Page:South African Geology - Schwarz - 1912.djvu/84

 which if spread over the surface would cover it 1 ft. deep, then in nine million years South Africa would be awash. There are forces which counterbalance this degradation of the continents, in that they cause the elevation of land surfaces, nevertheless in the course of

geological time the land has been covered repeatedly by the sea.

Mountains of Circumdenudation. — Weathering and erosion go on most vigorously along the river courses, and hence it frequently happens that blocks of land between two river systems remain comparatively free from attack, while the surrounding country is cut away. These blocks stand out as mountains of circumdenudation. The Karroo kopjes are the type of this kind of mountain. Their sides are said to be scarped or to form an escarpment. If the rock is sufficiently hard to turn aside the rivers, as in the case of a granite