ON DUNE-FORMATION AND SANDY THRESHOLDS. 367
to an end. In order to ensure their continuance, it is necessary that several successive stones should strike the water one after the other at the same spot. This is what takes place in our desert, each succeeding windy period breathing, as it were, fresh life into the sandy waves, the only difference between them and the waves set up by the stone being that the former travel with exceeding slowness.
Fig. 319. BAJIR NO. 20 AND ITS IMMEDIATE NEIGHBOURS. WHERE THE SKETCH IS DARKEST
THE SAND LIES DEEPEST.
But one might suppose that some time or other the transportation of sand from the east would cease, especially as the curving streams of the Tarim and the Kontsche-darja constitute an almost impassable barrier in the way of the sand of the Desert of Lop, preventing it from crossing over and mingling with the sand of the Desert of Tschertschen. And indeed, if the Tarim continues in the future to maintain
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