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.

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

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.]