596 OROGRAPHY OF CENTRAL TIBET.
Tibet, namely the water-partings which separate that vast protuberance from East Turkestan and Tsajdam on the north, from the Pacific Ocean on the east, and from the Indian Ocean on the south. Equally difficult would it be — indeed even after the said map is finished it will be in part still impossible — to attempt to define the boundaries of the great basins of internal drainage which have been crossed over by various explorers, as also to determine their areas. Still, that would be in a high degree an interesting investigation, for it would be found that those basins, varying greatly as they do in size, are intimately connected with the general hypsometrical relations of the highlands. It would probably be found, that as a general rule the larger self-contained basins lie lower than the smaller ones.
It would however be of even greater interest and importance to ascertain what is the morphological connection of southern Tibet with central Tibet. We have seen that the Arka-tagh may be regarded as an important orographical boundary between central Tibet and the northernmost parts of the country, and that this last-mentioned region breaks up into orographical systems which descend step-wise to lower levels. There undoubtedly exists a similar transitional region between the plateau proper and southern Tibet, with the valley of the Tsangpo. A yet further task is to establish the connections between this river and the northern transverse valleys of the Himalayas, also to ascertain the position of the water-divide between the Indus and the Tsangpo, and what relations it bears to the Himalaya and to the system which I believe to unite the Tang-la with the Kara-korum.
Another problem of great interest, and especially to climatology, is the desiccation of the Tibetan lakes. We know that the lakes all over the highlands are shrinking in area, but we do not know why they are doing so. Beside some of them there exist no distinct terraces ; beside others there are low terraces arranged in
Fig. 374. VIEWS OF THE NAKTSONG-TSO.
IIIF Color HighRes
