Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/62

38 from here to the surface must always weigh the same, whether it be taken in the continental area or in the oceanic. If we call the specific gravities of sial x and sima y, and consider that that of sea-water, which must also be taken into account, is 1.03, the equation will therefore become

Now, since sima rocks like basalt, diabase, melaphyre, gabbro, peridotite, andesite, porphyrite, diorite, among others, have mostly a specific gravity of about 3.0 (only rarely up to 3.3), we can put y = 3.0 and then obtain x = 2.9. Whitman Cross and Gilbert actually obtained the specific gravity of 2.615 for gneiss as an average from twelve specimens. Other observations gave values between 2.5 and 2.47. This small difference, however, can easily be explained, in that the specific gravity in the sial zone as well as in the sima increases with depth, and that basalt is derived from great depths, whilst the specimens of gneiss came from close to the surface. To be sure, this is not mathematically proved, since we do not know the extent of the increase of specific gravity with depth. We only know that, according to research on earthquakes, the average value of 3.4 is yielded for the whole of the approximately 1500 km. thick silicate mantle of the earth. These figures agree, in any case, qualitatively with our assumptions.

Finally, in this connection the smoothness of the ocean floor must be mentioned, because it also confirms the accuracy of our ideas. It has been long known that the ocean floor often shows astonishingly slight