Page:South African Geology - Schwarz - 1912.djvu/59

 mean sea level and just about 6 ml. below. The question arises: Is there any limit to these irregularities? I take the simplest case, that of the mountains. Can the earth's crust support an elevation greatly exceeding 6 ml.? An investigation into the crushing strengths of rocks proves that if a mountain stands over 6 ml. high, the weight of the mass will exceed that which the strength of the rocks will stand. The sea acts to a certain extent as a help to sustain the weight, but still, where there are great differences of elevation between the mountains on land and the bottom of the sea off shore, there is unstable equilibrium and movement takes place, evidenced by continuous earthquakes. Two such areas are very marked. In South America, south of the bend in the west coast, there is a very deep hollow in the ocean floor close inshore, in which soundings occur of 25,052 ft., and the Andes adjacent rise to a maximum of 23,100 ft. at Aconcagua, so that there is difference of level of over 48,000 ft. (9 ml.) within a short distance, and this area is celebrated for its earthquakes. Off the coast of Japan there is a great long hollow, following the eastern coast line, with depths exceeding 26,000 ft., while the mountains on the mainland reach a maximum in the sacred cone of Fujiyama, 12,390 ft. high, a total difference of level of about 38,500 ft. (7 ml.), and here again the land is shaken with incessant earthquakes. These instances show that the crust of the earth cannot be loaded beyond a certain limit without very marked effects being produced. In the continents of less-pronounced relief the same thing goes on, though by imperceptible stages, and continents are continually tending to sink; though this may be more than balanced by forces causing elevation.

The Rocks of the Earth's Crust. — There are three types