Sec. iii.] THE OASES OF THE TARIM BASIN 45
In the southern marginal belt of the basin, stretching along the foot of the K'un-lun from Karghalik to the Lop tract, conditions differ in various ways. southern belt of oases. Here only one oasis of real importance is found, that of Khotan, and the rest in most cases are separated from each other by considerable expanses of true desert. To the east of Niya, over a marching distance of some 350 miles to Vish-shahri, the westernmost hamlet of Lop, there is met now only the single small oasis of Charchan, and our historical records show even this to have been in abeyance again and again during intervals of centuries. 6
Geographical conditions duly reflected by the map fully account for the thinness, or else the complete absence, of cultivated areas along the whole belt.
Khotan oasis. With the single significant exception of Khotan, all oases in the plains
are separated from the debouchure of the rivers which supply their irrigation by great stretches of absolutely sterile gravel forming the glacis of the K'un-lun. But in the case of the Khotan tract the vast volume of water brought down by the Yurungkish and Kara-kish rivers has led to the formation of large and extremely fertile beds of alluvial loess almost reaching their debouchure from the mountains. The combined presence of an extensive area of fertile soil and of abundant water, aided by the configuration of the ground particularly favourable to its full use for irrigation, adequately accounts for the presence here of a large, and throughout historical times a very important, oasis. 6
Elsewhere along the foot of the K'un-lun the big glacis causes much of the water brought down by the rivers to be lost on its way from the foot of the
Irrigation by kara•sa mountains through evaporation or absorption in its huge gravel de-
and ak•sa. gravel P huge
posits. Near its northern edge however, some of the water thus absorbed comes to the surface again in the form of springs. The supply of irrigation furnished by them, known as kara-su, or black water,' is of great importance. Without it cultivation would be greatly restricted in all oases of this region and in many altogether impossible;„ for it is only during the height of the summer, when the snow and ice on the big mountain ranges melt, that flood water ( distinguished as ak-sn, ' white water' ) passes across the broad gravel glacis in such volume as to provide ample irrigation, and more, for the oases beyond. ? The same subterraneous drainage from the rivers debouching on the gravel glacis makes it possible for desert vegetation to subsist in the sandy tracts
Snbterraneous along the southern edge of the bare dune-covered wastes of the Takla-
makin or skirting the oases. s It also accounts for the rare wells which permit traffic to be maintained through, or along, these areas of desert vegetation east of the line of existing oases.
Excepting Khotan and Karghalik, all oases of the southern belt are ' terminal oases,' i.e., they occupy the furthermost ground at which the water supply
Location of southern derived from the rivers of the K'un-lun can be utilized for cultivation. oases.
Their location is primarily determined by the presence of fertile loess soil, and this itself owes its existence to the moisture brought there, either by the summer floods or by subsoil drainage; for it is only this moisture and the vegetation it supports which can retain the fine alluvium washed down from the mountains as well as the sub-aerial deposits of fine ' sand' constantly brought by the dust-laden desert winds from the north and north-east, and then protect the loess thus formed from the powerful erosive action of the same winds. 9
When discussing elsewhere this process constantly at work in the region here described,
6 For the ground from Niya to Vnsh•shahri, see Sheets Nos. 19, 22, 23, 26. Regarding the chequered history of the oasis of Charchan, cf. Serindia, i. pp. 296 sqq.
6 See Sheets No. 9. D. 2; 14. A. 2. For a detailed analysis of the physical aspects of the Khotan oasis, cf. Ancient Khotan, i. pp. 124 sqq.
7 Regarding the general conditions governing irrigation in the region from Karghalik to Niya, cf.
my observations in Ancient KSotan, i. pp. 94 sqq., 125 sq.; for a particularly instructive local instance as to the importance of the supply of kara-su, cf. Serindia, i, pp. 202 sqq.
6 See Sheets No. 9, 14, 19, 23.
9 Cf. the remarks, Ancient Khotan, i. pp. 204 sq., regarding the formation of the loess soil of Khotan, as determined from microscopically analyzed specimens; also i. pp. 688 sq. for Prof. de Lo'czy's analysis.