Page:South African Geology - Schwarz - 1912.djvu/49

 depression which may become filled with water, as at Lake Tanganyika in the Great Rift Valley, and the Dead Sea, but these will be discussed later on in connection with faults.

The salt of pans and lakes is usually common salt with an admixture of gypsum; in Palestine the two are mixed, so that a block of salt may "lose its savour", that is, all the salt may be sucked out and only gypsum may be left behind. In all the South African salt pans the salt crystallizes out separately in a layer above the gypsum. In North Africa there is a great quantity of sodium carbonate in the lakes; this comes up in springs, and is not of superficial origin. Such a sodium carbonate spring, occurs at Palapye, in Bechuanaland. In Egypt the ordinary washing soda or natron has been obtained from time immemorial from the natron lakes in the Suez region. A natron lake occurs also in the Western Rift Valley, east of Tanganyika. In the Lake Chad region the substance is the bicarbonate of soda, trona. Elsewhere, as in Tibet, in the Great Salt Lake, of Utah, and in the lagoons of Tuscany, borax is contained in the water, but in these cases the springs which contribute the substance to the lake waters are connected with volcanoes.

The highest lake in the world is Titicaca, 12,576 ft., lying under the giant volcanoes of the Andes, Illimani, and Sorata, each 21,000 ft. The lowest lake is the Dead Sea, 1286 ft. below sea level. There are not many lakes below sea level — Lake Assal (-566 ft), in Abyssinia, is an example. If the flow of river water into the Black Sea were to be stopped by desert conditions coming over the land, the level of the sea might fall far below sea level, as the Black Sea lies in a basin over 7000 ft. deep