Chapter VIII: Tertiary Volcanic Action

Absence of pre-Tertiary volcanic rocks within the Protectorate: two periods of Tertiary volcanic activity. The early Tertiary period: the Maradu vent; the nepheline basalts of Wase and Tangalto; the phonolites of Wase Rock, Tangale Peak and Bantaji; the plateau lavas of southern Bornu, Bauchi and Nassarawa; petrographical character of the lavas; early Tertiary lavas of the Sahara, Sudan and Adamawa. The later Tertiary volcanic period: the puys of northern Yola, the middle Benue valley and the Bauchi plateau; petrographical character of the lavas; petrographical provinces.

It may be presumed that the intrusion of the alkali granites and dyke rocks throughout the Protectorate was accompanied in pre-Cretaceous times by considerable superficial volcanic activity (p. 142). M. Chautard[172] has, moreover, described the intercalation of volcanic rocks in the Upper Cretaceous of Senegal. All trace of pre-Tertiary effusive rocks, however, within the Protectorate appears to have been removed in the course of denudation. The fracturing and folding of the crust at the close of the Cretaceous was apparently accompanied in places by the formation of mineral veins within the fissures, but no contemporaneous igneous activity has been definitely identified within the Protectorate (p. 140). Within the Tertiary era itself two separate periods of volcanic activity may be recognised, one in the middle Eocene and probably to some extent contemporaneous with the accumulation of the Eocene sandstones within the Protectorate, and one in late Pliocene or early Pleistocene times and contemporaneous with the later Tertiary oscillations of the crust. For the most part the eruptions were of an isolated character and gave rise to the formation of detached pipes, cones and puys of igneous material. More rarely, as in southern Bornu and on the borders of Bauchi and Nassarawa, the extruded lavas united to form a continuous sheet of volcanic rock, which covered a considerable area of the underlying sedimentary and crystalline rocks.

The earliest outburst of volcanic activity appears to have taken place towards the close of the post-Cretaceous erosion or possibly in the middle Eocene at a period contemporaneous with the accumulation of the lower marine beds of Sokoto, and is represented now by the extensive neck or vent of fragmental material near Maradu in Sokoto province whose levelling was accomplished during the later extension of the middle Eocene sea. The greater number of the boulders in the agglomerate are composed of a microgranitic quartz porphyry with phenocrysts of orthoclase, biotite and brown hornblende in addition to quartz (1323). To the same period is probably also to be referred the line of well-worn stumps of basalt, phonolite and trachyte which stretches from the Gongola valley westward through Tangale and Angass to Namu in northern Muri. The Tangale Peak (Pl. XIII, Fig. 2) and the Wase Rock (Pl. V, Fig. 2), which still retain a portion of the original flow attached to the rock in the vent, were probably among the latest eruptions of the series. The intrusive dolerites of Bashar (41) and Awe (1014) may be ascribed to the same phase of igneous activity, while the Rock of Bantaji in Southern Muri may be taken to represent contemporaneous volcanic action to the south of the Benue.

In petrographical character the basaltic pipe rocks are always olivine- bearing and usually porphyritic with olivine and augite. Felspar is found only in the ground-mass which at times contains a little intersertal glassy base. The basalt of Wase Topa is remarkable for the number and size of the nodules of olivine which it encloses. A worn vent immediately south of the town of Wase is filled with a fine-grained nepheline basalt (32) containing glomeroporphyritic aggregates of augite crystals and well-formed crystals of olivine. Felspar is entirely wanting, the ground-mass being composed of small rods of augite and granules of magnetite set in a brown glassy base studded with minute crystals of nepheline. A somewhat similar but coarser-grained rock from Tangalto (1661) contains large crystals of decomposed olivine and purple and well-zoned augite in a holocrystalline ground-mass of rods and granules of augite, magnetite grains, flakes of biotite and abundant nepheline in well-formed and irregular crystals. The nepheline was evidently the last to crystallise, and to some extent takes the place of matrix, being in part moulded upon the other constituents of the ground- mass.

The phonolite of the Wase Rock (31) is of a trachytoid type, containing columnar phenocrysts of sanidine, ragged crystals of ægirine augite and scattered pseudomorphs after nepheline in a ground-mass consisting predominantly of minute felspar prisms with a well-marked fluxional arrangement, mingled with magnetite grains and rods and granules of green pyroxene. The phonolite of Tangale Peak (1665–6) contains phenocrysts of sanidine and soda pyroxene in a ground-mass of a more orthophyric type, composed of minute idiomorphic crystals of nepheline, imperfect prisms of ægirine and ægirine augite and elongated microlites of felspar which pass into an ill-defined felspathic matrix. Traces of brown hornblende in irregular porous growths may also be observed in the ground-mass. A similar rock from the immediate neighbourhood of Tangalto (1663) contains soda pyroxene, but no evident nepheline in the ground- mass, and may be grouped provisionally as a trachyte. The phonolite of Bantaji Rock is of a more nephelinitoid type, with the ground-mass enclosed in a somewhat intersertal manner between the abundant but much decomposed phenocrysts of felspar. With the felspar are associated as phenocrysts hypidiomorphic crystals of ægirine and ægirine augite frequently in zonal growths and smaller idiomorphic and abundant crystals of somewhat weathered nepheline. The ground-mass is composed of lath-shaped felspars, granular pyroxene and irregular patches of analcime probably of secondary origin. The association of phonolite and nepheline basalt, observed in the neighbourhood of Wase and the Peak, has not been recognised around Bantaji.

