116 MY FIRST JOURNEY IN NORTH-EASTERN TIBET.
But there exist also great differences between the same two basins. One difference is that in the northern basin the salt lake is much deeper than the freshwater lake, and its maximum depth is ten times greater than the maximum depth of the salt lake in the southern basin. In the latter it is undoubtedly the freshwater lake which is the deeper. Moreover the two salt lakes are at very different stages of development. The northern, Ajagh-kum-köl, is a large lake, and its area and level certainly change but little during the course of the year, while its water is a long way from being fully impregnated with salt; hence it is only under certain specially favourable circumstances that concentrated salt deposits are found around its shores, e. g. in the shallow lagoons that are at times cut off from the lake. But we have seen that the southern lake has advanced much nearer to that stage in its existence in which it will have ceased to act as a salt reservoir, because its basin will be entirely filled up with crystallized salt. The function which this peculiar salt lake performs in relation to its freshwater twin bears no little resemblance to the function which the vast lagoon of Kara-bughas or Adschi-darja (the Bitter Sea) exercises in relation to the Caspian Sea. Fresh water flows constantly into the lagoon through a narrow opening, the Kara-bughas Sound, situated between two flat, natural pier-like arms, and as the lagoon is surrounded on every side by arid deserts, it is consequently exposed to a very active evaporation. This lagoon and salt-collecting basin is extraordinarily shallow in relation to the vaster area of the great sea which feeds it. The comparison halts however in consequence of the disproportionately great difference in the dimensions of the two basins. The Kara-bughas exerts no influence upon the salinity of the Caspian Sea, which is certainly increasing, because the evaporation exceeds the inflow, causing the sea to shrink in area. Our salt-lake however acts upon the freshwater lake like a distilling apparatus, extracting from it its salt; and the thoroughness with which it does this must be great, because of the small area of the freshwater lake coupled with the copious volumes of fresh water that flow into it. Thus, even though we might with some show of reason compare the salt lake with the Kara-bughas, it would nevertheless be absurd to compare the freshwater lake with the Caspian Sea. I shall however return to this matter, to other geographical homologies that exist, and to a comparison between this basin and its neighbours, when I proceed to deal with the general résume' of the hypsometrical and morphological relations of the Tibetan highlands. All that I will add here is, that in those parts of it which I visited I nowhere found hard rock, nothing but sand and soft yellow soil. As rocks approach nowhere quite close to the shore, it may be assumed that the same circumstances prevail throughout, or in other words, that these lakes have been formed amongst the extremely finely pulverised, disintegrated materials which during countless centuries have been at work filling up this basin; the basin itself having originated when the mountain-ranges to north and south of it, which are now for the greatest part planed away, formed gigantic folds of the earth's crust, folds which are far better preserved on the north and on the south, i. e. in the peripheral parts of the Tibetan protuberance; in fact so far is this the case that in the south they still support the loftiest peaks that exist on the face of the earth. Now the range which borders our twin-lakes on the south is, as we shall see in our next day's march, ridiculously unimportant, with regard to both height
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