Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/160

134 but nevertheless the most fluid. Sial preserves its form (as continental blocks) so long as the forces remain under the limit, but becomes folded or fractured when this is exceeded.

We have not in the foregoing pages considered the temperature relationships in the body of the earth. These also are of importance with regard to the question of the possibility of displacement. The composite silicate rocks have no sharp melting-points, as the researches of Doelter and Day have shown, but only a range, occasionally very great, of melting temperature. It can be said that diabase melts at 1100° C. and Vesuvian lava at about 1400° to 1500° C. These temperatures apply to atmospheric pressure, so that some 100° C. must certainly be added for a depth of 100 km. On the other hand, the deepest boreholes to-day of Czuchov II. and Paruschowitz V. in Upper Silesia give an increase of temperature of 3.1° C. for each 100 m. depth for the uppermost 2 km. of the earth’s crust. These measurements were carried out in sedimentary rocks which possess a smaller thermal conductivity than igneous rocks, in consequence of which the isotherms therein are closer together. In the primitive rocks of the tunnels of St. Gotthard, Mönch and Simplon the temperature gradient is only 2.2, 2.2 and