Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/196

170 the marginal parts of both portions of the block sink into the rift as it opens, to the accompaniment of numerous local earthquakes, so that only a rift-valley of moderate depth is exposed to view, with its floor formed of faulted blocks of the same series of rocks as those which occur in situ high up on the margin of the trough. At this stage the rift-valley is not yet isostatically compensated, and this is the case, according to E. Kohlschütter, with a large portion of the recent troughs of East Africa. An uncompensated mass-defect exists; therefore a corresponding abnormality of gravity is observed, and, what

is more, both margins of the rift rise in isostatic compensation, so that the impression is produced that the trough passes exactly longitudinally through an anticline. The Black Forest and the Vosges on the two sides of the Rhine Valley are well-known examples of this marginal elevation. Finally, if the split travels right through the block, the sima will rise up into it, so that the previous mass-defect is lost, and the trough is henceforth found to be, as a whole, isostatically compensated. In most places the floor of the trough is completely covered with fragments from the margins of the rift, but naturally, by further widening, the time comes when the free surface of the sima is also