streams grow more sluggish farther away from the mountains; and at last, the very finest particles are deposited in the terminal salt lake, if the streams are able to persist so far. By the long continuance of these processes, the basin floor has been provided with two chief parts. The first is a somewhat sloping zone of gravel, a characteristic piedmont deposit, forming a peripheral ring from five to forty miles wide at the base of the mountains. The second is a vast inner plain of sand and clay, nine hundred miles long by three hundred miles wide, and so flat that it is everywhere sensibly horizontal. The plain is not featureless, however, for it is clearly divided into three parts, characterized, as the map shows, by vegetation, by sand, and by Iacustrine deposits.
Vegetation is confined chiefly to a narrow but continuous zone encircling the desert areas of sand and lake deposits, and lying just within the piedmont zone of gravel, to which it is closely related. Occasional narrow tongues or bands of vegetation accompany the larger rivers far out into and even across the sand, but they are relatively unimportant. The relation of the piedmont gravels to the zone of vegetation must be clearly understood, as it is perhaps the most important fact in determining the habitability of the Lop basin. On entering the zone of gravel, the streams deposit their loads of pebbles, and thereby fill their beds, making it necessary for the water to seek new channels, which naturally branch often and are broad and shallow. Thus a large surface is exposed to evaporation, and the streams lose in volume. A much greater loss takes place, however, because of the vast quantities of water which sink
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