Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/180

154 masses of sial. The original structure, at least, of the sial crust will have corresponded more or less to this scheme. By compression the crust can purify itself from the sima; most of the sima will then be pressed downwards, but some rises (in volcanoes), and spreads out as flat sheets. In great continental displacements, a kind of gliding plane, which is characterized by the fact that there the mineralogical composition changes especially rapidly, will be formed on the under-margin of the block of sial.

Such a structure of the continental blocks offers an explanation of many phenomena; it is, for example, in this way comprehensible that in the track of many drifting blocks (as Australia) the deep-sea floor is studded with numerous elevations, which may cause uncertainty as to whether they are to be considered as part of the deep-sea floor or of the continental block. The fact that the upper edge of the block retains its outline, whilst the deeper portions are drawn out, as in the neighbourhood of Iceland, also appears to be easily explicable in this way. Finally, it is perhaps possible to explain the frequently described “lability” of the geosynclinals, by the fact that, in the upper part of the sial crust, very numerous and large inclusions of sima are present. The surface of such a block would lie lower than its surroundings through the greater specific gravity of such masses, and the greater fluidity of these inclusions would also increase its mobility in the vertical direction, so that it easily sinks under the load of sediments. If, then, mountain-building compressive forces occur, such a part of a block, for the same reason, will be predestined for folding. The great eruptions of lava, by which mountain-building is always accompanied, appear to confirm the correctness of this idea. The inclusions will be forced out by the folding.