Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/190

164 remained at the same elevation. This was explained by Hall by means of isostatic movements of compensation on account of the weight of the deposits, in a similar manner to the submergence of a continental block beneath the burden of land-ice. But why is it just those portions of the blocks that have especially thick sedimentary deposits which are later involved in the folding? Such regions with heavy sedimentation are known as geosynclinals (basins). Haug formulated the law thus: mountain chains are formed from geosynclinals. Probably it might be better expressed: from continental shelves; because a marginal shelf—as, for example, that from which the South American Andes are built—can hardly be called a basin. There are a variety of reasons why the shelves are favoured by folding. It has already been mentioned that they perhaps contain especially numerous and large inclusions of sima, and are on that account more plastic. It may also be possible that here the sial crust possesses a lesser total thickness, and thus also a lesser power of resistance. Reade suggested that the primitive rock was forced down by the thick deposits into the region of higher temperature, and was therefore made more plastic. Perhaps all these causes work in co-operation.

If we consider the regional distribution of the folded mountain systems, we see that two regions are favoured by them: the anterior margin of drifting blocks and the equatorial belt. This strikes us particularly forcibly in the case of the last great period of folding; it is that of the Tertiary, in which, on the one hand, the chief folding is found on the anterior margin of the American blocks as well as of that of Australia-New Guinea, and on the other in the zone of the