Prior to the moment at which the lake was cut off, it may be assumed, that its level remained constant for a very long interval of time, and that that level was prescribed by the height of the western sill or threshold barrier. No matter how great or how little the amount of water that the lake then received, the surplus would always find its way out at the western end, the sill therefore serving as a regulator of the level. If now, as I suspect, the altitude of this sill corresponds to the 54 m. terrace, or even if it lies rather lower, the depth of the outgoing stream being added, we obtain a plausible means of accounting for the absence of observable terraces above the 54 m. altitude, as also for the fact of this terrace being more distinctly developed than those which lie at a lower level than itself. It is the memorial of a level at which the lake remained constant for an exceptionally long period of time. The Tso-ngombo is at the present day in a precisely similar situation, for its efferent stream regulates its level and keeps it constant, so that the strand-terrace which is now being formed by the action of its waves has every prospect of being powerfully developed. If at any time the inflow into the Tsongombo should be, as I have assumed it will be, too small to occasion an overflow, the same fate will then overtake that lake which has already overtaken the Panggong-tso: it will be cut off, and will turn salt. Judging from the slight degree of salinity which the water of the Panggong-tso exhibits, it cannot be very long since that lake was cut off: the water is not too salt but that yaks and dogs will drink it. The existence of mollusc-shells on its shores is evidence that the lake actually was fresh once. Possibly molluscs are still living in its fresher parts, for instance in those localities in which there are several springs or in the vicinity of the mouth of the stream from the Tso-ngombo; but as the salinity will increase, the molluscs are doomed to die.
According to von Richthofen, the peripheral regions are increasing in area at the expense of the self-contained inland-drainage parts, as indeed is natural, seeing that erosion is eating its way back into the heart of the continent, proceeding from the coast upwards. But if my observations and deductions are sound, we have here an instance of an exception of a peculiar character, namely a region which formerly had an outlet to the sea, but subsequently, in consequence of climatic changes, has passed over to the central regions of self-contained drainage. And I have no doubt that similar instances could be met with elsewhere.
The subjoined observations of Strachey are interesting when compared with the reflections which I have made above, although he gives to the lake an altitude of only 13,400 feet (4085 m.) and puts the beach-lines at only 70 feet, while he considers that the water-dividing threshold lies too feet above the lake. According to Rawling, the altitude of the lake is 14000 feet or 4268 m., which comes nearer to my result of 4318 m. Strachey says: »The Panggong has no effluence what-
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