assume their characteristic lustrous carapace, and the removal of the
vegetation and of the surface soil can be accomplished only by a
climatic change. Mr. Campbell[131] in Upper Guinea, and Mr. Maufe[132]
in East Africa, would assign the deforestation very largely to the
influence of man, and the subsequent removal of the soil to the agency
of the tropical rains.[133] It is difficult to believe, however, that
the characteristic ironstone platforms of Hausaland can possess such a
comparatively recent origin, and there can be little doubt that the
highest platforms of which traces still remain should be referred to at
least the later Tertiary period. Once formed, the hard resistant crust
may be covered by later alluvial or æolian accumulations or by the
products of disintegration and re-arrangement of the ironstone itself,
upon the return of more humid conditions. Wherever covered by later
deposits, however, the ironstone is softer and lacks its characteristic
varnish. Moreover, where a scattered vegetation is now found creeping
over a surface of lustrous ironstone, it is frequently a difficult
matter to decide how much of the scanty soil is a recent accumulation,
and how much of it represents the original covering of alluvium which
had not been entirely removed.
PLATE XIV
[Illustration: DOZI HILL, KERRI-KERRI.]
[Illustration: A TRENCH-LIKE VALLEY NEAR LELE, IN KERRI-KERRI.]
The formation of the surface ironstones of the plains of Hausaland is
thus by no means readily explicable. Their origin is probably due, in
part, to many different causes, and their fragmental occurrence at
various levels, sometimes upon weathered rock, sometimes upon
unweathered rock, and sometimes upon drifted alluvium, points to
repeated formation and repeated erosion at several different periods. It
is believed that, if thoroughly investigated, their distribution and the
distribution of the surface drift would afford some clue to those
repeated climatic and crustal changes to which within comparatively
recent times the surface of the central Sudan has been subjected. The
evidence already detailed appears to be sufficient to establish the
following succession of events upon the plains of Hausaland:—
(1) A period of minimum erosion during which the superficial rocks were
deeply decomposed to a variable depth and covered with a ferruginous
cap.
(2) A period of maximum erosion during which the earlier sheet of
decomposed rock was largely removed, with the exception of such relics
as Mounts Dala and Kogon Dutsi.
(3) A period of minimum erosion and accumulation during which the
surface drift was spread out upon the plains.
(4) The present period of erosion of the drift during which, in places,
successive platforms have been cut in the drift and floored with
concretionary ironstone.
The cause of the alternation of these periods of accumulation and
erosion is probably to be sought in crustal changes of a regional
character. If a condition of low relief be granted to begin with, a
general depression of the surface to the neighbourhood of base level may
be postulated during the periods of accumulation, and a corresponding
elevation of the surface above base level during the periods of erosion.
The various periods indicated above may therefore be termed
respectively:—
(1) The first period of depression.
(2) The first period of elevation.
(3) The second period of depression.
(4) The second period of elevation.
The history of the earlier periods is somewhat obscure and it is
possible that there may have been many minor oscillations of the crust
during the first period of depression and the first period of elevation.
It is possible also that the first period of depression may have been
represented merely by a general base levelling of the surface through
erosion, and that the original sheet of weathered rock may have been
formed in part underneath sedimentary rocks which were afterwards
removed. The flat-topped hillocks of consolidated granitic grit and clay
between Kano and Kazaure may perhaps represent contemporaneous
accumulations during the first period of erosion, which were themselves
subsequently exposed to denudation. There is some reason to believe,
also, that there has been within the various periods a minor oscillation
in climatic conditions. These minor oscillations would naturally be most
clearly recognisable within the second period of elevation which is
continuous with and includes the present. There is little doubt that the
formation of the surface ironstones was favoured by the successive
occurrence of periods of maximum evaporation, and it is highly probable
that these periods assumed at times a typically desert character, and
that the partially eroded surface of the drift was covered up at
intervals by æolian accumulations. One such period at least immediately
preceded the present more humid conditions, and is clearly recognisable
in the worn and rounded dunes of the dead _erg_ of northern Hausaland.
The dunes are now grassed over and covered with scattered trees, but
their origin is still sufficiently indicated in their composition and
general outlines as well as in the characteristic banking of the sand
against rocky exposures. The surface soil and the recent alluvium of the
plains of northern Hausaland probably contain much wind-blown material,
and the swampy character of much of the surface of the plains during the
rainy season is probably to be in part explained as the result of the
interference of the accumulations of this recent arid period with the
earlier system of drainage. At the same time, however, it should be
remembered that the erosion of the drift has in most places not yet
become very intense and that the average slope of the plains of
Hausaland is as a rule so gentle that a considerable volume of water
must accumulate before a perceptible flow can be established. Much
swampland is found on the high plains around and to the east of Zaria,
where Dr. Baikie[134] first recognised the drifted character of the
alluvium.
2. _Bornu and Katagum._
A striking feature of the plains of Bornu and Katagum and of the Gongola
valley between Nafada and the Yola border is the absence of any
superficial crust of vesicular ironstone. The drift which may reach in
Bornu a depth of more than 300 feet differs considerably in composition
from the characteristic drift of Hausaland. Upon the open plains of
Bornu the drift is composed for the most part of a mixture of white or
yellow sand and light or dark grey clay. A red ferruginous staining of
the drift is comparatively rare to the east and north-east of Gujba. The
clays in places yield irregular concretions of kankar and small nodules
of concretionary ironstone are dug from the surface sands in the
neighbourhood of Kukawa, but no evidence has been anywhere obtained of
the formation or earlier existence of any continuous sheet of vesicular
ironstone. Well sections exhibit below the surface alternations of sand
and clay in beds of very varying thickness.[135] The sand varies much in
coarseness of grain and is in part angular and of drifted fluviatile
origin and in part well rounded and of æolian origin, the latter
apparently predominating largely towards the north-east. The clays are
finely divided and flocculent in water and mixed with minute grains of
blown sand. In places they become darker coloured from the inclusion of
organic matter and pass into the “firki” (p. 52) or so-called black
cotton soil of Chad and the lower Shari. Where exposed at the surface,
the clay becomes plastic and very tenacious in the rains, but hard and
brittle and much cracked and fissured in the dry season. These clays are
rich in azote which contributes much to their fertility.[136] This
advantage, however, is counterbalanced by the almost complete absence of
lime, potash, magnesia, and phosphoric acid, and by the presence of
large quantities of silicate of alumina and of salts of soda. Fragments
of broken shells of land snails and lacustrine molluscs are common in
the clays of eastern Bornu, while thin beds of calcareous marl[137] with
abundant remains of recent molluscs are intercalated among the sands and
clays of Mongonu and Kukawa. It is evident that the conditions of
accumulation of the drift in Bornu were different from those under which
the drift of Hausaland was accumulated. The latter was preeminently a
terrestrial or sub-aerial accumulation while the former in the finer-
grained character of the sand, the abundance of clay and the absence of
pebbles and of any intense ferruginous staining affords indications of
having been deposited to a great extent under water in a shallow
lacustrine or swampy region. It is probable, however, that the
accumulation of the drift of Bornu was, at least in the earlier stages,
contemporaneous with the accumulation of the drift of the plains of
Hausaland and may therefore be referred to a considerable extent to the
second period of depression. No traces are left, however, upon the
plains of Bornu of the earlier periods of depression and elevation, and
it may therefore be presumed that the surface upon which the drift was
deposited was entirely cleared of earlier accumulations during the first
period of erosion and that its minor irregularities have been completely
obscured by the later drift. The second period of elevation which
brought about the erosion of the drift in Hausaland appears to have
resulted in comparatively little erosive action upon the drift of Bornu.
It is highly probable indeed that Bornu retained its swampy and
lacustrine and therefore receptive character to a comparatively recent
period and that the oscillations of less and more humid conditions which
probably prevailed during the accumulation and erosion of the drift of
Hausaland are reflected in Bornu in the intermingling of æolian and
fluviatile and lacustrine material. The local abundance of alkaline
salts in the drift of Bornu may also be taken as indicating an
occasional and local desiccation.
The presence of swampy and lacustrine conditions in Bornu dates,
therefore, from the second period of depression and there is no reason
to believe that there has been any marked break in continuity between
these later Tertiary swamps and the present Lake Chad. There is,
however, no evidence that the present lake or the former more extensive
swamps in any way represent the shrunken relics of the early Tertiary
seas. The later swamps indeed, with which the lake is more or less
continuous in point of time, appear never to have reached any great
depth and to have been liable to repeated desiccation. Desert periods
may have changed them partially or wholly into sand fields, but the
return of humid conditions constantly re-established their former
character.[138] It may be presumed also that during periods of
exceptional rainfall the swamps became united into a shallow lake of
considerable extent which may even at times have covered the greater
part of central and Eastern Bornu. There is no reason to believe,
however, that this was to any extent a normal condition of things, or
that the drift was primarily accumulated in a large permanent lake. So
far as known, marly beds occur in Bornu only in the neighbourhood of the
present lake, and whatever the extent of the marginal swamps may have
been, the deepest and clearest water seems always to have been confined
to eastern Bornu. The thinness and local character of the marls would
seem to imply, however, that the requisite conditions for their
formation were by no means common or universal. Captain Freydenberg has
described in sufficient detail the former extension of the swamps to the
north, east, and south of Chad.
The character and thickness of the drift deposits appear to indicate
that Bornu has in the past formed part of an area of predominant
depression. The shelly marl of Kukawa which lies at least fifty feet
above the present level of Chad, undoubtedly indicates that the floor of
the lake was at one time at a higher level than it is at present. The
long-continued accumulation was probably conditioned at first by the
second period of depression and later by the gradual uprise of the Congo
watershed during the latest period of elevation. In the neighbourhood of
Kukawa, however, the low hillocks of drift show evident signs of
erosion, and it may therefore be presumed that the present floor of the
lake has been excavated out of the earlier lacustrine and drift
deposits. This erosion which M. Garde[139] would presumably ascribe
entirely to æolian activity, was probably in great part conditioned by
the continued relative subsidence of the Bodele depression, the
consequent eastward draining of the lacustrine region and the
establishment of the Bahr el Ghazal as a waterway from west to east. The
lower relative level of Bodele has now been definitely established, and
M. Chudeau’s[140] arguments in favour of the eastward flow of the Bahr
el Ghazal may be provisionally accepted until fuller information has
confirmed or contradicted them. Desert conditions, however, followed
close upon erosion. The worn and rounded hillocks of drift around Kukawa
were partially buried in blown sand and the Bahr el Ghazal was blocked
by the action of the wind. On the establishment of the present
conditions the lake as we now know it was formed and limited towards the
west by the line of sand dunes, now covered with abundant vegetation,
which stretches from Kukawa to the river Yo along the margin of the
lake. There is no reason, however, to postulate the presence of an
earlier lake actually on the site of the present Chad at a period
immediately anterior to the coming of the desert.
It may be presumed that during the period of erosion which preceded the
advent of the desert, shallow watercourses were cut in the drift of
Bornu and that they were in part dismembered and filled up again during
the succeeding period of desiccation. One such watercourse may be
clearly traced from Murguba to Maigumeri in central Bornu. The present
channels of the Yo, the Maidugari river, the Yedseram, and the Shari
were probably also established at the same period. The cuvettes of Manga
probably originated at the same epoch and represent portions of earlier
watercourses detached and isolated by æolian accumulations. The sites of
the cuvettes may possibly have been determined in part by the presence
of springs in the earlier river beds. While, therefore, there is little
reason to believe with Captain Freydenberg[141] that the bottoms of the
cuvettes represent detached portions of the original floor of an
extensive lake almost entirely filled up with æolian accumulations,
there is equally little reason to believe with M. Chudeau[142] that the
cuvettes have arisen through the local subsidence of the underlying
rocks brought about by the dissolution of beds of soluble salts,
hypothetically contained within the sandstones of the Tegama below. M.
