These older ranges have been for the most part worn down by denudation, . and all that remains are flat ridges rising the merest trifle above the level of the contiguous latitudinal valleys. Add to this, that the two big latitudinal valleys possess no effluent, either to the ocean, or to East Turkestan, or to Tsajdam, but on the contrary are themselves divided into a number of small self-contained drainage-basins; and in consequence of that the solid material which is washed down by the streams is carried into the bottoms of the valleys, where it becomes deposited in the deeper parts of the depressions, raising their level continuously. In the Tschimen valley on the other hand the material is carried away partly to East Turkestan, partly to Tsajdam. From the Kum-köl depression it is not carried away, it is true, but that hollow may not only have been originally relatively deeper, but it is also bigger, and a longer time will be required to fill up its basin to the same degree as the relatively much smaller basins farther south. The recently enunciated law, that the big latitudinal valleys grow shallower and shallower the farther you proceed south does not however hold good right across the highland region of Tibet; for we shall soon reach one or two valleys which are rather deeply trenched in relation to the ranges that border them on each side.
No sooner do we get on the south side of Wellby's route than we are confronted with great uncertainty with regard to the position and direction of the mountain-ranges. Upon studying his map, we do, it is true, get the impression, that the part of his route which we are now discussing is bordered on the south by a fairly compact and continuous range; but when we compare his route with other routes in the same region, it turns out that the orography is by no means so simple. In this locality, again, we can no more talk of a single range than we can in the case of the Arka-tagh or the Koko-schili; but we can talk, legitimately enough, of a separate mountain-system, which according to the combined routes can be nothing else except a westward continuation of the Dungbure. The Dungbure proper lies immediately south of Wellby's route, though its crest is there highly irregular. My route of the summer of 1900 intersects Wellby's in two places. His lake of 6th August, which he calls »Salt Lake», is unquestionably identical with my big salt lake; the reason that Wellby does not show the western, freshwater lake, which empties itself by a broad sound into the salt lake, is simply this, that his route ran to the north of it and he really did not see it. Wellby's salt lake of 28th July corresponds ro my salt lake of Camp LV. When a map of Wellby's itinerary is placed upon my map on the scale of I : i,000,000, the two lakes are seen practically to coincide in position, though not completely so, for Wellby's two lakes overlap a few minutes to the east. That portion of my route which describes an arc of a circle from the eastern lake to the western, south of Wellby's route, proves, that it would be altogether inappropriate to speak about a distinct mountain-range, for the Dungbure is there very flat and split up into a number of small parallel ranges. In the interval between my big salt lake and the two freshwater lakes, that is to say between Camp XXXIII and Camp XXXVI, I did not cross over anything that could be called a mountain-range, the highest point along that section of the journey being 4934 m. The surface was, it it true, markedly undulating, with detached ridges and lines of heights, but still on the whole relatively level.
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