Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/202

176 this excess of pressure in the continental platform:—

The excess of pressure thus increases very rapidly in the uppermost portions because there rock is in contrast to air; in the next section only about two-thirds as quickly, since here water is present in the oceanic area. The maximum excess of pressure is reached at the depth of the ocean-floor. At still greater depths it will become smaller again, since now the heavy sima lies in the oceanic area, and causes there an accelerated increase of pressure; and on the under-surface of the continental blocks the pressures must naturally have been equalised.

This difference of pressure produces a region of stress on the vertical continental margin, which is struggling to press out the material of the continental platform into the oceanic space, and most of all in the layer that forms the deep-sea floor of the ocean. If the sial were mobile, it would spread out in this layer. Now that is not the case. But the sial is, however, plastic enough to yield noticeably to these enormous forces, as is well shown in the step-like faults, which as a rule accompany the margin of a continent (Fig. 38). This lateral forward flow of the deeper plastic layers is also the reason for the fact that the margins of blocks split asunder and widely