larity to the Triassic (Gondwana) life-forms of India, and also to those of Australia. They were probably deposited during the Permian and Triassic periods. Underlying them unconformably in places are the Dwyka conglomerate, a peculiar rock that often has the appearance of a volcanic breccia, and the Ecca mudstones and sandstones, constituting a group some 4000 feet in thickness. Volcanic rocks are represented by diabase and basalt, which are spread out over the surface in large sheets, being especially prominent along the eastern edge of the Drakenberg Mountains in the Transvaal. The diamond mines of South Africa are located in the vents of old volcanoes through which a basic rock (peridotite) 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. Livingstone mentioned the presence of sandstones and coal seams along the Zambezi River (lat. 16° 40' to 15° 50' S.), and somewhat further south crystilline rocks of Archæan type appear, as also along the shores of Lake Nyassa. The Rovuma River flows for a considerable distance (about lat. 11° 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 Rovuma River sandstone strata, possibly of Carboniferous age, are developed on a large scale along two general lines, one extending 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, inclosing Lake Victoria Nyanza and reaching northward nearly to Lado on the Nile. They have been broken through and are overlaid by volcanic rocks, especially around Lake Rudolf, where volcanoes are still in eruption, and in the region east of Victoria Nyanza, where there are many inactive cones. Volcanic action has been accompanied here by great vertical displacements, to which allusion has already been made. (See also article on Great Rift-Valley.) The west side of Central Africa, from the Kunene River 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 eastern coast. On the shore of Angola there is a narrow fringe of Cretaceous sandstones, and in the interior crystalline rocks, mostly granite and gneiss, and fossiliferous sandstones of undetermined age predominate. It seems probable that these formations extend into the interior toward the Congo Basin, and they may reach also northward into the Sudan. In the Congo Basin there comes into prominence a peculiar superficial deposit called “laterite,” which also covers wide areas in Sudan and the Sahara Desert. It is a porous, yellow or reddish rock, formed by the disintegration and weathering of the underlying strata.
The plateau of Abyssinia has been found to consist of gneisses and granites as a basal formation, with overlying sandstone strata in nearly horizontal position. This region is especially characterized by the enormous development of volcanic rocks, which at different times have spread out over the surface. Westward, between Khartum and Fashoda on the Nile, there is a large area of Paleozoic sediments, extending on the eastern Nile bank as far south as Lado, where it sweeps around to the west. In central Sudan, crystalline rocks have been found along the Benue River and in the region between this river and the Niger. In the extreme western Sudan, sedimentary strata with Devonian and Carboniferous fossils prevail; they are also developed to a lesser extent on the Gold Coast, where they overlie gneisses and schists. The interior of Liberia and Sierra Leone is supposed to be composed largely of crystalline rocks. The Sahara Desert presents a monotonous stretch of horizontal eroded beds of Paleozoic age resting upon eruptives and gneisses. After the Carboniferous times, the whole Sahara region appears to have been elevated above sea-level and to have maintained this position until the beginning of the Cretaceous, when there was a subsidence, and the eastern part of the Sahara, including Egypt, was formed. Volcanic rocks are found in certain parts of the interior, but they are relatively unimportant. In Lower Egypt, the ridge that forms the western border of the great rift or fault of the Red Sea is made up of gneisses, granites, and basic igneous rocks, with a sedimentary cap called the “Nubian” sandstone. The last-named constitutes the banks of the Nile at Assuan, and also extends for a considerable distance into the desert region. To the north, the Nubian sandstone is succeeded by Cretaceous and Tertiary limestones.
The Atlas region of Morocco, Algeria, and Tunis offers a striking contrast to the remainder of Africa, in that it is the only present representative of a mountain system formed by crustal folding. It is composed of eruptives, including trachyte and basalt, along the northern edge, with interfoliated gneisses, schists, granite, limestone, and sediments of Carboniferous, Jurassic, and Triassic age. Suess divides the region into parallel zones; the first is composed of volcanic rocks on the coast; the second consists of granite, gneiss, and schist; the third is a belt of sandstone and limestone, reaching southward into the Sahara Desert.
The continental islands, including the Canary, Madeira, and Cape Verde groups, and many isolated islands, are mostly of volcanic origin. Madagascar, however, is an exception, and represents the remnant of a larger area that once extended from southern Africa to lower India. The central part of Madagascar is made up of granites and gneisses similar in character to those found on the mainland, while the western shore is formed by Jurassic and Tertiary sediments. See also articles on countries of Africa.
Hydrography. The great river systems of Africa, excepting the Niger, have their sources in the mountains of the south and southeastern parts. At the Gulf of Suez a line of highlands crosses to Africa from Syria, which follows the coast line of the Red Sea to its southern extremity, then bends to the south, passes the equator, and joins the broad plateaus that extend over South Africa. As there is no prominent interior mountain range, this long line of coastal highlands forms the most important water-parting of the continent. Within its bounds are the upper courses of the Nile, Congo, and Zambezi, as well as of the Orange and of most of the smaller streams. The Nile, Niger and Congo
