Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/187

Rh same. Since we assume that the continental block rising 5 km. out of the sima is 100 km. thick, we can take this ratio to be about 1:20. Hence the downwardly directed part of the compressed sial must be 20 times greater than that directed above. What we see in the mountain chains is thus only a very small portion of the whole squeezed up mass, and this consists only of those strata which already lay above the ocean-floor even before the compression. All that lay below this level remains there after the compression, if disturbances are disregarded. If thus the upper structure of the block consisted of a layer of sediments 5 km. thick, then the whole mountain mass would originally consist of sedimentary deposits. Only when this becomes worn away by erosion does a central chain of

primitive rocks rise up to isostatic compensation, until finally a broad massive of practically the same average height is visible after the more complete peeling of the sedimentary cover.

The Himalayas and their neighbouring mountain chains can be considered as examples of the first stage. The erosion is enormous in these sedimentary folds, so that glaciers are practically buried beneath the débris—as, for example, the Baltoro glacier, the greatest in the Kara-Korum range, which, with a breadth of 1$1⁄2$ to 4 km. (length 56 km.), carries no fewer than 15 median moraines. The second stage, in which the central chain is already composed of primitive rocks, but is still flanked on both sides with a zone of sediments, is found in the Alps. Since erosion is much slighter in primitive rocks, the Alpine glaciers are poor in moraines, one of