Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/47

Rh extremely viscous, all such isostatic movements of compensation must lag greatly. The strand-lines were formed chiefly after the melting of the ice, but before the elevation. Indeed, Scandinavia is still rising, as levelling shows, at the rate of about 1 m. in 100 years. Also deposition of sediments, as was first recognized by Osmond Fisher, results in the depression of the block. Each loading leads to a somewhat delayed sinking of the platform, and thus the new surface lies again at practically the old elevation. Sediments many kilometres thick may originate in this manner, which have, however, all been formed in shallow water.

Gravity measurements are the physical basis of the doctrine of isostasy as deduced by Pratt (the word having been coined by Dutton in 1892). In 1855 Pratt had already ascertained that the Himalayas did not exert the expected attraction in the plumb-line experiments and correlated with it the fact, which was universally confirmed, that the force of gravity in great mountain chains does not deviate by the expected amount from the normal values. Thus the mountain massifs appear to be compensated in some way by subterranean defects of mass, as the work of Airy, Faye and Helmert, among others, showed, and also as worked out recently by Koszmat in a very lucid article. It has been pointed out that on the oceans gravity possesses almost its normal value in spite of the evident defects of mass formed by the great ocean basins. The earlier measurements on islands, however, gave scope for differing interpretations, but after Hecker