Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/46

22 practically unaltered, the fact that in all geological periods extensive portions of the continental blocks have remained dry land, forces us to the conclusion that the total oceanic area has been essentially constant. This would mean that if the position of the continents is unchanged, hitherto considered as obvious, the several ocean basins have also been permanent features on the face of the earth.

Further, the advocates of the permanence doctrine base it on the geophysical facts of isostasy or the equilibrium in the earth’s crust. According to this theory, the relatively light crust of the earth swims in a somewhat heavier magmatic lower shell. Just as a piece of wood is immersed more deeply into water by loading it, so this uppermost rocky crust sinks deeper into the heavy magma, according to the law of Archimedes, at the place where it is burdened. This happens, for example, with an ice-cap, in which case the shore-lines formed during the depression will be elevated after the melting of the ice. Thus the isobase maps of de Geer, founded on raised beaches, show a depression of the central portion of Scandinavia of at least 250 m. for the last glaciation, an amount which becomes gradually less as we recede from the area, but which assumes still higher values during the Great Ice Age. De Geer has detected the same phenomena in the case of the glaciated area of North America. Rudzki has shown that on the assumption of isostasy plausible values for the thickness of the inland ice-sheet can be calculated, namely, 930 m. for Scandinavia and 1670 m. for North America, where the depression amounted to 500 m. Since the magma of the underlying shell is not so mobile as water, but is