CHAPTER XXV.
COMPARISON BETWEEN THE TARIM AND OTHER
CENTRAL ASIAN BASINS.
I have attempted to show that, although the terminal lake of the Tarim is, like most of the lakes of Central Asia, undergoing a process of desiccation, the process in its case, at all events at the present time, advances at a slower rate than in most other lakes; and the reason for this I trace to the vast extent of its source-regions, and to the fact that climatic changes which take place there could hardly make themselves noticeable in such a relatively short period as the historical epoch, or at any rate within a space of z 600 years. In this respect the Tarim, together with its terminal lake, occupies an exceptional place amongst the rivers of the earth; and I know no other instance that admits of full comparison with it, i. e. I know no other river that flows through a self-contained central basin, after gathering up its waters from a girdle of mountains that are beyond parallel gigantic, and then, in an ex-centrically situated part of its basin, forms an ambulatory lake on what is practically a dead level. There does indeed exist a certain degree of resemblance between the basin of the Tarim and the basin of Tsajdam; for the latter, although its area is only one-fourth of the area of the basin of the Tarim, likewise forms an elliptical depression, shut in by extensive ranges of mountains stretching from east to west. Yet in respect of height these mountains fall a long way short of those that encircle the basin of the Tarim, the reason being, at any rate in part, that the basin of Tsajdam lies nearly 2000 m. in absolute altitude above its neighbour. The result of the denudation processes which in the course of countless thousands of years have been going on in the border-ranges of the Tsajdam basin, and of all the gravel, sand, and sediment which have been transported by the rivers, streams, brooks, and torrents after the rains down towards the central parts of the basin, has been to fill it up to such an extent that the bordering ranges, as compared with the general surface, appear incomparably lower than the mountain-chains that shut in the basin of the Tarim. Taking the basin of Tsajdam as a whole, the surface slopes towards the east, just as it does in its larger neighbour; but taking the actual salt-marsh of Tsajdam proper, the fall is to the west Or north-west, the direction in which the
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