Page:South African Geology - Schwarz - 1912.djvu/22

 reversed action goes on: chemical changes occur which absorb heat.

(b) Movements of extraordinary magnitude have occurred in the earth's crust. At Worcester, Cape Colony, for instance, a segment of the earth has slipped down vertically 2 ml., and faults of like nature occur throughout the country. In the Highlands of Scotland and in the Alps great segments of rock have been bodily thrust 10 ml. horizontally over other rocks, and in every mountain range similar movements have taken place. These movements are very slow, so that the heat produced by them is spread over a vast period of time; nevertheless they are in progress to-day as they have been in the past, and are yielding a constant supply of heat to the earth's crust. All movements in the crust do the same; earthquakes, for instance, which heave the ground, produce a certain amount of frictional heat. In the Charlestown earthquake of 1886 it has been calculated that a force of 1,300,000 h.p. was expended over an area of 100 sq. ml., and all this energy was dissipated as heat.

(c) Radium occurs in all rocks of the earth's crust. The Hon. R. J. Strutt found that granite from Rhodesia contained 9'56 tons of radium in every billion tons of rock, or, say, 200 lb. of radium per cubic mile.

A ton of radium will give out, during its half-life of 1760 years, heat equivalent to that produced by a million and a half tons of coal burnt during that time. Taking granite as the typical rock forming the crust of the earth below the covering of sedimentary rocks, and assuming the crust to be 30 ml. thick and the area of South Africa, including Rhodesia, to be 1,235,000 sq. ml., the slice of the earth represented by South Africa is being