| Leaving behind us Northern Tibet with the Arka-tagh and its system of border-ranges, let us turn our attention to the highlands of Tibet, where, by way of a beginning, we will first study the two principal features of a physico-geographical character that are best known and most distinctly developed, namely the two pregnant trenches in the surface relief which are traversed on the map by my route and Wellby's, both of the year 1896. The mean absolute altitudes which we possess for these two latitudinal valleys are incomparably more trustworthy than all the means for the latitudinal valleys that we have already discussed, owing to the fewness of the data upon which these last are based. I have already quoted in a previous paper* one or two mean values for this region; but in these I am now constrained to make a slight alteration, not only because the new calculations are more accurate, but also because some of the altitudes taken in 1896 have now been properly corrected. In my latitudinal valley the mean value of thirty-five fixed points, situated neither on the threshold passes nor yet in the depressions, amounts to 4892 m. The list of these measurements gives varying altitudes for two-thirds of the length of the latitudinal valley, though the variations are not great, ranging only between 5098 and 48 r o ni.; but in the remaining third, towards the east end, a distinct downward slope is noticeable in that same direction, where the lowest point lies at 4616 m. And the same observations apply to the corresponding parts of Wellby's latitudinal valley, except that they are more accentuated owing to his having crossed the last cross-threshold that serves as a boundary between the region of internal drainage and the source-region of the Tschumar river. Hence with regard to the extreme east of my latitudinal valley, we may as a general rule say that its internal-drainage basins lie increasingly lower towards the east (although there are exceptions) and the differences of level are but slight The lakes in my latitudinal valley that were measured for altitude lie, going from west to east, at elevations of 4906, 4937, 4946, 4896, 4932, 4920, 481o, and | | | | | | | | |
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