88 FROM KÄSHGAR TO THE KHOTAN RIVER [Chap. III
ever that the attempt, if it is to succeed—on foot, of course, not by aeroplane—will have to be made from the latter point. For the traveller starting from the Khotan river, the foot of the Mazar-tagh would afford relatively easy going for the first twenty miles, if not farther. Slight as the elevation of the Khotan Mazar-tagh is, as far as Lai Singh reconnoitred it in 1908, it would yet give a fair chance of sighting any isolated rock islands which might be found to continue its direction farther on. After crossing some further sixty miles of difficult ground, the crest of the Chok-tagh would be sighted and offer safe guidance for the rest of the desert crossing. January or February would probably be the best season for good visibility as well as for the transport of water in the form of ice.
Survival of And here I may in conclusion offer a few conjectural observations of geological interest. It
desert range is evidently deserving of notice that the north-west and the south-east ends of the ancient hill range
river. Khotan that may be assumed to have once diagonally crossed the great basin now covered by the sands
river. Y g Y g Y
of the Taklamakan, both survive in the immediate vicinity of large rivers. The fact cannot be due to mere chance, and its explanation is not far to seek. It is clear that the large river beds, and still more the broad belts of vegetation that moisture must always have maintained along them, would inevitably make the accumulations of drift-sand less heavy, and consequently their corrosive effect when in movement less great, on that side which, in respect of the prevailing wind direction, would lie under the lee of the river beds. Now all the surface features due to wind-erosion that I have observed, from the Lop desert to the fragments of the old transverse range about Maral-Bashi, conclusively prove the prevailing direction of the winds at work in the Tarim basin to have always lain from NE. or ENE. to SW. or WSW. Keeping this fact in view, it is easy to realize from the map that the Khotan river flowing approximately from south to north could afford that protection only to such portions of the old hill range as lay near to the west of it. Thus the survival of the Mazar-tagh, to a distance as actually traced of some fourteen miles from the river's left bank, is fully accounted for.
Protection Turning to the area where the range is adjoined by the Yarkand river we find conditions some-
afforded by what different. Here the general bearing of the river's course is approximately from S. 243° W.
belts. to N. 6 ° E., or roughly WSW. to ENE., and thus approximately the same as the prevailing
belts. 3 g Y PP Y P a
wind direction. If account is taken of the width of the riverine belts of vegetation—and owing to the close vicinity of the Kashgar river delta it is particularly great on the left or northern bank of the Yarkand river—it is evident that the hills near either side of the Yarkand-darya must receive some protection from corroding drift-sand ; but this would necessarily be more effective on the left bank, where the vegetation belt is greatly increased by the added moisture from the end of the Kashgar river. The map shows us topographical facts in close agreement with this theoretical assumption ; the high and relatively broad massif of the Maral-bashi Mazar-tagh lies near the left bank, and the distinctly lower and more attenuated ridges of Chok-tagh and Kum-tagh lie on the right. It is further fully in keeping with the above theory that we find the detached ridges to the north and north-east of the Marâl-bashi Mazar-tagh sinking lower and lower the farther away they lie from the protection of the riverine belts of the Kashgar-darya. Thus the Bel-tagh is distinctly lower and more eroded than the Ukur-mazar-tagh of which it is a prolongation (Map No. 8. A, B. 1), and the Lal-tagh lower than the hills near Tumshuk. A probable explanation is thus found for the almost total effacement of the ancient range where its line approaches at right angles the outermost Tien-shan or Kelpin-tagh, presumably of geologically later origin ; here the small Shikarwai knoll (Map No. 8. A. 1) appears to form its only remnant.
Persistence These observations are offered with all the reserve which my want of systematic geological
of factors training necessarily imposes. But it may at least be claimed for them that they are consistent affecting
desert with that method, the application of which, as a safe guide to the genetic history of present desert