law of equilibrium, which effectually precludes all idea of the Desert of Lop being the place of origin of the masses of sand that have accumulated farther to the west. Accordingly we have no alternative but to seek for the origin of the sand in the Kuruk-tagh in the first place, and then, further, in the Tschöl-tagh and the swelling which lies between those two ranges. And as an actual fact the whole of that region does bear plain indications of being but the ruins of what it once was. As you travel from the Chan-tengri to the Bei-schan, you find that the ranges and their crests grow continuously lower, as well as both more arid and more disintegrated. In proportion as water and vegetation diminish towards the east, the more do the materials liberated by disintegration fall a prey to the winds and their transporting power. It is not however the wind that has levelled down the former summits of the Kuruk-tagh; they have been broken down by the forces of disintegration, and the process of demolition has been accelerated by corrasion. The storms then swept, and still continue to sweep, the loose material farther and farther towards the west-south-west. Thus a river of drift-sand has for countless thousands of years been streaming across the basin of East Turkestan, and when it has been unable to advance farther, the masses of sand have gone on increasing in volume and the desert has extended more and more. When the migratory lake of Lop-nor shall finally have disappeared, and the extreme tentacle of the Tarim shall have died away in the sand higher up its course than it does now, the wind will then be able to excavate the Desert of Lop unchecked, producing a deep depression, the centre of a never-ending aridity.
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