Page:South African Geology - Schwarz - 1912.djvu/110

 by the intrusion of igneous rocks, exert sufficient thrust to form mountains.

Faults.—The earth's crust is like an immense arch spanning the central nucleus. If one pulls a little apart the two sides of an arch, the keystone will fall downwards; if the earth's crust be pulled apart, a strip representing the keystone will fall downwards. The block that has sunk is said to be faulted, and the breaks on either side are called faults. In the matter of folds we saw that the beds became inclined, and the inclination or dip was measured from the horizontal; that is because folds are most conspicuous in mountains and on the surface of the earth, where the most important plane by which we orientate ourselves is the horizontal. Faults, on the other hand, are seldom very well marked on the surface,