THE HEART OF ASIA 103
important as showing that in times past, under different conditions, the same regions were more favorable to life. In the eastern part of the plain, widespread deposits of broken and blistered salt, and beds of variegated clay, proclaim that long ago the lake of Lop-Nor was much larger than now. Farther west, the waste of sand known as the Takla-makan desert illustrates the work of the wind. Acting upon the materials deposited by the streams in broad flood-plains and playas, the wind heaps the coarser grains of sand into an endless succession of waves. It carries off the finer particles as a hateful haze, which is finally deposited on the mountain slopes to the south, covering them with soft beds of loess, excellent for pasture-land wherever there is rain enough. In many places the sands of Takla-makan have buried the ruins of ancient villages, or the remnants of ancient vegetation, which could have existed only when the climate was moister than now.
Few parts of the world are so simple in structure as the Lop basin with its marked concentric zones. In few do contiguous regions differ so greatly and change so abruptly in physical character, and hence in their relation to life. The lofty, well-watered plateau zone stands like a continental ring around a sea forever dry. In their colder, more elevated portions the plateaus are absolutely desert; but in slightly lower regions they are covered with rich grass, and in summer support the numerous flocks of pastoral nomads. Sometimes the nomads pass over the edges of the plateaus, and go part way down the long slope leading to the basin floor. In general, however, and especially where the break between the plateaus and the floor takes the form of a fault,
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