Page:South African Geology - Schwarz - 1912.djvu/111

 but they are conspicuous in mining operations, where they often displace the lode; and in a mine, where one is constantly thinking of the depth that one is below the surface, in other words, where the vertical is the more important plane, all features are referred to this plane, and hence the inclination of faults is measured from the vertical. It is called the hade, to distinguish it from the dip. Faults, then, are planes along which rocks have broken across; one side has sunk, the other remained stationary. The side that has sunk is called the downthrow side, and the amount of the sinking is called the throw of the fault. The fault plane hades towards the downthrow side, as is obvious from the illustration of the keystone of the arch; if the keystone were to be cut with the top narrower than the base, when the arch is pulled apart the block would not fall in and occupy the vacant space, but would leave that space empty. Faults which run parallel to axis of folding are called strike faults, those which cut across the axis of folding are called dip faults. The largest fault in South Africa is the Worcester Swellendam fault, with a throw of over 2 ml. vertical on the south.

Rift Valleys. — Faults were illustrated by the pulling apart of an arch, when the keystone would sink. The earth's crust is a vast arch, and it has been pulled apart in places, allowing extended strips to sink downwards between parallel faults; the sunken region constitutes a trough fault, or, in the larger cases, a rift valley. This structure, in its grandest development, exists in East Africa: the rift begins along the Shire River, extends northwards, includes Lake Nyasa, and then branches. The western branch includes Lake Tanganyika, Lakes Kivu and Albert, and then turns north-eastwards to