292 The Realm of Nature CHAP. 



differ in their nature from the cataracts of the plateau rivers 

 of the south. 



376. Congo Basin. Shut in between the Central and 

 West Coast Ridges, the equatorial basin was probably at one 

 time a great inland sea several times larger than the Caspian. 

 Its waters found an outlet across a comparatively low part of 

 the West Coast Ridge, which they eroded into a deep gorge 

 and so drained the lake into the Atlantic, leaving a basin 

 of fertile soil now covered in great part with dense forests. 

 Rivers flow into the circular basin from the high ground on 

 every side and become tributaries to the giant Congo. 

 This river descends from the Great Plateau at the equator 

 foaming over the cataracts of Stanley Falls, sweeps through 

 the basin in a magnificent curve as a navigable stream for 

 1000 miles, and bursts in a far grander chain of cataracts 

 over the plateau edge through the gorge of Yellala. The 

 source of the Congo lies somewhere in the Great Plateau 

 about 13 S., Lake Bangweolo, 4000 feet above the sea, 

 serving as a reservoir to collect the head- waters. In its 

 northward course the river is joined by the Lukuga from 

 Lake Tanganyika in the centre of the Great Hollow, 2600 feet 

 above the sea ; but it is only when the level of that lake is 

 raised considerably above its average height that it overflows. 

 Tanganyika, like most continental lakes, was once much 

 larger, and appears to be shrinking into a basin of internal 

 drainage, destined ultimately to become a small salt lake. 



377. Tsad Basin and Sahara. The northern basin 

 enclosed by the West Coast and Central Ridges is even 

 larger than that of the Congo, and so far as this very 

 inaccessible region has been explored it appears to have 

 no outlet. Lake Tsad, a sheet of shallow water varying in 

 size from 4000 to 10,000 square miles according to the 

 rainfall, and 800 feet above the sea, receives a number of 

 great rivers from the south, and overflows in the rainy season 

 to a much lower enclosed basin in the north-west, where 

 excessive evaporation leaves only a crust of salt upon the 

 ground. To the north nearly the whole breadth of Africa 

 forms the internal drainage area of the Sahara, a sandy 

 high plain broken by the rugged mountains of the Tibesti 



