Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/162

136 distribution of radium in the earth’s crust. For the temperature of the centre of the earth, which, in contrast to the earlier much higher estimates, is assumed to be about 3000° to 5000° C., we consequently possess only very scanty fundamental information. Still it is probable that temperatures between 1000° and 2000° C. are to be expected at 100 km. depth, so that the assumption that a temperature of about the melting-point is reached on the under-margin of the continental blocks is not contradictory to our previous ideas.

It must certainly not be thought that this melting temperature is at the same depth all over the world, and that the depth is constant at all times. On both these questions the phenomenon of “granite fusion” is very instructive. The observations by Cloos in South Africa has freed its significance from previous doubts, and shown that the fusion-isotherms can at times penetrate up to the surface of the earth. It is probable that to these places with an abnormally high position of the fusion isotherm are opposed others with an abnormally deep position. We do not know the causes of the variations in the course of time. In this case also the radio-active transformations perhaps play a part.

In any case, the mobility of the sima must be increased by high temperature. We do not know, yet, however, how these relations vary with the depth, and whether there is especially a zone of greatest mobility at the under surface of the continental blocks.

But in any case the separation of the blocks of sial will be favoured by the fact that, according to Doelter, the melting-point of sial rocks is in general 200° to 300° higher than that of the sima, so that magmatic