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.
[Footnote 28: Hubert, _Mission Scientifique au Dahomey_, 1908, p. 448.]
[Footnote 29: Chudeau, _Sahara Soudanais_, 1909, p. 18.]
[Footnote 30: The numbers in brackets refer to microscopic preparations
in the Imperial Institute Collection, NN. F.]
[Footnote 31: Parkinson, _Q. J. G. S._, Vol. LXIII, p. 315.]
[Footnote 32: _Mem. Geol. Sur. Scotland_, Expl. of Sheet 55, 1905, p.
13.]
[Footnote 33: Van Hise, _Treatise on Metamorphism_, 1904, p. 696.]
[Footnote 34: Parkinson, _Q. J. G. S._, Vol. LXIII, 1907, p. 308; Vol.
LXIV, 1908, p. 313.]
[Footnote 35: Chudeau, _Sah. Soud._, p. 2.]
[Footnote 36: Gentil in _Doc. Sc. Miss. Sah._, 1905, p. 721.]
[Footnote 37: Haug, _C. Rd. Ac. Sc._, 7 Aug., 1905.]
[Footnote 38: Haug in _Doc. Sc. Miss. Sah._, 1905, p. 753.]
[Footnote 39: Flamand, _C. Rd. Ac. Sc._, 140, p. 954.]
[Footnote 40: Flamand in Voinot, _Bull. Com. Afr. Fr._, 1908, p. 218.]
[Footnote 41: Hubert, _C. Rd. Ac. Sc._, 146, 3 Feb., 1908; _Miss. Sc. au
Dahomey_, 1908, p. 253.]
[Footnote 42: Chautard, _Le Fouta Djalon_, 1905.]
[Footnote 43: Parkinson, _Q. J. G. S._, 1908, LXIV, p. 313.]
[Footnote 44: Chevalier, _La G._, XVII, 1908, p. 201.]
[Footnote 45: Lenz, _G. M._, 1877, p. 27; Knox, _Geology of Africa_,
1905, p. 93.]
[Footnote 46: Von Ammon, _Geologie von Togo_, _M.H. Geog. Gessell.
Mün._, 1903, I, p. 393.]
[Footnote 47: Parkinson, _Q. J. G. S._, 1907, LXIII, p. 308.]
[Footnote 48: Courtet in Chevalier, _L’Afr. Cent. Fr._, 1908, p. 646.]
[Footnote 49: Passarge, _Adamaua_, 1895, p. 312; Esch, _Geol. von
Kameruns_, 1904, p. 23.]
[Footnote 50: Barratt, _Ann. des Mines_, VII, 1895.]
[Footnote 51: _Cf._ Horwood and Wade, _G. M._, V, 6, 1909, p. 455.]
[Footnote 52: “The Geology of the North-West Highlands of Scotland,”
_Mem. Geol. Sur._, 1907, pp. 75, 214, 595, 607.]
[Footnote 53: Voit, _Trans. Geol. Soc. S. A._, 1905, VIII, pp. 106,
141.]
[Footnote 54: _Cf._ Flett, “Geology of Land’s End District,” _Mem. Geol.
Sur. Eng._ 1907, p. 25.]
[Footnote 55: _Cf._ Gentil et Freydenberg, _Bull. Soc. Geol. Fr._, 4th
Ser., VIII, 1908, p. 44.]
[Footnote 56: Gentil, _C. Rd. Ac. Sc._, 8 Aug., 1904; Foureau and
Gentil, _C. Rd. Ac. Sc._, 2 Jan., 1905; Lacroix, _C. Rd. Ac. Sc._, 1
May, 1905; Gentil, _Doc. Sc. Miss. Sah._, 1905, p. 697.]
[Footnote 57: Chudeau, _Sah. Soud._, 1909, p. 265; _C. Rd. Ac. Sc._ 1
July, 1907.]
[Footnote 58: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 59: Hubert, _C. Rd. Ac. Sc._, 145, 1907, p. 764.]
[Footnote 60: Freydenberg, _Chad et Shari_, 1908, p. 107; Gentil et
Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII, 1908, p. 44.]
[Footnote 61: Lacroix, _C. Rd. Ac. Sc._, 150, 1910, p. 18.]
[Footnote 62: Gentil, in Foureau’s _Miss. Sahar._, 1905, p. 720.]
[Footnote 63: Chudeau, _Sah. Soud._, 1909, p. 256.]
[Footnote 64: Passarge, _Adamawa_, 1895, p. 382.]
[Footnote 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.]
[Footnote 66: Hubert, _C. Rd. Ac. Sc._, 1904 (139), p. 378.]
