JOURNEYS OF BONVALOT, DUTREUIL DE RHINS AND GRENARD. 50I
nous nous retrouvâmes dans un de ces paysages désolés d'autrefois, dans une vaste vallée, couverte d'efflorescences salines, avec, au pied des grands monts blafards, un lac salé et pris par les glaces, qui s'étendait au loin, indéfiniment morne et triste. C'était le premier lac gelé que nous rencontrions, comme aussi le plus grand que nous eussions encore vu. On l'appelle Gya-ring tso, nom qu'il mérite par sa longueur exceptionnelle de cent kilomètres. C'est sur sa rive méridionale que M. Bower fut arrêté et obligé à retrousser chemin ... Continuant notre marche, nous arrivâmes au bord même du Gya-ring tso. Il neigeait et nous étions enveloppés d'une brume épaisse à travers laquelle nous nous dirigions à la boussole. Nous nous engageâmes ainsi sur la glace du lac, mais des craquements, accompagnés d'un tremblement de la surface, nous fit revenir précipitamment sur nos pas, et après de longs tâtonnements causés par l'impossibilité de distinguer nettement, dans l'obscurité blanche qui nous entourait, l'eau gelée de la terre ferme, nous réussîmes à contourner le lac par l'est, nous franchîmes la glace d'un étang, traversâmes des collines et parvînmes au pied de la grande chaîne, dans un vallon vert où se détachaient en noir plusieurs tentes et la muraille vivante d'un gros troupeau de yaks domestiques. Ce lieu s'appelait Tag-sta pou (24 Novembre).»
It is interesting to learn that the Selling-tso (Gya-ring tso) was frozen as early as the end of November, and this in spite of the lake being salt and of its large area. It is true the salinity is not very great; but one would expect that the lakes Addan-tso and Tschargut-tso, that lie to the west of it, both perfectly fresh and both very much smaller, would freeze before it does. In the end of November they were however open, while the Selling-tso, the biggest lake that the French expedition had hitherto encountered, was also the first frozen lake they saw. The explanation must lie in the depth. The part of the Selling-tso which I sounded was extraordinarily shallow, and if the depth throughout does not exceed a few meters, the water, being so slightly saline, ought by the end of November to be so cooled right down to the bottom that it needs but one or two still days and nights for a thin crust of ice to cover the entire lake, and by the middle of January and February this ought to reach a not inconsiderable thickness. In the Tschargut-tso, in which we obtained depths of fully 48 m., a longer time will be required for the cooling of the water, and the ice will therefore form later. The same thing would appear to be true of the Addan-tso.
The map of the French expedition, notwithstanding that it marks a vast advance upon Bower's map of the same region, does not show the hydrographical interrelations of the three lakes in question. The expedition kept to their northern shores and was therefore unable to see the sounds that connect them. Grenard's sketch of the Addan-tso and the Tschargut-tso bears a very close resemblance to my map, and the isthmus between the two lakes is shown as very narrow, though not cut through by a river-arm. The western lake is represented as lying 5o m. higher than the eastern, whereas the real difference of altitude between them barely reaches I m. In Grenard's Lac Sinueux it is easy to recognise my Tschargut-tso with its projecting peninsulas and deeply penetrating bays. The distance between the Tschargut-tso and the Selling-tso is as nearly as possible correct, but the river Jagjurapga, the stream which carries the water from the former into the latter, is wanting.
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