along the frontier of the Protectorate to the
neighbourhood of Nikki in Dahomey.
The arches throughout are asymmetrical in character, and vary
somewhat in elevation from point to point. The sinuous character of
the crests is probably due to the mutual influence of the
complementary forces which produced the flexuring at right angles.
The intensity of the flexuring appears also to have varied, and in
the case of two intersecting flexures one or other usually
predominates. In the central region, for example, the α-arches
clearly predominate at their intersection with the β-arch (β₁),
while in the west the β-arch (β₂) has evidently been the
determining factor in the establishment of the present course of
the Niger. The interference of the β-arch (β₂) with the western
extension of the α-flexures is clearly reflected in the repeated
bending of the river. Where the α-arches were crossed by the
β-arch, dome-shaped elevations of a regional character were formed,
whose effect may still be traced in the westwardly concave curves
which the Niger describes at Yelwa and above Lokoja. Where the intervening α-trough,
however, was crossed by the β-arch, the resulting curve was concave
eastward as at Jebba. The asymmetrical position of the Niger valley
indicates a shorter radius for the western portion of the β-arch,
while the fact that the river was able to pursue a continuous
course across the α-arches and troughs makes it sufficiently
obvious that the β-arch (β₂) must have reached originally a
somewhat greater elevation than the western portion of the
α-arches. The influence of the formation of the western β-trough,
now occupied by the Niger valley, may also be traced in the
southward deflection of the lower courses of the Kebbi, Kaduna and
Gurara. It is obvious also from the height of the sandstone
plateaux above Lokoja that the greatest and perhaps most prolonged
movement of elevation in the Niger valley took place in the region
of the confluence of the Niger and the Benue. This movement,
moreover, was evidently sufficiently slow to allow of the Niger
cutting a continuous trench across the rising arch, a result,
however, which was rendered possible only by the presence of
sedimentary rocks in the river bed. There is no reason to believe
with Gurich[155] that, during the formation of their
trench-like valleys in the region of the confluence, the middle
courses of the Niger and the Benue were ever converted into a
series of detached basins or inland seas. It should be remembered
also that the relative elevation of the arches was for the most
part insufficient to produce any marked divergence from the apparent horizontality
of the sandstones in the Niger valley.
NORTHERN NIGERIA
Axes of Elevation.
MAP No. 3.
London. Macmillan & Co., Ltd.
Face p. 236.
The symmetrical elevation of the southern α-arch and the
southward prolongation of the central β-arch were modified and
interrupted by the formation of lines of weakness longitudinally
along the former and in the region of the intersection of the two
arches. The relative influence of the forces which produced the
flexures can be traced in the varying direction and sinuous
character of the lines of weakness. Along these lines differential
movements took place which resulted in the disproportionate
elevation of the Nassarawa tableland and of the Bauchi plateau with
reference to the middle Benue valley and the northern plains of
Bauchi. On the north the fracturing of the crust, as already
described (p. 39), was confined only to the
central region, while to the west and east the tableland and the
plateau merged respectively into the general slope of the arch. On
the south, however, a continuous line of fracture extended from
western Nassarawa by way of Darroro and Assab to Sura and Angass in
southern Bauchi. There is no reason to believe that the southern
α-arch as a whole ever reached the elevation of the Bauchi plateau
or that the plateau originated in any sense as a “horst,”[156] separated from the adjoining tableland and the
plains of Nassarawa and Muri by the later differential subsidence
of the latter. On the contrary, the movement appears to have been
in one direction only, and characterised by the differential
elevation of the
plateau and the tableland and the lagging behind of the lower
plains of Nassarawa, Muri and northern Bauchi. The trough of the
Benue above the confluence of the Niger affords clear evidence
indeed both of the comparatively stationary character of the middle
Benue valley and of the gradual relative elevation of the Lokoja
sandstones in the region of the confluence.
In the east the prolongation of the northern α-arch through
Manga and the region to the north of Chad extends beyond the limits
of the Protectorate. The southern arch, on the other hand, is
prolonged as a simple flexure of varying elevation through eastern
Bauchi into Yola province. In the trough between the northern and
southern arches, the river Yo occupies the same relative position
to the east of the central β-arch as the Kaduna does to the west.
The presence in the east, however, of the former watershed of the
first period of elevation has introduced complications into the
otherwise simple relations of the α-axes and troughs. It may be
presumed that on the formation of the southern α-axis and the
differential elevation of the Bauchi plateau, the river Gongola,
like the Shidya and the Delime, would have followed a general
north-easterly course and ultimately joined the Yo but for the
intervention of the higher ground along the earlier watershed which
diverted the river to the south. It should be remembered that the
whole of this region had been deeply trenched and eroded during the
first period of elevation (p. 231) and that
the earlier valleys had been largely filled with drift during the
second period of
depression. It seems probable, therefore, that on the subsequent
elevation of the southern α-arch the Gongola on leaving the high
plains of Bauchi was able to make its way for long distances
through the earlier river valleys and stream courses and ultimately
to escape into the Benue valley through the gap at Kombo. The
trench of the Gongola above Gombe may possibly be to some extent
the product of recent erosion, but there is little doubt that the
plain of the Gongola below Nafada was excavated at an earlier
period and is in no way to be regarded as the result of the
activity of the present river, whose energy has been almost
entirely directed to the removal of the drift, which formerly
obscured the hummocky surface of the plain.
The prolongation of the southern α-axis into eastern Bauchi
appears to have been characterised by the formation of a flexure of
a flattened monoclinal type, which resulted in the elevation of the
plains of eastern Bauchi and the middle Gongola to a height of 500
feet above the valley of the Benue. The actual summit of the
flexure is probably situated in the vicinity of the rapids of
Kombo. The hydrography of northern Yola is very imperfectly known
and complicated by the presence of the earlier watershed. It is
probable, however, that the present watershed between the Benue and
Lake Chad on the eastern frontier corresponds approximately with
the prolongation of the α-axis, which here resumes its normal
character of an asymmetrical arch. The last trace of the flexure
may be recognised to the east of the Mandara hills in the swampy
watershed between
the Benue and the Logone. The configuration of southern Yola
suggests an ancient topography, and its physical character is
probably very similar to that which prevailed at the close of the
first period of elevation. The Chad area to the north remained a
region of relative depression during all the later flexures, and it
seems probable that accumulation continued and the escape southward
of the surplus waters remained possible until the final elevation
of the Congo watershed along a parallel flexure to the south. The
contemporaneous formation of the Bodele depression, however,
brought about the establishment of the effluent of Chad through the
Bahr el Ghazal and the partial erosion of the drift and lake
deposits of Bornu. The subsequent advent of arid conditions during
the period immediately preceding the present resulted, as already
indicated (p. 219), in the diminution of
the water supply, the closure of the Bahr el Ghazal, and the
restriction of the lake to its present limits.
This attempted reconstruction of the more recent hydrographical
changes affords an explanation of many of the more striking
peculiarities in the physical geography of the Protectorate.
Attention has already been directed to the fact that the larger
tributaries of the Niger and the Benue offer throughout their
courses a reversal of the normal sequence of mountain tract, valley
tract and plain tract. The Kebbi, the Kaduna, and the Gongola rise
upon lofty plains and flow for long distances in broad sandy
channels bounded by imperceptible slopes until they enter, in their
lower courses, well-defined trench-like valleys bounded
on either hand by
steep walls of rock. The watersheds are marked by no ranges of
jagged hills. On the contrary, the lofty plains on which the rivers
rise are frequently more deeply buried in drift than the flood
plains of their lower courses. The rivers flow peacefully around
and between the lofty granite domes which rise abruptly from the
upper plains, and even such larger groups of hills as those of
Ningi form merely local centres of dispersion for the smaller
streams. Moreover, the hydrographical centre of the Protectorate is
not a rugged mountain mass, but an open treeless plain, a plateau
4,000 feet in height, bounded by steep cliffs on the south and west
and provided with worn and flattened hillocks of drifted alluvium
perched in places upon the very margin of the cliffs. A complete
reorganisation and rejuvenation of the drainage system is clearly
indicated, and it is possible only in some such manner as that
already described. The crust has been flexured and fractured during
a recent regional elevation, and from the crests of the arches the
consequent streams have been thrown off to pick a course for
themselves over the drift-covered plains and between the solitary
hills and finally to unite to form the major rivers in the bottoms
of the shallow troughs. Dr. Passarge[157] has frequently called attention to the
youthful age of the rivers of Africa, and his suggestion would
appear to be amply confirmed by the whole character of the present
river-system of Nigeria.
The sinuous course of the Niger within the Protectorate still
represents in a general way the original adaptation of the drainage system to the
tectonic conditions. The diversity of scenery in the present valley
is, however, largely due to the influence of the lithological
character of the underlying rocks. There is little doubt that at
first the Niger flowed throughout upon a sandstone surface, relics
of which now lie from 500–1,000 feet above its present level, and
that it eroded its channel originally as a continuous, deep and
narrow trench. Complications first ensued when the river succeeded
in cutting through the overlying sandstones between Yelwa and
Jebba, and in reaching the crystalline rocks below. The greater
thickness of sandstones below Jebba conditioned the cutting back of
the gorge between Jebba and Yelwa with which the river is still
occupied. The resistant character of the crystalline rocks brought
about the formation of falls and rapids, while the consequent
lateral erosion at the head of the gorge gave rise to the broad
sandy plains between Yelwa and Ilo. Below Jebba the prolonged
elevation of the southern α-arch brought about the continued
trenching of the sandstones above Lokoja and the lateral erosion of
the banks in Nupe and Ilorin, where the level of the river remained
stationary for considerable intervals of time. The irregular
degradation of the channel of the Niger conditioned also the
formation of the broad, flat-bottomed valleys (dallols) of Sokoto
and the narrow canyon valleys of southern Kontagora, the dissection
of the sandstone plateaux of Nupe, the removal of the Eocene rocks
from northern Kabba and the cutting of the lower gorge of the Gurara. Everything,
indeed, points to the recent erosion of the Niger valley, while the
rapids and falls of the Kaduna and the Gurara beyond the limit of
the sandstones and the ill-defined valleys of their upper and
middle courses clearly indicate that they have only just begun to
cut their way backwards and downwards into the central crystalline
plains.
