594 OROGRAPHY OF CENTRAL TIBET.
of the self-contained drainage-basins of western Tibet, but neither the denudation nor the levelling up has there advanced so far, not only because the folding of the earth's crust was there more energetic from the beginning, but also because the building up of mountain-ranges, a process which is still being continued, though with but a faint reflection of the pristine energy, is more active in the west than in the east. This is the very circumstance which causes the altitudinal difference between the western and eastern ranges to be so great, while the altitudinal difference between fundamental plateau in west and east is considerably less and the altitudinal difference between the lakes in west and east is least, being not more than 26 m. For if the process of building up the mountain-ranges is still operative in however slight a degree, it will counteract not only the levelling down of the crests but also the filling up of the valley-basins. And supposing that we may in even the feeblest acceptation of the word speak of mountain-building as being still active in the east, it is in any case infinitesimally small in comparison with the active agencies of denudation. In that quarter not only are the crests being more rapidly lowered, but the valleys are also being more rapidly filled up, than they are respectively in the west. In conformity with the assumptions made above, we might also infer, that the lakes of western Tibet ought to be deeper than those of eastern Tibet, but we do not possess the materials necessary for a solution of this problem. It would appear however as though any such law could hardly exist, for in the Panggong-tso we reached the same depth of close upon 48 m. that we obtained for the more easterly of the two freshwater lakes which we sounded in eastern Tibet. On the other hand one or two of the English explorers mention that this or the other lake in western Tibet is particularly shallow. Nor can we speak either of any noteworthy difference of altitude between the fresh and the salt lakes. In general the salt lakes lie lower, because they are always the final reservoirs in each basin, while the freshwater lakes usually have an emissary, and in the majority of cases it is precisely into a salt lake that that emissary empties itself. But on the other hand we find salt lakes at all possible altitudes, and freshwater lakes likewise at very varying elevations.
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