292 THE LAKES BESIDE THE LOWER TARIM.
of I o minutes only we obtain a volume of 7, 200,000 cub. m. of drift-dust, or if the storm continues 24 hours, a volume of 1036 million cubic meters, equivalent to a loess deposit over the whole of East Turkestan of between 2 and 3 mm. in thickness; or the same number of centimeters if we assume that ten to twelve such severe tempests blow in the year. In the course of a century this would be equal to an elevation of the surface to the extent of from 2 to 3 meters. In several parts of the country, where the ground is free from sand, or barren, or sheltered from the wind, as in the bajirs and in certain parts of the Lop Desert, the traveller is often in danger of being buried in the loose dust; and in the desert just named he always prefers any surface to the dust-choked depressions to travel on. That similar deposits of loess never succeed in forming in exposed situations is quite self-evident. The dust which is dropped by one storm is carried on farther by the next. In a word, it is in never-resting circulation, giving rise to the desert haze or dust-gloom which is so characteristic of East Turkestan; it is for this reason that you are seldom able to distinguish the mountains which girdle it round, even from a comparatively short distance away. The dust which falls on the windward side of the dunes is blown away, that which falls on the leeward side has every prospect of lying there, at any rate in part, and of being covered by the down-flowing sand; nor will it come to light again until after the dune-mass has taken a full step in advance, when after its century-long rest it once more begins its peregrinations. If the ground is moist, the dust which falls into the bajirs remains in great part where it falls; otherwise it is whirled up again by the north-east and south-west winds. The dust which falls among the belts of vegetation is retained by the roots, and so becomes an essentially contributing factor to the circumstance noted by Prschevalskij, that the lower Tarim flows as it were between ramparts, and that at a higher level than the adjacent country on either side. Finally, the dust which falls directly into the river, or which is immediately caught up and carried along by its erosive power, finds a resting-place partly in its marginal lakes, partly on its countless sedimentary deposits, and partly in its terminal reservoir. The larger quantity finds its way into the last-named, that being the goal towards which the entire system is continually moving with concentrated energy. To the deposition of sediment there exists a limit; it is reached when the erosive power of the stream and the power of deposition implicit in its water counterbalance one another. The sediment which is laid down at the season of low water is swept away in the succeeding high-water period; thus during these two seasons it is in a state of intermittent movement, and only comes to rest during the former. Fresh material is continually being brought down from above, but the banks of sediment gravitate on the whole towards the terminus of the river, the Kara-koschun marshes. There it is finally yielded up by the stationary, filtering water and falls to the bottom, and like the dust under the dunes enters upon its century-long rest. Nor will it resume its peregrinations until the distribution of the water undergoes an alteration. However small a particle of dust may be, it can never be destroyed and can never disappear. Its destiny is inexorably knitted to the history of the development of the Tarim basin. So long as it is exposed, it must follow the movements of the atmosphere and the water; when brought to rest, it finds its own special niche under the law of the earth's attraction. But it can never
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