Compared with the Niger valley, the plains of the middle and
upper Benue present all the characteristics of a more ancient
topography, and it is probable that they retain to a great extent
the configuration which they possessed at the close of the first
period of erosion. Only in its lower course has the Benue cut a
trench in the Eocene sandstones at all comparable with the gorge of
the Niger above Lokoja, and this has probably been accomplished, as
already explained (p. 238), during the slow
differential elevation of the region of the confluence. There seems
reason to believe, also, that during the earlier stages of the more
recent erosion the Benue was provided with a greater volume of
water than the Niger, and that to this influence is to be ascribed
the greater imperfection of the trench of the lower Benue and the
removal of the sandstones from the hummocky crystalline plain of
eastern Kabba. This increase of volume, however, was probably only
relative and contemporaneous with the diminution of the Niger
during the recent arid period in the Sudan. The return of more
humid conditions, however, together with the capture of
the upper Niger by
a tributary of the Taffassasset, as described by M. Chudeau,[158] restored the activity of the Niger and enabled
it to cut its present channel in the surface of the crystalline
plains above and below Lokoja.
The northern tributaries of the Benue, with the exception of the
Gongola, take their rise on the southern border of the Nassarawa
tableland and the Bauchi plateau. Their headwaters alone, however,
exhibit evidence of any recent intense erosive activity. Behind
Jagindi and in the neighbourhood of Assab the headwaters of the
Modu river have trenched and worn the margin of the plateau. The
Wase river has laid bare in Yergum and Angass the hummocky and
irregular slopes which separate the upper from the lower plains.
The headwaters of the Kudu river have cut picturesque steep-sided
valleys in the rocks of Duguri, and narrow canyon-like trenches in
the central plains of Bauchi. In the Gongola valley, on the other
hand, the only traces of recent erosion are to be found in the
rapids of Baddera and Kombo and, in part, in the imperfect trench
which the river has cut in the sandstones between Gombe and Deu.
From its source on the plateau until it enters the plains to the
east of Gombe, the topography of the river valley recalls in every
detail the characteristic features of the valleys of the Kaduna and
the Gurara. Nevertheless, although of similar age and origin, the
Gongola flows from Nafada southward through a valley which presents
all the features of a more intense erosion than the
Kaduna or the
Gurara or even the Niger itself has been able to accomplish in its
lower course. It is difficult to escape the conclusion that upon
the rearrangement of the drainage system and the establishment of
the present upper Gongola, the river entered to the east of Gombe,
an older river valley which had been excavated during some former
period of erosion. The interlacing valleys of the margin of the
Kerri Kerri plateau may possibly have arisen during the latest
period of erosion, but if so, the streams which excavated them have
never regained the activity which they possessed before the last
appearance of the desert in the Sudan.
Plate XVI
Upper Benue Sandstones and Conglomerates upon a
Hummocky Granitic Floor.
The isolated domes and kopjes (Plates III., XII.) and detached
groups of hills which form the most striking feature of the higher
plains are characteristic also of all the older gneissic areas of
Africa, and the question of their origin has given rise to
considerable difference of opinion. The particular type of scenery
to which they give rise has been termed by German writers the
“Inselberglandschaft” (Plate XVII., Figs. 1,2). Passarge[159] sees in them the product
of arid conditions, and concludes that the surfaces which they
characterise assumed their present character for the most part in
early Mesozoic times when the greater part of Africa was a barren
desert. Bornhardt,[160] on the other hand, believes that the
inselberge have arisen as the result of the superposition of
several cycles of erosion. There is, however, no reason
to believe that
this peculiar topography cannot arise simply through an alternation
of periods of weathering and periods of erosion, brought about
either by a gentle oscillation of the crust or by a repeated base
levelling of the plains and rejuvenation of the drainage system. A
plane surface of granite and gneiss subjected to long-continued
weathering at base level would be decomposed to unequal depths,
mainly according to the composition and texture of the various
rocks. When elevation and erosion ensued, the weathered crust would
be removed, and an irregular surface would be produced from which
the more resistant rocks would project. Those rocks which had
offered the greatest resistance to chemical weathering beneath the
surface would upon exposure naturally assume that configuration of
surface which afforded the least scope for the activity of the
agents of denudation. In this way would arise the characteristic
domes and turtle-backs which suffer further denudation only through
insolation and exfoliation.[161] Their general elliptical
outlines, which Merrill[162] would ascribe very
largely to the influence of crustal stress and strain, are probably
in great part due simply to the modification by weathering of
original phacolitic intrusions. A slight subsequent movement of
depression would suffice to cover with drift large areas of the
irregular surface between the domes and to produce the
characteristic topography of isolated hills rising abruptly from
the level surface of the plain (Passarge’s Kordofan Type). That such indeed has been
the mode of formation of the kopjes of Hausaland can be
demonstrated within the walls of Kano. Kogon Dutsi, the larger of
the two flat-topped hills of diorite, although deeply decomposed,
still preserves in its lower part detached boulders or cores of
unweathered rock. If the subsequent erosion had continued until the
weathered material had been entirely removed, the flattened hill
would have been replaced by a typical kopje of loose boulders
resting upon a smooth and rounded surface of rock below. On the
plains around ample evidence can be found of the levelling of the
surface by means of the accumulation of drift. A repetition of
these conditions of formation would give rise to an accentuation of
the earlier domes and kopjes and the formation of others of lower
level. A similar origin to that of the domes may be ascribed also
to the detached groups of hills and even to the larger granite
massifs like those of the Ningi, Ruruma, Kagoro, and Vere Hills.
There is no reason to believe with Passarge[163] that the latter partake in any way of the
nature of “horsts,” which have assumed their present position
through the differential subsidence of the surrounding plains.
Their flattened though hummocky summits evidently bear witness to
an earlier regional level of erosion, but their characteristic mode
of occurrence is to be ascribed not to marginal faulting, but to a
succession of periods of decomposition and accumulation and periods
of erosion which
has brought about the relative lowering of the surrounding and less
resistant gneisses and schists.
The pre-Cretaceous land surface, as exposed around Tangalto, was
marked like the present with domes and inselberge. The pre-Eocene
surface was somewhat broken and irregular, but probably also
characterised, as in the Niger valley below Lokoja, by detached and
solitary hills. There is no reason, however, to suppose that the
actual pre-Cretaceous configuration of the plains of Hausaland is
still retained. It is much more probable that domes and solitary
hills have been formed in the manner indicated at many different
periods, and are a characteristic product of sub-aerial tropical
weathering of a crystalline surface when exposed to oscillations of
level.
Dr. Passarge,[164] in explanation of the structure of
Adamawa, has assumed the existence of two tectonic directions, an
E.–W. or Benue line, and a S.S.W.–N.N.E. or Kamerun line, along
both of which extensive faulting has taken place at various periods
and in such a manner as to leave a numerous series of isolated
hills and ranges of hills standing out as horsts upon the lower
plains. According to him, also, the Bagele and other hills of Yola
have assumed their present position through the subsidence of the
surrounding plains, while the Benue valley between Yola and Garua
is of tectonic origin, and bounded by faulted margins to the north
and south. While, however, there is some reason to believe that the
Cretaceous rocks of
Yola have been faulted down against the crystallines to the north
and south, and that in pre-Eocene times an earlier river occupied a
tectonic valley in the region of the upper Benue (p. 190), there is no evidence to show that there
has been a recurrence of faulting on the upper Benue in post-Eocene
times, or that the slopes of any of the hills of “Upper Benue
Sandstone” have been defined by later Tertiary or recent movements.
Passarge’s hypothesis is applicable only to pre-Eocene topography,
and cannot be extended in explanation of those features of the
upper Benue valley which are of more recent origin.
M. Hubert[165] has suggested in explanation of the
hydrography of Dahomey that the courses of the present streams bear
a direct relationship to the strike of the gneisses and schists,
and that the surface of the colony has remained stationary since
the close of the Eocene. His hypothesis, however, is admittedly
inadequate to explain the transverse character of the Niger valley.
In Nigeria, moreover, all the principal rivers without exception
flow across the strike of the gneisses, while the drift-covered
character of the lofty plains and of the watersheds and the nature
of the whole hydrographical system clearly indicate the recent
elevation of the present surface of the Protectorate. It is very
probable indeed that Dahomey has participated in all the recent
crustal movements which have affected Nigeria, and that the central
watershed of the colony corresponds with the westward prolongation
of the northern α-arch and the western β-arch of the Protectorate. A
recent rejuvenation of the drainage system is postulated, moreover,
by M. Chudeau[166] in the French Sudan, by Mr.
Parkinson[167] in Southern Nigeria, and by Dr. Esch[168] in the Kameruns, and it is improbable that
Dahomey should have escaped the general movement of elevation.
