Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/147

Rh discussed isostasy in, and have also referred to Scandinavia and North America as examples of isostatic movements of compensation. They were submerged about 250 and 500 m. respectively in the Pleistocene by the load of the inland ice, but have been elevated these same amounts after the melting of the ice. Rudzki has shown that it is not a matter of elastic deformations, for he calculated the very plausible thickness of 933 m. for the former ice-sheet from Airy’s theory of isostasy on the basis of the assumed elevation of Scandinavia of 280 m. (the figure for North America being 1667 m. with an elevation of 500 m.). The supposition of an elastic deformation, on the other hand, would lead to a wholly improbable thickness of ice of 6 to 7 km. The lag in the movement of elevation also testifies to the fact that flow-movements must be taken into account. Scandinavia is still rising about 1 m. in a century, although 10,000 years have passed since the maximum temperature, when the ice was removed. W. Köppen has recently made it seem probable that the area depressed by the burden of ice is surrounded by a zone with slight opposite vertical movement, and explains this by the lateral squeezing out of the sima beneath the depressed block. All this naturally presupposes viscosity.

Not only do the vertical movements for the restoration of isostasy, but also the horizontal movements of the continents, positively demand that the earth be viscous. We need not consider this point any further, since it has been dealt with sufficiently in the preceding pages.