Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/186

160 with the received normal value for the geographical latitude in question and the difference, the gravity anomaly, is given on the map. It shows us immediately the mass-defect under mountain chains through which the latter are isostatically compensated. “One can only arrive at the conception already expressed by many geophysicists, and by Heim, that it was not a dilatation that caused the defect, but that the upper relatively light parts of the earth’s crust are greatly thickened by the folding, and that this accumulation, during its formation, sank into the plastic lower layer. A folded mountain chain grew not only upwards, but, owing to its weight, downwards also: the folded elevation, is, as Heim expressed it, opposed by a still greater folded depression.” We can thus see directly in the map the approximate topography of the underside of the sial crust; beneath the Alps, where the gravity anomalies reach the greatest negative value, the lower side of the sial crust also sinks deepest into the sima.

If, on the other hand, we had not taken away from the observed values the influence of the masses above the sea-level, we should not have obtained such gravity anomalies in the region of the mountains, but—although with minor deviations—have found normal values there. Subterranean mass-defect and mass-excess above the sea-level therefore mutually counterbalance, isostasy thus prevailing in mountain masses. Since this is proved to be the same in ancient as well as recent mountains, we arrive at the principle: the folding of mountains is a compression subject to the preservation of isostasy.

It is advisable to make clear, as in Fig. 32, what this means. In the compression of a block swimming in the sima, the ratio of the portions above and below the mean level of the sima must always remain the