and the hypsometric relations make it possible that this has been the case. The length of the valley, some 12 6o km., should not be surprising, remembering that the length of the Tsangpo from its source to the point where it begins to break through the Himalaya is nearly exactly the same, or 1200 km., in a fairly straight line. Very long latitudinal valleys are characteristic and a natural feature in a folded land as Tibet. The Upper Indus flows to the north-west for some 850 km.
The Sachu-tsangpo which takes its origin from the Tang-la mountains far in the east, would thus have been the source branch of this long river which then proceeded westwards from the present region of Selling-tso to Dagtse-tso, Tongka-tso and other lakes on the way to Panggong-tso. At the time of the existence of the river, the appearance of its valley must have been like that of the Tsangpo valley as it now is. The difference in altitude between the valley and the mountain range north and south of it was greater than now, as the destruction of the mountains by denudation had not proceeded so far, and as the present self-contained basins had not yet been filled with debris. And still the difference in altitude between the lowest points on the ranges, 1. e. the passes, and the lowest points in the valley, 1. e. the lakes, is, as we have seen, some 700 and Boo m. From the beginning of the desiccation period which still is proceeding in the direction of increasing aridity, the atmospheriles; wind, weather, frost, insolation, etc., act to diminish the relative altitudes, destroying the rocks and demolishing the ranges into mere ruins, and, because of the isolation from the sea, filling up the basins with solid material in the finest degree of division. The ultimate goal of this activity is to obliterate the hills altogether and to bring them at one level with the former depressions, then filled with the very debris of the former mountains. The interior of Tibet would then become an ideal plateau with slightly undulated, soft ground, without hills, lakes or rivers.
The only agencies that could counteract this kind of development would be an increasing folding activity of the crust and an increasing precipitation such as prevailed during the early post-glacial epoch. Even the latter of these agencies alone would bring about the most revolutionary consequences. The lake basins of the plateau-land would get filled to their brims, and overflow across the lowest threshold in their peripheries. Neighbouring lakes would become joined by river arms, and new hydrographic systems would come into existence. And finally every one of these systems would find its way to the ocean. By the increasing erosive action of the rivers, tremendous fluvial terraces would become carved out through the mighty beds of gravel, sand and dust now filling the basins. The exterior peripheric rivers, 1. e. the feeders and source branches of the Indus, the Tsangpo, the Indo-Chinese rivers, the Tsaidam and Tarim rivers, would erode their beds with an energy that gradually would remove the different sources nearer and nearer to the heart of the present plateau-land. On their way thither they would sooner or later meet the
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