Page:South African Geology - Schwarz - 1912.djvu/118

 irregularities of the surface; this line is called the outcrop of the bed. If the bed outcrops on a horizontal plain, the outcrop becomes the strike; the strike is at right angles to the dip. A geological map of a district gives the outcrops of the various beds seen from a bird's-eye point of view; the smaller the scale the nearer the outcrops approach the strikes of the beds, for the undulations in the course of the outcrop become too small to be inserted.

Both are hollowed out as by a river course. In A the "outcrop" points upstream, and in B the outcrop points downstream.

Igneous Action.—Rocks become molten in the deeper layers of the crust; whether by the central heat or by solution does not immediately concern us. The liquid rock moves upwards, thrusting apart the sedimentary or other rocks, or melting out cavities for itself by absorbing the walls of its chamber. The molten rock is said to be intruded into the earlier rocks. If the liquid magma or fluid rock does not rise very far, but solidifies in an enormous lump in the deeper portions of the earth's crust, the mass is called a plutonic boss. Examples of these are the bosses of granite in the Western Province, such as that forming the base of Table Mountain, the Paarl and Paarde Bergs, the great masses of granite at George, and that which occurs between Johannesburg and Pretoria. If the granite is harder than the rock it has intruded, denudation causes it to