Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/189

Rh the depressed sial masses, is especially distinct in the case of the Apennines. But the mass deficit of the Alps also extends far on the north-east to the northern boundary of Bohemia and towards Central Germany. And conversely a zone of excess of mass extends from the south under the Alpine chains, which indicates that no depression of the sial masses to great depths corresponds to the superficial folds. At this place, however, a considerable deviation from isostasy occurs, which can be easily explained, for the abnormally high position of the base of the crust according to the map should, if isostasy prevails, correspond to a smaller thickness of the block; that is, the upper surface of the block should be of exceptionally low level in the area, lying everywhere under water. If, on the other hand, considerable altitudes above sea-level are here reached, this can only be rendered possible by deviation from isostasy, these belts of crust being held considerably above their position of equilibrium by their rigid connection with the neighbouring rocks. The map of Koszmat does not by any means give us direct information of these deviations from isostasy.

Sir James Hall was the first to note that the thickness of the sediments in folded mountain chains is always greater than in the neighbouring undisturbed areas. In other words, in this region, before the folding, these sedimentary strata were deposited in greater thickness than in the neighbouring area. This rule has been found to be universally true, and has given geologists much to ponder over. Since the deposits under consideration, which are often many kilometres in thickness, must have all been formed in shallow water, it must be assumed that the block sank simultaneously with the deposition, whilst the sedimentation continued, so that the surface for the time being always