Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/194

168 of the earth’s crust was connected with a certain upward movement of the broken margins when suddenly set free. On this peculiar turned-up form of the edges of the plateau depends the fact that the sources of the Nile arise immediately to the east of the slope down to the Tanganyika valley, whilst the lake itself discharges itself into the Congo.” A third well-marked trough commences east of the Victoria Nyanza, contains farther north Lake Rudolph, and turns off to the north-east near Abyssinia, where it continues on one side into the Red Sea, and on the other into the Gulf of Aden. In the coastal region and in the interior of German East Africa these faults usually assume the form of step-faults, with down-throws to the east.

The great triangle in the angle between Abyssinia and the Somaliland peninsula (between Ankober, Berbera and Massowa), shown dotted in Fig. 34 in a similar manner to the floor of the troughs is of special interest. This relatively level and low-lying area consists entirely of late volcanic lavas. Most authors consider it to be an enormous widening of the floor of the rift. This idea is suggested chiefly by the course of the coast-lines on both sides of the Red Sea, the parallelism of which is disturbed by this projection; if it be cut away, the opposite corner of Arabia exactly fits into the notch. It has already been mentioned that we are obviously dealing with masses of sial from the underside of the Abyssinian mountains, which have spread out unilaterally towards the north-east and have thus emerged at the margin of the block. Perhaps the rift was already filled with sima, so that the rising mass of sial carried with it a cap of this material; or else there might have been great inclusions of sima in this welling-up mass of sial, which