Africa — страница 8

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formations in many regions must be regarded as subject to considerable revision. Rocks of Archean age cover wide areas in the interior, in West and East Africa and across the Sahara. Along the coastal margins they underlie the newer formations and appear in the deep valleys and kloofs wherever denudation has laid them bare. The prevailing types are granites, gneisses and schists. In the central regions the predominant strike of the fohae is north and south. The rocks, for convenience classed as pre-Cambrian, occur as several unconformable groups, chiefly developed in the south where alone their stratigraphy has been determined. They are unfossiliferous, and in the absence of undoubted Cambrian, Ordovician and Silurian strata in Africa they may be regarded as of older date than

any of these formations. The general occurrence of jasper-bearing rocks is of interest, as these are always present in the ancient pressure-altered sedimentary formations of America and Europe. Some unfossiliferous conglomerates, sandstones and dolomites in South Africa and on the west coast are considered to belong to the Cambrian, Ordovician and Silurian formations, but merely from their occurrence beneath strata yielding Devonian fossils. In Cape Colony the Silurian age of the Table Mountain Sandstone is based on such evidence. The Devonian and Carboniferous formations are well represented in the north and south and in northern Angola. Up to the close of the palaeozoic period the relative positions of the ancient land masses and oceans remain unsolved; but the absence of

marine strata of early palaeozoic age from Central Africa points to there being land in this direction. In late Carboniferous times Africa and India were undoubtedly united to form a large continent, called by Suess Gondwana Land. In each country the same succession of the rocks is met with; over both the same specialized orders of reptiles roamed and were entombed. The interior of the African portion of Gondwana Land was occupied by several large lakes in which an immense thickness—amounting to over 18,000 ft. in South Africa—-of sandstones and marls, forming the Karroo system, was laid down. This is par excellence the African formation, and covers immense areas in South Africa and the Congo basin, with detached portions in East Africa. During the whole of the

time—-Carboniferous to Rhaetic—that this great accumulation of freshwater beds was taking place, the interior of the continent must have been undergoing depression. The commencement of the period was marked by one of the most wonderful episodes in the geological history of Africa. Preserved in the formation known as the Dwyka Conglomerate, are evidences that at this time the greater portion of South Africa was undergoing extreme glaciation, while the same conditions appear to have prevailed in India TABLE OF FORMATIONS Sedimentary. Igneous. Recent Alluvium; travertine; coral; sand dunes; continental } Some volcanic islands; dunes. Generally distributed } rift-valley volcanoes. Pleistocene. Ancient alluviums and } gravels; travertine. } Generally distributed. } A

long-continued Pliocene. N. Africa; Madagascar. } succession in the } central and northern Miocene. N. Africa. } regions and among } the island groups. Oligocene. N. Africa. } Doubtfully represented } south of the Zambezi. Eocene. N. Africa, along east and } west coasts; Madagascar. } Cretaceous Extensively developed in } Diamond pipes of S. N. Africa; along coast } Africa; Kaptian and foot-plateaus in east } fissure eruptions; and west; Madagascar. } Ashangi traps of } Abyssinia {Jurassic N. Africa; E. Africa; K{ Madagascar; Stormberg } Chief volcanic period a{ period (Rhaeric) in S. } in S. Africa r{ Africa } r{Trias. Beaufort Series in S. } o{ Africa; Congo basin; } o{ Central Africa; Algeria; } { Tunis. } {Permian. Ecca Series in S. Africa. } Feebly, if anywhere } developed.

Carboniferous. N. Africa; Sabaki Shales } in E. Africa; Dwyka } and Wittebery Series in } South Africa } Devonian. N. Africa; Angola; Bokkeveld } Not recorded. Series in S. Africa } Silurian. {Table Mountain Sandstone } { in S. Africa, Silurian(?). } Ordovician. { Doubtfully represented } Klipriversberg and { in N. Africa, French } and Ventersdorp Series Cambrian { Congo, Angola. and by } of the Transvaal (?). { Vaal River and Waterberg } { Series in S. Africa } Pre-Cambrian. Quartzites, conglomerates } phyllites, jasper-bearing } S. Africa and generally. rocks and schists. } Generally distributed. } Archeaan. Gneisses and schists of the } Igneous complex of continental platform. } sheared igneous } rocks;granites. and Australia. At the close of the Karroo period there was a