Chudeau’s objection that an approximately level surface such as that of
the plains of Bornu cannot be produced under the influence of the wind
is clearly untenable in view of the known occurrence of extensive level
sandfields in modern deserts. Moreover, in Katagum solitary dunes or
isolated hummocks of blown sand formed during the recent arid period may
be found alone in the midst of an open undulating plain. There is some
reason also to believe that the winds which never attained any great
intensity varied in direction in different parts of the Protectorate. In
Bornu the dunes are elongated in a N.–S. direction while in Katagum they
are more generally E.–W. and it is possible that a successive variation
in the direction of the winds over the same area may have been
responsible for much of the levelling of the present surface of the
plain.
The lacustrine conditions which prevailed over central and eastern Bornu
during the deposition of the drift extended also for a considerable
distance westward through Katagum. The drift of Katagum is very similar
in character to that of Bornu and the abundance in it of wind-blown
material, as in the drift of Bornu, bears witness to the constant
struggle between humid and desert conditions during the period of its
accumulation. The predominance of the latter at a period immediately
preceding the present is indicated as in Bornu by worn and rounded dunes
and banks of sand. There is every reason to believe that Bornu and
Katagum have been for long on the margin of the desert and that the
history of the repeated advance and retreat of the desert can be clearly
read in the varied character and composition of the drift.
3. _The Gongola Valley._
To the west of Gujba, thick accumulations of red sandy loam effectually
obscure the irregularities of the topography of eastern Kerri Kerri. The
drift has evidently been largely derived from the disintegration of the
red sandstones and ironstones of Kerri Kerri, but the surface of the
original deposits has been in great part rearranged by subsequent æolian
action. Upon the plateau itself the æolian accumulations reach in places
a very considerable thickness. In the valley of the middle Gongola to
the south of Kerri Kerri there is as in Bornu and Katagum, a remarkable
absence of surface ironstone. Around Nafada and southward upon the
narrow plain to the west of the Gongola the drift is predominantly a red
sandy loam. To the east of Ashaka, however, the drift consists very
largely of a stiff tenacious black or dark grey clay, very similar in
appearance to the “firki” of eastern Bornu. This material also covers
large areas upon the central plain of Bauchi to the south and south-east
of Deba Habe. It is clear from the manner in which the red loam and the
dark coloured clay obscure the irregularities of the surface and creep
up the flanks of the sandstone hills that the superficial accumulations
are here also, as on the plains of Hausaland, of drifted or travelled
origin and not primarily of the nature of material weathered and
disintegrated in situ. The local origin of the drift may however be
presumed. The red sandy loam of the northern and western plains has been
derived very largely from the disintegration of the Kerri Kerri and
Gombe sandstones, while the clays of the east and south have probably
arisen in great part from the decomposition of the lavas of the Burra
plateau. The original depth of the drift is unknown, but must have been
very considerable. The Gongola in places has not yet succeeded in
cutting its way through the drift to the rock below, while the highest
portion of the plain which is still buried in drift lies about 300 feet
above the present river bed. The total thickness, however, may well have
been much greater and there is every reason to believe that what is now
the valley of the middle Gongola between Nafada and Gasi formed, along
with that portion of the central plain of Bauchi which lies between Deba
Habe and Putu, an area of special accumulation throughout the second
period of depression and that the activity of the river and its
tributaries during the most recent period of elevation has been directed
mainly towards the removal of the drift which had accumulated in the
earlier valley.
4. _The Benue Valley._
The alternation of the major periods of accumulation and of erosion as
established for the plains of Hausaland was probably to some extent of a
regional character, although the relative amount of respective
depression or elevation may have varied much in different parts of the
Protectorate. The irregular movements to which the Benue valley as a
whole was subjected during the latest period of elevation renders the
correlation of the superficial deposits with those of the northern
States a matter of considerable difficulty. There is good reason to
believe, however, that the lower Benue valley and the greater part of
the provinces of Kabba and Ilorin were exposed during the same period to
very intense erosion, in consequence of which the original drift
deposits have been almost entirely removed and the underlying Eocene
sandstones deeply trenched and worn. The middle Benue, on the other
hand, appears to have been long a region of relative depression and
accumulation, and although platforms of surface ironstone may be found
in places even on the plains of Muri,[143] the requisite conditions for
its formation appear to have been for the most part wanting. It is
possible that its absence may be due, as in Bornu and the Gongola
valley, to a prolonged water-logged condition of the subsoil. Wherever,
on the other hand, the surface of a hillock of red sandstone was exposed
above the drift, there seems to have been a rearrangement of the
ferruginous material and a concentration of the iron ores in the upper
part which has given rise upon exposure to a typical honeycombed
limonitic cap. The ferruginous crusts of the flat-topped hills and
plateaux of the lower Benue and the Niger have a somewhat similar origin
and have arisen in part through the rearrangement and deposition in the
subsoil of the original iron of the sandstones and oolites and in part,
like the ferruginous conglomerate on the summit of Mount Patti, through
the cementation of travelled material of a similar composition. Where
the summits are bare and lustrous it may be presumed, as in the case of
the ironstone platforms of Hausaland, that the surface soil, originally
overlying, has been removed in the course of denudation. At Awe, on the
middle Benue, a sheet of sintery calcareous material appears to have
been deposited upon the upturned edges of the Cretaceous before the
accumulation of the drift.
PLATE XV
[Illustration: CRETACEOUS SANDSTONES AND SHALES AT AWE.]
[Illustration: A FLAT-TOPPED HILLOCK OF DRIFT ABOVE NARAGUTA.]
Attention has been already directed (pp. 27, 77) to the occurrence in
Kabba, Nassarawa, and Southern Zaria of belts of felspathic muscovite
magnetite schists whose deeply decomposed crusts support a luxuriant
vegetation. Where partially eroded, it may be observed that the iron
ores have a tendency to accumulate in the subsoil, and it is possible
that if the overlying soil and vegetation were removed, the subsoil
might harden into a typical vesicular ironstone. It is difficult to
decide, however, to what extent the decomposed crusts may be considered
the result of weathering under present conditions. It seems probable in
view of the absence of any deep decomposition of the adjoining rocks
that they may to a large extent represent the irregular base of the
earlier sheet of decomposed rock to which the presence of Mounts Dala
and Kogon Dutsi bears witness upon the plains of Hausaland and which may
possibly have extended at the same time over the remainder of the
Protectorate. In some places, moreover, as at Kanna, there is evidence
that the crystalline floor had been locally and deeply decomposed as the
result of differential weathering underneath the Eocene sandstones. A
similar occurrence has been noted by M. Hubert[144] at Youri in the
Niger Valley.
Little consolidated accumulations of late Tertiary age, frequently
provided with a ferruginous cap like the drift of Northern Nigeria, have
been described under the designation of “Benin Sands” by Mr.
Parkinson[145] from Southern Nigeria, of “Ossa Sandstones” by Dr.
Esch[146] from the Kameruns, of “terre de barre” by M. Hubert[147] from
Dahomey, and of _reg_ from the Northern Sahara by numerous French
authorities.[148] It is yet premature, however, to attempt any
correlation of these deposits with the drift of Northern Nigeria.
[Footnote 102: Chudeau, _Sah. Soud._, 1909, p. 97.]
[Footnote 103: De Lapparent, _C. Rd. Ac. Sc._, 26 Dec., 1904.]
[Footnote 104: Chudeau, _op. cit._, p. 273.]
[Footnote 105: See Chamberlain and Salisbury’s _Geology_, 1905, p. 174.]
[Footnote 106: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 102.]
[Footnote 107: _Cf._ Lenz, _G. M._, 1879, p. 172.]
[Footnote 108: _Cf._ Passarge, Rumpffläche und Inselberge (Kordofan-
type), _Zeit. Deut. Geol. Gesell._, 56, 1904, p. 193.]
[Footnote 109: Andrews and Bailey, _Q. J. G. S._, Vol. LXVI, 1910, p.
228.]
[Footnote 110: Walther, _Das Gesetz der Wüstenbildung_, 1900.]
[Footnote 111: _Cf._ Chudeau, _Sah. Soud._, 1909, p. 285.]
[Footnote 112: Rohlfs evidently mistook the surface ironstones of Bauchi
and Zaria for rocks of sedimentary origin. _Pet. Mitt. Erg._, VII, 1872,
p. 64.]
[Footnote 113: Hubert, _Miss. Sci. au Dahomey._, p. 160, Plate VIII.]
[Footnote 114: Hubert, _Miss. Sci. au Dahomey_, 1908, pp. 109, 123.]
[Footnote 115: Passarge, _Adamaua_, 1895, p. 400.]
[Footnote 116: _Cf._ also Mennell, _G. M._, VI, 1909, p. 350.]
[Footnote 117: Chautard, _Le Fouta-Djallon_, 1905, p. 126.]
[Footnote 118: Hatch and Corstorphine, _The Geology of South Africa_,
2nd Ed., 1909, p. 330. _Cf._ also Hatch, _Mineral Resources of Natal_,
1910, p. 73; Rogers and Du Toit, _Geology of Cape Colony_, 1909, p.
390.]
[Footnote 119: Freydenberg, _Tchad et Chari_, 1908, p. 21.]
[Footnote 120: Lacoin, _Bull. Soc. Geol. Fr._, 1903, p. 484. _Cf._
Foureau, _Miss. Sah._, p. 674 _et seq._]
[Footnote 121: De Lapparent, _Traité de Géologie_, 4th Ed., p. 333.]
[Footnote 122: Walther, _Das Gesetz der Wüstenbildung_, 1900, p. 19.]
[Footnote 123: Chautard, _Le Fouta Djallon_, 1905, p. 137; _C. Rd. Ac.
Sc._, 146, p. 239.]
[Footnote 124: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 199.]
[Footnote 125: Holland, _Geol. Mag._, X, 1903, p. 59.]
[Footnote 126: Passarge, _Adamaua_, 1895, p. 396.]
[Footnote 127: Maufe, _The Geology of East Africa_, Col. Reps.
(Miscell.), No. 45, 1908, p. 53.]
[Footnote 128: Maclaren, _Geol. Mag._, 3, 1906, p. 536.]
[Footnote 129: Mennell, _Geol. Mag._, 6, 1909, p. 350.]
[Footnote 130: Campbell, _Tr. Inst. M.M._ 19, 1910, p. 432.]
[Footnote 131: Campbell, _op. cit._, p. 209.]
[Footnote 132: Maufe, _op. cit._, p. 208.]
[Footnote 133: _Cf._ also Mennell, _op. cit._, p. 209.]
[Footnote 134: Baikie, _A Journey from Bida to Kano_, 1866.]
[Footnote 135: _Cf._ Freydenberg, _Tchad et Chari_, 1908, p. 1.]
[Footnote 136: Courtet, _C. Rd. Ac. Sc._, 140, p. 163.]
[Footnote 137: _Cf._ Courtet, _C. Rd. Ac. Sc._, 140, p. 160; Garde, _C.
Rd. Ac. Sc._, 148, pp. 1616, 1698.]
[Footnote 138: Freydenberg, _Tchad et Chari_, 1908, p. 63.]
[Footnote 139: Garde, _C. Rd. Ac. Sc._, 148, pp. 1698, 1616.]
[Footnote 140: Chudeau, _Sah. Soud._, 1909, p. 232.]
[Footnote 141: Freydenberg, _Tchad et Chari_, 1908, p. 61.]
[Footnote 142: Chudeau, _Sah. Soud._, p. 84.]
[Footnote 143: Passarge, _Adamaua_, 1895, p. 400.]
[Footnote 144: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 368.]
[Footnote 145: Parkinson, _Q. J. G. S._, 63, 1907, p. 309.]
[Footnote 146: Esch, _Geol. von Kamerun_, 1904, p. 14.]
[Footnote 147: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 219.]
[Footnote 148: See Gautier, _Sahara Algerien_, 1908.]
