LEVEL OF PERPETUAL SNOW.
Captain R. Strachey has estimated (from the mean of several observations) the snow-level on the southern slope of the cis-Sutlej Himalaya at 15,500 feet. This elevation is, no doubt, as near as possible correct. Captain Herbert, in his geological report, had fixed upon 15,000 feet, which is a little too low even in the district of Basehir, to which his estimate, I believe, refers. In the trans-Sutlej Himalaya, from the diminished amount of summer cloudy weather, the snow-level is probably a little higher, but we are not yet in possession of any accurate determinations of heights in that range in those parts which are in close contact with the plains of India. Two of its ramifications are extremely well adapted for determining the height of perpetual snow. First, the Chumba range, which, as has been pointed out to me by Major Cunningham, is barely snow-tipped throughout the year; and second, the Pir Panjal range south of Kashmir, the northern slopes of which have perpetual snow and glaciers, while on the south side the snow has entirely melted before the end of summer. The elevation of the Pir Panjal has not been determined with accuracy, the heights given by Baron Hügel and by Mr. Vigne being estimated from their measurement of the pass over which they crossed[40].
SNOW-LEVEL IN TIBET.
In the interior of north-west Tibet every principal range attains the elevation of perpetual snow, but only a few peaks rise much above it. There is therefore no very great mass of snow during the summer months to lower the temperature of the air, and consequently circumstances are the most favourable possible for the elevation of the snow-line to an extreme degree; a dry, stony, desert, treeless country, violent winds, clear sky, and powerful sun, being all combined. In the most central part of the country, the Lanak pass, near Hanle, and the Sabu pass, near Le, both elevated as nearly as possible 18,000 feet, are without perpetual snow, but the Parang pass, between 18,400 and 18,600 feet, has a glacier on its north face, and therefore exceeds in elevation the snow-line. The snow-level in central Tibet must therefore be sought between these heights, but nearer that of the Parang pass, which has no perpetual snow towards the south: it is, therefore, certainly not below 18,000 feet.
In the Kouenlun, on the northern border of Tibet, where the mountains are again much more elevated, the snow-level descends no lower. Even on the 19th and 20th of August, the mass of snow, which was on the northern face of its highest peaks continuous down from 20,000 feet and upwards, did not descend below 17,500 feet, and the open level plain of the upper Shayuk had at that height only trifling patches of snow. On the Karakoram pass (18,200 feet) there were only large patches of snow, the south face of the ridge being quite bare for some distance in both directions.
LEVEL ON OPPOSITE SIDES OF PASSES.
The vexata quæstio of the difference of the level at which snow lies on the north and south slopes of the Himalaya, affords a singular instance of misconception. Enunciated originally in an obscure and somewhat incorrect form, when little was known of the structure of the inner part of the chain, the fact has been repeatedly contradicted by those who thought they found it contrary to their experience. Both parties were to a certain extent right. On each individual range the snow-level will at all times be found lower on the north face than on the south, except when the range which we are crossing happens to coincide with a very marked and abrupt change of climate, which will only be the case when it is extremely elevated. When this is the case, the proposition, otherwise true of the mountains en masse, or the inner ranges compared with the outer, becomes applicable to a particular range. This is probably the case in the very pass in Kamaon (I know not which it was) from which the law was first inferred. It is certainly so in the great passes north of the Chenab, where, on the Indian face, I found in June snow at 11,500 feet, while on the north side, only twenty miles distant, it had already receded beyond 15,000 feet.
From the rapid nature of my journey, and the great number of objects to which I was obliged to devote my attention, the geological observations which I was enabled to make were much more imperfect than I could have wished. It appeared, however, desirable, hurried as they were, to enumerate them, for the purpose of drawing the attention of future travellers to the subject; and for the same reason I shall here recapitulate the general conclusions which appear to result from the facts observed.
GEOLOGY OF TIBET.
The greater part of Tibet consists of plutonic and metamorphic rocks; and from the gigantic scale on which the sections are exposed, and the general bareness of the mountains, which enables their structure to be seen, that country probably presents the finest field in which these classes of rocks could be studied. Granite occurs in great abundance, sending immense veins in all directions into the metamorphic rocks, which are seen to be everywhere upheaved and dislocated by the injected mass. In the immediate vicinity of the plutonic masses, all traces of the direction of the strata of the superposed rocks are lost; but elsewhere, with every variety of dip, it is very generally found that the stratified rocks strike in a direction which varies between north-west and south-east, and north-north-west and south-south-east. As all my observations were made roughly and unconnectedly, and without my discovering this identity till after my return to India, the strike is probably very uniform throughout a great extent of country.