Page:South African Geology - Schwarz - 1912.djvu/96

 itself controls the rocks on the surface by gravitational pull. When the weight of the rocks is removed from a continental area, there is less material to pull in towards the centre, and the earth's crust consequently bulges; whereas, when the sea floor is weighed with sediments, it sinks, for the earth, though solid, allows long-sustained forces to operate on its form as if it were a liquid, while to sudden jars, like that of earthquakes, it is perfectly rigid. The relief from unloading and overloading is usually obtained by fits and starts, not continuously, so that there is a long period of quiescence, then the strain becomes too great and the earth's crust gives along a line of fault. Not only so, but it usually happens that when movement does occur of this nature, the segment of the earth springs up more than equilibrium demands, and hence there is an adjustment afterwards, when the portion elevated sinks a certain amount. Sinking and rising go on continuously, now one predominating, now the other. South Africa at the present time is on the rise, but the movement is one following a period of subsidence. Resting stages in the progress of the elevation of land masses are shown by the rivers cutting down to base level, consequently South Africa should show peneplains at various elevations, and these are everywhere extraordinarily well developed. The first great peneplain — perhaps, rather, in this case, an original plain of deposition in the Karroo grading into a peneplain on the north — was at what is now 6000 ft. above sea level. The Witwatersrand, the Central Karroo hills, Nieuweveld, Cambdeboo, Winterberg, &c., and the coastal mountains, the Zwartberg, Cederberg, Langberg, Outeniquas, Cockscomb, &c., all rise to this level. Cut into this great plain are all the valleys of the