Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/13

Rh by Wegener and others at a hundred kilometres, a figure which I am inclined to think is excessive. However this may be, it is Professor Wegener’s belief that the drifting of the continents is in effect a movement of masses of sial through the sima which slowly yields to their passage. He compares the physical character of the sima to sealing wax which may be regarded as an extremely viscous fluid. The viscosity (that is to say, the resistance to change of form) would, of course, be very much greater in sima, but in the course of the vast periods of the earth’s history it would yield in the same manner to continuously acting forces.

I am inclined to think that the most important distinction between the sial and sima lies in the fact that the magma from which the sial originally crystallised out owed its fluidity to the very large amount of magmatic water and other volatile constituents that it contained and when these have once been lost in the course of crystallisation, a far higher temperature than that of the original magma will be required to bring it once more into a fluid state. The sedimentary and metamorphic rocks are also as a whole difficult to fuse.

These considerations do not apply to the same extent to the sima. Basic magmas contain less water and there is not so much difference between the temperature of the original crystallisation of the rock and that required for remelting.

The sima could therefore be more easily brought into a molten or semi-molten state by a rise in temperature which might be brought about through the blanketing of the rocks by the deposition of sediments above them and by the depression that follows such accumulations, or simply, as suggested by Professor Joly, as the result of radioactivity.

Professor Wegener supposes that the sial originally