Madagascar, off the eastern coast, is the only large island near the continent; it was at a distant period of geological time an integral part of the mainland, but it is now separated from it by the Mozambique Channel, which appears to be a rift valley analogous to that of the Red Sea. The Seychelles, the islands in the vicinity of Zanzibar (Mafia, Zanzibar, and Pemba), and Socotra, off the apex of the Somali Peninsula, may be considered as fragments of the continental mass, while many of the small islands along the east coast, including those in the Red Sea, are of volcanic and coral reef origin, and rise apparently from submerged portions of the continental plateau. On the Mediterranean coast the islands of Djerba and Kerkinah in the Gulf of Cabes were formerly united to the mainland, and in past geological times even the island of Sicily was part of a chain of folded mountains that extended from the Tunisian highlands northeastwardly across the Mediterranean Sea. Off the western extension, the Madeira, Canary, and Cape Verde archipelagoes are of volcanic origin, and appear to lie on the outer submerged slope of the continent, perhaps marking lines of folding and fracture that are extended under the ocean level. The Bissagos group, thirty in number, lying a short distance south of Cape Verde, are small fragments of the mainland. From the Bissagos group, the coast is free from islands as far as the head of the Bight of Biafra, where four volcanic islands, Fernando Po, Prince, St. Thomas, and Annobon, extend in a southwestward direction from Mount Kamerun on the coast. Southward from this point the coast has but few islands, and these of small size, all the way to the Cape of Good Hope; and this same condition, in even more marked degree, is continued along the eastern coast for 2500 miles to the island of Mafia. The small extent of Africa's island territory is expressed by its proportion to the mainland area, which is as 1 to 48.
Topography. The typical expression of African topography is that of a plateau that rises here and there by successive terraces to increasing elevations up to and beyond 4000 feet, which altitude is the general level of the highland region that covers a large part of the southern and eastern portion of the continent. The edges of the continental mass are as a rule somewhat more elevated than is the interior, and the plateau rims approach close to the sea. Only along the eastern part of the Mediterranean shore and along that part of the Atlantic seaboard between Cape Juby, near the Canaries, and Freetown, can there be said to exist a coastal plain that extends for any considerable distance toward the interior. Swampy districts of limited extent are found along the upper Guinea shore and on the east coast about the mouths of the Zambezi River, and a lowland borders the south side of the Somali Peninsula. The mean elevation of Africa, obtained by a reduction of all irregularities of the surface, has been estimated to be about 2100 feet, which is about equal to that of South America and somewhat less than that of North America, while it is greatly exceeded by the mean elevation of the Eurasiatic continent.
The topography of the interior presents over large areas a marked uniformity of expression, though different regions exhibit distinctive features. The general plateau character of the surface is broken in the interior of the continent by four areas of depression which in the south and north are occupied by basins of internal drainage. In the southern highland is the Kalahari-Ngami Desert (altitude 2250-3000 feet); the central plateau falls toward its middle to form the Congo Basin (altitude 600-1600 feet); in the central Sudan the Lake Chad (altitude 900 feet) and Bodele (altitude 500 feet) depressions receive the drainage of a great interior region that has no outlet to the sea; and in the northwestern Sahara several inclosed basins lie at altitudes of from 400 to 600 feet above the ocean.
Africa is divided topographically into the following regions: (1) the elevated Southeastern Highlands, (2) the Sahara and Sudan plateau of lower level that covers the entire central and most of the northern part of the continent, and (3) the narrow, comparatively small area of the Atlas Mountains on the extreme northwest coast. On the whole, the general slope of the surface is from the southeast to the northwest.
The highest portions of the continent, called the Southeastern Highlands, lie near the eastern coast and in the lower end of the southern limb. They are limited on the north by an irregular line that may be drawn from Loanda on the west coast, at the mouth of the River Kuanza, eastward to Ankoro on the Upper Congo, thence northward to Daruma, and through Lado and Kassala to Suakin on the Red Sea. Northward from Suakin the eastern highland is continued as a narrow ridge of lower elevation along the western shore of the Red Sea almost to Cairo. This great highland region may be topographically considered to form the backbone of the continent, though it is scarcely that in a geologic sense, for the rocks of which it is composed lie generally horizontal, and the differences of topography are the result of long continued erosion and denudation rather than of mountain-making forces. This highland has an elevation of over 4000 feet, and above this height rise numerous isolated and grouped peaks to altitudes of 10,000 feet and over. The majority of these high peaks are remnants of a dissected plateau of still higher level, while others are volcanic mountains that rest upon the table-land and rise above it to still greater heights of from 12,000 to 20,000 feet. The central depression of the Kalahari Desert and Ngami Basin in the southern part of the highland, and the deep valleys cut by the rivers that drain this interior basin, serve to divide this southern region into four well-marked isolated plateaus. The most southerly plateau occupies the Cape, Natal, Orange River, and Transvaal colonies, and their seaward edges, known as the Roggeveld, Schnee, Zwarte, and Drakenberg mountains, rise in single peaks of 9000 to 11,000 feet. North of the Transvaal, between the Limpopo and the Zambezi valleys, is the less extensive plateau of Matabeleland. with an average level of 4500 feet and a single peak (Mashona Mountain, 7300 feet), near its eastern edge. On the western side of the continent, between the Kalahari-Ngami Basin and the Atlantic coast is the plateau of German West Africa, covering Damara and Great Namaqualand. This plateau rises to somewhat lesser single heights than does the plateau of British South Africa: Kara (6500 feet), Awas (6530 feet), and Omatoka (8700 feet). Northward of all these, and extending from west to east through Angola and British Central Africa to the vicinity of lakes Nyassa and Tanganyika,