M. Chudeau[169] has recognised in the Sudan the recent
period of arid conditions of which traces are still to be found
within Northern Nigeria. He believes, however, that the dallols
were excavated during the period of desert conditions by rivers
which drew their supplies from the mountains of Air. It is possible
that there may have been repeated alternations of desert and humid
conditions within the latest period of erosion; but it is clear in
Nigeria that the dallols of Sokoto were already excavated before
the advent of the latest arid period, and that the rivers which
formed them have not regained the activity which formerly
characterised them. Capt. Meynier’s suggestion, quoted by M.
Chudeau, that the rivers of Hausaland turned their backs to the
desert, though picturesque, is little in harmony with the facts.
The courses they follow within the Protectorate were defined long
before the advent of the desert, and not by any adaptation to
climatic conditions, but by the direct influence of the early
Pleistocene crustal movements which brought about the rejuvenation
of the drainage
system of Nigeria and the formation of the Bauchi plateau. These
movements were probably also responsible for the establishment of
the dallols of Tahoua and the quaternary wadis of the northern
Sahara, as well as for the elevation of the plateaux of Air and
Adr’ar’ des Ifor’as. These movements, moreover, were probably
contemporaneous with the later Tertiary movements in the
Mediterranean basin and in East Africa.
M. Chudeau[170] has pointed out the analogy between the
pre-Devonian and the post-Carboniferous movements to the south of
the Atlas and the Caledonian and hercynian movements to the north
of the Alps. In Nigeria and the Sudan some analogy may perhaps be
permitted between the general meridional strike and vertical
folding of the gneisses and schists and the Caledonian folding of
Scotland and Scandinavia. Of hercynian movements, however, there is
little or no trace in the Sudan, while the presence of an
unconformability at the base of the Eocene and the absence of any
later folding makes the post-Cretaceous and early Tertiary
movements in the Sudan more nearly parallel with the sequence of
events in northern Europe than with the sequence in the Alps and
the Atlas. The period of maximum movement in the Alps would appear
to be represented in Nigeria and the Sudan only by minor
post-Eocene oscillations of the crust, while the major movements in
the Sudan which gave birth to the present drainage system appear to
be contemporaneous with the later Mediterranean stages of the Alpine
movements.[171] At the same time, however, it should be
emphasised that, while the movements may have been contemporaneous,
there was probably little or no genetic connection between them.
The Tertiary period was everywhere a period of great instability of
the crust, and the movements would appear to have varied both in
intensity and in kind, and to have been localised now in one place
and now in another, at different periods of their history.
[149]Cf. De Lapparent, C. Rd. Ac.
Sc., 136, p. 1118.
[171]Suess, The Face of the Earth, Vol.
I., Oxf. Trans., 1904, p. 279.
CHAPTER VIII
TERTIARY VOLCANIC ACTION
Absence of pre-Tertiary volcanic rocks within the
Protectorate: two periods of Tertiary volcanic activity. The early
Tertiary period: the Maradu vent; the nepheline basalts of Wase and
Tangalto; the phonolites of Wase Rock, Tangale Peak and Bantaji;
the plateau lavas of southern Bornu, Bauchi and Nassarawa;
petrographical character of the lavas; early Tertiary lavas of the
Sahara, Sudan and Adamawa. The later Tertiary volcanic period: the
puys of northern Yola, the middle Benue valley and the Bauchi
plateau; petrographical character of the lavas; petrographical
provinces.
It may be presumed that the
intrusion of the alkali granites and dyke rocks throughout the
Protectorate was accompanied in pre-Cretaceous times by
considerable superficial volcanic activity (p.
142). M. Chautard[172] has, moreover, described
the intercalation of volcanic rocks in the Upper Cretaceous of
Senegal. All trace of pre-Tertiary effusive rocks, however, within
the Protectorate appears to have been removed in the course of
denudation. The fracturing and folding of the crust at the close of
the Cretaceous was apparently accompanied in places by the
formation of mineral veins within the fissures, but no
contemporaneous igneous activity has been definitely
identified within
the Protectorate (p. 140). Within the
Tertiary era itself two separate periods of volcanic activity may
be recognised, one in the middle Eocene and probably to some extent
contemporaneous with the accumulation of the Eocene sandstones
within the Protectorate, and one in late Pliocene or early
Pleistocene times and contemporaneous with the later Tertiary
oscillations of the crust. For the most part the eruptions were of
an isolated character and gave rise to the formation of detached
pipes, cones and puys of igneous material. More rarely, as in
southern Bornu and on the borders of Bauchi and Nassarawa, the
extruded lavas united to form a continuous sheet of volcanic rock,
which covered a considerable area of the underlying sedimentary and
crystalline rocks.
The earliest outburst of volcanic activity appears to have taken
place towards the close of the post-Cretaceous erosion or possibly
in the middle Eocene at a period contemporaneous with the
accumulation of the lower marine beds of Sokoto, and is represented
now by the extensive neck or vent of fragmental material near
Maradu in Sokoto province whose levelling was accomplished during
the later extension of the middle Eocene sea. The greater number of
the boulders in the agglomerate are composed of a microgranitic
quartz porphyry with phenocrysts of orthoclase, biotite and brown
hornblende in addition to quartz (1323). To the same period is
probably also to be referred the line of well-worn stumps of
basalt, phonolite and trachyte which stretches from the Gongola
valley westward through Tangale and Angass to Namu in northern Muri. The
Tangale Peak (Pl. XIII, Fig. 2) and the Wase
Rock (Pl. V, Fig. 2), which still retain a
portion of the original flow attached to the rock in the vent, were
probably among the latest eruptions of the series. The intrusive
dolerites of Bashar (41) and Awe (1014) may be ascribed to the same
phase of igneous activity, while the Rock of Bantaji in Southern
Muri may be taken to represent contemporaneous volcanic action to
the south of the Benue.
In petrographical character the basaltic pipe rocks are always
olivine-bearing and usually porphyritic with olivine and augite.
Felspar is found only in the ground-mass which at times contains a
little intersertal glassy base. The basalt of Wase Topa is
remarkable for the number and size of the nodules of olivine which
it encloses. A worn vent immediately south of the town of Wase is
filled with a fine-grained nepheline basalt (32) containing
glomeroporphyritic aggregates of augite crystals and well-formed
crystals of olivine. Felspar is entirely wanting, the ground-mass
being composed of small rods of augite and granules of magnetite
set in a brown glassy base studded with minute crystals of
nepheline. A somewhat similar but coarser-grained rock from
Tangalto (1661) contains large crystals of decomposed olivine and
purple and well-zoned augite in a holocrystalline ground-mass of
rods and granules of augite, magnetite grains, flakes of biotite
and abundant nepheline in well-formed and irregular crystals. The
nepheline was evidently the last to crystallise, and to some extent
takes the place of
matrix, being in part moulded upon the other constituents of the
ground-mass.
The phonolite of the Wase Rock (31) is of a trachytoid type,
containing columnar phenocrysts of sanidine, ragged crystals of
ægirine augite and scattered pseudomorphs after nepheline in a
ground-mass consisting predominantly of minute felspar prisms with
a well-marked fluxional arrangement, mingled with magnetite grains
and rods and granules of green pyroxene. The phonolite of Tangale
Peak (1665–6) contains phenocrysts of sanidine and soda pyroxene in
a ground-mass of a more orthophyric type, composed of minute
idiomorphic crystals of nepheline, imperfect prisms of ægirine and
ægirine augite and elongated microlites of felspar which pass into
an ill-defined felspathic matrix. Traces of brown hornblende in
irregular porous growths may also be observed in the ground-mass. A
similar rock from the immediate neighbourhood of Tangalto (1663)
contains soda pyroxene, but no evident nepheline in the
ground-mass, and may be grouped provisionally as a trachyte. The
phonolite of Bantaji Rock is of a more nephelinitoid type, with the
ground-mass enclosed in a somewhat intersertal manner between the
abundant but much decomposed phenocrysts of felspar. With the
felspar are associated as phenocrysts hypidiomorphic crystals of
ægirine and ægirine augite frequently in zonal growths and smaller
idiomorphic and abundant crystals of somewhat weathered nepheline.
The ground-mass is composed of lath-shaped felspars, granular
pyroxene and irregular patches of analcime probably of secondary
origin. The association of phonolite and nepheline basalt, observed
in the neighbourhood of Wase and the Peak, has not been recognised
around Bantaji.
The period of formation of the lava fields in southern Bornu and
on the borders of Bauchi and Nassarawa may be provisionally
referred to the close of the middle Eocene, on the strength of the
apparent intercalation of a friable red sandstone amongst the lavas
in the escarpment of the Barbur plateau behind Dukshi. The
extensive erosion which the plateau lavas themselves have undergone
indicates, moreover, that they are of no very recent age. The
eastern lava field is the more extensive of the two and perhaps
somewhat younger, so far as one can judge from the less denuded
character of the cones. The rich content in iron of the upper part
of the Eocene accumulations may possibly have been conditioned to a
considerable extent by the distribution of much fragmental basic
igneous material upon the surface of the waters.
In petrographical character the lavas are basaltic throughout
and rich in olivine, and vary from felspathic doleritic types to
limburgitic varieties in which felspar is greatly reduced in
amount.[173] Nepheline-bearing types have nowhere been recognised, nor any
effusive rocks more acid than basalt. MM. Chudeau and Gentil[174] have, however, indicated the occurrence of
ægirine rhyolites and phonolites, as well as of basalts, tephrites
and limburgites, amongst the older lavas of Ahaggar and Air, which
are probably of similar age to those of Nigeria. Dr. Passarge[175] has described the occurrence of basalt, augite
andesite and phonolite on the summit of the Shebshi hills on the
borders of Nigeria and the Kameruns. MM. Gentil and
Freydenberg[176] believe that the ægirine porphyries of Mounio
and of Hadj el Hamis on the southern shore of Chad are of volcanic
origin. M. Garde,[177] however, is of opinion that they possess
an intrusive character and belong to the hypabyssal phase of the
alkaline magma which gave rise to the Mounio and Melfi
granites.
