Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/60

36 pumice … then fragments of sanidine, plagioclase, hornblende, magnetite, volcanic glass and its decomposition product palagonite, also pieces of lava, basalt, augite-andesite, etc., are found.” Volcanic rocks are in fact distinguished by greater specific gravity and greater iron content, and are generally considered as derived from greater depths. Suess called all this basic group of rocks, the chief member of which is basalt, “Sima,” from the initial letters of the principal constituents, silicon and magnesium, in contrast to the other more siliceous group of “Sal” (silicon and aluminium), the chief representatives of which, gneiss and granite, form the substructure of our continents. Following a short communication from Pfeffer, I should like to write “Sial” instead of “Sal,” in order that there may be no confusion with the Latin word for salt. From the preceding the reader will have already drawn for himself the conclusion that the rocks of the sima group, which we only know as eruptive rocks in the sialic continental blocks where they appear as foreign bodies, have their own place beneath these blocks, and form at the same time the floor of the ocean. Basalt has all the necessary properties for the material of the ocean floors. In particular its specific gravity is in harmony with the thickness of the continental blocks, calculated by other means.

It is not without profit to obtain a few more exact figures on the matter. The thickness of the continental blocks has been calculated by Hayford and Helmert by different methods. Hayford derived the so-called “depth of compensation level” (that of