CHAPTER VII
TERTIARY CRUSTAL MOVEMENTS AND DRAINAGE CHANGES
Post-Cretaceous erosion in the Sudan; the Middle Eocene depression;
western and eastern basins of sedimentation; crustal oscillations of the
Middle Tertiary period; later Tertiary oscillations; the first period of
depression; the first period of elevation; early drainage system of the
Protectorate; the second period of depression; lacustrine conditions in
Bornu; the second period of elevation; flexuring of the crust; α- and
β-flexures; differential elevation of the Nassarawa tableland and the
Bauchi plateau; deflection of the Gongola; depression of the Chad area.
Peculiarities of the watersheds and river valleys of the Protectorate:
explanation of the characteristic features of the valleys of the Niger,
Benue, and Gongola; the origin of inselberge; M. Hubert’s explanation of
the hydrography of Dahomey; the period of formation of the “dallols” of
Sokoto; correlation of the Tertiary crustal movements in the Sudan with
those of the Mediterranean area.
The close of the Cretaceous was marked by regional upheaval, accompanied
by fracturing and folding of the crust and by subsequent intense
erosion. The Cretaceous sandstones, shales, and limestones, which may be
presumed to have at one time covered the greater part of the
Protectorate, were largely removed from the underlying hummocky and
irregular surface of crystalline rocks. The plateau of southern Yola, as
described by Mr. Longbottom (p. 32), and the eastern cliffs of the
central tableland of Kabba (p. 16), would appear to have originated at
this period. The final character of the post-Cretaceous drainage system
and of the early Eocene land surface is, however, very imperfectly
known. Mr. Longbottom believes that there were earlier river valleys in
the place of the Upper Benue and Lower Gongola of the present day (p.
190), and there is some reason to believe that there was also an earlier
river in the present valley of the Lower Niger. At all events it is in
these localities that the greatest thickness of Eocene sediments is now
preserved, although there is little doubt that the later crustal
movements have also, to some extent, affected their present
distribution.
The movement of depression in the Middle Eocene which resulted in the
establishment of open sea in the north-west and in the deposition of the
marine beds of Sokoto province, brought about also the gradual
submergence of the land surface and the predominance of shallow water
conditions over the greater part of the Protectorate. In the earlier
river valleys the greatest thickness of sediments would naturally be
deposited, while only the ferruginous sandstones and ironstones of the
latest lacustrine phase would be deposited over the higher lying
portions of the early Eocene land surface. The formation of the lava
fields of Bauchi and Burra was probably contemporaneous with the latest
phase of sedimentation. There is some reason to believe, moreover, that
during the greater part of the period occupied by the movements of
depression, there were two major regions or basins of sedimentation, a
western and an eastern, separated by a stretch of higher ground which
extended from Zinder and the northern frontier of Hausaland to the
southern border of the Benue valley, and of which the Murchison Range
formed the only striking physical feature. The thinning out of the
Eocene sandstones to the east of the Niger valley, on the middle Benue,
and to the west of the Gongola, lends some support to this hypothesis.
The accumulations within the western basin, which was probably
continuous westward with the Atlantic, would then include the lacustrine
series of Southern Nigeria, the shallow water deposits of the middle
Niger, the marine limestones of Sokoto and the sandstones of Dahomey and
the Futa Djallon, while the Duguri, Gombe, and Kerri Kerri sandstones,
the Upper Benue sandstones and their prolongation into the northern
Kameruns, together with the ferruginous sandstones of the Upper Shari
and the Congo, would represent the accumulations within the eastern
basin. The outliers of Eocene upon the Kanna hills and the relics of
ferruginous sandstones upon the plains of Muri indicate, however, that
the western and eastern basins became finally united, and it seems
probable that towards the close of Eocene sedimentation, shallow water
and lacustrine conditions prevailed over the greater part of the
southern Sudan. Free communication may even have been established at
this time over Central Africa,[149] between the western seas of the
Upper Niger and the eastern seas of Lybia and Egypt. There is no reason
to believe, however, that the lower marine facies of Sokoto ever
extended eastward much beyond its present limits.
There can be little doubt that the close of the Eocene was marked by the
cessation of the predominant movement of depression and the beginning of
those crustal oscillations which culminated in late Pliocene or early
Pleistocene times in the formation of the Bauchi plateau and the
establishment of the present river system. No deposits of later
recognisable age than Eocene have been discovered within the
Protectorate and it may therefore be presumed that until the close of
the Miocene, when the sea is believed to have finally retired from the
Sahara (p. 194), the surface of the Protectorate remained near or at the
level of the sea. The predominant movement, however, was probably that
of elevation, with the result that the uppermost members of the Eocene
suffered some considerable erosion and were entirely removed from the
greater part of the surface of the central provinces. The later Tertiary
oscillations, which may be conveniently grouped as Pliocene, began with
a negative or downward movement, which has already been termed the
“first period of depression” (p. 213), and during which the crystalline
rocks of the central provinces were deeply decomposed and probably also
covered with a layer of drift. The “first period of elevation” which
followed was marked by a somewhat irregular upheaval of the surface of
the Protectorate. The maximum movement appears to have been attained in
the east, where the Eocene and underlying rocks were elevated in a low
extended arch whose axis formed the primary watershed and ran
approximately from Eastern Yola to Zinder and Air. As a result of the
erosion which followed the movement of elevation the greater part of the
sheet of decomposed rock formed during the first period of depression
was removed from the surface of the central plain. The crystalline areas
assumed very largely their present physical features and characteristic
scenery. The plains of Hausaland, however, were continuous with the
plains of Nassarawa and Muri across the site of the Bauchi plateau and
the Nassarawa tableland. The Murchison Range, the Wadai Hills, and the
Ningi and Kagoro massifs remained then as now relics of a former
erosion, and beyond acting as secondary watersheds exerted comparatively
little influence upon the general system of drainage. The relics of
higher beds occasionally found on the sandstone plateaux of the Niger
may, however, be referred to this period of erosion. The larger rivers
probably drained south-westward into Southern Nigeria and Dahomey across
the present valley of the lower Benue and the Niger. The gorges of the
Niger above Lokoja and between Jebba and Yelwa clearly indicate the much
more recent origin of the present river valley. In the east, however,
along the watershed the sedimentary rocks were deeply trenched and worn
on either side by the headwaters of the rivers. The valleys of the
middle Gongola and of the Upper Benue present when compared with the
Niger Valley all the characteristics of a more ancient topography. It
seems probable indeed that towards the close of erosion the rivers which
drained westward over the plains of Bauchi had already established the
escarpments of Kerri Kerri, the narrow plains of the middle Gongola, and
the cliffs of Waja and Tangale. An earlier river, moreover, flowed
westward from the hills of Yola with its valley limited as now by the
highlands of Wurkum and Waja, which for a time interposed a continuous
barrier between the rivers of Bauchi and the headwaters of the Benue.
The establishment of the gap at Kombo was probably accomplished by a
tributary of the Maio Haul towards the close of erosion. To the east of
the watershed the plains of eastern and central Bornu were likewise
established at this period by rivers which probably drained eastward and
southward over the site of Chad and the Shari valley into the Congo
basin.
There followed, however, the “second period of depression,” during which
the drift was accumulated upon the irregular surface of the plain. The
maximum movement would appear to have taken place in the east in the
region of the former elevation. It seems probable that not only was the
earlier watershed depressed to such an extent that the lateral valleys
in Bauchi and Bornu were filled with drift to a depth of 400 to 500
feet, but that lacustrine conditions prevailed farther east over the
plains of Bornu and Chad and the present valleys of the Logone and the
Shari. Another area of special depression was apparently situated in
southern Zaria and western Bauchi, where the remains of a covering of
drift 200 to 300 feet in thickness forms a series of flat-topped hills
on the highest portion of the plateau. The older Benue gravels, as
described by Passarge[150] and Edlinger,[151] were probably also
accumulated at this period. The drainage in the earlier stages may be
presumed to have been, in general, towards the areas of maximum
depression, while the close of the period was marked by the prevalence
of conditions of accumulation and of uncertain drainage over the whole
surface. The inland seas of the Upper Niger, as described by MM.
Chevalier,[152] Gautier and Chudeau,[153] should probably also be
referred to this period, but it should be remembered that, although they
may have been at times connected with the open sea to the west, there
is, as in the case of Lake Chad (p. 217), no reason to postulate any
kind of continuity between them and the earlier Miocene or Eocene
seas.[154]
Radical changes ensued upon the cessation of the movement of depression
and the return of a positive movement of elevation in late Pliocene or
early Pleistocene times. It should be remembered, however, that the
irregular upheaval of the surface during the “second period of
elevation” and the consequent changes of drainage were in no sense
catastrophic. On the contrary, the movements and changes were gradual,
and possibly to some extent successive and spread over a considerable
interval of time. The regional elevation of the Protectorate, with the
exception of the middle Benue valley, to an average height of 1,000 feet
was accompanied and complicated by a gentle flexuring of the crust into
low arches and shallow troughs along axes which ran approximately
W.S.W.–E.N.E. and N.N.W.–S.S.E. (see map, p. 236). The two sets of
flexures may be termed respectively the α-flexures and the β-flexures,
and it may be presumed that they were produced for the most part
simultaneously and only to a small extent successively by crustal
movements, which were themselves contemporaneously functional. The
irregular elevation and depression of the surface which gave rise to the
present system of drainage may therefore be looked upon as the result of
the interference of the two sets of flexures. Two axial arches of the
α-series, a northern (α₁) and a southern (α₂), are readily
distinguishable within the Protectorate, the southern being the more
important and of the greater elevation. The northern (α₁) runs from
Borgu over the high plains of Kontagora, Sokoto, and Katsina towards
Machina and Zinder, and forms the watershed between the Gulbin Kebbi and
its tributaries on the one hand, and the Kaduna and the Yo on the other.
These rivers may be presumed to occupy the shallow troughs on either
side of the axial ridge. The axis of the southern arch (α₂) runs from
central Kabba through Nassarawa and southern Bauchi into northern Yola,
and the Benue may be presumed to occupy the trough to the south. The two
α-arches are not strictly parallel, but diverge somewhat towards the
east. Of the β-flexures two axial arches are also distinguishable within
the Protectorate, a central (β₁) and a western (β₂), the latter being
the more important of the two. The central arch (β₁) is of a very
partial character, and is recognisable only in the sinuous watershed
which runs northward between the two α-arches from the summit of the
plateau towards the Sokoto-Kano frontier and separates the headwaters of
the Kaduna from those of the Yo. The western arch (β₂) forms, in part,
the western watershed of the Niger, and runs northward from the high
plains to the south of Ilorin approximately along the frontier of the
Protectorate to the neighbourhood of Nikki in Dahomey.
The arches throughout are asymmetrical in character, and vary somewhat
in elevation from point to point. The sinuous character of the crests is
probably due to the mutual influence of the complementary forces which
produced the flexuring at right angles. The intensity of the flexuring
appears also to have varied, and in the case of two intersecting
flexures one or other usually predominates. In the central region, for
example, the α-arches clearly predominate at their intersection with the
β-arch (β₁), while in the west the β-arch (β₂) has evidently been the
determining factor in the establishment of the present course of the
Niger. The interference of the β-arch (β₂) with the western extension of
the α-flexures is clearly reflected in the repeated bending of the
river. Where the α-arches were crossed by the β-arch, dome-shaped
elevations of a regional character were formed, whose effect may still
be traced in the westwardly concave curves which the Niger describes at
Yelwa and above Lokoja. Where the intervening α-trough, however, was
crossed by the β-arch, the resulting curve was concave eastward as at
Jebba. The asymmetrical position of the Niger valley indicates a shorter
radius for the western portion of the β-arch, while the fact that the
river was able to pursue a continuous course across the α-arches and
troughs makes it sufficiently obvious that the β-arch (β₂) must have
reached originally a somewhat greater elevation than the western portion
of the α-arches. The influence of the formation of the western β-trough,
now occupied by the Niger valley, may also be traced in the southward
deflection of the lower courses of the Kebbi, Kaduna and Gurara. It is
obvious also from the height of the sandstone plateaux above Lokoja that
the greatest and perhaps most prolonged movement of elevation in the
Niger valley took place in the region of the confluence of the Niger and
the Benue. This movement, moreover, was evidently sufficiently slow to
allow of the Niger cutting a continuous trench across the rising arch, a
result, however, which was rendered possible only by the presence of
sedimentary rocks in the river bed. There is no reason to believe with
Gurich[155] that, during the formation of their trench-like valleys in
the region of the confluence, the middle courses of the Niger and the
Benue were ever converted into a series of detached basins or inland
seas. It should be remembered also that the relative elevation of the
arches was for the most part insufficient to produce any marked
divergence from the apparent horizontality of the sandstones in the
Niger valley.