The plutonic rocks of this early Tertiary volcanic period are
nowhere exposed within the Protectorate, and hypabyssal types are almost unknown.
The doleritic sills of Awe and Bashar were probably injected during
one of the earlier phases of igneous activity. Dr. Passarge[178] has indicated the occurrence of basaltic dykes
in the sandstones of Adamawa and a boss of nepheline syenite
between Yola and Garua on the upper Benue. The latter, however,
more probably belongs to the underlying crystalline floor. M.
Chudeau[179] believes that the alkali granites of Zinder
and Mounio have been intruded since the close of the Cretaceous;
but while there is every reason to believe that the nepheline
basalts and ægirine rhyolites of Tertiary age drew their origin
from a magma rich in soda, there is, as already stated (p. 140), no evidence within the Protectorate upon
which any immediate connection between the younger soda granites
and the Tertiary volcanic rocks can be established. On the
contrary, there would appear to have been in the middle Eocene
within the Protectorate merely a renewal of volcanic activity from
the same alkaline source from which the pre-Cretaceous granites and
dyke rocks originated. If, however, further investigation should
confirm M. Chudeau’s suggestion of the Tertiary age of the granites
and porphyries of Zinder, Mounio and Goure, these rocks may then be
taken to represent the plutonic and hypabyssal phases of this
middle Eocene period of igneous activity.
To the later Tertiary period of volcanic activity, which may be referred to the
close of the Pliocene and which perhaps also extended into the
early Pleistocene, are to be assigned the puys of Kereng (Plate X,
Fig. 2) on the southern margin of the Bauchi
plateau, the puys of Song and Mboi (Plates VI, Fig.
2; VII, Figs. 1,2) in
northern Yola, and somewhat doubtfully also those of the Awe
district on the middle Benue (Plate V, Fig. 1).
The puys of the Kereng group are remarkably perfect and exhibit
steep crater walls of fragmental material breached by a single
short flow of olivine basalt. Elsewhere on the plateau also, as
between Vom and Bukuru, there are small puys with breached craters
which are probably of a similar recent age. From the cliffs of
Assab, moreover, later flows of lava may in places be seen on the
worn slopes of the plateau edge, and there is every reason to
believe that the erosion of the drift and of the margin of the
plateau was considerably advanced before the final close of
volcanic activity. The presence of craters and the local character
of the associated flows serve to distinguish these later products
of igneous activity from the earlier lavas of the plateau. A
similar superposition of later volcanic material upon earlier lavas
has been observed by M. Chudeau[180] in the Ahaggar and in
Air. Unlike the central Sahara, however, there is in Nigeria no
reason to believe that there has been any actual continuity in
volcanic action between the earlier and the later periods.
In Yola province the Mboi Hills are crossed by a line of small
craters four or five miles long, running roughly in a N.W.–S.E. direction. To the
south-east between the Mboi Hills and Song the line is continued by
a number of hillocks of basalt for a few more miles, but how far it
extends to the N.W. is not known. Only a relatively small amount of
basalt was erupted from these vents. In the case of the largest
crater with a crater floor of 250 yards in diameter the volcanic
material was not sufficient to conceal the granite through which it
came. The granite was broken away during the eruption so as to form
a cliff extending one-third round the crater, while the other
two-thirds is formed of agglomerate and tuff with very many
fragments of granite. The crater thus resembles a swallow’s nest
attached to its support. With one exception, where the wall is
entire, all the craters were breached by a lava flow. They are in
excellent preservation, and the cliff-like walls, with
insignificant talus heaps at their bases, indicate a very recent
origin. (A. L.)
Plate XVIII
The Salt-fields of Awe; Volcanic Hills in the
background.
Akiri Saltings.
The vents on the plain to the south-east are not marked by
craters or by fragmental materials, but by mounds of basalt less
than fifty feet high spreading out into lava flows, which cover
most of the surface near by. Here the eruptions seem to have
consisted in a much quieter extrusion of lava alone. The basalts
are all vesicular and very commonly quite light and cellular. (A.
L.)
The basalts of the Benue plain likewise form low hillocks or
mounds with no tuff or fragmental material to be seen. Generally
there is no lava flow proceeding from them, and the extrusion of
quite a small quantity of lava seems to have sufficed to relieve
the pressure. Kaddemi Hill is an exception; an old basalt flow
leads from it for two to three miles to the south-east. (A. L.)
All the basalt flows around Awe are raised considerably above
the level of the plain, and protect the limestone shales below from
denudation, though elsewhere the shales are very badly exposed. In
consequence the flows rest on “pedestals” from thirty to fifty feet
in height. Assuming that the lava chose the level surface of the
plain rather than ridges upon it, it follows that since the
eruptions around Awe there has been a general lowering of the
surface by that amount. From this it appears that the Awe eruptions
were not quite contemporaneous with those of Mboi, for at the
latter place only a small amount of talus has been broken off from
the crater cliffs. (A. L.)
The ash cones and puys of the middle Benue are much more
imperfectly preserved than those of Kereng or Mboi and are
certainly considerably older. Their comparatively recent age is
presumed, however, as in the case of the later volcanic period in
Air and Tibesti,[181] from the presence of thermal springs in
the immediate neighbourhood. It seems probable that they were
established as the result of the crustal movements anterior to the
accumulation of the drift, while the puys of Kereng and Mboi owe
their origin to the differential movements of the latest period of
elevation.
In petrographical character these later rocks are very similar
to the earlier Tertiary lavas. All are basalts rich in olivine and augite and
varying in the same locality from types with abundant felspar in
the ground-mass to thoroughly limburgitic[182] varieties (12) from which felspar is entirely
wanting. The ash of the Kereng puy is remarkable for the number and
size of the rounded augite crystals and olivine nodules which it
contains. Recent flows of olivine basalt and limburgite are known
also from Ahaggar and Air[183] and from the
Kameruns.[184] M. Chudeau[185] has described rhyolites
from Inzize to which he assigns a later Tertiary age, Dr.
Passarge[186] has noted a nepheline basalt from an isolated
cone of recent origin in the Benue valley between Yola and Garua,
and Dr. Esch[187] has described nephelinites and leucitites from
the Kamerun Mt. which is still periodically in active eruption. No
rocks of similar composition, however, have been recognised amongst
the products of the most recent volcanic period within the
Protectorate.
MM. Chudeau[188] and Chautard,[189] following M. Lacroix,[190] have suggested that the alkaline rocks of
North Africa may be grouped into three petrographical provinces, a
western province including the Azores, Canaries, Cape Verde and
Senegal, a central province including Ahaggar, Inzize, Air, Zinder
and Mounio, Chad and the Shari valley, the Kameruns, and
Dahomey, and an eastern province including Abyssinia, Arabia and
Somaliland. Upon this hypothesis the alkaline rocks of Northern
Nigeria belong to and complete the central province of Lacroix, as
extended by M. Chudeau. It is probable, however, that this central
province includes intrusive and volcanic rocks of many different
ages, and that alkaline types have been a characteristic product of
differentiation from the earliest times to the present day,
(p. 142). There is, moreover, every reason
to believe that the progress of investigation will reveal many
connecting links between the various provinces, which are
themselves somewhat too extensive and comprehensive to warrant
their recognition as such. A petrographical province in the
stricter sense of the term may perhaps be set up to include the
alkaline plutonic rocks of Bauchi province and their associated
hypabyssal types, but there can be little doubt that the general
alkaline character of the igneous rocks of northern Africa, to
which Lacroix has given expression, may be quite sufficiently
indicated by emphasising their natural classification within the
Atlantic branch of igneous rocks.[191]
[173]These rocks present in microscopical
structure a decided resemblance to the Carboniferous lavas of the
Bathgate Hills (Falconer, Trans. Roy. Soc. Edin., Vol.
XLV., 1906, p. 133) in Scotland. With the exception that the
doleritic types are somewhat less weathered, the description and
classification of the Bathgate rocks may be applied almost word for
word to these Tertiary lavas of Nigeria. Doleritic rocks (Falconer,
op. cit., Plate I., Fig. 2) with phenocrysts of augite and
olivine, and more rarely of felspar are common on the Morrua
plateau in eastern Nassarawa and along the base of the escarpment
between Darroro and Assab (928, 1573, 1630). Felspar is abundant in
the ground-mass and the augite occurs in granules, granulitic
aggregates, small ophitic plates, or minute idiomorphic crystals
between the felspars. The basalts of Zammagan (1578) and Buratai
(1640) show a typical fluxional arrangement of the felspars
(Falconer, op. cit., Pl. I., Fig. 3). The basalt of Kwoia
(1654) exhibits an intimate mixture of felspar and augite in the
ground-mass (Falconer, op. cit., Pl. I., Fig. 4). The
basalt of Assab (934) contains scattered felspars with abundant
minute granular aggregates of augite, while a similar rock from
Buratai and Dukshi (1641) contains some residual brown glassy
material and passes into limburgitic varieties with phenocrysts of
augite and olivine in an ill-defined microlitic base.
