Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/188

162 the chief reasons for their beauty. Finally, the Norwegian mountains represent the third stage: the sedimentary cover is entirely removed and the elevation of the massive completed. Thus the denudation of the sedimentary capping of a mountain chain is effected without disturbing isostatic adjustment.

We must deal shortly with the asymmetry of folded mountain chains, which can nearly always be recognized. It is a general rule that, on approaching a mountain mass from one side, one has to pass a general gradual rise of ground, often also foothills and similar features, whilst on the other side the “foredeep” reaches close up to the main system of folds. Much has been written about this. A very simple explanation is yielded by our hypothesis. The masses of sial, pushed deep down by the folding, spread themselves out, and to some extent penetrate beneath the unfolded crust, near the fold, bearing it up, naturally at the expense of the altitude of the mountains themselves. This spreading out can only be symmetrical on both sides when the crust has no progressive movement over the sima. But if the crust is displaced—not taking into account its folding movements—as a whole over the sima (which will almost always be the case), then the spreading out of the sial masses must take place unilaterally. The European and Asiatic blocks have doubtless for a long time endeavoured to reach the equator, and thus are thrust southwards relative to the sima. Moreover, they probably take part in the general westerly drift of the continents. Therefore the total movement relative to the sima will be directed to the south-west. The spread of the masses of sial beneath them must follow unilaterally to the north-east. This, as a matter of fact, is the case, and is shown on the map of Fig. 31. The displacement of the gravity anomaly, or, what amounts to the same thing, of