[Illustration: NORTHERN NIGERIA
Axes of Elevation.
MAP No. 3.
London. Macmillan & Co., Ltd.
_Face p._ 236.]
The symmetrical elevation of the southern α-arch and the southward
prolongation of the central β-arch were modified and interrupted by the
formation of lines of weakness longitudinally along the former and in
the region of the intersection of the two arches. The relative influence
of the forces which produced the flexures can be traced in the varying
direction and sinuous character of the lines of weakness. Along these
lines differential movements took place which resulted in the
disproportionate elevation of the Nassarawa tableland and of the Bauchi
plateau with reference to the middle Benue valley and the northern
plains of Bauchi. On the north the fracturing of the crust, as already
described (p. 39), was confined only to the central region, while to the
west and east the tableland and the plateau merged respectively into the
general slope of the arch. On the south, however, a continuous line of
fracture extended from western Nassarawa by way of Darroro and Assab to
Sura and Angass in southern Bauchi. There is no reason to believe that
the southern α-arch as a whole ever reached the elevation of the Bauchi
plateau or that the plateau originated in any sense as a “horst,”[156]
separated from the adjoining tableland and the plains of Nassarawa and
Muri by the later differential subsidence of the latter. On the
contrary, the movement appears to have been in one direction only, and
characterised by the differential elevation of the plateau and the
tableland and the lagging behind of the lower plains of Nassarawa, Muri
and northern Bauchi. The trough of the Benue above the confluence of the
Niger affords clear evidence indeed both of the comparatively stationary
character of the middle Benue valley and of the gradual relative
elevation of the Lokoja sandstones in the region of the confluence.
In the east the prolongation of the northern α-arch through Manga and
the region to the north of Chad extends beyond the limits of the
Protectorate. The southern arch, on the other hand, is prolonged as a
simple flexure of varying elevation through eastern Bauchi into Yola
province. In the trough between the northern and southern arches, the
river Yo occupies the same relative position to the east of the central
β-arch as the Kaduna does to the west. The presence in the east,
however, of the former watershed of the first period of elevation has
introduced complications into the otherwise simple relations of the
α-axes and troughs. It may be presumed that on the formation of the
southern α-axis and the differential elevation of the Bauchi plateau,
the river Gongola, like the Shidya and the Delime, would have followed a
general north-easterly course and ultimately joined the Yo but for the
intervention of the higher ground along the earlier watershed which
diverted the river to the south. It should be remembered that the whole
of this region had been deeply trenched and eroded during the first
period of elevation (p. 231) and that the earlier valleys had been
largely filled with drift during the second period of depression. It
seems probable, therefore, that on the subsequent elevation of the
southern α-arch the Gongola on leaving the high plains of Bauchi was
able to make its way for long distances through the earlier river
valleys and stream courses and ultimately to escape into the Benue
valley through the gap at Kombo. The trench of the Gongola above Gombe
may possibly be to some extent the product of recent erosion, but there
is little doubt that the plain of the Gongola below Nafada was excavated
at an earlier period and is in no way to be regarded as the result of
the activity of the present river, whose energy has been almost entirely
directed to the removal of the drift, which formerly obscured the
hummocky surface of the plain.
The prolongation of the southern α-axis into eastern Bauchi appears to
have been characterised by the formation of a flexure of a flattened
monoclinal type, which resulted in the elevation of the plains of
eastern Bauchi and the middle Gongola to a height of 500 feet above the
valley of the Benue. The actual summit of the flexure is probably
situated in the vicinity of the rapids of Kombo. The hydrography of
northern Yola is very imperfectly known and complicated by the presence
of the earlier watershed. It is probable, however, that the present
watershed between the Benue and Lake Chad on the eastern frontier
corresponds approximately with the prolongation of the α-axis, which
here resumes its normal character of an asymmetrical arch. The last
trace of the flexure may be recognised to the east of the Mandara hills
in the swampy watershed between the Benue and the Logone. The
configuration of southern Yola suggests an ancient topography, and its
physical character is probably very similar to that which prevailed at
the close of the first period of elevation. The Chad area to the north
remained a region of relative depression during all the later flexures,
and it seems probable that accumulation continued and the escape
southward of the surplus waters remained possible until the final
elevation of the Congo watershed along a parallel flexure to the south.
The contemporaneous formation of the Bodele depression, however, brought
about the establishment of the effluent of Chad through the Bahr el
Ghazal and the partial erosion of the drift and lake deposits of Bornu.
The subsequent advent of arid conditions during the period immediately
preceding the present resulted, as already indicated (p. 219), in the
diminution of the water supply, the closure of the Bahr el Ghazal, and
the restriction of the lake to its present limits.
This attempted reconstruction of the more recent hydrographical changes
affords an explanation of many of the more striking peculiarities in the
physical geography of the Protectorate. Attention has already been
directed to the fact that the larger tributaries of the Niger and the
Benue offer throughout their courses a reversal of the normal sequence
of mountain tract, valley tract and plain tract. The Kebbi, the Kaduna,
and the Gongola rise upon lofty plains and flow for long distances in
broad sandy channels bounded by imperceptible slopes until they enter,
in their lower courses, well-defined trench-like valleys bounded on
either hand by steep walls of rock. The watersheds are marked by no
ranges of jagged hills. On the contrary, the lofty plains on which the
rivers rise are frequently more deeply buried in drift than the flood
plains of their lower courses. The rivers flow peacefully around and
between the lofty granite domes which rise abruptly from the upper
plains, and even such larger groups of hills as those of Ningi form
merely local centres of dispersion for the smaller streams. Moreover,
the hydrographical centre of the Protectorate is not a rugged mountain
mass, but an open treeless plain, a plateau 4,000 feet in height,
bounded by steep cliffs on the south and west and provided with worn and
flattened hillocks of drifted alluvium perched in places upon the very
margin of the cliffs. A complete reorganisation and rejuvenation of the
drainage system is clearly indicated, and it is possible only in some
such manner as that already described. The crust has been flexured and
fractured during a recent regional elevation, and from the crests of the
arches the consequent streams have been thrown off to pick a course for
themselves over the drift-covered plains and between the solitary hills
and finally to unite to form the major rivers in the bottoms of the
shallow troughs. Dr. Passarge[157] has frequently called attention to
the youthful age of the rivers of Africa, and his suggestion would
appear to be amply confirmed by the whole character of the present
river-system of Nigeria.
The sinuous course of the Niger within the Protectorate still represents
in a general way the original adaptation of the drainage system to the
tectonic conditions. The diversity of scenery in the present valley is,
however, largely due to the influence of the lithological character of
the underlying rocks. There is little doubt that at first the Niger
flowed throughout upon a sandstone surface, relics of which now lie from
500–1,000 feet above its present level, and that it eroded its channel
originally as a continuous, deep and narrow trench. Complications first
ensued when the river succeeded in cutting through the overlying
sandstones between Yelwa and Jebba, and in reaching the crystalline
rocks below. The greater thickness of sandstones below Jebba conditioned
the cutting back of the gorge between Jebba and Yelwa with which the
river is still occupied. The resistant character of the crystalline
rocks brought about the formation of falls and rapids, while the
consequent lateral erosion at the head of the gorge gave rise to the
broad sandy plains between Yelwa and Ilo. Below Jebba the prolonged
elevation of the southern α-arch brought about the continued trenching
of the sandstones above Lokoja and the lateral erosion of the banks in
Nupe and Ilorin, where the level of the river remained stationary for
considerable intervals of time. The irregular degradation of the channel
of the Niger conditioned also the formation of the broad, flat-bottomed
valleys (dallols) of Sokoto and the narrow canyon valleys of southern
Kontagora, the dissection of the sandstone plateaux of Nupe, the removal
of the Eocene rocks from northern Kabba and the cutting of the lower
gorge of the Gurara. Everything, indeed, points to the recent erosion of
the Niger valley, while the rapids and falls of the Kaduna and the
Gurara beyond the limit of the sandstones and the ill-defined valleys of
their upper and middle courses clearly indicate that they have only just
begun to cut their way backwards and downwards into the central
crystalline plains.
Compared with the Niger valley, the plains of the middle and upper Benue
present all the characteristics of a more ancient topography, and it is
probable that they retain to a great extent the configuration which they
possessed at the close of the first period of erosion. Only in its lower
course has the Benue cut a trench in the Eocene sandstones at all
comparable with the gorge of the Niger above Lokoja, and this has
probably been accomplished, as already explained (p. 238), during the
slow differential elevation of the region of the confluence. There seems
reason to believe, also, that during the earlier stages of the more
recent erosion the Benue was provided with a greater volume of water
than the Niger, and that to this influence is to be ascribed the greater
imperfection of the trench of the lower Benue and the removal of the
sandstones from the hummocky crystalline plain of eastern Kabba. This
increase of volume, however, was probably only relative and
contemporaneous with the diminution of the Niger during the recent arid
period in the Sudan. The return of more humid conditions, however,
together with the capture of the upper Niger by a tributary of the
Taffassasset, as described by M. Chudeau,[158] restored the activity of
the Niger and enabled it to cut its present channel in the surface of
the crystalline plains above and below Lokoja.
The northern tributaries of the Benue, with the exception of the
Gongola, take their rise on the southern border of the Nassarawa
tableland and the Bauchi plateau. Their headwaters alone, however,
exhibit evidence of any recent intense erosive activity. Behind Jagindi
and in the neighbourhood of Assab the headwaters of the Modu river have
trenched and worn the margin of the plateau. The Wase river has laid
bare in Yergum and Angass the hummocky and irregular slopes which
separate the upper from the lower plains. The headwaters of the Kudu
river have cut picturesque steep-sided valleys in the rocks of Duguri,
and narrow canyon-like trenches in the central plains of Bauchi. In the
Gongola valley, on the other hand, the only traces of recent erosion are
to be found in the rapids of Baddera and Kombo and, in part, in the
imperfect trench which the river has cut in the sandstones between Gombe
and Deu. From its source on the plateau until it enters the plains to
the east of Gombe, the topography of the river valley recalls in every
detail the characteristic features of the valleys of the Kaduna and the
Gurara. Nevertheless, although of similar age and origin, the Gongola
flows from Nafada southward through a valley which presents all the
features of a more intense erosion than the Kaduna or the Gurara or even
the Niger itself has been able to accomplish in its lower course. It is
difficult to escape the conclusion that upon the rearrangement of the
drainage system and the establishment of the present upper Gongola, the
river entered to the east of Gombe, an older river valley which had been
excavated during some former period of erosion. The interlacing valleys
of the margin of the Kerri Kerri plateau may possibly have arisen during
the latest period of erosion, but if so, the streams which excavated
them have never regained the activity which they possessed before the
last appearance of the desert in the Sudan.
PLATE XVI
[Illustration: UPPER BENUE SANDSTONES AND CONGLOMERATES UPON A HUMMOCKY
GRANITIC FLOOR.]
The isolated domes and kopjes (Plates III., XII.) and detached groups of
hills which form the most striking feature of the higher plains are
characteristic also of all the older gneissic areas of Africa, and the
question of their origin has given rise to considerable difference of
opinion. The particular type of scenery to which they give rise has been
termed by German writers the “Inselberglandschaft” (Plate XVII., Figs.
