Page:South African Geology - Schwarz - 1912.djvu/74

 million meteorites fall on the earth in twenty-four hours, and in a year the weight of these amounts to some 20,000 tons.

SECTION II

DYNAMIC GEOLOGY

Weathering. — Passing now from the descriptive part of Geology to that part which deals with the natural forces in action on the earth, that is to say, to Dynamic Geology, the first and most important is weathering, or the action of heat, water, and air upon the rocks exposed on the surface.

The simplest case is where the action of heat alone is operative, as in the desert, and, as the desert is at our doors nearly everywhere in South Africa, this type of weathering may be taken here as at least of equal importance to the weathering under humid conditions, which is the only type usually represented in Europe.

Take, as an example, a mass of dolerite exposed on the top of a hill on the Karroo. The sun's rays cause the rock to become very hot during the day, and when the night comes all the heat absorbed during the day is radiated into space, and the rock cools. When material is heated it expands, and when it cools it contracts; therefore the mass of dolerite will expand and contract, just as much of it, at least, as has been warmed by the sun, for the inner portions remain at a more or less constant temperature. The outer portion, then, is moving over the inner, and after a time the rock will split at the junction. If the mass is exposed at the edge of the hill,