THE HEART OF ASIA 99
and the basin floor takes the form of a monocline or bend. Farther to the east, from Keriya to beyond Lop-Nor or farther, and from Korla to Ying-pen, where the uplifted portion of the northern mountains comes to an end, it takes the form of a fault or break. The result is a pronounced topographic difference between the more gently descending western borders of the basin, on the one hand, and the sharply cut eastern border, on the other. To the west, the mountains merge somewhat into the plain, and, however grudgingly, invite man, as it were, to enter and occupy them : to the east, the mountains, though battered and dissected, rise steep and repellent, offering no opportunity for roads or settlement. Here, as on the borders of other sharply depressed seas like the Mediterranean or Caribbean, there are evidences of volcanic activity at times no farther distant than the early part of the glacial epoch, as I saw on the Keriya River near Polu.
In order to understand the distribution of life in the Lop basin, we must remind ourselves of a familiar geological process. Ever since the beginning, of the differentiation between the plateaus and the basin, streams from the highlands have brought to the basin floor rock waste of all sizes, and have deposited it to a depth of probably thousands of feet. Among the mountains, the streams descend steeply and rapidly, and are heavily loaded with rock-waste, much of which consists of cobbles and pebbles. On reaching the edge of the basin, their velocity is suddenly checked because of the change in grade; and they are compelled to deposit the coarser materials, and to go on loaded only with sand and clay. Little by little these too are deposited, as the
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