Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/64

continental blocks are wrinkled in all directions by ancient and recent folded mountain chains, we do not know, in spite of all the soundings that have been made, a single feature from the enormous area of deep sea which we could claim with any certainty as a chain of mountains. Some, of course, would conceive the elevations of the floor of the Middle Atlantic, and also the ridge between the two troughs lying in front of Java, as corresponding to folded mountains; nevertheless, this view counts so few adherents that we can content ourselves here with a reference to Andrée’s criticism.^[1] How can this absence be explained, since compression must be also assumed in the sima? The answer is obvious if we consider isostasy in relation to mountain building. Mountain building is folding subject to the preservation of isostasy. Since by far the greatest portion of the continental block, a thickness of 100 km., is submerged in the sima, the greatest portion of the thickening of the block by folding must be in a downward direction. Only a very small portion of the compression will be visible as elevation, a subject to which we will return later in Chapter XI. But while the greatest part of the compression of the continental blocks is downward, a compression in the sima cannot in any case lead to an elevation. The material in this case is squeezed out below or to the side, just as water between two approaching icebergs. The objection of A. Penck,^[2] that “the absence of sima folding on the anterior side of the drifting continents appears to be a decisive proof against his conception of the constitution of the crust and the mobility of the continents,” is not valid. On the contrary, we see a

1. K. Andrée, Über die Bedingungen der Gebirgsbildung, p. 86, etc. Berlin, 1914.
2. A. Penck, “Wegeners Hypothese der kontinentalen Verschiebungen,” Zeitschr. d. Ges. f. Erdk. zu Berlin, pp. 110–120, 1921.