5 THE DEPRESSIONS ALONG THE NORTHERN AND THE SOUTHERN BASE OF THE TRANSHIMALAYA.
of the Great Himalaya. A comparison between this valley and the two already considered is difficult on account of the difference in the stages of development reached by the three great valleys. The northernmost has reached farthest in maturity, as its old river all the way from Selling-tso to the neighbourhood of Tso-ngombo is dead and buried under subaerial and sedimentary deposits, and only its uppermost course from Tang-la to Selling-tso is still alive.
In the second valley to the south, the Tengri-nor—Nganglaring-tso depression, the upper courses of the rivers flowing to the west and east are still in function, viz. the Soma-tsangpo or possibly a yet unknown river from the Targo-gangri to the eastern part of Teri-nam-tso, and the Targo-tsangpo. But still there is more hydro-graphical life and a greater abundance of water in this second valley. The lakes are more numerous and generally larger than those in the first valley, where only Selling-tso exceeds in size all lakes except Tengri-nor; and all the larger tributaries of the two old main rivers are still in function, though the volume of water they now bring down from the heigths of the Transhimalaya is not sufficient to give rise to any great rivers with an outflow to the ocean.
The third great valley, the one of the Upper Brahmaputra or Tsangpo, has still a drainage that is strong enough to flow the whole year round and to reach the ocean. In these relations we get a very clear illustration of the distribution of the precipitation which is carried by the south-west monsoon towards the world of high mountains situated to the north of India. By far the greatest portion of the moisture is caught by the southernmost and highest mountains, the Himalayan ranges. But a considerable number of wet clouds find their way across the Himalaya and deliver to the northern sides of the water-parting Himalaya and the southern sides of the Transhimalaya, a sufficient quantity of precipitation to give rise to the river Tsangpo. Even on the northern side of the Transhimalaya the precipitation is abundant enough to feed such rivers as the Buptsang-tsangpo , Targo-tsangpo and others. Having crossed the Himalaya and Transhimalaya, the monsoon winds arrive comparatively dry at the mountain ranges bordering the Panggong—Selling-tso valley, which therefore has only small lakes and rivers, the greatest being the Tsanger-shar, the Bogtsang-tsangpo and the Yagyü-rapga. The Selling-tso and its principal feeder , the Sachu - tsangpo , occupy an exceptional position as they derive their water from the gigantic Tang-la Ranges which probably receive precipitation not only from the south - west monsoon, but also from other prevailing winds. Further, the same condition as in the south, viz. that the precipitation decreases towards the interior of Tibet, also obtains in the east. From whatever quarter the winds come they have already got rid of the greathest portion of their moisture on the high peripheric mountain ranges surrounding the interior plateau-land as ramparts.
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