Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/176

150 could actually become sufficiently increased, by the smoothing out of all the folds in the continental blocks, for it to embrace the whole earth. The continental blocks, however, with their shelves nowadays only occupy a third of the earth’s surface, but we obtain for the Carboniferous a considerable increase in area (to about a half of the earth’s surface). But the further back we go into the earth’s history, the more wide-spread are the foldings. E. Kayser writes: “It is of great importance that the most ancient Archæan rocks are strongly disturbed and folded everywhere on the earth. It is only from the Algonkian onwards that we find, besides folded rocks, here and there non-folded, or but slightly folded deposits. If we pass to the post-Algonkian period, we see how the extent and number of the rigid unyielding masses increase more and more, and correspondingly the area of the foldable portions of the crust become increasingly limited. This applies especially to the Carboniferous and Permian thrusting. The folding forces gradually weaken more and more in the post-Palæozoic, in order, however, to awake again in the Upper Jurassic and the Cretaceous, and to attain a new maximum in the Lower Tertiary periods. But it is very significant that the area affected by this most recent great mountain thrusting is considerably less than that of the Carboniferous folding.”

According to this, our assumption that the sialsphere once surrounded the whole earth is not at all in opposition to previous ideas on the subject. This movable and plastic skin of the earth was then, on the one hand, torn apart, and on the other thrust together by forces the nature of which will be discussed later. Thus the origin and the enlargement of the deep-sea basins is only one aspect of this process, the other of which consists of