Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/195

Rh would be subsequently pressed out in the same way as in Iceland. In any case, the great elevation above the level of the ocean betrays the presence of sial masses beneath the lava-flows.

The date of the origin of these faults, which are arranged in a mesh-like manner in East Africa, is to be placed in a geologically late period. At many places they cut late basaltic lavas, and at once place Pliocene fresh-water formations. Thus in any case they cannot have been formed before the close of the Tertiary period. On the other hand, they appear to have existed in the Pleistocene, as has been deduced from the raised beaches marking higher water-levels in the lakes without outlet, lying in the bottom of the troughs. In Lake Tanganyika, its obviously earlier marine fauna, which later adapted itself to fresh-water conditions (relict fauna), points to a somewhat long existence. But the frequent earthquakes and the strong vulcanicity of the fault zone certainly indicate that in any case the process of separation is similar to that in progress to-day. The only new fact regarding the mechanical significance of such rift-valleys is that they form the first steps of a complete separation of the two parts of the block, whether it is a matter of recent still unfinished rifts, or earlier attempts at one, which have become quiescent again in consequence of the weakening of the tensile forces. According to our ideas, a complete separation would take place somewhat in the manner diagrammatically shown in Fig. 35. First an opening cleft arises in the more brittle upper strata, whilst the more plastic layers below are stretched. Since steep vertical walls, of the height here in question, would make much too great demands on the resistance to pressure of the rocks, inclined gliding planes of slipping are formed simultaneously with the rift, or in place of it, along which