they are only moderately developed. As this arrangement seems to be the rule, the question naturally arises, what can be the cause of this? The answer is obviously, the same cause that is operative to-day, namely the westerly wind, which is prevalent during the greater part of the year, or at all events during the colder half of it, and which blows with the regularity of a trade-wind. Under this constant and violent west wind the waves of the lake are driven against the eastern shore, where they are unceasingly active, filing, washing away, and abrading the shelving acclivities and rocks that dot the shore-line, the whole weight of the lake beating as it were upon them ; whereas the western shore is sheltered behind big mountain-masses, so that there the effects of the westerly trade wind are almost entirely neutralized. The northern and southern shores are also exposed to the action of the waves, though in a less degree than the eastern. But since there do exist beach-lines and terraces, still distinct, on the west, as for instance above our Camp CIX, though in comparison with those on the opposite slopes they are very slightly developed, their existence proves that other winds besides those from the west do sometimes prevail, and may possibly even be specially characteristic of some other season of the year, say early summer.»
By means of a detailed investigation such as that which I have just indicated it would be possible to determine with almost mathematical certainty the relations which the east and the west winds hold with respect to one another, that is the percentage of all the winds visiting that region which blow from the east and from the west respectively; and the result would go to show, I feel certain, that the latter are, both as regards frequency and as regards strength, far superior to the former. It would of course be essential to compare the beach-lines at identically the same level, although it is scarcely conceivable that the proportion between the winds from the different quarters can have in the slightest degree changed during what is, geologically speaking, the short period in which the lake has been subsiding. From that point of view the percentage would no doubt be the same at every level that is indicated by the successive beach-lines. And considering the accentuated character of the physical geographical features which distinguish the whole of the Tibetan highlands, that vast upheaval of the earth's crust which is bordered on the south by tropical lowlands and a warm ocean, it is very reasonable to suppose that the wind has not changed in such a relatively brief period as that, even though the moisture of the atmosphere and the precipitation have both diminished during the same period, as they naturally have done in consequence of the lake's own shrinkage. It is self-evident, that we should arrive at erroneous conclusions, if we failed to investigate and compare together beach-lines at the same absolute altitudes; for, as we have seen, lines at different elevations above the lake have reached different degrees of development, so that one line will indicate the level that was maintained by the lake for twice, or even several times twice, as long a period as another line. But one thing there is that does not admit of a shadow of a doubt, both here and in other parts of western Tibet in which I observed similar beach-lines, and that is, that even at the period when the lake stood at the 133 m. level, the westerly winds blew with a frequency that was incomparably greater than all other winds put
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