1, 2). Passarge[159] sees in them the product of arid conditions, and
concludes that the surfaces which they characterise assumed their
present character for the most part in early Mesozoic times when the
greater part of Africa was a barren desert. Bornhardt,[160] on the other
hand, believes that the inselberge have arisen as the result of the
superposition of several cycles of erosion. There is, however, no reason
to believe that this peculiar topography cannot arise simply through an
alternation of periods of weathering and periods of erosion, brought
about either by a gentle oscillation of the crust or by a repeated base
levelling of the plains and rejuvenation of the drainage system. A plane
surface of granite and gneiss subjected to long-continued weathering at
base level would be decomposed to unequal depths, mainly according to
the composition and texture of the various rocks. When elevation and
erosion ensued, the weathered crust would be removed, and an irregular
surface would be produced from which the more resistant rocks would
project. Those rocks which had offered the greatest resistance to
chemical weathering beneath the surface would upon exposure naturally
assume that configuration of surface which afforded the least scope for
the activity of the agents of denudation. In this way would arise the
characteristic domes and turtle-backs which suffer further denudation
only through insolation and exfoliation.[161] Their general elliptical
outlines, which Merrill[162] would ascribe very largely to the influence
of crustal stress and strain, are probably in great part due simply to
the modification by weathering of original phacolitic intrusions. A
slight subsequent movement of depression would suffice to cover with
drift large areas of the irregular surface between the domes and to
produce the characteristic topography of isolated hills rising abruptly
from the level surface of the plain (Passarge’s Kordofan Type). That
such indeed has been the mode of formation of the kopjes of Hausaland
can be demonstrated within the walls of Kano. Kogon Dutsi, the larger of
the two flat-topped hills of diorite, although deeply decomposed, still
preserves in its lower part detached boulders or cores of unweathered
rock. If the subsequent erosion had continued until the weathered
material had been entirely removed, the flattened hill would have been
replaced by a typical kopje of loose boulders resting upon a smooth and
rounded surface of rock below. On the plains around ample evidence can
be found of the levelling of the surface by means of the accumulation of
drift. A repetition of these conditions of formation would give rise to
an accentuation of the earlier domes and kopjes and the formation of
others of lower level. A similar origin to that of the domes may be
ascribed also to the detached groups of hills and even to the larger
granite massifs like those of the Ningi, Ruruma, Kagoro, and Vere Hills.
There is no reason to believe with Passarge[163] that the latter partake
in any way of the nature of “horsts,” which have assumed their present
position through the differential subsidence of the surrounding plains.
Their flattened though hummocky summits evidently bear witness to an
earlier regional level of erosion, but their characteristic mode of
occurrence is to be ascribed not to marginal faulting, but to a
succession of periods of decomposition and accumulation and periods of
erosion which has brought about the relative lowering of the surrounding
and less resistant gneisses and schists.
The pre-Cretaceous land surface, as exposed around Tangalto, was marked
like the present with domes and inselberge. The pre-Eocene surface was
somewhat broken and irregular, but probably also characterised, as in
the Niger valley below Lokoja, by detached and solitary hills. There is
no reason, however, to suppose that the actual pre-Cretaceous
configuration of the plains of Hausaland is still retained. It is much
more probable that domes and solitary hills have been formed in the
manner indicated at many different periods, and are a characteristic
product of sub-aerial tropical weathering of a crystalline surface when
exposed to oscillations of level.
Dr. Passarge,[164] in explanation of the structure of Adamawa, has
assumed the existence of two tectonic directions, an E.–W. or Benue
line, and a S.S.W.–N.N.E. or Kamerun line, along both of which extensive
faulting has taken place at various periods and in such a manner as to
leave a numerous series of isolated hills and ranges of hills standing
out as horsts upon the lower plains. According to him, also, the Bagele
and other hills of Yola have assumed their present position through the
subsidence of the surrounding plains, while the Benue valley between
Yola and Garua is of tectonic origin, and bounded by faulted margins to
the north and south. While, however, there is some reason to believe
that the Cretaceous rocks of Yola have been faulted down against the
crystallines to the north and south, and that in pre-Eocene times an
earlier river occupied a tectonic valley in the region of the upper
Benue (p. 190), there is no evidence to show that there has been a
recurrence of faulting on the upper Benue in post-Eocene times, or that
the slopes of any of the hills of “Upper Benue Sandstone” have been
defined by later Tertiary or recent movements. Passarge’s hypothesis is
applicable only to pre-Eocene topography, and cannot be extended in
explanation of those features of the upper Benue valley which are of
more recent origin.
M. Hubert[165] has suggested in explanation of the hydrography of
Dahomey that the courses of the present streams bear a direct
relationship to the strike of the gneisses and schists, and that the
surface of the colony has remained stationary since the close of the
Eocene. His hypothesis, however, is admittedly inadequate to explain the
transverse character of the Niger valley. In Nigeria, moreover, all the
principal rivers without exception flow across the strike of the
gneisses, while the drift-covered character of the lofty plains and of
the watersheds and the nature of the whole hydrographical system clearly
indicate the recent elevation of the present surface of the
Protectorate. It is very probable indeed that Dahomey has participated
in all the recent crustal movements which have affected Nigeria, and
that the central watershed of the colony corresponds with the westward
prolongation of the northern α-arch and the western β-arch of the
Protectorate. A recent rejuvenation of the drainage system is
postulated, moreover, by M. Chudeau[166] in the French Sudan, by Mr.
Parkinson[167] in Southern Nigeria, and by Dr. Esch[168] in the
Kameruns, and it is improbable that Dahomey should have escaped the
general movement of elevation.
M. Chudeau[169] has recognised in the Sudan the recent period of arid
conditions of which traces are still to be found within Northern
Nigeria. He believes, however, that the dallols were excavated during
the period of desert conditions by rivers which drew their supplies from
the mountains of Air. It is possible that there may have been repeated
alternations of desert and humid conditions within the latest period of
erosion; but it is clear in Nigeria that the dallols of Sokoto were
already excavated before the advent of the latest arid period, and that
the rivers which formed them have not regained the activity which
formerly characterised them. Capt. Meynier’s suggestion, quoted by M.
Chudeau, that the rivers of Hausaland turned their backs to the desert,
though picturesque, is little in harmony with the facts. The courses
they follow within the Protectorate were defined long before the advent
of the desert, and not by any adaptation to climatic conditions, but by
the direct influence of the early Pleistocene crustal movements which
brought about the rejuvenation of the drainage system of Nigeria and the
formation of the Bauchi plateau. These movements were probably also
responsible for the establishment of the dallols of Tahoua and the
quaternary wadis of the northern Sahara, as well as for the elevation of
the plateaux of Air and Adr’ar’ des Ifor’as. These movements, moreover,
were probably contemporaneous with the later Tertiary movements in the
Mediterranean basin and in East Africa.
M. Chudeau[170] has pointed out the analogy between the pre-Devonian and
the post-Carboniferous movements to the south of the Atlas and the
Caledonian and hercynian movements to the north of the Alps. In Nigeria
and the Sudan some analogy may perhaps be permitted between the general
meridional strike and vertical folding of the gneisses and schists and
the Caledonian folding of Scotland and Scandinavia. Of hercynian
movements, however, there is little or no trace in the Sudan, while the
presence of an unconformability at the base of the Eocene and the
absence of any later folding makes the post-Cretaceous and early
Tertiary movements in the Sudan more nearly parallel with the sequence
of events in northern Europe than with the sequence in the Alps and the
Atlas. The period of maximum movement in the Alps would appear to be
represented in Nigeria and the Sudan only by minor post-Eocene
oscillations of the crust, while the major movements in the Sudan which
gave birth to the present drainage system appear to be contemporaneous
with the later Mediterranean stages of the Alpine movements.[171] At the
same time, however, it should be emphasised that, while the movements
may have been contemporaneous, there was probably little or no genetic
connection between them. The Tertiary period was everywhere a period of
great instability of the crust, and the movements would appear to have
varied both in intensity and in kind, and to have been localised now in
one place and now in another, at different periods of their history.
[Footnote 149: _Cf._ De Lapparent, _C. Rd. Ac. Sc._, 136, p. 1118.]
[Footnote 150: Passarge, _Adamaua_, 1895, p. 385.]
[Footnote 151: Edlinger, _Niger-Tsad Expedition_, 1904, p. 157.]
[Footnote 152: Chevalier, _C. Rd. Ac. Sc._, 15 April, 1901, p. 926.]
[Footnote 153: Gautier and Chudeau, _Sah. Soud._, 1909.]
[Footnote 154: _Cf._ Suess, _The Face of the Earth_, Vol. IV., Oxf.
Trans., p. 92.]
[Footnote 155: Gurich, _Zeit. Deut. Geol. Gesell._, xxxix, 1887, p.
126.]
[Footnote 156: _Cf._ Passarge, _Adamaua_, 1895, p. 377.]
[Footnote 157: Passarge, _Die Kalahari_, 1904, p. 637.]
[Footnote 158: Chudeau, _Sah. Soud._, 1909, p. 222.]
[Footnote 159: Passarge, _Rumpffläche und Inselberge_, _Z. D. G. G._,
56, 1904, p. 193.]
[Footnote 160: Bornhardt, _Deutsch Ost Afrika_, Bd. VII, 1900, p. 37.]
[Footnote 161: _Cf._ Harrison, J. B., _Handbook of British Guiana_, p.
87.]
[Footnote 162: Merrill, _Rocks and Soils_, 1897, p. 245.]
[Footnote 163: Passarge, _Adamaua_, 1895, p. 377. _Cf._ also Suess, _The
Face of the Earth_, Vol. IV, Oxford Trans., 1909, p. 283.]
[Footnote 164: Passarge, _Adamaua_, 1895, p. 387. See also Suess, _The
Face of the Earth_, Vol. IV, 1909, p. 280.]
[Footnote 165: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 443.]
[Footnote 166: Chudeau, _Sah. Soud._, 1909, p. 220.]
[Footnote 167: Parkinson, _G. J._, XXIX., 1907, p. 56.]
[Footnote 168: Esch, _Geologie von Kamerun_, 1904.]
[Footnote 169: Chudeau, _Sah. Soud._, 1909, p. 220.]
[Footnote 170: Chudeau, _Sah. Soud._, 1909, p. 1.]
[Footnote 171: Suess, _The Face of the Earth_, Vol. I., Oxf. Trans.,
1904, p. 279.]
CHAPTER VIII
TERTIARY VOLCANIC ACTION
Absence of pre-Tertiary volcanic rocks within the Protectorate: two
periods of Tertiary volcanic activity. The early Tertiary period: the
Maradu vent; the nepheline basalts of Wase and Tangalto; the phonolites
of Wase Rock, Tangale Peak and Bantaji; the plateau lavas of southern
Bornu, Bauchi and Nassarawa; petrographical character of the lavas;
early Tertiary lavas of the Sahara, Sudan and Adamawa. The later
Tertiary volcanic period: the puys of northern Yola, the middle Benue
valley and the Bauchi plateau; petrographical character of the lavas;
petrographical provinces.
It may be presumed that the intrusion of the alkali granites and dyke
rocks throughout the Protectorate was accompanied in pre-Cretaceous
times by considerable superficial volcanic activity (p. 142). M.
Chautard[172] has, moreover, described the intercalation of volcanic
rocks in the Upper Cretaceous of Senegal. All trace of pre-Tertiary
effusive rocks, however, within the Protectorate appears to have been
removed in the course of denudation. The fracturing and folding of the
crust at the close of the Cretaceous was apparently accompanied in
places by the formation of mineral veins within the fissures, but no
contemporaneous igneous activity has been definitely identified within
the Protectorate (p. 140). Within the Tertiary era itself two separate
periods of volcanic activity may be recognised, one in the middle Eocene
and probably to some extent contemporaneous with the accumulation of the
Eocene sandstones within the Protectorate, and one in late Pliocene or
early Pleistocene times and contemporaneous with the later Tertiary
oscillations of the crust. For the most part the eruptions were of an
isolated character and gave rise to the formation of detached pipes,
cones and puys of igneous material. More rarely, as in southern Bornu
and on the borders of Bauchi and Nassarawa, the extruded lavas united to
form a continuous sheet of volcanic rock, which covered a considerable
area of the underlying sedimentary and crystalline rocks.
