362 THE TSCHERTSCHEN DESERT.
the same. True, the north-north-east wind is a good deal the less powerful, but then the sand-masses of the thresholds are also smaller. The part played by the south wind would seem to be altogether subordinate; at all events the south wind which we experienced in the winter of 1899-1900 was incapable of making any appreciable impression upon the relief of the dunes. I possess no data whatever for estimating the rates at which the two systems advance, and I prefer to abstain from guesses. At all events, it is certain that such immense masses can only travel very slowly.
With regard to the large accumulations of sand, we have ascertained the following laws — (I) In the north of the desert they turn their steep faces towards the north-west, in its middle towards the west-north-west, and in the south towards the west and west-south-west ; (2) their eastern slopes ascend rather slowly towards their crests ; (3) on the other side their steep leeward faces go down sheer at an angle of 33°, or else in two or three steps; (4) their mass diminishes-towards the south; (5) they are each built up of an innumerable number of individual dunes; (6) although their relief is influenced by winds from other quarters than the predominant,' their mass is unaffected by them; (7) it is their varying breadths which give rise originally to the thresholds, and consequently to the formation of the bajirs.
The following general laws apply to the bajirs: (I) They constitute so many links in the chain of depressions which in the northern part of the desert is continuous in one straight line, but subsequently in some places deviates, forms elbows, and bends like a river that flows nearly straight with few curves. (2) In regard to size they vary in a capricious way, from tiny basins of two or three score meters in extent to one of no less than 19.5 km. in length, this being bajir No. 33. (3) Of the entire distance of 284.5 km. almost exactly one-half, or 143 km., indicates the length of the bajirs, the other half being the measure of the sandy thresholds, though it is to be observed that, while bajir ground preponderates in the north, sandy ground is in excess in the south. (4) The bajir ground, or the soil in the spaces encircled by sand-waves, varies greatly in composition in different parts of the desert: in the north it consists of fine, loose, moist saliferous matter (schor), making the middle of the bajir as soft as a morass; in the next zone the same formation occurs again, with this difference only, that the surface is dry, and consequently can be travelled upon; in the middle of the desert the bajirs are often dotted over with small dunes lying north-east to south-west, and here a sparse and languishing vegetation makes its appearance; whereas in the south the bajirs are for a considerable distance completely sanded up, though they make their appearance again in the extreme south, smaller indeed than in the north, but possessing a hard flooring of sedimentary clay, and generally some vegetation. (5) Their general shape is that of an ellipse or an oval, though there exist numerous varieties: for instance, bajir No. 28 is horse-shoeshaped, No. 31 almost rectangular, No. 4 is somewhat curved, whilst 33 makes almost a straight line. (6) They are deepest on the east side, close to where the sand avalanches take place. (7) They occupy the lowest parts of the intervals between two sandy waves, and correspond to the »troughs» of the ocean waves. (8) On the east and north each bajir is bounded by perpendicular sandy walls, but on the south and west the sand has a gentle slope. (9) The bajirs travel along with
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