not however say a single word, though it is shown on his map. Since then that lake has shrunk year by year, until by the year 1901 nothing of it was left except the detached and extremely insignificant, as well as shallow sheets of water, the Harschu, Luma-ring-tso, Tsolla-ring-tso, Oring-tso, and Batsa-singi. In this there is intrinsically nothing remarkable, when it is recollected that a rise of a couple of meters would be sufficient to unite at all events the middle ones of these sheets of water. But it is more difficult to explain the existence of the transverse thresholds and the sharply defined strand-terraces, which under no circumstances could be formed in the course of 3o years, unless they were a yrzori existent, and were originally nothing more than moraine ridges which date from an even earlier epoch, and have been gradually left behind by the ice-stream as it step by step retreated. This possibility is by no means inconceivable; but the problem cannot be solved without fresh and exhaustive investigations, which I had at that time no opportunity to make, especially as it was then winter, with the thermometer down to — 2o° C.; besides, grass was seldom to be found and the strength of my caravan animals was exhausted.
The other explanation is to be sought in the fact that Nain Singh visited the region in the end of August, whereas it was the beginning of November when I was there. I have already mentioned, that the isthmus between the Luma-ring-tso and the Tsolla-ring-tso narrows very considerably during the rainy season, owing to the fact that the level floor-like bottom of each lake is then completely under water, which also rises and encroaches upon the isthmus from both sides. It was this circumstance which I took to be a proof of the great sensitiveness that these shallow lakes exhibit to the varying amounts of rainfall during the course of the year, in that in the actual rainy season they are biggest, but contract during the winter and spring. We have assumed, that all the salt lakes of Tibet are drying up, but different lakes are at different stages of desiccation. Some, like the Sellingtso and the Lakor-tso, are still constant, even though their level does vary with the seasons; others, like the Luma-ring-tso, dry up in all probability in the spring, but fill again at the beginning of the rainy season; others again have dried up for good, leaving merely a salt-pan behind them — of these we shall come across examples presently. The Luma-ring-tso and its neighbours are seasonal lakes, and exist only half the year. They are thus well advanced towards those of the third category, namely the class of lakes which have entirely disappeared. Nain Singh however — and this is the point I wanted to come to — visited the lake just at the height of the rainy season, when the quantity of water in these elongated depressions is greatest. I found, long after the conclusion of the rainy season, a mere chain of wretched salt pools where he witnessed one single long continuous lake. We cannot therefore assert without qualification that his map is in this respect incorrect, because the differences within the thirty years admit of a perfectly natural explanation. But in this instance we have a proof, without looking for it, of how important it is that pioneers should in their journeys make and record as accurate measurements as they possibly can. A traveller who visits, say, the Panggong-tso for a month, and makes a number of reliable measurements, is far more entitled to the gratitude of the scientific world than a man who traverses the whole of Asia
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