[174]Chudeau, Sah. Soud., 1909, p. 258;
Gentil, Doc. Sc. Miss. Sah., 1905, p. 726.
[175]Passarge, Adamaua, 1895, p. 374.
See also Downes, Geol. Rep. Yola-Cross River Boundary
Commission, 1910, p. 35.
[176]Gentil and Freydenberg, Bull. Soc.
Geol. Fr., 4th Ser., VIII., 1908, p. 35; Lacoin, C. Rd.
Ac. Sc., 22 June, 1903.
[177]Garde, C. Rd. Ac. Sc., 149, 5
July, 1909, p. 43.
Salt — Iron — Gold — Monazite — Galena —
Tinstone.
Detailed information
respecting the principal mineral resources of Northern Nigeria will
be found in Professor Dunstan’s Reports on the Results of the
Mineral Survey.[192] The following is a brief summary of the
more important facts.
Salt has long been a staple article of inter-tribal commerce on
the middle Benue, and its manufacture has in the past been
extensively carried on in the province of Muri. The introduction of
European salt has, however, adversely affected the native industry,
and salt-making is now confined only to those localities in which
the natural conditions are most favourable. In Muri province the
salt is prepared by the concentration and evaporation of the waters
of weak brine springs which are distributed over a western and an
eastern area. In the western area the town of Awe is the centre of
the industry, while Akiri and Azara are subordinate in both
production and population. (Plate XVIII, Figs.
1,2.) Keana on the Nassarawa border forms
the western limit of the salt-producing area, while Akwana to the south of the Benue
marks its southernmost extension. In addition to these towns which
still manufacture a surplus for export, salt-making for local
purposes is carried on at Kanji, Doya, Abuni, Ribi and Arofu within
the same area. At a former period also salt was made at Lafin
Gisheri, between Tunga and Ibi, and this locality may be taken to
mark the eastern limit of the salt area. In eastern Muri the
salt-producing district is of less importance than that in the
west. The centres of the industry are Bomanda and Jenoe, but small
quantities of salt are also prepared at Jebjeb, Jebero and
Langa.
With regard to geological occurrence it is believed, as already
indicated, that the weak brine from which the salt is made in Muri
province is located within the upper part of the “Muri sandstones”
and immediately underneath the passage beds and the Turonian
limestone. Mr. Longbottom found that at Awe the springs supplied
from the sandstones of the passage beds are entirely free from salt
and cease flowing before the end of the dry season, while the
neighbouring brine springs which rise from rocks lying below the
surface drainage maintain a constant flow throughout the year.
Moreover, wherever the shaly bands within the passage beds were
pierced by boreholes, the brine rose with a steady flow under
considerable pressure. The sandstones and grits underneath the
passage beds appear to be saturated with weak brine which is
retained, as in a reservoir, by the overlying shales. It is
believed that the western and eastern salt-bearing districts form
detached and irregular synclinal areas affected by a subordinate
folding. The brine springs at Awe, Azara and Bomanda are especially
associated with the subordinate anticlinal axes along which the
salt-bearing beds within the synclinals approach the surface. Some
of the minor salt towns, such as Ribi, Keana and Jenoe, are
probably located on or near the outcrop of the salt-bearing beds.
Wherever the brine springs are found, bubbles of a colourless gas,
probably carbon dioxide, rise with the water. Calcareous
incrustations round the orifices of the springs are found at Awe
and Jebero. Some of the springs of Awe are, moreover, slightly
thermal and possess a temperature a few degrees above sun heat.
This is probably to be correlated with the recent volcanic action
in the neighbourhood.
NORTHERN NIGERIA
Distribution of Salt, Tinstone,
Gold and Galena.
MAP No. 4.
London. Macmillan & Co., Ltd.
Face p. 266.
The present springs rise in stream courses and are unavailable
during the rainy season. Boring, however, within the synclinal
areas would afford a copious and continuous supply. Professor
Dunstan has analysed the salt of various localities and discussed
the possibilities of increasing the native output. The general
weakness of the brine, however, would appear to be unfavourable to
European exploitation.
Salt is won from brine pools in south-eastern Bauchi over an
extensive belt of country which stretches northward from the Pero
Hills and passes to the east of Chongwom and Tangalto. The pools
are presumably fed by springs and the salt is said to be abundant
and easily prepared. The whole area, however, is very imperfectly
known and largely unexplored. Scanty incrustations of natron,
utilised by the local herdsmen, are known from the Gongola valley
in the neighbourhood of Nafada.
In Bornu salt is made from the ash of the siwak tree
(Salvadora persica) which grows abundantly on the ancient
dunes which fringe the western shore of Chad. The slaves of Mongonu
and Musara prepare salt also from the ash of three varieties of
grass, “pagam, kalaslim and kanido,” which are found in the
immediate neighbourhood of the lake. Over the whole of central
Bornu the waters of the wells are charged with salts of soda in
very varying amount. The ancient alluvium of Bornu is evidently
impregnated with soda, and incrustations are found in places
upon the surface of
desiccated swamps. Natron pools, similar to those of Manga and
presumably fed by springs, are unknown in central Bornu.
In Sokoto province incrustations of salt are found in the dry
season at Zauro in the valley of the Gulbin Kebbi, and natron
(kungwa) occurs in a similar manner between Gulma and Argungu.
Knots and streaks of gypsum are common in the shales and clays
around Sokoto, while efflorescences of gypsum and alum are found on
the cliffs of Wurnu to the north-east. Scanty incrustations of
natron are found upon desiccated swamps in many localities in the
provinces of Sokoto, Kano, Zaria and Ilorin, but never in
sufficient quantity to give rise to even a native industry.
Hausaland has been in the past supplied with salt and natron from
Asben, Bilma and Manga in the French Sudan. European salt, however,
is rapidly replacing the native article, and many caravans now
bring only natron from the north to the markets of Kano.
As in the case of salt, so in the case of iron, the native
manufacture has been very largely superseded by the importation of
European bar-iron. Only in the more inaccessible districts or in
those places where the ore is abundant do the natives still smelt
the local ironstones in their primitive fashion. The surface
ironstones were at one time generally utilised throughout the
Protectorate as a source of iron. The magnetite sands are still
washed and smelted by the pagans of the Mumuye hills in eastern
Muri, while in northern Nassarawa the Koro tribe around Kao is
still noted for its proficiency in the manufacture of iron from the
local limonitic ores which occur within the schists in veins of
varying thickness.[193] Similar ores occur in veins in the sandstones around
Akiri in western Muri, but are no longer utilised by the natives.
The oolitic and earthy ironstones of sedimentary origin and of
Eocene age give rise to an extensive native industry around Akwi
and Ojerami in central Kabba, around Mahorro in eastern Kontagora,
around Aliero in southern Sokoto, and around Kantana in southern
Bauchi, as well as in the Kerri Kerri country in western Bornu and
in the Tagum hills in southern Bornu. So far as European
exploitation is concerned, the sedimentary ores of Mt. Patti and
the lower Niger are most favourably situated for transport. The
ores, however, while abundant, are of a somewhat low grade
character and relatively rich in phosphoric acid. Magnetite and
hæmatite schists are extensively developed in eastern Kabba, but
their composition would render necessary costly crushing and
separating processes before the ores could be available for
export.[194]
Traces of gold have been found in some of the quartz veins which
are so widely distributed throughout the Protectorate, but no
payable reefs have yet been discovered. Small quantities of free
gold occur in the streams between Koriga and Birnin Gwari in
northern Zaria and around Tagum in southern Muri. In Zaria the gold
is evidently located within the northern prolongation of the
Zungeru belt of slates and schists, and further exploration may
possibly lead to valuable discoveries. All the slaty belts already
described within the Protectorate are, indeed, worthy of careful
prospecting for gold. Small quantities of gold are also found
associated with cassiterite in some parts of Bauchi province.
Monazite is widely distributed throughout the Protectorate, and
appears to be a fairly constant constituent of the gneisses and
schist. Small quantities are contained in almost every river and
stream within the crystalline areas. It has been found most
abundantly in central Kabba and in northern Nassarawa, but, so far
as known, it occurs nowhere in sufficient quantity to afford any
hope of profitable European exploitation.
A slightly argentiferous galena occurs in veins within the
Cretaceous rocks around Arofu and Akwana in western Muri, and in
the Kudu valley to the west of Jebjeb in northern Muri. In the
Arofu district the galena is associated with fluorspar and calcite,
and the formation of the veins has been accompanied as already
described by much silicification of the adjoining limestones. The
Arofu galena is excavated by the natives for use as a cosmetic, and
from time to time small pockets of silver are discovered within the
veinstuff. To the west of Jebjeb the galena is no longer dug by the
natives, and the old workings are now levelled and grassed over.
The galena appears to have been associated with quartz, hæmatite
and blende, but there is no evidence of silicification of the
country rock.
Tinstone is the most valuable asset of the Protectorate from the
point of view of mineral resources. Its occurrence in western
Bauchi was known to the natives long before the advent of
Europeans. Recent investigation, however, by the Mineral Survey has
proved its presence over considerable areas in widely separated
localities within the Protectorate. The most extensive area over
which tinstone is found in the surface alluvium occurs in western Bauchi,
and includes the summit of the plateau and the greater part of its
northern margin. The alluvial deposits of Naraguta, which lie
within this area, were originally developed by the Niger Co., but
have now been acquired by the Consolidated Gold Fields, Ltd.
