the river-bed and its accompanying masses of sand into the direction north-west to south-east, that is at right angles to the line of the prevailing wind. On the whole the Ettek-tarim may be said to lie north-north-west to south-south-east, whereas the great furrows of the western depressions run from north-north-east to south-southwest. Thus they lose their parallelism in proportion as they advance westwards; a deviation being produced owing to the southern portions of the sandy ridges advancing more rapidly than the northern, no matter whether this be due to the relative mass of the sand, the strength of the wind, or the conformation of the ground. As I have already said, Keng-lajka lies 30o m. higher than Jangi-köl; possibly this increase along the line of the prevailing wind is just sufficient to increase the sandlifting power of the wind. The dunes lie, as it were, more exposed, they do not screen one another, which they would do to a greater extent if the surface sloped downwards instead of upwards along the line of the prevailing wind. Relatively insignificant though the increase of elevation is, it must nevertheless exercise some influence.
Fig. 363. THE DIFFERENCE BETWEEN DUNES SLOPING UPWARDS AND DOWNWARDS AS COMPARED WITH THE PREVAILING WIND.
Seen from the projecting angle of the Tagh-kum, the zone of vegetation which accompanies the Ettek-tarim presents the appearance of a brown strip, pretty broad, but scarcely winding, across the universal yellow of the sand. Towards its edges the brown colour dies away and finally passes over into yellow. Looking westwards, one sees nothing but an illimitable expanse of sand. Only one bajir is visible, a small one lying due west; all the rest are hidden behind the sand-waves. Now we have found, that the bajir formation exists not only in that part of the Desert of `I'schertschen where we crossed it, but also along its extreme eastern margin. It is there that these depressions originate, as well as the sand-waves, and from that cradle of their being they begin their millennial migration towards the west, until they reach the region in which the east-north-east wind is less constant, or alternates with other winds, so that consequently the depressions there become filled with sand, are blotted out, and finally disappear under the overwhelming inundation of the drifting sand. Hence in process of time the valley of the Ettek-tarim will likewise disappear: the sand-wave which lies to the east of it will march on over it, and soon this long trench will become divided, like its congeners farther west, by thresholds or sand-isthmuses into a chain of bajir depressions.
IIIFカラー高解像度
