598 OROGRAPHY OF CENTRAL TIBET.
Asiatic continent; for this lacustrine desiccation is by no means confined to Tibet alone, but is attacking most of the other lakes of inner Asia, altogether apart from their absolute altitude and situation: the Sea of Aral, for instance, is drying up at a relatively rapid rate. Under these circumstances it would be difficult to unravel the special causés which conduce to the slow disappearance of the Tibetan lakes, and even if we possessed an exhaustive knowledge of the manner, and the rate, in which their desiccation is proceeding, it would not be sufficient to enable us to solve the problem entirely. But that should not deter us from endeavouring to get nearer to the kernel of the problem by exploring these lakes. What is required for this purpose is a series of surveys and detailed explorations of as many of these dried up basins as possible, and my plans for the projected journey embrace the collection of as much material of this kind as possible.
It is only when we have become acquainted with a large number of Tibetan lakes and have investigated the morphology of as many of them as may be, that we shall be warranted in drawing sound conclusions as to their origin and formation, and in the light of the results so obtained in dividing them into different groups and families, with regard to both their morphology and their geographical distribution. In the preceding pages I have given a list of ninety-two lakes, all of them relatively large and all of them with definitely ascertained absolute altitudes, although these last data vary in trustworthiness. How great is the number of other lakes which explorers have not touched, but merely seen in the distance as they marched along, and which are consequently missing in my lists, and how great, again, is the number of small lakes that every traveller passes every day between the points the altitudes of which he does ascertain! Besides, it is difficult to draw the line between lakes and pools, or quite insignificant accumulations of water. The number of the latter is legion ; they occur of every shape and every size.
Inferring from those lakes which I have myself seen, I can however provisionally suggest certain varieties amongst them. To begin with, it may be said that as a general rule the Tibetan lakes are situated in the latitudinal valleys and stretch parallel to the adjacent mountain-ranges. In eastern Tibet their longer axis runs therefore from west to east, . whereas in western Tibet it lies from north-west to south-east, and of this the Panggong-tso furnishes the most striking instance. The twin lakes of Kum-köl in the extreme north are a beautiful example of the parallelism between lakes and mountain-ranges. Throughout the east side of the plateau between 86° and 92° practically every lake stretches from east to west; any way exceptions are very rare, and when they do occur it is usually easy to explain the departure from the rule as due to some irregularly curved mountain-range or to a spur from such. Even in the extreme south we have another beautiful instance of this same parallelism in the lacustrine chain of the Selling-tso, Tschargut-tso, Addantso, Dagtse-tso, and several others; and the chain may be said to be continued in the Luma-ring-tso, the Tsolla-ring-tso etc. In both Wellby's and my own latitudinal valleys we may justly speak of real chains of lakes, or an entire series of them, all elongated from east to west and all situated in identically the same valley. As great numbers of similar lakes have been discovered south of Wellby's valley, and as they possess the same character and shape as their more northerly congeners;
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