travels towards the south-west. The Kontsche-darja follows step by step hard after it. The rivers shift their courses towards the right, that is to say precisely in the directions in which one would expect the greatest hindrances to lie. The Tarim has to wash away the gigantic dunes of the sandy desert; the Kontsche-darja to chisel away the elevated inter-riverine tract, overgrown, as it is, with vegetation. The movement of the river is illustrated in the accompanying vertical section (fig. 8). The question naturally arises, what is the cause of this movement towards the right. The reason the river shifts its .course is that it is gradually filling up with sedimentary mat-
ter, and is thus being elevated too high relatively to the adjacent country. But if that were all, the river would deviate now to the one side, now to the other; instead of doing that however, it always moves towards the right. For this, then, there must exist some other cause. No doubt the general slope of the country as a whole is towards the south-west, from the Kuruk-tagh towards the central parts of the desert. Secular changes of level, the effect (according to von Baer's law) of the rotation of the earth, the results of wind erosion, which is especially active in certain parts of the desert, and perhaps other causes as well, may all co-operate to produce the actual result we see before our eyes. In this particular part that we are now considering the migration seems to be taking place pretty swiftly, for huts were still standing beside watercourse No. I. With each step that the river takes towards the southwest, it leaves behind it a strip of forest, and this, owing to the sudden stoppage
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