in and out. Here the same struggle goes on between the Kevir and the driftsand as between driftsand and water in Eastern Turkestan ; but it is the sand which gains ground, and the level surface of the Kevir which is contracted and grows less. If we compare the conditions in the Kevir with the Takla-makan we shall involuntarily come to the conclusion that in both cases the same effective work of transmutation is in progress, caused by weathering in association with changes of climate and the transporting action of wind. But in the two lands the transformation of the earth's surface is at different stages. In Eastern Turkestan the blown sand has spread and piled itself up in huge quantities, so that only an insignificant and vanishing part of the ground is bare. In Persia the bottom, that is, the ground of the Kevir, is still incomparably more extensive than the sand-belts on its margin. If the change of climate continues in the same direction as now, that is to say, towards a higher degree of drought, it may be taken for granted that the slough of the Kevir will lose moisture
s and afflux of water, and in time will become firmer, and
that the driftsand will with greater ease extend and firmly establish itself. Undoubtedly the physical geographical changes now in progress will end in entirely converting the Kevir into a sandy desert of the same kind as in Eastern Turkestan. And we can, on the other hand, draw the inference that Eastern Turkestan, after having been at one time a part of the Central Asiatic mediterranean sea, was
: gradually filled up with finely disintegrated products of
weathering of the same kind as in the present Kevir, and that its solidified lake of wet mud and clay was finally dried and hardened to such a degree that it could bear the weight of the encroaching sand. And that the sand was formerly of less extent than now is proved by the archæological discoveries of myself and other travellers. The substratum laid bare in the bayirs of the Cherchen desert strongly reminds one of the ground of the Kevir. In both cases it is dark, fine matter, forming an almost plane surface. In both cases this material, when mixed with water, is transformed into a slough in which a man sinks beyond recovery ; but in Eastern Turkestan the ground-water
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