Africa — страница 4
outflow of Lake Nyasa, flow down the southern slopes of the band of high ground which stretches across the conbnent in 10 deg. to 12 deg. S. In the south-west the Zambezi system interlaces with that of the Taukhe (or Tioghe), from which it at times receives surplus water. The rest of the water of the Taukhe, known in its middle course as the Okavango, is lost in a system of swamps and saltpans which formerly centred in Lake Ngami, now dried up. Farther south the Limpopo drains a portion of the interior plateau but breaks through the bounding highlands on the side of the continent nearest its source. The Rovuma, Rufiji, Tana, Juba and Webi Shebeli principally drain the outer slopes of the East African highlands, the last named losing itself in the sands in close proximity to the sea. Another large stream, the Hawash, rising in the Abyssinian mountains, is lost in a saline depression near the Gulf of Aden. Lastly, between the basins of the Atlantic and Indian Oceans there is an area of inland drainage along the centre of the East African plateau, directed chiefly into the lakes in the great rift-valley. The largest river is the Omo, which, fed by the rains of the Abyssinian highlands, carries down a large body of water into Lake Rudolf. The rivers of Africa are generally obstructed either by bars at their mouths or by cataracts at no great distance up-stream. But when these obstacles have been overcome the rivers and lakes afford a network of navigable waters of vast extent. The calculation of the areas of African drainage systems, made by Dr A. Bludau (Petermanns Mitteilungen, 43, 1897, pp. 184-186) gives the following general results:— Basin of the Atlantic . . . . . 4,070,000 sq. m. '' '' Mediterranean . . . 1,680,000 '' '' '' Indian Ocean . . . . 2,086,000 '' Inland drainage area . . . . . 3,452,000 '' The areas of individual river-basins are:— Congo (length over 3000 m.) . . 1,425,000 sq. m. Nile ( '' fully 4000 m.) . . 1,082,0004 '' Niger ( '' about 2600 m.) . . 808,0005 '' Zambezi ( '' '' 2000 m.) . . 513,500 '' Lake Chad . . . . . . . . . 394,000 '' Orange (length about 1300 m.) . . 370,505 '' '' (actual drainage area) . . 172,500 '' The area of the Congo basin is greater than that of any other river except the Amazon, while the African inland drainage area is greater than that of any continent but Asia, in which the corresponding area is 4,000,000 sq. m. The principal African lakes have been mentioned in the description of the East African plateau, but some of the phenomena connected with them may be spoken of more particularly here. As a rule the lakes which occupy portions of the great rift-valleys have steep sides and are very deep. This is the case with the two largest of the type, Tanganyika and Nyasa, the latter of which has depths of 430 fathoms. Others, however, are shallow, and hardly, reach the steep sides of the valleys in the dry season. Such are Lake Rukwa, in a subsidiary depression north of Nyasa, and Eiassi and Manyara in the system of the eastern rift-valley. Lakes of the broad type are of moderate depth, the deepest sounding in Victoria Nyanza being under 50 fathoms. Apart from the seasonal variations of level, most of the lakes show periodic fluctuations, while a progressive desiccation of the whole region is said to be traceable, tending to the ultimate disappearance of the lakes. Such a drying up has been in progress during long geologic ages, but doubt exists as to its practical importance at the present time. The periodic fluctuations in the level of Lake Tanganyika are such that its outllow is intermittent. Besides the East African lakes the principal are:—-Lake Chad, in the northern area of inland drainage; Bangweulu and Mweru, traversed by the head-stream of the Congo; and Leopold II. and Ntomba (Mantumba), within the great bend of that river. All, exceot possibly Mweru, are more or less shallow, and Chad appears to by drying up. The altitudes of the African lakes have already been stated. Divergent opinions have been beld as to the mode of origin of the East African lakes, especially Tanganyika, which some geologists have considered to represent an old arm of the sea, dating from a time when the whole central Congo basin was under water; others holding that the lake water has accumulated in a depression caused by subsidence. The former view is based on the existence in the lake of organisms of a decidedly marine type. They include a jelly-fish, molluscs, prawns, crabs, &c., and were at first considered to form an isolated group found in no other of the African lakes; but this supposition has been proved to be erroneous. Islands.—With one exception—-Madagascar—the African islands are small.