Rockhill has thus described the country that he travelled through in brief, but striking, words, and given us a very clear idea of its general geographical characteristics. He had an opportunity to explore the transitional country between the self-contained drainage region of inner Tibet and the peripheral region which drains to the ocean; whereas I only grazed, as it were, the self-contained drainage region of eastern Tibet. The first salt lake that Rockhill touched was the Dzurkenula-nor, and subsequently he noticed the broad, shallow channels in soft ground by which the internal drainage waters make their way into the salt lakes. In the case of the Koko-schili, Dung-bure, and Tang-la Rockhill believed that he had reached the western extremity of each range, or at any rate that they severally terminated not very far west of his route. He supposes therefore, that these chains, running from west-north-west to east-south-east, are characteristic of the border regions of eastern Tibet, and that they are backed on the west by a highland country, in which plateau characteristics .predominate. In the meantime he may be in so far right, that the ranges in question do in general increase in relative height towards the east, where the latitudinal valleys have been more deeply excavated, and where the disintegration material and sedimentary matter are carried by the streams down to lower regions; whereas in the west, on the plateau, the latitudinal valleys are being increasingly filled up with the material washed down off the flanks of the mountain-ranges between them. The reason of his thinking that he had reached the western ends of these ranges was therefore his having crossed over them in parts where they happen to be of low elevation. Other journeys through Tibet prove that they do continue towards the west and west-north-west through practically the whole of Tibet. This is peculiarly true of the Marco Polo range, which is only an eastward continuation of the gigantic Arka-tagh. And this too is the case with the Koko-schili, a system which for a distance of close upon ten degrees of longitude separates the latitudinal ranges in which I and Wellby travelled, and it was no doubt the westward continuation of the Dung-bure range which Wellby had on the immediate south of his route. And as for the Tang-la, I assume that the big range which I crossed over north of our headquarters camp
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