540 THE DEPRESSIONS ALONG THE NORTHERN AND THE SOUTHERN BASE OF THE TRANSHIMALAYA.
depressions and finding its way through Bongba-changma in the region north of Chunit-tso where, not far from Nila-yung-karpo, the altitude is only 4580 m., and N. W. of which there is a lake probably at a still lower altitude. The water would continue to flow further W. N. W. in the direction of the depression indicated by the salt lake Tabie-tsaka , where the altitude at any rate is lower than at Tarok-tso (4627 m.). Then we are lost in unknown country. But this country is no doubt filled up with N. W.—S. E. stretching ranges and valleys between them, and there are probably depressions and perhaps even lakes. The superfluous water would seek its way through the region north of Nganglaring-tso (4746 m.) where probably depressions as low as Tabie-tsaka will be found. The next region crossed by the escaping water would be the district of Yumba-matsen and the depressions of the lakes Dhubuk-tso and Karmo-tso, and finally the Singi-kamba or uppermost Indus would be reached—perhaps at Shildong, where the Indian maps give an altitude of 4465 m.
This, or something of the same kind, is what probably would happen upon the return of a pluvial epoch. The reason why I have left Chunit-tso, Tarok-tso, Shovo-tso and Nganglaring-tso to the south of the presumed river is that the course I have indicated seems to follow a latitudinal valley, of which the Nevem valley is a part, and which is for the rest unknown. However, on account of the increasing precipitation, every one of these lakes as well as the Karong-tso and Poru-tso would be captured by the new river system. The Tarok-tso would be filled, and its effluent would join the new river, and the same would sooner or later be the case with the other lakes.
It is a curious fact that the Soma-tsangpo and the Sumdang-tsangpo flow from S. E. to N. W. as if they had once been southern tributaries to a great westwards flowing river with which their courses formed more or less acute angles. On account of the proceeding desiccation and of the differential movements of the surface, the Soma-tsangpo and the Sumdang-tsangpo have, in their lower courses, been forced to make sharp bends and return to the east.
From its sources on the western sides of Targo-gangri and the Shuru-Range to the region of Shildong, this old branch of the Upper Indus has been some 55o km. in length. The Soma-tsangpo which now is 147 km. in length, was during the moist period only 1 2 7 km. ; the Buptsang-tsangpo, on the other hand, which now is 150 km. in length, was then 195 km., and the Sumdang-tsangpo, now 87 km., was then about 120 km. in length.
Directing now our attention to the eastern half of the great depression at the northern base of the Transhimalaya, we meet exactly the same hydrographical changes as in the western half. The rivers and lakes we now find there are only the fragments and remnants that remain of a considerable river system that has been de-
1 stroyed by desiccation and differential movements of the surface.
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