376 THE LOP-NOR PROBLEM.
which the river that entered it at one end quitted again at the other, and it disappeared in consequence of the erosive action of its drainage stream. The Lop-nor had at the dawn of historical time pretty much the same dimensions as the Kara-koschun has now. The Chua-chai-tsi existed prior to the beginning of the historical epoch, and by the year i 368 had for the most part disappeared. In this circumstance we have a direct point of comparison for estimating the length of the period during which both lakes were drying up. In the case of the Lop-nor we have ascertained that this period must have been very long, because the climatic changes in the vast area drained by the river would necessarily demand an immense amount of time. In the case of the Chua-chai-tsi the period must have been considerably shorter, because the process of erosion advances at a very much more rapid rate. The former lake shifts its position periodically whenever its bed gets filled up; whereas the basin of the latter becomes only in part filled with solid material, and would continue to occupy its bed for some considerable time, were it not that there is a gap in the side of its trough-shaped depression.
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