[Footnote 67: Chudeau, _Sah. Soud._, 1909, p. 267.]
[Footnote 68: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 69: Horwood and Wade, _Geol. Mag._ V, 6, 1909, p. 455.]
CHAPTER IV.
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.
_Muri Province._
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
[Illustration: HILLOCKS OF DRIFT ON THE BAUCHI PLATEAU NEAR VOM.]
[Illustration: 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.
_Yola Province._ (A. L.)
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.)
_Bauchi and Bornu._
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 | |Sandstones, grits, clays,
grits and | |and ironstones of Waja,
sandstones| |Ture, Gulani and Wuyo.
| |
Limestones| Turonian limestones and |Turonian limestones of the
| shales of the Benue valley |Gongola valley.
| |
|{ Passage beds (white |
|{ sandstone and carbonaceous |
|{ shales at Awe: calcareous |
Lower |{ grits and green calcareous |
grits and |{ marls at Bomanda). |
sandstones|{ |
|{ Muri sandstones (upper part|Bima sandstones and grits.
|{ salt-bearing). |
|{ |
|{ 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.
[Footnote 70: See Chudeau, _Sah. Soud._, 1909, pp. 47, 11, for pre-
Cretaceous rocks of the Sahara.]
[Footnote 71: See Appendix II, _Palæontology_, p. 273.]
[Footnote 72: De Lapparent, _C. Rd. Ac. Sc._, 135, 1903, p. 1298;
Chudeau, _Sah. Soud._, 1909, p. 89.]
[Footnote 73: De Lapparent, _C. Rd. Ac. Sc._, 132, 1901, p. 388.]
[Footnote 74: Chudeau, _Sah. Soud._, 1909, p. 91.]
[Footnote 75: _Ibid._, p. 76.]
[Footnote 76: Parkinson, “The Geology of the Oban Hills,” _Q. J. G. S._,
63, 1907, p. 314.]
[Footnote 77: Esch, _Geologie von Kamerun_, 1904, p. 4.]
CHAPTER V
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.
1. _The Sokoto and Lokoja series._
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
[Illustration: A GRANITE MOUNT NEAR BAUCHI.]
[Illustration: 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.
(2). _The Duguri, Gombe, and Kerri Kerri sandstones._
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.
3. _The Upper Benue sandstones._ (A. L.)
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
[Illustration: THE SANDSTONE HILLS OF TURE.]
[Illustration: 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.)
[Footnote 78: _Cf._ Suess, _The Face of the Earth_, Vol. IV, Oxford
Trans., 1909, p. 89.]
[Footnote 79: Chudeau, _Sah. Soud._, 1909, p. 104.]
[Footnote 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.)]
[Footnote 81: Fossiliferous limestone has, however, been reported from
Yelu at the confluence of the Dallul Mauri and the Niger valley.]
[Footnote 82: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 384.]
[Footnote 83: Chudeau, _Sah. Soud._, 1909, p. 274.]
[Footnote 84: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 112.]
[Footnote 85: Chudeau, _op. cit._, p. 275.]
[Footnote 86: _Cf._ Gurich, _Zeit. Deut. Geol. Gesell._, XXXIX, 1887, p.
122.]
[Footnote 87: Barth, _Central Africa_, 1857, II, p. 368.]
[Footnote 88: See Section IV on margin of coloured map.]
[Footnote 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.]
[Footnote 90: Passarge, _Adamaua_, 1895, p. 363.; Edlinger, _Niger-Tsad
Expedition_, 1904, p. 154.]
[Footnote 91: _Cf._ Passarge, _Adamaua_, 1895, p. 390. Passarge did not
recognise the presence of Cretaceous rocks in the Upper Benue valley.]
[Footnote 92: Chudeau, _Sah. Soud._, 1909, p. 97.]
[Footnote 93: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 399.]
[Footnote 94: Passarge, _Adamaua_, 1895, p. 363.]
[Footnote 95: Esch, _Geologie von Kamerun_, 1904, p. 11.]
[Footnote 96: Parkinson, “The post-Cretaceous Stratigraphy of S.
Nigeria,” _Q. J. G. S._, 63, 1907, p. 309.]
[Footnote 97: Parkinson, _op. cit._]
[Footnote 98: Chudeau, _Sah. Soud._, 1909, p. 85; and map.]
[Footnote 99: Chudeau, _Sah. Soud._, 1909, p. 81 _et seq._]
[Footnote 100: Barth, _Central Africa_, 1857, Vol. I, p. 531.]
[Footnote 101: Courtet, _C. Rd. Ac. Sc._, 140, 1905, p. 162.]
CHAPTER VI
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.
1. _Hausaland and Borgu._
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_.