336 TO THE SU-LO-HO DELTA [Chap.IX
fed by them subterraneously that are found along the Limes line from the Khara-nôr to the south of Toghrak-bulak (Maps No. 35. c, D. 4 ; 38. A. 4).
Subsoil It is by subsoil absorption, too, that we can account on the one hand for the fact that these
drainage depressions within the old lacustrine basin are drained of their temporary sheets of water before
trine basin, evaporation encrusts their surface with salt, and on the other for the presence in the BEsh-toghrak valley of the water that is found both in its wells and open bogs. The downward slope of the ground from the terminal course of the Su-lo-ho to the old Mesa-girt lacustrine basin is attested only by the general configuration and some aneroid readings consistent in their relative bearing. But farther on the regular line of measured levels supplies direct proof of a considerable and steady descent of the ground from the western edge of the basin down to the great eastern inlet of the ancient Lop sea.
Connexion Thus the conclusion imposes itself that even now water from the Su-lo-ho finds its way beneath
between the surface into the great terminal basin of Lop, and that a surface connexion between the drainage
Su-lo-ho of the Su-lo-ho and that of the Tarim during an earlier but geologically recent period may safely be
and Tarim.assumed by way of the Bésh-toghrak valley.10a How far back that period lies is a question which it is beyond my competence to discuss. Nor do I think that without a further detailed examination of the whole area, preferably with the help of a trained geologist, such a discussion is likely to prove profitable. Here it will suffice to call attention to the fact that the evidence which proves the Su-lo-ho to have at an earlier epoch discharged its waters, wholly or partly, into the basin of the Lop sea, may claim distinct geographical importance. It means that the extent of the drainage-less basin, conveniently known as that of the Tarim, already a rival to that of the Sea of Aral, is greatly increased by its eastern limit being shifted from about the 92nd degree of longitude to the 99th degree ; for there on the watershed towards the Pacific lie the easternmost glacier sources of the Su-to-ho."
Geographi- But the extension of the limits of the chief drainageless basin of innermost Asia is of geogra-
cal interest phical interest in another respect, and one moreover with a quasi-historical bearing. It makes us of nexus.
realize better that the big area drained by the Su-to-ho, whether we consider its deserts of bare stone or gravel, its oases, or its high Pamir-like mountain valleys, shares most of the essential physical characteristics of the Tarim basin. This similarity of geographical conditions has its importance for history. It explains why the wide open trough through which the lower Su-lo-ho flows westwards could never within historical times support a large nomadic population nor form a convenient passage for great migrations of races.
Natural To a great and highly civilized power, however, like the China of Han times, the lower Su-lo-ho
corridorTArim valley offered a most convenient ` corridor ', created by nature, for that great trade route to Central
basin. Asia and the West which it wished to open, and for the systematic advance of the political control
and military protection which that route was soon found to require. Thus the oasis of Tun-huang and the smaller ones scattered conveniently along the route leading to it from Su-chou came to serve the same purpose, in connexion with the expansion of Chinese trade and political influence westwards, as the strings of oases stretching along the foot of the Tien-shan and the K`un-lun, without which the Tarim basin could not have become the great natural highway for the interchange of the civilizations of China, India, and the West.