Page:South African Geology - Schwarz - 1912.djvu/86

 fore if we bore through the cracked portion of the surface rocks down to the uncracked portion we shall find there all the water that has soaked in from the rains. In Rhodesia and Bechuanaland the depth of this level is from 300 to 350 ft. The water in the cracked portion of the rocks is always moving from the higher to the lower levels, and care must be exercised, in locating a site for boring, that there is no outlet by which the water can be drained away. In well-watered countries the cracked portion of the rocks is always well supplied with water, and there is no definite water table or underground surface of the water, which lies above the impervious zone, and which rises and falls according to the annual rainfall. In South Africa, however, the rainfall is so small that the level for striking water practically coincides with the zone of division between the cracked and uncracked rocks.

There are exceedingly few pervious rocks in South Africa, and these mostly confined to the Cretaceous and Recent rocks of the coast; the chances for artesian water in South Africa, therefore, are few, except in these formations. There are deep-seated sources of water, however, which depend firstly on the rocks below the zone of surface-splitting being fissured by earth movements, and in these deep crevices water actively circulates. The Uitenhage spring comes from such a structural fissure. Secondly, the dolerite dykes form walls which bring up the waters of these deep crevices to the surface, as at De Aar and Beaufort West.

Volcanoes. — The most terrifying of geological activities is that of volcanoes; the earth is riven, and up the crevice boulders, ash, and eventually molten rock are vomited. The cause of volcanic action is undergoing