The earliest outburst of volcanic activity appears to have taken place
towards the close of the post-Cretaceous erosion or possibly in the
middle Eocene at a period contemporaneous with the accumulation of the
lower marine beds of Sokoto, and is represented now by the extensive
neck or vent of fragmental material near Maradu in Sokoto province whose
levelling was accomplished during the later extension of the middle
Eocene sea. The greater number of the boulders in the agglomerate are
composed of a microgranitic quartz porphyry with phenocrysts of
orthoclase, biotite and brown hornblende in addition to quartz (1323).
To the same period is probably also to be referred the line of well-worn
stumps of basalt, phonolite and trachyte which stretches from the
Gongola valley westward through Tangale and Angass to Namu in northern
Muri. The Tangale Peak (Pl. XIII, Fig. 2) and the Wase Rock (Pl. V, Fig.
2), which still retain a portion of the original flow attached to the
rock in the vent, were probably among the latest eruptions of the
series. The intrusive dolerites of Bashar (41) and Awe (1014) may be
ascribed to the same phase of igneous activity, while the Rock of
Bantaji in Southern Muri may be taken to represent contemporaneous
volcanic action to the south of the Benue.
In petrographical character the basaltic pipe rocks are always olivine-
bearing and usually porphyritic with olivine and augite. Felspar is
found only in the ground-mass which at times contains a little
intersertal glassy base. The basalt of Wase Topa is remarkable for the
number and size of the nodules of olivine which it encloses. A worn vent
immediately south of the town of Wase is filled with a fine-grained
nepheline basalt (32) containing glomeroporphyritic aggregates of augite
crystals and well-formed crystals of olivine. Felspar is entirely
wanting, the ground-mass being composed of small rods of augite and
granules of magnetite set in a brown glassy base studded with minute
crystals of nepheline. A somewhat similar but coarser-grained rock from
Tangalto (1661) contains large crystals of decomposed olivine and purple
and well-zoned augite in a holocrystalline ground-mass of rods and
granules of augite, magnetite grains, flakes of biotite and abundant
nepheline in well-formed and irregular crystals. The nepheline was
evidently the last to crystallise, and to some extent takes the place of
matrix, being in part moulded upon the other constituents of the ground-
mass.
The phonolite of the Wase Rock (31) is of a trachytoid type, containing
columnar phenocrysts of sanidine, ragged crystals of ægirine augite and
scattered pseudomorphs after nepheline in a ground-mass consisting
predominantly of minute felspar prisms with a well-marked fluxional
arrangement, mingled with magnetite grains and rods and granules of
green pyroxene. The phonolite of Tangale Peak (1665–6) contains
phenocrysts of sanidine and soda pyroxene in a ground-mass of a more
orthophyric type, composed of minute idiomorphic crystals of nepheline,
imperfect prisms of ægirine and ægirine augite and elongated microlites
of felspar which pass into an ill-defined felspathic matrix. Traces of
brown hornblende in irregular porous growths may also be observed in the
ground-mass. A similar rock from the immediate neighbourhood of Tangalto
(1663) contains soda pyroxene, but no evident nepheline in the ground-
mass, and may be grouped provisionally as a trachyte. The phonolite of
Bantaji Rock is of a more nephelinitoid type, with the ground-mass
enclosed in a somewhat intersertal manner between the abundant but much
decomposed phenocrysts of felspar. With the felspar are associated as
phenocrysts hypidiomorphic crystals of ægirine and ægirine augite
frequently in zonal growths and smaller idiomorphic and abundant
crystals of somewhat weathered nepheline. The ground-mass is composed of
lath-shaped felspars, granular pyroxene and irregular patches of
analcime probably of secondary origin. The association of phonolite and
nepheline basalt, observed in the neighbourhood of Wase and the Peak,
has not been recognised around Bantaji.
The period of formation of the lava fields in southern Bornu and on the
borders of Bauchi and Nassarawa may be provisionally referred to the
close of the middle Eocene, on the strength of the apparent
intercalation of a friable red sandstone amongst the lavas in the
escarpment of the Barbur plateau behind Dukshi. The extensive erosion
which the plateau lavas themselves have undergone indicates, moreover,
that they are of no very recent age. The eastern lava field is the more
extensive of the two and perhaps somewhat younger, so far as one can
judge from the less denuded character of the cones. The rich content in
iron of the upper part of the Eocene accumulations may possibly have
been conditioned to a considerable extent by the distribution of much
fragmental basic igneous material upon the surface of the waters.
In petrographical character the lavas are basaltic throughout and rich
in olivine, and vary from felspathic doleritic types to limburgitic
varieties in which felspar is greatly reduced in amount.[173] Nepheline-
bearing types have nowhere been recognised, nor any effusive rocks more
acid than basalt. MM. Chudeau and Gentil[174] have, however, indicated
the occurrence of ægirine rhyolites and phonolites, as well as of
basalts, tephrites and limburgites, amongst the older lavas of Ahaggar
and Air, which are probably of similar age to those of Nigeria. Dr.
Passarge[175] has described the occurrence of basalt, augite andesite
and phonolite on the summit of the Shebshi hills on the borders of
Nigeria and the Kameruns. MM. Gentil and Freydenberg[176] believe that
the ægirine porphyries of Mounio and of Hadj el Hamis on the southern
shore of Chad are of volcanic origin. M. Garde,[177] however, is of
opinion that they possess an intrusive character and belong to the
hypabyssal phase of the alkaline magma which gave rise to the Mounio and
Melfi granites.
The plutonic rocks of this early Tertiary volcanic period are nowhere
exposed within the Protectorate, and hypabyssal types are almost
unknown. The doleritic sills of Awe and Bashar were probably injected
during one of the earlier phases of igneous activity. Dr. Passarge[178]
has indicated the occurrence of basaltic dykes in the sandstones of
Adamawa and a boss of nepheline syenite between Yola and Garua on the
upper Benue. The latter, however, more probably belongs to the
underlying crystalline floor. M. Chudeau[179] believes that the alkali
granites of Zinder and Mounio have been intruded since the close of the
Cretaceous; but while there is every reason to believe that the
nepheline basalts and ægirine rhyolites of Tertiary age drew their
origin from a magma rich in soda, there is, as already stated (p. 140),
no evidence within the Protectorate upon which any immediate connection
between the younger soda granites and the Tertiary volcanic rocks can be
established. On the contrary, there would appear to have been in the
middle Eocene within the Protectorate merely a renewal of volcanic
activity from the same alkaline source from which the pre-Cretaceous
granites and dyke rocks originated. If, however, further investigation
should confirm M. Chudeau’s suggestion of the Tertiary age of the
granites and porphyries of Zinder, Mounio and Goure, these rocks may
then be taken to represent the plutonic and hypabyssal phases of this
middle Eocene period of igneous activity.
To the later Tertiary period of volcanic activity, which may be referred
to the close of the Pliocene and which perhaps also extended into the
early Pleistocene, are to be assigned the puys of Kereng (Plate X, Fig.
2) on the southern margin of the Bauchi plateau, the puys of Song and
Mboi (Plates VI, Fig. 2; VII, Figs. 1, 2) in northern Yola, and somewhat
doubtfully also those of the Awe district on the middle Benue (Plate V,
Fig. 1). The puys of the Kereng group are remarkably perfect and exhibit
steep crater walls of fragmental material breached by a single short
flow of olivine basalt. Elsewhere on the plateau also, as between Vom
and Bukuru, there are small puys with breached craters which are
probably of a similar recent age. From the cliffs of Assab, moreover,
later flows of lava may in places be seen on the worn slopes of the
plateau edge, and there is every reason to believe that the erosion of
the drift and of the margin of the plateau was considerably advanced
before the final close of volcanic activity. The presence of craters and
the local character of the associated flows serve to distinguish these
later products of igneous activity from the earlier lavas of the
plateau. A similar superposition of later volcanic material upon earlier
lavas has been observed by M. Chudeau[180] in the Ahaggar and in Air.
Unlike the central Sahara, however, there is in Nigeria no reason to
believe that there has been any actual continuity in volcanic action
between the earlier and the later periods.
In Yola province the Mboi Hills are crossed by a line of small craters
four or five miles long, running roughly in a N.W.–S.E. direction. To
the south-east between the Mboi Hills and Song the line is continued by
a number of hillocks of basalt for a few more miles, but how far it
extends to the N.W. is not known. Only a relatively small amount of
basalt was erupted from these vents. In the case of the largest crater
with a crater floor of 250 yards in diameter the volcanic material was
not sufficient to conceal the granite through which it came. The granite
was broken away during the eruption so as to form a cliff extending one-
third round the crater, while the other two-thirds is formed of
agglomerate and tuff with very many fragments of granite. The crater
thus resembles a swallow’s nest attached to its support. With one
exception, where the wall is entire, all the craters were breached by a
lava flow. They are in excellent preservation, and the cliff-like walls,
with insignificant talus heaps at their bases, indicate a very recent
origin. (A. L.)
PLATE XVIII
[Illustration: THE SALT-FIELDS OF AWE; VOLCANIC HILLS IN THE
BACKGROUND.]
[Illustration: AKIRI SALTINGS.]
The vents on the plain to the south-east are not marked by craters or by
fragmental materials, but by mounds of basalt less than fifty feet high
spreading out into lava flows, which cover most of the surface near by.
Here the eruptions seem to have consisted in a much quieter extrusion of
lava alone. The basalts are all vesicular and very commonly quite light
and cellular. (A. L.)
The basalts of the Benue plain likewise form low hillocks or mounds with
no tuff or fragmental material to be seen. Generally there is no lava
flow proceeding from them, and the extrusion of quite a small quantity
of lava seems to have sufficed to relieve the pressure. Kaddemi Hill is
an exception; an old basalt flow leads from it for two to three miles to
the south-east. (A. L.)
All the basalt flows around Awe are raised considerably above the level
of the plain, and protect the limestone shales below from denudation,
though elsewhere the shales are very badly exposed. In consequence the
flows rest on “pedestals” from thirty to fifty feet in height. Assuming
that the lava chose the level surface of the plain rather than ridges
upon it, it follows that since the eruptions around Awe there has been a
general lowering of the surface by that amount. From this it appears
that the Awe eruptions were not quite contemporaneous with those of
Mboi, for at the latter place only a small amount of talus has been
broken off from the crater cliffs. (A. L.)
The ash cones and puys of the middle Benue are much more imperfectly
preserved than those of Kereng or Mboi and are certainly considerably
older. Their comparatively recent age is presumed, however, as in the
case of the later volcanic period in Air and Tibesti,[181] from the
presence of thermal springs in the immediate neighbourhood. It seems
probable that they were established as the result of the crustal
movements anterior to the accumulation of the drift, while the puys of
Kereng and Mboi owe their origin to the differential movements of the
latest period of elevation.
In petrographical character these later rocks are very similar to the
earlier Tertiary lavas. All are basalts rich in olivine and augite and
varying in the same locality from types with abundant felspar in the
ground-mass to thoroughly limburgitic[182] varieties (12) from which
felspar is entirely wanting. The ash of the Kereng puy is remarkable for
the number and size of the rounded augite crystals and olivine nodules
which it contains. Recent flows of olivine basalt and limburgite are
known also from Ahaggar and Air[183] and from the Kameruns.[184] M.
Chudeau[185] has described rhyolites from Inzize to which he assigns a
later Tertiary age, Dr. Passarge[186] has noted a nepheline basalt from
an isolated cone of recent origin in the Benue valley between Yola and
Garua, and Dr. Esch[187] has described nephelinites and leucitites from
the Kamerun Mt. which is still periodically in active eruption. No rocks
of similar composition, however, have been recognised amongst the
products of the most recent volcanic period within the Protectorate.
