Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/179

We know very little about the inner structure of the blocks of sial.
Fig. 29.—Former and future hypsometric curves of the earth’s surface.
····· for the future ——for the present,—·— for the past, - - - - - original surface (coinciding with the mean crustal level).The fact that volcanoes exist at numerous places on the continental blocks, which emit magmas of sima composition, has been explained by Stübel on the widely accepted assumption that in the interior of the blocks, surrounded on all sides by solid or at least rather stiff sial, fluid, or relatively fluid, inclusions of sima (peripheral magma reservoirs), occur, which feed the volcanoes. On the other hand, no reason is to be seen why such slightly different materials as the sial and sima should completely separate, or have separated, in the body of the earth; much more probably there has been from the beginning a gradual transition from one to the other.
Fig. 30.—Section through a sial block. I imagine, therefore, the structure of the sial crust to be of such a form as is shown diagramatically in Fig. 30—uppermost a zone of continuous sial, with isolated inclusions of sima; below this a dovetailed zone in which each of the two portions are continuous; and beneath all a zone of continuous sima, in which lie a few isolated