From east to west the first four of these lakes show an increasing altitude. In the region of Lakes XIV, XIII and XII, a rise of ground seems to have taken place. The difference in altitude between the lowest and the highest lake in the series is no less than 474 m. The altitudes of the two easternmost lakes directly indicate the affinity of this valley for the upper feeders of the Yang-tse River. The relative altitudes of the thresholds situated between the lakes is very small , viz. between Lake XIX and Lake XVIII, 5085 m., between Lake XVIII and Lake XVI, 5099 m., between Lake XVI and Lake XV, 5116, thus increasing in altitude from east to west, whereas the threshold between Lake IV and Lake II is only 5026 m. From Lake XVIII and westwards the valley may be said to be very level, 2 20 m. being the greatest difference in altitude between a lake and a threshold.
The valley just described is a very regular and typical depression between two gigantic folds, the Arka-tagh and the Koko-shili. Lac de Corbeaux and Lac de l'Antilope of de Rhins clearly indicate its western prolongation. Then follows an extensive area of terra incognita. It may be regarded as pretty certain—by conclusions of analogy — that the latitudinal valley continues through the whole of this region, is nearly parallel with the valley of Wellby, and is in uninterrupted connection with the valley of the Antelope-plain, with the itineraries of WELLBY, DEASY, RAWLING and myself, and still farther west with the latitudinal valley of Yeshil-köl, Lighten Lake, the Aksai-chin Lake and the Salt marsh of 4663 m. The lakes of this enormous valley are fed from the snow fields and glaciers on the very high and mighty range of Kwen-lun—Arka-tagh. It is surprising that such considerable rivers as the Yurun-kash, Keriya-darya and Cherchen-darya take their origin from the northern side of this range which faces the desert, whereas only small brooks flow to the lakes along its southern side which ought t0 catch the greatest amount of the moisture brought hither by the southern winds.
It is of course impossible to tell, with any degree of certainty, how the drainage of this great latitudinal valley was arranged at an epoch where the whole of Tibet drained off to the ocean and to the Tarim basin. Very likely, however, the whole eastern part of the valley, from the region of Lac de l'Antilope and including all my lakes of 1896, drained to the Di-chu, King-sha-kyang, and Yang-tse; and the western part, from Yeshil-köl or Lighten Lake, to the Kara-kash. As to the precipitation falling in the yet unknown part of the great valley, it may have found its way out either to the west or the east, or, providing that it possessed the same force as the Kara-kash to break through the Kwen-lun in transverse valleys, — to the Tolan-khoja, Bostan-toghrak or Kara-muran.
An interesting problem which can only be slightly discussed in this connection is the question of the struggle between the rivers and the sand dunes of the Takla-makan Desert during the epoch of abundant precipitation in Tibet. If we consider
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