Western Tibet, then, is a highly mountainous country, lying on both sides of the river Indus, with its longer axis directed like that river from south-east to north-west. It is bounded on the north-east by the Kouenlun chain of mountains, by which it is separated from the basin of Yarkand. On the south-east its boundary is formed by the ridge which separates the waters of the Indus from those of the Sanpu. To the north-west and south-west its boundaries are somewhat arbitrary, unless the political division of the country be had recourse to, which, depending on accidental circumstances entirely unconnected with physical geography or natural productions, is so liable to change, that its adoption would be extremely inconvenient. The best mode of drawing a line of separation between India and Tibet, in those parts where mountain chains are not available for the purpose, appears to consist in regarding the latter to commence only at the point where the aridity of the climate is too great to support forests of trees, or any coniferous tree except juniper.
As limited by these boundaries, West Tibet includes the whole of the valley of the Indus and its tributaries, down to about 6000 feet above the level of the sea, a considerable portion of the upper course of the Sutlej down to between 9000 and 10,000 feet, and small portions of the upper course of the Chenab, of the Ganges (Jahnavi), and of the Gogra.
MOUNTAIN RANGES.
Every part of Tibet is traversed by ranges of mountains which have their origin either in the Kouenlun on the north, or in the trans-Sutlej Himalaya on the south. These mountain ranges are generally extremely rocky and rugged, but as a general rule it may be said that they are less so in the upper part of the course of the different rivers, than in their lower parts. This rule applies not only to the Indus and to the Sutlej, but with scarcely an exception to all the tributaries of these rivers. There are no extensive open plains in any part of the country, the only level portions being in the valleys of the rivers, the width of which is usually not more than one or two miles, and very seldom exceeds five miles.
To this general description of the surface of the country I have met with no exception in those parts of Tibet which I have had an opportunity of examining. I have not, however, had an opportunity of seeing the extreme south-west portion, my knowledge of the course of the Indus not extending further up than Hanle[35].
The height of the mountain ranges which traverse West Tibet is in all parts pretty much the same, and, as a consequence, the depth of the valleys in the lower portion of the course of the Indus and of all its tributaries is very much greater than near the sources of these rivers. In the higher valleys therefore the mountains are apparently much less lofty; they are also frequently rounded and sloping, or at all events less rocky and precipitous than lower down, though to this there are many exceptions.
ELEVATION OF PASSES.
The elevation of the passes in a mountainous region represents in general the height of the lowest part of the chain. In the mountain ranges of Tibet the average height of the ridges does not exceed from 1000 to 2000 feet above the passes, many of which indeed are scarcely at all lower than the highest crest of the ridge in which they are situated. I believe that in estimating the principal ranges of mountains at 19,000 feet, and the minor ranges at from 17,000 to 18,000 feet, I approximate very closely to the truth. This estimate applies to all parts of the country, the height of the ranges being remarkably uniform; but peaks occur at intervals in every one of the principal mountain ranges, which considerably exceed the elevation just stated, rising very generally (so far as can be judged by the eye from known heights of 17,000 and 18,000 feet) to twenty-one or twenty-two thousand feet; some peaks appearing to exceed even this.
It is generally supposed that the great peaks of the Himalaya on the southern border of Tibet are much more lofty than the mountains of the interior of that country. I do not think, however, that the facts of the case are such as to warrant this assumption. West of the Sutlej, in which district only the mountains of Tibet may be said to be at all known, many peaks of the interior of that country are probably much more lofty than any of those near the plains of India, and if inaccessibility is to be any criterion, the chain of the Kouenlun is beyond a doubt a much more elevated mass than any part of the Western Himalaya. Of Tibet east of the Sutlej little is known, except that between Ruduk and Lassa no road into the interior of Asia appears to exist.
CLIMATE.