512 THE DASH FOR LHASA.
cuss the absolute altitudes, we shall have an opportunity to investigate this important water-divide more closely. For the present it must suffice to say, that, like the water-divide between the Indus and the Brahmaputra, it is a flat swelling that forms a more or less curved meridional line, through the lower part of which the Brahmaputra breaks. True, although there do undoubtedly exist immense latitudinal ranges between the separate source-regions, still an integral part of the water-divide with which I am dealing consists of flat swellings in the latitudinal valleys between the mountain-ranges. Now these swellings are of exactly the same morphological rank as those which on the Tibetan plateau constitute the east-west water-divides between the several self-contained drainage-basins. With some of these we have already come into contact in the preceding pages. And with the view of rendering this characteristic morphological feature of the plateau-country of Northern and Central Tibet still more intelligible, I propose to conclude this present volume with a condensed account of the journey which I made along the great latitudinal valley south of the Arka-tagh in 1896. The latitudinal valley that lies next to this on the south has been traversed from end to end by Wellby and Malcolm. Both these valleys consist throughout of an unbroken chain of self-contained basins, with in almost every case a salt-lake in the middle. Each of these basins is bordered on east and west by a very flat threshold. To the south of the latitudinal valley which Wellby has described I crossed over a whole series of similar latitudinal valleys, the only difference between them being, that the farther south one advances the more indistinct they become, often indeed the valley is continuous. At all events from the scanty information which we have regarding these regions, it appears that each of them possesses towards its eastern end a terminal basin, bordered on north and south by mountain-chains more or less distinct, on the west by a flat threshold and on the east by a similar flat threshold. At the same time this last constitutes also an element of the great and important water-divide of which I am speaking and off the eastern versant of which the water streams down to the sea. The river Satschutsangpo, which gave occasion to this digression, belongs presumably to the latitudinal valley that we crossed over in the course of the day's march which I have recently described, and to the most easterly of its self-contained basins. I say it belongs presumably, for little or nothing is known with regard to the source-region of this river; for Bonvalot's, Rockhill's, Bower's, Littledale's, and this present hurried excursion of mine, which I am now describing, all have to do with the lower part of the river only. However I had an opportunity subsequently of studying the region of its estuary more thoroughly than I was able to study any other part of its course, and Rockhill has surveyed a part of it which cannot be very far from its source (see Vol. IV).
Where I crossed over the Satschu-tsangpo, a short distance below the confluence of the Gartschi-sängi, its latitudinal valley appeared to extend to the N. 82° E. and to the S. 85° W. At that point the river-bed is, as we have seen, very shallow and broad, and had an erosion terrace on its left side only; this was very seldom steep and hardly more than 2 m. in height, as a rule indeed it is lower and has a rounded face. Seeing however that it is just at this point that the Lhasa road crosses the river, it is fair to assume that this ford is the best and the most
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