Elsewhere, however, tinstone becomes equally abundant, and there is
little doubt that further investigation will reveal many localities
within this extensive area in which alluvial workings may be
profitably carried on. Several syndicates indeed are now engaged in
exploiting certain portions of the tinfield. In the neighbourhood
of Ngell the tinstone, accompanied by sulphide ores of copper, zinc
and lead, occurs, as already described, in part as a pneumatolytic
modification of the Bukuru granite, and in part as a true lode or
pegmatitic formation. This particular occurrence is being developed
by the Niger Co., but there is every reason to believe that similar
occurrences will be found throughout the area as investigation
proceeds.
Apart from this extensive tin-field in the centre of the
Protectorate, cassiterite is also found in Bauchi province in many
places throughout the Ningi and Burra hills, and detailed
prospecting will probably here also reveal several payable
localities. The completion of the Baro-Kano railway[195] within the next two years will afford an easy
means of access to the tin-producing areas of Bauchi, and will at
the same time largely overcome the difficulty of transport which is
at present the greatest obstacle in the way of the development of
the tin fields.
Beyond the limits of Bauchi province small quantities of
tinstone have been found in the neighbourhood of Gadama and Fagam
on the borders of Bauchi and Kano, around Gantam and Aribi on the
Nassarawa tableland as well as on the eastern slopes of the Vere
hills in Yola, the western slopes of the Shebshi hills in southern Muri, and in the
neighbourhood of Eri in eastern Ilorin. In the latter locality the
tinstone was found to be derived from a richly stanniferous
pegmatite vein, but the actual extent and possibilities of the
occurrence are as yet unknown. The same may be said also of the
other localities indicated, and there can be little doubt that all
of them are worthy of further investigation. Tinstone has been
reported to occur abundantly in the Mada country in eastern
Nassarawa, but this discovery likewise awaits confirmation.
[192]Dunstan, Col. Reps.
(Miscell.), Nos. 32 (1906), 46 (1908), 47 (1908). A fourth
report on the results of the Mineral Survey will shortly be
issued.
[193]Sir William Wallace has reported an
extensive native iron industry at Fawa in Katsina, where the ore
appears to occur in a similar manner to that in the neighbourhood
of Kao.—G. J., VIII., 1896.
[194]In The Iron Ore Resources of the
World, Vol. II., 1910, p. 1029, Dr. J. W. Evans has given a
résumé of the results of the investigations of the Mineral
Survey into the extent and value of the ferruginous deposits of
Northern Nigeria.
[195]And the branch line to Leri in Zaria
province.
THE PALÆONTOLOGY OF THE UPPER CRETACEOUS DEPOSITS
OF NORTHERN NIGERIA
By Henry Woods, M.A.,
University Lecturer in Palæozoology, Cambridge
The fossils collected by
Dr. J. D. Falconer and Mr. A. Longbottom from the Upper Cretaceous
deposits of Northern Nigeria include Fishes, Ammonites,
Gasteropods, and Lamellibranchs. The localities from which these
fossils were obtained are: Awe, Deba Habe, Jibaru, Gongila,[196] Gulani, Kumberi, Kunini, and Reme. At present
the number of species which can be identified is comparatively
small, but the collection is sufficient to show that under more
favourable conditions a large and important fauna could probably be
obtained from the deposits of this region.
Teeth, vertebræ, &c., of fishes were collected at Kumberi
and Jibaru, and have been examined by Dr. A. S. Woodward who
identifies them as Gigantichthys, a genus which is known
from the Chalk of Egypt and the Sahara.[197] Ammonites have been found at Gongila, Reme,
Kunini, and Kumberi, and they show that the deposits are of
Turonian age—probably Lower Turonian. The Ammonites
belong to the genera Vascoceras, Mammites
(Pseudaspidoceras), and Hoplitoides.
Vascoceras is confined to the Turonian.
Pseudaspidoceras is mainly of Turonian age. The species of
Hoplitoides belongs to the section of that genus which
occurs in Turonian deposits. The species of Vascoceras and
Mammites (Pseudaspidoceras) found in Nigeria are
closely allied to forms which occur in the Lower Turonian of
Northern Africa and the Iberian peninsula.
The larger number of the Gasteropods and Lamellibranchs were
found at Awe, but no Ammonites were obtained at that locality;
these fossils, so far as they can be identified, do not give any
definite evidence that the deposit at Awe is of earlier age than
the Turonian.
At Deba Habe Ostrea (Exogyra) olisiponensis, Sharpe, is
abundant and is the only fossil obtained at that place. In Portugal
that species occurs in both Turonian and Cenomanian deposits. In
Nigeria it has also been found at Gongila, where the Turonian
Hoplitoides occurs. So that it is probable that the deposit
at Deba Habe is of the same age as the one at Gongila.
At Gulani Ostrea prælonga, Sharpe, is abundant, but no
other fossil was found at that locality. In Portugal O.
prælonga occurs in the Bellasian—an horizon which is believed
to come between the Aptian and the Cenomanian. The same species is
found at a similar level in Tunis and Algeria. The occurrence of
this one species can, I think, hardly be taken as sufficient
evidence that the deposit at Gulani is of Bellasian age, since it
is not unlikely that, as is the case with O. (Exogyra)
olisiponensis, this form may recur at later horizons when a
favourable facies of deposit again appears.
At present, then, we can state that the deposits in which
Ammonites occur are of Turonian age, and that there is not sufficient evidence to show
that beds of earlier age are represented.
The fauna of the Turonian deposits of Northern Nigeria resembles
closely that found in beds of similar age in Portugal, Tunis, and
Algeria. This is shown especially by the Ammonites and oysters. At
present the genus Vascoceras is known only from Portugal,
Spain, Algeria, Tunis, Egypt, Nigeria and Brazil.
Hoplitoides occurs in Algeria, Tunis, Nigeria, the Kameruns,
and probably in Egypt. The species of Pseudaspidoceras found
in Nigeria is allied to forms found in Portugal, Tunis and Brazil.
The oysters likewise belong to species characteristic of Northern
Africa and South-western Europe, and one of them has also been
found as far south as Angola. The Ammonites also show, as
Pervinquière has already pointed out is the case in Tunis, that the
fauna is related to that found in the Upper Cretaceous of
Brazil.
Amongst other places in West Africa, Cretaceous deposits have
been found in the Kameruns, and are regarded by Solger as of Upper
Turonian and Lower Senonian age, but by Harbort[198] they are referred to the Emscherian (Lower
Senonian). They certainly appear to be of later date than the
Nigerian deposits with which they are connected chiefly by the
presence of Hoplitoides.
Until the fauna of the Nigerian Cretaceous is more completely
known any further discussion of its relationship to the faunas of
other areas must be postponed.
The list on the following page shows the distribution of the
species found in Nigeria, and is followed by palæontological notes
and descriptions of some of the species.
Several specimens of Trigonoarca were found, but are not
sufficiently well-preserved for specific identification. In general
form they resemble some examples of T. trichinopolitensis
(Forbes)[199] from the Trichinopoli Beds of Southern
India.
Locality.—Awe.
Two specimens of a species belonging to the Scabræ section of
Trigonia are in the collection, but are not sufficiently
well-preserved for identification.
Locality.—Awe.
Examples of a small form of Modiola occur at Awe.
Three specimens with the shell preserved were found at Awe.
An imperfect specimen from Awe resembles Lima (Mantellum)
gaultina, Woods.[200]
1850. Ostræa prælonga, D. Sharpe, Quart. Journ. Geol.
Soc., VI., p. 187, Pl. XX., Fig. 4.
1869. Ostrea prælonga, H. Coquand, Mon. Ostrea. Terr.
Crét., p. 169, Pl. LXVI., Figs. 1, 2.
1890. Ostrea prælonga, A. Peron, Moll. Foss. Terr.
Crét. Tunisie, p. 110, Pl. XXIII., Figs. 1–6.
Remarks.—About twenty more or less perfect specimens of
an Ostrea, agreeing closely with O. prælonga, Sharpe,
were found. O. pentagruelis, Coquand,[201] is considered by Peron to be only a very old
individual of O. prælonga.
This species occurs in the Bellasian of Portugal. In Algeria and
Tunis it is characteristic of beds which occur below the
Cenomanian, and are referred by Peron to the Upper Albian.
1850. Exogyra olisiponensis, D. Sharpe, Quart. Journ.
Geol. Soc., VI., p. 185, Pl. XIX., Figs. 1, 2.
1862. Ostrea Overwegi, H. Coquand, Géol. pal. rég.
sud. prov. Constantine, p. 226, Pl. XIX., Figs. 1–6.
1862. Ostrea Coquandi, Julien, in Coquand,
ibid., p. 324, Pl. XXXIII., Figs. 10–12.
1869. Ostrea
olisiponensis, H. Coquand, Mon. Ostrea. Terr. Crét.,
p. 125, Pl. XLV., Figs. 1–7.
1869. Ostrea Overwegi, Coquand, ibid., p. 140,
Pl. XLIV., Figs. 1–9; Pl. XLVI., Figs. 14, 15.
1873. Ostrea olisiponensis, L. Lartet, Ann. Sci.
géol., Vol. III., p. 59, Pl. XI., Fig. 1.
1882. Exogyra olisiponensis, G. Seguenza, Atti d. R.
Accad. dei Lincei, Ser. 3; Mem. Scien. Fis. Math.,
Vol. XII., p. 180, Pl. XVII., Fig. 2.
1882. Exogyra oxyntas, Seguenza, ibid., p. 178,
Pl. XVIII., Fig. 1.
1888. Ostrea olisiponensis, P. Choffat, Mém. Soc.
Phys. et d’hist. nat. de Genève, XXX., 2, p. 95.