MM. Chudeau[188] and Chautard,[189] following M. Lacroix,[190] have
suggested that the alkaline rocks of North Africa may be grouped into
three petrographical provinces, a western province including the Azores,
Canaries, Cape Verde and Senegal, a central province including Ahaggar,
Inzize, Air, Zinder and Mounio, Chad and the Shari valley, the Kameruns,
and Dahomey, and an eastern province including Abyssinia, Arabia and
Somaliland. Upon this hypothesis the alkaline rocks of Northern Nigeria
belong to and complete the central province of Lacroix, as extended by
M. Chudeau. It is probable, however, that this central province includes
intrusive and volcanic rocks of many different ages, and that alkaline
types have been a characteristic product of differentiation from the
earliest times to the present day, (p. 142). There is, moreover, every
reason to believe that the progress of investigation will reveal many
connecting links between the various provinces, which are themselves
somewhat too extensive and comprehensive to warrant their recognition as
such. A petrographical province in the stricter sense of the term may
perhaps be set up to include the alkaline plutonic rocks of Bauchi
province and their associated hypabyssal types, but there can be little
doubt that the general alkaline character of the igneous rocks of
northern Africa, to which Lacroix has given expression, may be quite
sufficiently indicated by emphasising their natural classification
within the Atlantic branch of igneous rocks.[191]
[Footnote 172: Chautard, _Bull. Soc. Geol. Fr._, 4th Series, VII., 1907,
p. 427.]
[Footnote 173: These rocks present in microscopical structure a decided
resemblance to the Carboniferous lavas of the Bathgate Hills (Falconer,
_Trans. Roy. Soc. Edin._, Vol. XLV., 1906, p. 133) in Scotland. With the
exception that the doleritic types are somewhat less weathered, the
description and classification of the Bathgate rocks may be applied
almost word for word to these Tertiary lavas of Nigeria. Doleritic rocks
(Falconer, _op. cit._, Plate I., Fig. 2) with phenocrysts of augite and
olivine, and more rarely of felspar are common on the Morrua plateau in
eastern Nassarawa and along the base of the escarpment between Darroro
and Assab (928, 1573, 1630). Felspar is abundant in the ground-mass and
the augite occurs in granules, granulitic aggregates, small ophitic
plates, or minute idiomorphic crystals between the felspars. The basalts
of Zammagan (1578) and Buratai (1640) show a typical fluxional
arrangement of the felspars (Falconer, _op. cit._, Pl. I., Fig. 3). The
basalt of Kwoia (1654) exhibits an intimate mixture of felspar and
augite in the ground-mass (Falconer, _op. cit._, Pl. I., Fig. 4). The
basalt of Assab (934) contains scattered felspars with abundant minute
granular aggregates of augite, while a similar rock from Buratai and
Dukshi (1641) contains some residual brown glassy material and passes
into limburgitic varieties with phenocrysts of augite and olivine in an
ill-defined microlitic base.]
[Footnote 174: Chudeau, _Sah. Soud._, 1909, p. 258; Gentil, _Doc. Sc.
Miss. Sah._, 1905, p. 726.]
[Footnote 175: Passarge, _Adamaua_, 1895, p. 374. See also Downes,
_Geol. Rep. Yola-Cross River Boundary Commission_, 1910, p. 35.]
[Footnote 176: Gentil and Freydenberg, _Bull. Soc. Geol. Fr._, 4th Ser.,
VIII., 1908, p. 35; Lacoin, _C. Rd. Ac. Sc._, 22 June, 1903.]
[Footnote 177: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 178: Passarge, _Adamaua_, 1895, pp. 363, 384.]
[Footnote 179: Chudeau, _Sah. Soud._, 1909, p. 266.]
[Footnote 180: Chudeau, _Sah. Soud._, 1909, p. 361.]
[Footnote 181: Chudeau, _Sah. Soud._, 1909, p. 264.]
[Footnote 182: _Cf._ Falconer, _Trans. Roy. Soc. Ed._, XLV., 1906, Pl.
I, Fig. 6.]
[Footnote 183: Chudeau, _Sah. Soud._, 1909, p. 361.]
[Footnote 184: Esch, _Geologie von Kamerun_, p. 40.]
[Footnote 185: Chudeau, _op. cit._, p. 259.]
[Footnote 186: Passarge, _Adamaua_, 1895, p. 384.]
[Footnote 187: Esch, _Sitz. Ber. Ak. Wiss. Berlin_, 1901, p. 277.]
[Footnote 188: Chudeau, _op. cit._, p. 268.]
[Footnote 189: Chautard, _Bull. Soc. Geol. Fr._, 4th Ser., VII., 1907,
p. 427.]
[Footnote 190: Lacroix, _C. Rd. Ac. Sc._, 24 May, 1899, p. 1353; 1905,
140, p. 22. _Nouv. Arch. Mus._, 4th Ser., IV., p. 156; V., p. 246.]
[Footnote 191: _Cf._ Prior, _Min. Mag._, 1903, XIII., p. 228; Harker,
_Nat. Hist. of Igneous Rocks_, 1910, p. 91.]
APPENDIX I
MINERAL RESOURCES
Salt — Iron — Gold — Monazite — Galena — Tinstone.
Detailed information respecting the principal mineral resources of
Northern Nigeria will be found in Professor Dunstan’s Reports on the
Results of the Mineral Survey.[192] The following is a brief summary of
the more important facts.
(1). _Salt._
Salt has long been a staple article of inter-tribal commerce on the
middle Benue, and its manufacture has in the past been extensively
carried on in the province of Muri. The introduction of European salt
has, however, adversely affected the native industry, and salt-making is
now confined only to those localities in which the natural conditions
are most favourable. In Muri province the salt is prepared by the
concentration and evaporation of the waters of weak brine springs which
are distributed over a western and an eastern area. In the western area
the town of Awe is the centre of the industry, while Akiri and Azara are
subordinate in both production and population. (Plate XVIII, Figs. 1,
2.) Keana on the Nassarawa border forms the western limit of the salt-
producing area, while Akwana to the south of the Benue marks its
southernmost extension. In addition to these towns which still
manufacture a surplus for export, salt-making for local purposes is
carried on at Kanji, Doya, Abuni, Ribi and Arofu within the same area.
At a former period also salt was made at Lafin Gisheri, between Tunga
and Ibi, and this locality may be taken to mark the eastern limit of the
salt area. In eastern Muri the salt-producing district is of less
importance than that in the west. The centres of the industry are
Bomanda and Jenoe, but small quantities of salt are also prepared at
Jebjeb, Jebero and Langa.
With regard to geological occurrence it is believed, as already
indicated, that the weak brine from which the salt is made in Muri
province is located within the upper part of the “Muri sandstones” and
immediately underneath the passage beds and the Turonian limestone. Mr.
Longbottom found that at Awe the springs supplied from the sandstones of
the passage beds are entirely free from salt and cease flowing before
the end of the dry season, while the neighbouring brine springs which
rise from rocks lying below the surface drainage maintain a constant
flow throughout the year. Moreover, wherever the shaly bands within the
passage beds were pierced by boreholes, the brine rose with a steady
flow under considerable pressure. The sandstones and grits underneath
the passage beds appear to be saturated with weak brine which is
retained, as in a reservoir, by the overlying shales. It is believed
that the western and eastern salt-bearing districts form detached and
irregular synclinal areas affected by a subordinate folding. The brine
springs at Awe, Azara and Bomanda are especially associated with the
subordinate anticlinal axes along which the salt-bearing beds within the
synclinals approach the surface. Some of the minor salt towns, such as
Ribi, Keana and Jenoe, are probably located on or near the outcrop of
the salt-bearing beds. Wherever the brine springs are found, bubbles of
a colourless gas, probably carbon dioxide, rise with the water.
Calcareous incrustations round the orifices of the springs are found at
Awe and Jebero. Some of the springs of Awe are, moreover, slightly
thermal and possess a temperature a few degrees above sun heat. This is
probably to be correlated with the recent volcanic action in the
neighbourhood.
[Illustration: NORTHERN NIGERIA
Distribution of Salt, Tinstone, Gold and Galena.
MAP No. 4.
London. Macmillan & Co., Ltd.
_Face p._ 266.]
The present springs rise in stream courses and are unavailable during
the rainy season. Boring, however, within the synclinal areas would
afford a copious and continuous supply. Professor Dunstan has analysed
the salt of various localities and discussed the possibilities of
increasing the native output. The general weakness of the brine,
however, would appear to be unfavourable to European exploitation.
Salt is won from brine pools in south-eastern Bauchi over an extensive
belt of country which stretches northward from the Pero Hills and passes
to the east of Chongwom and Tangalto. The pools are presumably fed by
springs and the salt is said to be abundant and easily prepared. The
whole area, however, is very imperfectly known and largely unexplored.
Scanty incrustations of natron, utilised by the local herdsmen, are
known from the Gongola valley in the neighbourhood of Nafada.
In Bornu salt is made from the ash of the siwak tree (_Salvadora
persica_) which grows abundantly on the ancient dunes which fringe the
western shore of Chad. The slaves of Mongonu and Musara prepare salt
also from the ash of three varieties of grass, “pagam, kalaslim and
kanido,” which are found in the immediate neighbourhood of the lake.
Over the whole of central Bornu the waters of the wells are charged with
salts of soda in very varying amount. The ancient alluvium of Bornu is
evidently impregnated with soda, and incrustations are found in places
upon the surface of desiccated swamps. Natron pools, similar to those of
Manga and presumably fed by springs, are unknown in central Bornu.
In Sokoto province incrustations of salt are found in the dry season at
Zauro in the valley of the Gulbin Kebbi, and natron (kungwa) occurs in a
similar manner between Gulma and Argungu. Knots and streaks of gypsum
are common in the shales and clays around Sokoto, while efflorescences
of gypsum and alum are found on the cliffs of Wurnu to the north-east.
Scanty incrustations of natron are found upon desiccated swamps in many
localities in the provinces of Sokoto, Kano, Zaria and Ilorin, but never
in sufficient quantity to give rise to even a native industry. Hausaland
has been in the past supplied with salt and natron from Asben, Bilma and
Manga in the French Sudan. European salt, however, is rapidly replacing
the native article, and many caravans now bring only natron from the
north to the markets of Kano.
(2). _Iron._
As in the case of salt, so in the case of iron, the native manufacture
has been very largely superseded by the importation of European bar-
iron. Only in the more inaccessible districts or in those places where
the ore is abundant do the natives still smelt the local ironstones in
their primitive fashion. The surface ironstones were at one time
generally utilised throughout the Protectorate as a source of iron. The
magnetite sands are still washed and smelted by the pagans of the Mumuye
hills in eastern Muri, while in northern Nassarawa the Koro tribe around
Kao is still noted for its proficiency in the manufacture of iron from
the local limonitic ores which occur within the schists in veins of
varying thickness.[193] Similar ores occur in veins in the sandstones
around Akiri in western Muri, but are no longer utilised by the natives.
The oolitic and earthy ironstones of sedimentary origin and of Eocene
age give rise to an extensive native industry around Akwi and Ojerami in
central Kabba, around Mahorro in eastern Kontagora, around Aliero in
southern Sokoto, and around Kantana in southern Bauchi, as well as in
the Kerri Kerri country in western Bornu and in the Tagum hills in
southern Bornu. So far as European exploitation is concerned, the
sedimentary ores of Mt. Patti and the lower Niger are most favourably
situated for transport. The ores, however, while abundant, are of a
somewhat low grade character and relatively rich in phosphoric acid.
