CH. LXII WIND AS A GEOLOGICAL FACTOR 141
which now blow over the desert with remarkable violence and persistence come mainly from the north-east and east. How often had Chiang and myself had reason to comment upon that terribly cutting wind which sweeps down the lower Su-lo Ho valley from the side of the Mongolian Gobi, and which, as he told me beforehand, Chinese travellers to and from Turkestan have learned to dread under the name of ' the wind of An-hsi ' ! The whole region seemed, indeed, a true home for Boreas and his sons. The extent and character of the damage which the wall has suffered in its various sections fully agreed with the evidence furnished, on a vastly larger material and chronological scale, by the jagged ridges and isolated terraces of this ancient lake shore.
This observation derives additional importance from the fact that the same prevailing winds make their effect felt even far away in the Tarim Basin, as I had ample occasion to observe in the climatic conditions and surface formations round Lop-nor. It is probable that many years will pass before that delectable region and the equally attractive desert tracts about Tun-huang are provided with their meteorological stations to supply exact data,—and to tax scientific devotion. But even without such data I may hazard the conjecture that a likely explanation for these prevailing winds is supplied by ' aspiration,' due to the higher temperatures which the atmosphere of the low-lying desert around and to the west of Lop-nor must generally attain as compared with the great barren plateaus of stone and gravel to the north of the Su-lo Ho depression.
For me there was something distinctly stimulating in
the bigger physical features of this desert of gravel and marsh-land, and in the expanding horizon as we moved from tower to tower south-westwards. The great marshy basin with its glittering salt efflorescence looked at times as if it were still one big lake. On clear days—and of such we now had several—I thought I could distinguish high ridges of drift sand beyond it to the north-west, the easternmost offshoots of the Kum-tagh sands we had skirted from the Lop-nor side. From the southern edge of the basin the
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