RELATIONS BETWEEN MARGINAL LAKES AND DUNES. 247
I assume therefore, that, previous to the Tarim's occupying its present situation, the dune-valleys which now contain lakes were already in existence, but in the shape of ordinary bajirs. I assume further, that the soil, where not covered by sand (see fig. 2 I 6), formed, theoretically, elevations as compared with those tracts which were overlain by dunes. That is to say, they would have formed elevations, had it not been for the counteracting influence of another factor which prevented them from doing so, namely a force which gradually came into play as the super-incumbent dunes advanced, and as a consequence of that advance, and which scooped them out as the on-moving dunes left them exposed. That force is the wind. The energy it puts forth can hardly be called deflation in the proper and usual acceptation of the term, but it is at any rate an expression of the wind's property of carrying away the finely pulverised, superficial, and perfectly dry dust, which, as we shall see subsequently, constitutes the bottom covering of the bajirs. To some extent corrasion is also operative, though as an effect not of the prevailing east wind, but rather of the south-west and north-east winds, which seem to blow sometimes during the winter. After this finely comminuted material has been lifted into the air by the last-named winds, it hangs suspended there until such time as the first dominating currents of the atmosphere from the east seize upon it, and sweep it westwards, creating in the interior of the great desert of East Turkestan, as I observed myself in February 1896,* a state of complete twilight even when there is an entire absence of wind.** A conclusive proof that this haze, which completely neutralizes the effect of the sun, is nothing but fine dust is to be found in the toJa jamgor, or »dust-rain», which deposits upon every object exposed to it a thin coating of fine, light yellow dust. If it be true, as it is true, that not all the fine dust which I saw beside the Kerija-darja originated from the bajirs, still it cannot be denied, that an integral portion of it must be traced to that source. In the Lop country the dust-haze is hardly so thick as beside the Kerija-darja. Consequently the latter can scarcely originate in any other region except the Tschertschen Desert, the greater part of it indeed from the bajirs of that desert, the floor of which consists of just such fine loose, imponderable material as we find in the deposits of the dust-rain. And we have a further confirmation of this in the circumstance that the dust-haze of the Kerija-darja appears every year in February, sometimes it may be a little earlier, sometimes a little later, it all depends upon when the first burans set in from the east. But the phenomenon which we are here considering owes its origin to a combination of circumstances, from amongst which it is not always easy to select those which have been chiefly instrumental in bringing it about. For instance, when we find that the surface of each bajir in the Tschertschen Desert is covered with a layer of fine dust, at least some decimeters thick, and that the gentlest puff of wind is sufficient to waft it away in clouds, it is equally open to us to say, that these sheltered depressions are just the places which the atmospheric dust chooses to settle in. And undoubtedly this does take place; but then it no more precludes the occurrence I have spoken of than the deposition of sediment by a river is prohibitive of its exercising erosive power. The only wind
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