1891. Ostrea olisiponensis, A. Peron, Moll. Foss.
Terr. Crét. Tunisie, p. 114, Pl. XXIII., Figs. 14–18.
1902. Ostrea (Exogyra) olisiponensis, P. Choffat,
Faune Crét. Portugal, Vol. I., Ser. 4, p. 166, Pl. VI.,
Figs. 17–19.
1903. Ostrea olisiponensis, R. Fortau, Bull. Instit.
Égypt., Ser. 4, Vol. IV., p. 283, Figs. 3–5.
1905. Ostrea (Exogyra) olisiponensis, P. Choffat,
Géol. Col. Portug. d’Afrique. II, Nouvelles données sur la zone
littorale d’Angola, p. 44, Pl. I., Figs. 4, 5.
Remarks.—This species is represented by numerous
examples, in a few of which the surfaces of the shell are
well-preserved and show a few coarse ribs. Sharpe and other writers
have noted the great variability in the ribbing of this species.
The Nigerian specimens agree with the varieties which possess only
a few ribs. Peron, Choffat, and Fortau have discussed fully the
affinities and synonymy of O. (Exogyra) olisiponensis. This
species is common in Northern Africa and Portugal; it occurs in the
Cenomanian of Algeria, Tunis, Egypt, and Palestine; in Portugal it
is found in both the Upper and Lower Turonian, as well as in the
Cenomanian, wherever the deposits are of the marly facies. South of
Nigeria this species has been discovered in the province of
Angola.
There are several internal casts resembling in form G.
(Pseudoptera) anomala (Sowerby),[202] but there is no evidence to show
whether radial ribs
were present on the shell or not, and the species cannot be
determined at present.
Specimens from the Cenomanian of the neighbourhood of Tebessa
(Algeria) have been identified with Sowerby’s species by
Coquand,[203] and Peron[204] states that it is
abundant and well-preserved at that locality.
Description.—Shell rather small, moderately convex,
with the postero-dorsal part somewhat compressed, outline
subquadrate or subtriangular, very inequilateral. Anterior margin
slightly concave; ventral margin convex; posterior margin short,
truncated, forming an obtuse angle with the long, slightly convex
postero-dorsal margin. Umbones pointed, rather prominent. Lunule
large, ovate, smooth, with a sharp edge. Escutcheon large, deep,
with a sharp edge. Ornamentation consists of about fourteen broad,
prominent concentric ribs, which are marked by fine concentric
lines. Length, 8·25; height, 7 mm.
Affinities.—In the character of its ornamentation A.
awensis resembles several small species of Astarte found
in the Cretaceous rocks. It is more inequilateral, more elongate,
and has the posterior margin more distinctly truncated than in
A. subsimilis, J. Böhm.[205] In similar respects it
differs from A. griesbachi, Woods,[206] from Pondoland.
Remarks.—Numerous casts of this species occur in
ferruginous nodules.
Description.—Shell nearly orbicular, convex, slightly
inequivalve; height and length nearly equal. Anterior and ventral
margins rounded; posterior margin truncated, forming an obtuse
angle with the straight or slightly convex postero-dorsal margin.
Umbones rather prominent. Ornamentation consists of regular,
concentric lamellar
ribs separated by broad, flat interspaces which are marked by fine,
concentric lines. Length, 11 mm.; height, 11·5 mm.
Affinities.—In general form, and in the character of
its ornamentation, this species resembles one of the varieties of
Lucina fallax, Forbes, described by Stoliczka[207] from the Utatúr Group, but the umbones are
more prominent, and the postero-dorsal angle is more distinct. It
also resembles L. saharica, Quaas,[208] and some forms of L. subnummismalis,
d’Orbigny.[209]
In the possession of numerous radial ribs and in the general
form of the shell, some internal casts resemble Cardium
cenomanense, d’Orbigny,[210] but the valves are less
convex.
Locality.—Awe.
Specimens of Corbula resemble C. elegans,
Sowerby,[211] in the character of their ornamentation, but
the posterior part of the shell is more produced.
Several specimens of a small gasteropod resemble
Pseudomelania Pallaryi, Peron and Fortau,[212] from the Cenomanian of Egypt, but they are of
much smaller size, being from 8 to 13 mm. long. The surface of the
shell is smooth, except for slightly curved growth-lines, and, on
the base, spiral grooves.
A species of Turritella resembles T. Choffati,
Thomas and Peron,[213] from the Turonian of Tunis. The anterior
part of each whorl is bent at an angle with the large flattened
posterior part, and the ornamentation consists of four strong
spiral ribs, one of which occurs at the angle, giving the
appearance of a carina. On the later whorls the ribs become
granular or tubercular. The ribs are much coarser than in T.
Choffati.
Description.—Shell subfusiform. Spire shorter than the
last whorl, consisting of about five whorls. Whorls carinate, the
carina being more or less distinctly nodular. Posterior part of the
whorls flattened, much larger than the anterior part in the spire.
Anterior part of last whorl convex. Ornamentation consists of
narrow spiral ribs with flat interspaces crossed by growth-lines;
on the later whorls the ribs become divided up into rows of
elongated tubercles.
Remarks.—This species, which is represented by several
imperfect specimens, shows some resemblance to Lagena
nodulosa, Stoliczka,[214] and L. secans,
Stoliczka,[215] from the Ariyalúr Group, and also to
Serrifusus dakotensis var. vancouverensis,
Whiteaves,[216] but possesses one carina only.
Description.—Shell inflated, rounded, subglobose.
Whorls thick, largely embracing; umbilicus deep, rather more than a
third of the
diameter of the shell, with nearly vertical walls, the umbilical
margin subcarinate, without tubercles. Flanks curving gradually
from the umbilical margin and not limited from the rounded
periphery. Surface of shell with growth-rings.
Sutures: Saddles large, broad, and low; external saddle
unsymmetrically divided and with small minor divisions; first
lateral smaller than the external saddle, more rounded, less
divided, with the outer margin sloping and the inner margin nearly
perpendicular; second lateral saddle broad, only slightly divided.
External lobe deeper than the first lateral lobe; the latter is
small with one main division; second lateral lobe smaller, with two
points.
(1)
(2)
Diameter
88
93 mm.
Thickness
58
54
Height of last whorl
37
34
Width of umbilicus
30
33
(1) Reme. (2) Kunini.
Affinities.—This species resembles the thicker forms of
V. durandi (Peron and Thomas),[217] but the walls of the umbilicus are steeper,
the umbilical margin is without tubercles, and the external and
first lateral saddles are broader and not so high. In general form,
and in the absence of tubercles at the umbilical margin, V.
nigeriensis resembles V. amierensis, Choffat,[218] but the whorls embrace to a larger extent, and
their sides are less flattened; there are also some differences in
the suture. In the form of the umbilicus, and in the breadth of the
saddles, this species resembles V. harttiformis,
Choffat,[219] but the shell is much less thick. Another
similar form but with a thicker shell, is V. hartti,
Hyatt,[220] from Brazil.
Description.—Shell moderately inflated. Umbilicus less
than a quarter of the diameter of the shell, with vertical walls
and subcarinate
margin. Flanks somewhat compressed. Periphery flattened. Inner
whorls with strong ribs on the flanks and prominent tubercles at
the umbilical margin and at the limit of the flanks and periphery;
on the last whorl the ribs and tubercles gradually become
obsolete.
Sutures: External saddle broad, subquadrate, with small
divisions; first lateral saddle broad, rounded, divided
unsymmetrically by a small lobe; second lateral saddle subquadrate;
one small auxiliary saddle on the flanks. External lobe deep and
larger than the first lateral lobe, which has only small divisions;
second lateral lobe small, its lower margin with pointed
divisions.
Diameter
108
mm.
Thickness
57
„
Height of last whorl
52
„
Width of umbilicus
24
„
Affinities.—This species resembles V.
subconciliatum, Choffat,[221] from the Turonian of
Portugal, but the ribs and tubercles become obsolete at an earlier
stage, the umbilicus is smaller, the divisions in the external
saddle are less deep, the first and second lateral saddles are more
rounded, and the first lateral lobe is narrower. V.
gongilensis also resembles some forms of V. cauvini,
Chudeau.[222]
Locality.—Gongila.
(Palæontographica, XXXIII., 1887, p.
229.)
Sub-genus—Pseudaspidoceras, Hyatt.
(“Pseudoceratites of the Cretaceous,” U. S.
Geol. Surv. Mon., XLIV., 1903, p. 106.)
In the collection there is one specimen of Mammites
(Pseudaspidoceras); it resembles the form figured by
Choffat[223] under the name Acanthoceras (?), cf.
Footeanus (Stol.), from the Turonian of Portugal, but
differs in having a smaller umbilicus and less distinct siphonal
tubercles. The sutures are not shown in the specimen from
Nigeria.
The Portuguese
form differs from A. footeanus, Stoliczka,[224] by the absence of secondary ribs on the
siphonal region of the inner whorls, and by the forward inclination
of the ribs between the lateral tubercles and the siphonal
tubercles.
Other allied species are M. (Pseudaspidoceras)
salmuriensis (Courtiller) var. byzacenica,
Pervinquière,[225] from the Lower Turonian of Tunis, and
M. (Pseudaspidoceras) Pedroanus (White),[226] from Brazil.
Locality.—Gongila.
(“Nachtrag Foss. Unter. Kreide Mungo in Kamerun,”
Abhandl. d. k. Gesellsch. d. Wiss. Göttingen, Math.-Phys. Kl.,
N.F. Band 1, 1898, p. 53. Emended F. Solger, Foss.
