The plateau of Abyssinia has been to eonsist of gueisses and granites as a basal forma tion, 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 hank 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 sedimentary strata with h 1)m-twin n and Carboniferous fossils prevail; they are also de veloped to a lesser extent on the Gold Coast, where they overlie gneisses and schists. The interior of Liberia and Sierra Leone is supposed to he 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 Carboni ferous times,• the whole Sahara region appears to have been elevated hot sea-level and to have maintained this position until the beginning of the Cretaceous, when there was 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 ignetius rocks, with a sedimentary van called the "Nubian" sand shme. The last-named constitutes the banks of the Nile at Assuan, and also extends for a con siderable distance into the desert region. To the north, the Nubian sandstone is succeeded by Cretaceous and Tertiary limestones.
The Atlas region of Aloroeco, Algeria, and Tunis offers a striking contrast to the remainder of Africa, in that it is the only present rep resentative of a mountain system formed by crustal folding. It is composed of eruptives, including trachyle and basalt, along the northern edge. with int erfol iat ed gneisses, schist. granite, limestone. and sediments of Carboniferous, Jur assic, and Triassic age. Si OSA divide"; the region into parallel zones; the first is composed of vol canic rocks on the coast ; the second consists of granite, gneiss, and schist ; the third is a belt of sandstone and limestone, reaehing southward into the Sahara Desert.
The continental islands, including the Canary, Madeira, and Cape Verde groups, and many isolated islands. arc mostly of volcanic ori gin. Madagascar, however, is an exception, and represents the remnant of a larger area that once extended front southern Africa to lower India. '(lie central part of Madagascar is made up of granites and gneisses similar in character to those found on the mainland, while the west ern shore is formed by Jurassie and Tertiary sediments. See also articles on countries of Africa.
11YDROGliAP111". 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 (Tosses to Africa from Syria, which follows the coast line 14 the Bed Sea to its southern ex tremity, 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-part ing of the continent. Within its bounds are the upper courses of the Nile, Congo. and Zam bezi, as well as of the Orange and of most of the smaller streams. The Nile. Niger and Congo rivers have their origin on the interior slopes of the highlands. and therefore discharge into
the Atlantic Ocean, while the Zambezi drainage basin, lying largely on the outer slopes. falls oil toward the Indian Ocean. The longest river system is that of the Nile. which rises in the lake region of Equatorial Africa and flows northward through the mountainous divide to the plateau region of eastern Sudan. where it receives an important affluent from the west in the Bahr-el-Ghazal, and is joined further north by the Bahr-el-Azrek (Blue Nile) and by the Atbara, both from the plateau of Abyssinia. In the middle portion of its course the Nile practi cally completes its vertical descent by numerous cataracts, after whieh it flows through a valley that is but little above the level of the sea. The drainage basin of the Nile includes an area of about 1.500,000 square miles. Next to the Nile in length and superior to it and to all other rivers of the world excepting the Amazon in volume is the Congo. which rises in the equato rial lake region and drains an area probably ex ceeding that of the Nile. The Congo flows north west, then describes a great are. with its chord formed by the equator, and finally turns south west, and pierces the coastal barrier of lower Guinea to enter the Atlantic. The tributaries of the Congo include many great rivers, such as the Ubangi, Kassai, and Knango. South of the Congo are the drainage basins of the Zambezi and Orange rivers, which extend nearly across the lower limb of the continent, and have an east ward and westward slope respectively. The great land-mass composing the western limb of the continent is poorly watered, the Niger being the only river of first importance lying wholly within the area. This river drains the northern slopes of the coastal highlands of Guinea, throm7h which it breaks after being joined by an important tributary from the east, the Benue. and enters the Gulf of Guinea. Of lesser rivers may be mentioned the Limpopo, Bovtuna, Sabi. Tana, and Joh, which enter the Ocean, and the K u n en e, Kit a n za , Ogowe, Volta. Gambia Senegal, and Draa on the western coast. Owing to the mountainous barrier through which they must pierce to reach the sea, the smaller rivers of Africa generally are unnavigable in their lower courses.
Between the drainage basins of the Nile, Niger, and Congo. and west of the north and south range of highlands of Sudan, is the interior basin of Lake Chad. This lake is fed by the Shari and Waube, and is subject to great variations of level. It is at the present time a shallow body of fresh water, with an area that is said to range at various times from 10.000 to 20,000 square miles. This phenomenon of sudden variations in level and eonsequently in area is peculiar to all the rivers and lakes of Africa within the equatorial regions. and is due to the seasonal distribution of rainfall. Between Abyssinia and the Zambezi River and within the hounds of the north and south highland region there is another inland drainage basin with sev eral large lakes, which together constitute one of the most striking physiographieal features of Africa. Apparently the lakes lie along a line of rifts or fissures which have been formed by sudden displacements of the earth's erust. Some of the lakes are, Margherita, Abaya. Stephanie. Rudolf, Manyara, Natron, Baringo, Eyassi, and Leopold ( llikwa), all but Rudolf being 51111111 bodies of water. The largest lakes (Victoria, Albert. Albert Edward, Kivu, Tanganyika, and Nyassa ) drain into the Nile, the Congo, or the Zambezi. and are fresh water bodies. Victoria, Tanganyika. and Nyassa rival in extent the great lakes of North America. For further details, see articles on CONGO. VICTORIA NYANZA, etc.