ORIGIN OF SAND IN THE TARIM BASIN. 453
It would require a layer, spread out over the whole of this area, of 5 m. in thickness, or rather less owing to its varying degrees of compactness, before the 4000 cub. km. of sand we have estimated to exist in the desert would be accumulated. One factor which does not enter at all into this calculation is the fine drift-dust, which for the most part settles in quite other regions than the drift-sand, after being sifted out from this latter by the wind. The amounts at which we have thus arrived are per se stupendous. The masses of sand in the Takla-makan would indeed form a ring all round the earth, or a wall of sand built all along the equator, one kilometer in breadth and one hundred meters high!
And yet the loss which the mountain girdle has thus suffered ought not to have altered its character to any noticeable extent. The elevations and valleys will still occupy the same position and present the same appearance, and the human eye would never suspect how great is the transportation of material that has taken place. If two maps could be drawn of those peripheral regions — peripheral, that is, to the catchment-basin of the Tarim — one before, and the other after, the transportation, and drawn with perfect accuracy, they would nevertheless be exactly alike, for the changes of level would not admit of being detected by ordinary instruments, unless it were a finely graded theodolite. When we remember how slowly denudation proceeds and disintegration becomes perceptible in relation, not to the average span of human life, but to the interval of historic time, we literally stand aghast when we endeavour to form a conception of the enormous period that has been necessary to effect a denudation over the area in question to the extent of 5 m. in depth. For the transportation of solid material from the highlands to the depressions and the basin proper also went on of course at a very lively rate even during the time that the Tarim basin was filled with water, though the main operative agency was then erosion.
After the great inland sea dried up and the climate became more arid, in other words in the period that is now elapsing, it has been, I am convinced, the wind which has been mainly operative in increasing the amount of the sand and in extending the sandy desert far beyond its older limits. Such a statement does not admit however of being proved with figures. But if, as I have endeavoured to show, the rivers at the present time not only carry too small a volume of water, but also move too slowly, to transport any noticeable amount of sand to the eastern parts of the basin, then the wind is the only force that does possess sufficient power to make an increase in the already existing quantities of sand. But that it must be increased is almost a matter of natural necessity, because it was precisely the general and steadily growing aridity of the climate which established the conditions for the origination of the sandy desert. Nobody who has had personal experience of the spring storms that blow along the southern foot of the Kuruk-tagh can any longer be in doubt as to the rôle • which the wind plays. Suppose your tent is standing in a bare, open part of the clay desert, that is perfectly free from sand, when one of these storms is raging. A few hours suffices to cover the interior of the tent with a thick layer of drift-sand, and there will be another layer to the leeward of the tent. And if, when you are out in the open, you stoop down and face the wind, you can feel the particles of sand striking against your skin, and
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