especially during the rainy season; hence we may say, that the Perutse-tso under all circumstances receives a far more abundant inflow than the Tsolla-ring-tso. Even though the Perutse-tso is, as its terraces betray, dropping and nearing extinction, it is doing so at a slower rate than its neighbour lake, because its more abundant inflow makes the balance between + and —, or between influx and evaporation, less than in the case of the Tsolla-ring-tso. The Perutse-tso is therefore more tenacious of life, and will certainly outlive its western neighbour, notwithstancling that the latter was once 69 m. the deeper. I do not of course assert that the figures which I have given above are absolutely unassailable: the essential thing is that they are relatively pretty nearly correct. Whether the respective values turn out to be greater or less is of no real importance, for the reasoning which I have adduced above would still hold good.
We have thus found that in the space of time which the Perutse-tso required in order to drop i o m., the Tsolla-ring-tso (to confine ourselves to the lowest part of the depression) has dropped no less than 69 m., counting from the moment when the two basins were finally separated at the point where the flat threshold-pass is now situated. The subsidence in the basin of the Tsolla-ring-tso has thus proceeded seven times faster than that of the Perutse-tso. At the beginning of this period of subsidence the Perutse-tso was of no very great size, although it was of course several times bigger than it is now, for it extended westwards close up to the threshold-pass. The terraces, counting from the 2nd to the 5th on its southern shore, date however from a very much older period, namely that in which the two basins of the depression were connected together. At the beginning of the same period of subsidence the Tsolla-ring-tso was a very large lake; not indeed much broader than it is now, but it stretched, as we shall see presently, a very long way towards the west. Its depth was also greater than that of any of the lakes which I sounded in Tibet; for at the moment when the threshold first emerged above the water the depth of this lake was equal to 69 m. + the maximum depth of the existing Tsolla-ring-tso, though this is indeed inconsiderable. Yet, however swiftly this western lake may have dropped, it nevertheless had sufficient time, and possessed sufficient power, to sculpture strand-terraces, as we saw, with the utmost distinctness at a distance of about 4 km. north-west of the threshold-pass. There were four of them, all running concentrically and quite close together, while there was also a fifth less distinct. But in the four km. that intervene between them and the pass we perceived no beach-lines, the reason probably being that the slope just there is almost imperceptible. Upon following the four terraces round towards the south-west, west, and west-north-west with our eyes, we discovered one or two of them indicated again, though faintly, at the foot of the mountains on the south side of the valley. But we failed to detect them at the foot of the northern mountains, though this may have been due solely to the greater distance and the less favourable light. These terraces, both those that our route directly crossed and those that run along the foot of the southern mountains, are of course formed in purely disintegrated material, though it is now hard and consolidated. There were no beach-lines visible in the hard rock.
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