Magnetite and hæmatite schists are extensively developed in eastern
Kabba, but their composition would render necessary costly crushing and
separating processes before the ores could be available for export.[194]
(3). _Gold._
Traces of gold have been found in some of the quartz veins which are so
widely distributed throughout the Protectorate, but no payable reefs
have yet been discovered. Small quantities of free gold occur in the
streams between Koriga and Birnin Gwari in northern Zaria and around
Tagum in southern Muri. In Zaria the gold is evidently located within
the northern prolongation of the Zungeru belt of slates and schists, and
further exploration may possibly lead to valuable discoveries. All the
slaty belts already described within the Protectorate are, indeed,
worthy of careful prospecting for gold. Small quantities of gold are
also found associated with cassiterite in some parts of Bauchi province.
(4). _Monazite._
Monazite is widely distributed throughout the Protectorate, and appears
to be a fairly constant constituent of the gneisses and schist. Small
quantities are contained in almost every river and stream within the
crystalline areas. It has been found most abundantly in central Kabba
and in northern Nassarawa, but, so far as known, it occurs nowhere in
sufficient quantity to afford any hope of profitable European
exploitation.
(5). _Galena._
A slightly argentiferous galena occurs in veins within the Cretaceous
rocks around Arofu and Akwana in western Muri, and in the Kudu valley to
the west of Jebjeb in northern Muri. In the Arofu district the galena is
associated with fluorspar and calcite, and the formation of the veins
has been accompanied as already described by much silicification of the
adjoining limestones. The Arofu galena is excavated by the natives for
use as a cosmetic, and from time to time small pockets of silver are
discovered within the veinstuff. To the west of Jebjeb the galena is no
longer dug by the natives, and the old workings are now levelled and
grassed over. The galena appears to have been associated with quartz,
hæmatite and blende, but there is no evidence of silicification of the
country rock.
(6). _Cassiterite (Tinstone)._
Tinstone is the most valuable asset of the Protectorate from the point
of view of mineral resources. Its occurrence in western Bauchi was known
to the natives long before the advent of Europeans. Recent
investigation, however, by the Mineral Survey has proved its presence
over considerable areas in widely separated localities within the
Protectorate. The most extensive area over which tinstone is found in
the surface alluvium occurs in western Bauchi, and includes the summit
of the plateau and the greater part of its northern margin. The alluvial
deposits of Naraguta, which lie within this area, were originally
developed by the Niger Co., but have now been acquired by the
Consolidated Gold Fields, Ltd. Elsewhere, however, tinstone becomes
equally abundant, and there is little doubt that further investigation
will reveal many localities within this extensive area in which alluvial
workings may be profitably carried on. Several syndicates indeed are now
engaged in exploiting certain portions of the tinfield. In the
neighbourhood of Ngell the tinstone, accompanied by sulphide ores of
copper, zinc and lead, occurs, as already described, in part as a
pneumatolytic modification of the Bukuru granite, and in part as a true
lode or pegmatitic formation. This particular occurrence is being
developed by the Niger Co., but there is every reason to believe that
similar occurrences will be found throughout the area as investigation
proceeds.
Apart from this extensive tin-field in the centre of the Protectorate,
cassiterite is also found in Bauchi province in many places throughout
the Ningi and Burra hills, and detailed prospecting will probably here
also reveal several payable localities. The completion of the Baro-Kano
railway[195] within the next two years will afford an easy means of
access to the tin-producing areas of Bauchi, and will at the same time
largely overcome the difficulty of transport which is at present the
greatest obstacle in the way of the development of the tin fields.
Beyond the limits of Bauchi province small quantities of tinstone have
been found in the neighbourhood of Gadama and Fagam on the borders of
Bauchi and Kano, around Gantam and Aribi on the Nassarawa tableland as
well as on the eastern slopes of the Vere hills in Yola, the western
slopes of the Shebshi hills in southern Muri, and in the neighbourhood
of Eri in eastern Ilorin. In the latter locality the tinstone was found
to be derived from a richly stanniferous pegmatite vein, but the actual
extent and possibilities of the occurrence are as yet unknown. The same
may be said also of the other localities indicated, and there can be
little doubt that all of them are worthy of further investigation.
Tinstone has been reported to occur abundantly in the Mada country in
eastern Nassarawa, but this discovery likewise awaits confirmation.
[Footnote 192: Dunstan, _Col. Reps._ (_Miscell._), Nos. 32 (1906), 46
(1908), 47 (1908). A fourth report on the results of the Mineral Survey
will shortly be issued.]
[Footnote 193: Sir William Wallace has reported an extensive native iron
industry at Fawa in Katsina, where the ore appears to occur in a similar
manner to that in the neighbourhood of Kao.—_G. J._, VIII., 1896.]
[Footnote 194: In _The Iron Ore Resources of the World_, Vol. II., 1910,
p. 1029, Dr. J. W. Evans has given a _résumé_ of the results of the
investigations of the Mineral Survey into the extent and value of the
ferruginous deposits of Northern Nigeria.]
[Footnote 195: And the branch line to Leri in Zaria province.]
APPENDIX II
THE PALÆONTOLOGY OF THE UPPER CRETACEOUS DEPOSITS OF NORTHERN NIGERIA
By HENRY WOODS, M.A., _University Lecturer in Palæozoology, Cambridge_
The fossils collected by Dr. J. D. Falconer and Mr. A. Longbottom from
the Upper Cretaceous deposits of Northern Nigeria include Fishes,
Ammonites, Gasteropods, and Lamellibranchs. The localities from which
these fossils were obtained are: Awe, Deba Habe, Jibaru, Gongila,[196]
Gulani, Kumberi, Kunini, and Reme. At present the number of species
which can be identified is comparatively small, but the collection is
sufficient to show that under more favourable conditions a large and
important fauna could probably be obtained from the deposits of this
region.
Teeth, vertebræ, &c., of fishes were collected at Kumberi and Jibaru,
and have been examined by Dr. A. S. Woodward who identifies them as
_Gigantichthys_, a genus which is known from the Chalk of Egypt and the
Sahara.[197] Ammonites have been found at Gongila, Reme, Kunini, and
Kumberi, and they show that the deposits are of Turonian age—probably
Lower Turonian. The Ammonites belong to the genera _Vascoceras_,
_Mammites_ (_Pseudaspidoceras_), and _Hoplitoides_. _Vascoceras_ is
confined to the Turonian. _Pseudaspidoceras_ is mainly of Turonian age.
The species of _Hoplitoides_ belongs to the section of that genus which
occurs in Turonian deposits. The species of _Vascoceras_ and _Mammites_
(_Pseudaspidoceras_) found in Nigeria are closely allied to forms which
occur in the Lower Turonian of Northern Africa and the Iberian
peninsula.
The larger number of the Gasteropods and Lamellibranchs were found at
Awe, but no Ammonites were obtained at that locality; these fossils, so
far as they can be identified, do not give any definite evidence that
the deposit at Awe is of earlier age than the Turonian.
At Deba Habe _Ostrea (Exogyra) olisiponensis_, Sharpe, is abundant and
is the only fossil obtained at that place. In Portugal that species
occurs in both Turonian and Cenomanian deposits. In Nigeria it has also
been found at Gongila, where the Turonian _Hoplitoides_ occurs. So that
it is probable that the deposit at Deba Habe is of the same age as the
one at Gongila.
At Gulani _Ostrea prælonga_, Sharpe, is abundant, but no other fossil
was found at that locality. In Portugal _O. prælonga_ occurs in the
Bellasian—an horizon which is believed to come between the Aptian and
the Cenomanian. The same species is found at a similar level in Tunis
and Algeria. The occurrence of this one species can, I think, hardly be
taken as sufficient evidence that the deposit at Gulani is of Bellasian
age, since it is not unlikely that, as is the case with _O. (Exogyra)
olisiponensis_, this form may recur at later horizons when a favourable
facies of deposit again appears.
At present, then, we can state that the deposits in which Ammonites
occur are of Turonian age, and that there is not sufficient evidence to
show that beds of earlier age are represented.
The fauna of the Turonian deposits of Northern Nigeria resembles closely
that found in beds of similar age in Portugal, Tunis, and Algeria. This
is shown especially by the Ammonites and oysters. At present the genus
_Vascoceras_ is known only from Portugal, Spain, Algeria, Tunis, Egypt,
Nigeria and Brazil. _Hoplitoides_ occurs in Algeria, Tunis, Nigeria, the
Kameruns, and probably in Egypt. The species of _Pseudaspidoceras_ found
in Nigeria is allied to forms found in Portugal, Tunis and Brazil. The
oysters likewise belong to species characteristic of Northern Africa and
South-western Europe, and one of them has also been found as far south
as Angola. The Ammonites also show, as Pervinquière has already pointed
out is the case in Tunis, that the fauna is related to that found in the
Upper Cretaceous of Brazil.
Amongst other places in West Africa, Cretaceous deposits have been found
in the Kameruns, and are regarded by Solger as of Upper Turonian and
Lower Senonian age, but by Harbort[198] they are referred to the
Emscherian (Lower Senonian). They certainly appear to be of later date
than the Nigerian deposits with which they are connected chiefly by the
presence of _Hoplitoides_.
Until the fauna of the Nigerian Cretaceous is more completely known any
further discussion of its relationship to the faunas of other areas must
be postponed.
The list on the following page shows the distribution of the species
found in Nigeria, and is followed by palæontological notes and
descriptions of some of the species.
--------------------+----+-----+--------+-------+--------+-------+-----
|Awe.|Deba |Gongila.|Gulani.|Kumberi.|Kunini.|Reme.
| |Habe.| | | | |
--------------------+----+-----+--------+-------+--------+-------+-----
_Nuculana_, sp. | | | | | * | |
| | | | | | |
_Trigonoarca_, sp. | * | | | | | |
| | | | | | |
_Trigonia_, sp. | * | | | | | |
| | | | | | |
_Modiola_, sp. | * | | | | | |
| | | | | | |
_Pecten (Neithea) | * | | | | | |
quinquecostatus_, | | | | | | |
Sow. | | | | | | |
| | | | | | |
_Lima (Mantellum)_, | * | | | | | |
sp. | | | | | | |
| | | | | | |
_Ostrea prælonga_, | | | | * | | |
Sharpe | | | | | | |
| | | | | | |
„ _(Exogyra) | | * | * | | | |
olisiponensis_, | | | | | | |
Sharpe | | | | | | |
| | | | | | |
_Gervillia | * | | | | | |
(Pseudoptera)_, sp. | | | | | | |
| | | | | | |
_Astarte awensis_, | * | | | | | |
sp. nov. | | | | | | |
| | | | | | |
_Lucina_, sp. | | | | | * | |
| | | | | | |
_Cardium_, sp. | * | | | | | |
| | | | | | |
_Corbula_, sp. | * | | | | | |
| | | | | | |
_Pseudomelania_, sp.| * | | | | | |
| | | | | | |
_Turritella_, sp. | | | | | * | |
| | | | | | |
_Semifusus | * | | | | | |
africanus_, sp. nov.| | | | | | |
| | | | | | |
_Vascoceras | | | | | | * | *
nigeriensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
„ | | | * | | | |
_gongilensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
_Mammites | | | * | | | |
(Pseudaspidoceras)_,| | | | | | |
sp. | | | | | | |
| | | | | | |
_Hoplitoides | | | * | | | |
nigeriensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
„ sp. | | | | | | | *
| | | | | | |
_Gigantichthys_, sp.| | | | | * | |
--------------------+----+-----+--------+-------+--------+-------+-----
LAMELLIBRANCHIA.
NUCULANA, sp.
Plate XIX., Fig. 4.
An internal cast resembles in form _N. Mariæ_ (d’Orbigny), but in the
absence of the shell the species cannot be identified.
_Locality._—Kumberi.
TRIGONOARCA, sp.
Plate XIX., Figs. 1–3.
Several specimens of _Trigonoarca_ were found, but are not sufficiently
well-preserved for specific identification. In general form they
resemble some examples of _T. trichinopolitensis_ (Forbes)[199] from the
Trichinopoli Beds of Southern India.
_Locality._—Awe.