Mungokreide in Kamerun, 1904, p. 127; L. Pervinquière,
Études Pal. Tunis, I. Ceph., 1907, p. 215.)
Description.—Shell discoidal, compressed, with
truncated periphery. Whorls largely embracing, with the flanks
slightly or moderately convex, the greatest convexity being at a
short distance from the umbilical margin. Umbilicus small, or very
small, about one-seventh of the diameter of the shell, its sides
nearly perpendicular. Periphery usually rather broad, bounded on
each side by a prominent rounded carina; the prominence of the
siphuncle gives rise to a median rounded ridge between which and
the carinas the shell is concave.
In fully-grown individuals the shell is smooth, but in smaller
examples the flanks bear a few strong, rounded ribs, which curve
slightly forwards near the periphery; the larger ribs start from
the umbilical margin, and one or two smaller ribs may be
intercalated on the flanks. Where the ribs join the carina the
latter may become more or less distinctly tuberculate. Usually the
ribs become obsolete when the shell reaches a diameter of about 50
mm., but occasionally they persist on larger individuals.
Sutures: External saddle broad, rather low, with a variable
number of small divisions. First and second lateral saddles
similar to, but
distinctly smaller than the external saddle; each has usually some
small divisions, but in some cases these are absent, and the saddle
is then rounded. One or two auxiliary saddles may occur. External
lobe deep, sometimes nearly as wide as the space between the two
carinæ. First lateral lobe usually rather narrower and not so deep
as the external lobe, with more or less numerous small divisions.
The second lateral lobe is similar to, but smaller than the
last.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Diameter
110
95
84
67
68
62
57
44
mm.
Thickness
47
42
34
28
28
27
28
20
„
Height of last whorl
61
49
45
37
38
38
34
25
„
Width of umbilicus
15
15
10
10
7
6·5
8
6
„
(1–8) From Gongila.
Fig. 1.—Hoplitoides
nigeriensis, sp. nov. Gongila. Sutures of six specimens.
Natural size.
Affinities.—This species appears to belong to the
bicarinate group of Hoplitoides. In general form it
resembles H. munieri, Pervinquière,[227] from the Lower Turonian of Tunis, but the
whorls, are thicker, the flattened periphery is broader and shows a
distinct siphuncular ridge. The sutures are similar to those of
H. munieri, but are symmetrically placed, and the first
lateral lobe is relatively narrow. The ribbing of the smaller
specimens is similar to that of Hoplites ingens, v. Kœnen
em. Solger.[228]
In general form,
H. nigeriensis is similar to Heterotissotia
neoceratoides, Peron,[229] from the Lower Senonian
of Algeria and the Upper Turonian of Tunis. Both possess a broad,
flattened periphery with a keel on each side and a prominent
siphuncular ridge, but in H. neoceratoides the ribs are
retained to a later stage. The figures given of the suture of
Peron’s species are not satisfactory, and I am indebted to Dr. L.
Pervinquière for comparing figures of the sutures of Hoplitoides
nigeriensis with the type of Heterotissotia
neoceratoides which is now in the Sorbonne. Dr. Pervinquière
states that the differences in the suture are too great to allow of
these two species being referred to the same genus.
Remarks.—The sutures of Hoplitoides nigeriensis
vary considerably in appearance on account of the difference in the
number and size of the small divisions of the saddles. A somewhat
similar variation is seen in the sutures of H. ingens, v.
Kœnen, em. Solger, and H. munieri, Solger. Variations also
occur in the thickness of the whorls, the breadth of the periphery,
the size of the umbilicus, and the period at which the ribs become
obsolete. In one specimen, which is less thick than the other, the
siphuncular ridge is absent.
This species is represented by numerous specimens, all of which
are internal casts or have only a small portion of the shell
preserved, and in nearly all cases the sutures are clearly
exposed.
[196]The occurrence of Turonian fossils at this
locality has been recorded by G. C. Crick, Geogr. Journ.,
Vol. XXIV., 1904, p. 522, footnote 1.
[197]Dames, Sitzungsber. Ges. nat. Freunde,
Berlin, 1887, pp. 69, 137.
[198]Guillemain and Harbort, “Profil der
Kreideschicht. am Mungo,” Abhandl. d. k. preuss. geol.
Landesanst. N.F., Heft 62, 1909, p. 405.
[199]Stoliczka, Cret. Fauna S. India,
1871, p. 353, Pl. XVIII., Figs. 12, 14; Pl. XIX., Figs. 2, 3; Pl.
XX., Figs. 2, 8, 9, 10.
[200]Mon. Cret. Lamell. England, Vol.
II., 1904, p. 31, Pl. V., Figs. 16–20.
[201]Mon. Aptien de l’Espagne, 1865, p.
166, Pl. XXVI., Figs. 1, 2.
[202]For figures see Woods, Mon. Cret.
Lamell. England, Vol. II., 1905, p. 64, Pl. IX., Figs. 2–4.
Compare also d’Orbigny, Pal. Franç. Terr. Crét., Vol.
III., 1846, p. 478, Pl. CCCXCII., Figs. 1–3. When describing this
species I overlooked Guéranger’s statement that he had seen
specimens showing ligament pits; from the presence of those pits it
is evident that this species cannot belong to Pteria. It
seems probable that Pseudoptera should be regarded as a
sub-genus of Gervillia. Guéranger, Album Paléont. de la
Sarthe, 1867, pp. 17, 20, Pl. XXII., Figs. 9, 10; Pl. XXV.,
Figs. 10, 11.
[216]Mesozoic Fossils, Vol. I., 1879,
p. 119, Pl. XV., Fig. 5. Compare also Woods, Ann. S. African
Mus., Vol. IV., 1906, p. 324, Pl. XL. Figs. 5, 6. Newton,
Trans. Roy. Soc. S. Africa, Vol. I., 1909, p. 28, Pl. IX.,
Fig. 6.
[217]Moll. Foss. Crét. Tunisie, 1889,
p. 27, Pl. XVIII., Figs. 5–8. Peron, Amm. Crét. supér.
Algérie, 1896–7, p. 44, Pl. IV., Fig. 1; Pl. V., Fig. 1; Pl.
XVII., Fig. 5. Pervinquière, Paléont. Tunisienne, Ceph.
secondaires, 1907, p. 332, Pl. XXI., Fig. 1. V.
Douvillei, Choffat, Faune crét. Portugal, Vol. I.,
1898, p. 59, Pl. X., Fig. 6; Pl. XI., Figs. 2–5; Pl. XXI., Figs.
13–16.
[218]Choffat, ibid., p. 61, Pl. XII.,
Figs. 1, 2; Pl. XIII., Figs. 1, 2; Pl. XXI., Figs. 17–21.
[219]Ibid., p. 61, Pl. XII., Fig. 3;
Pl. XIII., Figs. 3–6; Pl. XXI., Figs. 22–24.
[220]“Contrib. Paleont. Brazil,” Archiv.
Mus. Nac. Rio de Janeiro, VII., 1887, p. 226, Pl. XIX., Figs.
1, 2; Pl. XX., Fig. 3. Hyatt, “Pseudoceratites of the Cretaceous,”
U. S. Geol. Survey, Mon., XLIV., 1903, p. 103, Pl. XIV.,
Fig. 16.
[221]Faune crét. Portugal, Vol. I.,
1898, p. 64, Pl. XV., Figs. 1–3; Pl. XVI., Fig. 4; Pl. XXII., Figs.
28–31.
[222]“Ammonites du Damergou (Sahara
meridional),” Bull. Soc. géol. de France (4), IX., 1909,
p. 68, Pls. I., II., III.
[223]Faune crét. Portugal, Vol. I.,
1898, p. 66, Pl. XVI., Figs. 9, 10; Pl. XXII., Figs. 3, 4.
[224]Palæont. Indica, Ceph. Cret. S.
India, 1864, p. 101, Pl. LII., Figs. 1, 2. Kossmat,
“Südindische Kreideformat,” Beitr. Pal. Geol. Osterr-ungarns u.
d. Orients, XI., 1897, p. 20. Petrascheck, “Amm. sächsich.
Kreideformat., ibid., XIV., 1902, p. 144. Pl. IX., Fig.
1.
[225]Études Paléont. Tunisienne, I. Ceph.
Second., 1907, p. 315, Pl. XIX., Fig. 1.
[226]“Contrib. Paleont. Brazil,” Archiv.
Mus. Nac. Rio de Janeiro, VII., 1887, p. 212, Pl. XXII., Figs.
1, 2.
[227]Études Pal. Tunis, I, Ceph., 1907,
p. 217, Fig. 83, Pl. X., Figs. 1, 2.
[228]Solger, Foss. Mungokreide in
Kamerun, 1904, p. 137, Pl. V., Figs. 8, 9.
[229]Amm. Crét. supér. de l’Algérie,
1897, p. 82, Pl. XVI., Figs. 9, 10; Pl. XVIII., Fig. 20.
Pervinquière, Études Pal. Tunis, I., Ceph., 1907, p. 379,
Pl. XXIII., Fig. 7.
[230]Specimens of Hoplitoides have also
been found at Reme, but are not sufficiently well-preserved for
specific identification. An imperfect Ammonite from Kumberi
resembles in the form of its shell and in the general character of
the sutures Coilopoceras lesseli, Brüggen, Neues Jahrb.
für Min., etc., Beil.-Band XXX., 1910, p. 734, Plate XXIX.
![[Logo]](/books/the-geology-and-geography-of-northern-nigeria/images/decor1.jpg)
