descended from the culminating summit of the dune, the immense height of which (90 m.) has caused the sand to pour down in greater volume, and consequently to encroach farther upon the lake at that particular spot. It is an equally natural and inevitable consequence of this more active and more voluminous downpour of sand, that a similar promontory is formed on the opposite shore, though one pointing in the opposite direction, that is towards the east. Without pausing to discuss the question, whether a high ponderous dune-mass travels faster or slower than a lighter mass of lower elevation, I will merely state it as a fact, that one and the same identical chain of dunes, which may stretch for many scores, or even hundreds, of kilometers through the desert, does not run in a straight line, but forms an undulating, or rather a sort of »festooned», line. The bulging parts of the line, or the curves which are in advance, that is to say the portions which travel at the fastest rate, are likewise those which are highest. It is so, at all events, on the east side of the Karaunelik-köl. The dune ascends to an altitude of go m., and the sound to which the lake contracts at its foot is only 30o m. broad. If now we imagine the depression to be dry like an ordinary bajir, then that portion of the dune-slope on the west side of the lake which lies immediately opposite to the east-shore projection will be sheltered, ' at any rate in part, against the wind, and consequently will not advance westwards at the same speed as the other parts of the same dune-chain. It lags, as it were, behind the rest, and fails to keep its dressing. And the farther the eastern promontory advances, the greater becomes the shelter it affords against the wind, and consequently the greater grows also the retardation of the opposite promontory. In the end the advancing eastern promontory overtakes its lagging vis-à-vis and overwhelms it with its on-pouring masses of sand. In this way the two basins of the bajir become entirely separated the one from the other by a transverse neck or ridge of sand, which continues to increase in height as time goes on, and is almost always bordered on the east by a steep wall of advancing sand. This is the stage which has been reached by the neck of sand that divides the southern basin of the Karaunelik-köl from the bajir which lies south-south-west of it. And as I shall show subsequently, similar conditions prevail throughout the interior of the Desert of Tschertschen.
Nevertheless there do exist exceptions to this rule. We have already seen that in both the Basch-köl and the Jangi-köl the shore-lines are parallel, and that these two elongated depressions possess neither narrow sounds nor projecting promontories in the middle. Indeed in the case of the second of these two lakes these characteristics are so pronounced that its basin resembles a gently winding river-bed, each concave and convex curve on the one side being matched by an exactly similar convex or concave curve on the opposite side. This may be due to the entire stretch of dunes on the east shore being of one uniform height, so that the whole line of dunes marches forward at an equal pace, or what amounts to the same thing, there exists no reason why any one part of the line should advance faster than the rest. But at the southern end of each of these lakes there is a neck of sand separating it from the bajir which forms the continuation of its longer axis. This neck of sand has however arisen in the way I have just described; and the various successive stages of its growth are depicted in the accompanying fig. (223).
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