Wherever I have seen glaciers in Tibet or the mountains of India, I have been able to trace their moraines to a level very considerably lower than their present termination; and when I find in those ranges of the Himalaya which do not at present attain a sufficient elevation to be covered with perpetual snow, series of angular blocks, evidently transported, because different from the rocks which occur in situ, and, so far as I can judge, exactly analogous in position to the moraines of present glaciers, I feel myself warranted in concluding that they are of glacial origin, and find it necessary to look about for causes which should render it probable that the snow-level should have formerly been lower than it is at present. In the rainy districts of the Himalaya, where forest covers the slopes of the hills, it is difficult to fix the lowest limits at which evident moraines occur, but in many places I have seen them at least three thousand feet lower than the terminations of the present glaciers. In the valley of the Indus, accumulations of boulders, which I believe to be moraines, occur in Rondu as low as 6000 feet.
Glaciers, as is well known, terminate inferiorly at the point where the waste by melting in any given time begins to exceed in amount the mass of solid ice which is in the same space of time pushed forward by the vis à tergo. In the mountains of Tibet the elevation of this point is very different in different places. It seems to depend principally on the mass of the glacier, as large glaciers invariably descend much lower than those of smaller size; the inclination of the bed has perhaps also some influence in determining the matter.
In comparing the glaciers of the Tibetan Himalaya with those on the Indian face of the same mountains, it will be found that, cæteris paribus, glaciers descend much lower on the Indian side, or in a moist climate, than in the dry and arid Tibetan climate. It is indeed impossible to ascertain with certainty that any two glaciers are of equal size, but it appears to me sufficiently accurate to compare the main glaciers on the opposite sides of the same pass. In the Umasi pass, which is situated in the main chain of the trans-Sutlej Himalaya, all the circumstances seem favourable for comparison. On the south side of this pass the principal glacier terminates at about 11,500 feet, while on the north side a much more massive glacier comes to an end abruptly at 14,000 feet. The difference then, on opposite sides of the same pass, where the pass coincides with the line of transition of climate, amounts to 2500 feet.
That I am justified in ascribing the cause of this difference to the change of climate appears from the fact, that in the interior of Tibet, where no such change is observed in crossing even very lofty passes, there is frequently a glacier on the north declivity when none exists on the south. This is the case, for instance, on the Parang pass, and on the pass immediately north of Le. It may therefore be inferred, that when glaciers occur on both sides of a pass, that on the northern exposure will, unless there be a marked alteration of climate, invariably descend lower than that on the south side. I have not had an opportunity of seeing glaciers on both sides of any pass in the most external ranges of the Himalaya, but I have been informed that in the range south of the Chenab river, glaciers frequently occur on the north sides of the passes, while none exist towards the south. If this were to be found universally the case, it would be an additional proof that the lower descent of glaciers on the south or Indian side of the mountain chain is an exceptional occurrence.
GLACIERS OF KOUENLUN.
The glaciers of the southern slope of the Kouenlun appear, from the descriptions of travellers, to be on a still more gigantic scale than those of the Himalaya. Five mountain ranges of great height, separated from one another by rivers of great size, descend from the axis of that chain towards the Indus and Shayuk, and attain so great an elevation, that, with scarcely an exception, there is no passage from one of these lateral valleys to another. All these ranges rise far above the line of perpetual snow, and in their valleys enormous glaciers descend to a level which is gradually lower as we advance westward in the direction of the source of the rain- and snow-fall. The range east of the Shayuk has comparatively few and small glaciers, but to the west of that river the glaciers of Sassar terminate at about 15,000 feet. A little further west, a glacier, overhanging the valley of Nubra, terminates at 14,700 feet, and the great glacier of Nubra was found, by Captain Strachey, to terminate at 13,000 feet. In the range between Nubra and the Machulu again there are vast glaciers, but their height has not been determined, nor do we know precisely to what level those of the Shigar valley descend; though it is evident, from their proximity to the main valley, and their small distance from Shigar, which is not more than 7200 feet above the level of the sea, that they must descend very low, perhaps to 10,000 feet. In the valley of Gilgit, I am informed by Mr. Winterbottom, the glaciers descend as low as 8000 feet.
LEVEL OF PERPETUAL SNOW.
In the mountains further east than the Shayuk it would appear that the snow-fall is so very small that the level of perpetual snow recedes to an enormous height. This has been found to be the case on the passes north of the Pangong lake, many of which were crossed by Captain H. Strachey. The great height of the mountains without snow, east of the Karakoram pass, confirms the fact; and it is probable, so rapidly does the snow-level rise in advancing eastward, that if we could penetrate a very short distance beyond the eastern extremity of the Pangong lake, an absolutely dry country might be reached, in which rain or snow never falls.
So much error has unfortunately taken place regarding the height above which the mountains of North-west India are covered with perpetual snow, that it appears necessary that travellers should put upon record the results of their observations, however limited. It is for this reason, and not because I expect to throw much additional light on the subject, that the following remarks are hazarded. The recent paper of Captain R. Strachey[39] has furnished facts which had hitherto been wanting, while the theoretical considerations which have been laid down by Humboldt are so accurate and comprehensive, that the undoubted mistake into which he has fallen is the more to be regretted.
The Indian and Tibetan Himalaya, west of Nipal, lies entirely within the temperate zone, and from that circumstance has its year divided into summer and winter. The periodical rains, which it is well known are principally confined to the outermost parts of the mountains, being derived from the Bay of Bengal, are excessive in the easternmost part of the chain, and gradually diminish as we advance westward; there is no reason, however, to believe that the winter monsoon, which is particularly dwelt upon by Captain Strachey in the valuable paper to which I have had occasion to refer, is so. Probably indeed it is the reverse, though I have no detailed observations to refer to in corroboration of this opinion; I may however recall to mind, that the winter is the season of heavy snow, and the spring of heavy rain, throughout the north of Affghanistan, and that in the Punjab frequent cloudy weather and rain occurs during the cold season, while in the plains of India the weather seems to become at that period less unsettled as we advance eastward.