The period of formation of the lava fields in southern Bornu and on the borders of Bauchi and Nassarawa may be provisionally referred to the close of the middle Eocene, on the strength of the apparent intercalation of a friable red sandstone amongst the lavas in the escarpment of the Barbur plateau behind Dukshi. The extensive erosion which the plateau lavas themselves have undergone indicates, moreover, that they are of no very recent age. The eastern lava field is the more extensive of the two and perhaps somewhat younger, so far as one can judge from the less denuded character of the cones. The rich content in iron of the upper part of the Eocene accumulations may possibly have been conditioned to a considerable extent by the distribution of much fragmental basic igneous material upon the surface of the waters.

In petrographical character the lavas are basaltic throughout and rich in olivine, and vary from felspathic doleritic types to limburgitic varieties in which felspar is greatly reduced in amount.[173] Nepheline- bearing types have nowhere been recognised, nor any effusive rocks more acid than basalt. MM. Chudeau and Gentil[174] have, however, indicated the occurrence of ægirine rhyolites and phonolites, as well as of basalts, tephrites and limburgites, amongst the older lavas of Ahaggar and Air, which are probably of similar age to those of Nigeria. Dr. Passarge[175] has described the occurrence of basalt, augite andesite and phonolite on the summit of the Shebshi hills on the borders of Nigeria and the Kameruns. MM. Gentil and Freydenberg[176] believe that the ægirine porphyries of Mounio and of Hadj el Hamis on the southern shore of Chad are of volcanic origin. M. Garde,[177] however, is of opinion that they possess an intrusive character and belong to the hypabyssal phase of the alkaline magma which gave rise to the Mounio and Melfi granites.

The plutonic rocks of this early Tertiary volcanic period are nowhere exposed within the Protectorate, and hypabyssal types are almost unknown. The doleritic sills of Awe and Bashar were probably injected during one of the earlier phases of igneous activity. Dr. Passarge[178] has indicated the occurrence of basaltic dykes in the sandstones of Adamawa and a boss of nepheline syenite between Yola and Garua on the upper Benue. The latter, however, more probably belongs to the underlying crystalline floor. M. Chudeau[179] believes that the alkali granites of Zinder and Mounio have been intruded since the close of the Cretaceous; but while there is every reason to believe that the nepheline basalts and ægirine rhyolites of Tertiary age drew their origin from a magma rich in soda, there is, as already stated (p. 140), no evidence within the Protectorate upon which any immediate connection between the younger soda granites and the Tertiary volcanic rocks can be established. On the contrary, there would appear to have been in the middle Eocene within the Protectorate merely a renewal of volcanic activity from the same alkaline source from which the pre-Cretaceous granites and dyke rocks originated. If, however, further investigation should confirm M. Chudeau’s suggestion of the Tertiary age of the granites and porphyries of Zinder, Mounio and Goure, these rocks may then be taken to represent the plutonic and hypabyssal phases of this middle Eocene period of igneous activity.

To the later Tertiary period of volcanic activity, which may be referred to the close of the Pliocene and which perhaps also extended into the early Pleistocene, are to be assigned the puys of Kereng (Plate X, Fig. 2) on the southern margin of the Bauchi plateau, the puys of Song and Mboi (Plates VI, Fig. 2; VII, Figs. 1, 2) in northern Yola, and somewhat doubtfully also those of the Awe district on the middle Benue (Plate V, Fig. 1). The puys of the Kereng group are remarkably perfect and exhibit steep crater walls of fragmental material breached by a single short flow of olivine basalt. Elsewhere on the plateau also, as between Vom and Bukuru, there are small puys with breached craters which are probably of a similar recent age. From the cliffs of Assab, moreover, later flows of lava may in places be seen on the worn slopes of the plateau edge, and there is every reason to believe that the erosion of the drift and of the margin of the plateau was considerably advanced before the final close of volcanic activity. The presence of craters and the local character of the associated flows serve to distinguish these later products of igneous activity from the earlier lavas of the plateau. A similar superposition of later volcanic material upon earlier lavas has been observed by M. Chudeau[180] in the Ahaggar and in Air. Unlike the central Sahara, however, there is in Nigeria no reason to believe that there has been any actual continuity in volcanic action between the earlier and the later periods.

In Yola province the Mboi Hills are crossed by a line of small craters four or five miles long, running roughly in a N.W.–S.E. direction. To the south-east between the Mboi Hills and Song the line is continued by a number of hillocks of basalt for a few more miles, but how far it extends to the N.W. is not known. Only a relatively small amount of basalt was erupted from these vents. In the case of the largest crater with a crater floor of 250 yards in diameter the volcanic material was not sufficient to conceal the granite through which it came. The granite was broken away during the eruption so as to form a cliff extending one- third round the crater, while the other two-thirds is formed of agglomerate and tuff with very many fragments of granite. The crater thus resembles a swallow’s nest attached to its support. With one exception, where the wall is entire, all the craters were breached by a lava flow. They are in excellent preservation, and the cliff-like walls, with insignificant talus heaps at their bases, indicate a very recent origin. (A. L.)

PLATE XVIII

The 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.]