GEOLOGY. The geological structure of Africa has been studied only in bare outline, but its broad features may be said to exhibit great simplicity and nniformity. The entire lower limb, with the Sudan and the western portion of the Sahara Desert, has a basal complex of crystalline rocks supporting sediments of Paleo zoic and Mesozoic age. Strata of more recent deposition, with but one exception (Lower Egypt), occur only along the sea coast and the rivers. The greater part of the land sur face, therefore, was formed in early geological times, and has remained above sea-level during succeeding periods. Owing to this nniformity, Africa cannot be divided upon a strictly geologi cal basis into more or less distinct units; such a division, however, has been, made from a combined geological and geographical stand point, separating the entire area into three prov inces. The first of these comprises South Africa, Madagasear, and a large portion of Central Af rica, which at one time was united with lower India by an easterly land extension through the area now occupied by the Indian Ocean; the second includes the Sahara Desert and Egypt, and is a continuation of Arabia and Syria; the third comprises the Atlas Mountains, and is really a part of the Eurasian continent and of the great system of upheaval that is represented in Europe by the Alps and the Apennines.
The most ancient rocks found in South Africa are granites, gneisses, and s•hists, which lie be low all fossil-bearing rocks, and may, therefore, he classed as Archaean. Above these are tilled and eroded beds of sandstones and slates, which form the rampart along the southern extremities of Cape Colony, and extend around to the west and north, spreading out over large areas in Nama qualand, Griqualaml. Rhodesia, and regions to the north, and which have special economical importance, as they include within their limits the rich gold deposits of the Transvaal. These rocks are mostly of Paleozoic age. 'Higher up in the series are the Kimberley shales and the Karoo formation of sandstones and slates, which attain great development in British South Africa. No remains of a sea fauna have been found in the Karon beds, but they are rich in amphibian and reptilian fossils that bear a striking simi larit• to the Triassic (Gondwana) life-Forms of and also to those of Australia. They were probably deposited during the Permian and Triassic periods. Underlying them uneonform ably in pimps are the Dwyka conglomerate, a peen' jar rock that often ha: the appearance of a volcanic breccia, and the Eeett mudstones aml sandstones. consituting a group some 4000 feet in thickness. Volcanic rocks are represented by diabase and basalt. which are spread tint over the
surface in large sheets, being, especially prom inent along the eastern edge of the Drakenberg Mountains, in the Transvaal. The diamond mines of Sonth Africa are located in the vents of old volemmes through which a basic rock (peri (1006.) was erupted. On the other edge of the plateau. along the sea coast, are small detached areas of sediments, more recent in origin than the foregoing.
The region of central Africa from the Sudan as far south as the Zambezi River includes large areas of which little or nothing is known. Liv ingstone mentioned the presence of stones mid coal seams along the Zambezi (liver flat, IV 40' to 1.1 50' S.), and somewhat fur ther sonth eryst•lline rocks of Arch.ran type appear. as also along the shores of Lake Nvassa. The Bovuina River flows for a considerable dis tanee (about lat. II° S.) over sandstone beds, that rest upon granite. The sandstones are found as high as 2500 feet above sea-level, and extend from near the coast to long. 39' E. North of the Rovinna River sandstone strata, possibly of Carboniferous age, are developed on a large scale along two general lines, one ex tending northwest beyond the shores of Lake Tanganyika, and the other extending north to near the equator. Between the diverging areas of sandstone, crystalline rocks predominate, inclos ing Like Victoria Nyanza and reaching north ward nearly to Lado on the Nile. They Kaye been broken through and are overlaid by volcanic rocks, especially around Lake Rudolf, where vol canoes are still in eruption, and in the region east of Victoria Nyanza, where there are many inact ive cones. Volcanic action has been accompanied here by great vertival displacements, to which allusion has already been Made. I See also article on GREAT InFr-VALLEY.) The west side of Central Africa, from the Kunene Liver to the Gulf of Guinea, has been only partly explored. Such information as is available would indicate that its structure is similar to that of the east ern coast. On the shore of Angola there is a narrow fringe of Cretinous and in the interior crystalline rocks, mostly granite and gneiss, and fossiliferons sandstones of undeter mined age predominate. It seems probable that these formations extend into the interior toward the Congo Basin, and they may reach also north ward into the Sudan. In the Congo Basin there comes into prominence a peenliar superficial de posit called which also covers wide areas in Sudan and the Sahara Desert. 11 is a porous, yellow or reddish roek, formed by the disintegration and weathering, of the underlying strata.