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].

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.

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.

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.

It is not a little remarkable that a belt twenty miles wide, in the direction of this line of strike, drawn from Iskardo to the Niti pass, would cover every place south of the Indus in which limestone has been observed in Tibet. It would pass through Molbil on the Pashkyum river, the limestone districts of Zanskar, and the Lachalang pass, where limestone was found by Gerard. It would also cover Piti, Hangarang, and Bekhar, all well-known limestone tracts. Of course the limestones of Nubra and the Karakoram on the one hand, and of Kashmir on the other, cannot in any way be connected with this line.

The sandstones, slates, and conglomerates, which so closely resemble in appearance those rocks which in Europe are chiefly members of the old red sandstone and greywacke series, appear to assume also the same direction. I bring forward these coincidences of direction only as a remarkable fact, worthy of investigation, without attaching any great weight to them, as more careful observation may show that they are merely accidental, and that rocks of very different ages exist among the limestones and associated rocks of the northern Himalaya.

The great extent and development of a very modern alluvium-like formation, composed of great masses of clay with boulders, and occasionally of very fine laminated clay, constitutes one of the most remarkable and striking features of Western Tibet. In every part through which I have travelled, and at all elevations, except on the highest passes, I have found these deposits in greater or less quantity. In their most common state they consist of loose earthy or clayey unstratified masses, containing boulders either angular or rounded. Very fine clay, distinctly and horizontally stratified, is also common; sandstone and hardened conglomerate are more rare, but also occur occasionally.

That some of these beds are of lacustrine origin, the occurrence of fresh-water shells appears to prove very clearly; and though here and there small portions may be terrestrial and of glacial origin, it cannot, I think, be doubted that the great mass of the boulder clay was deposited under water.

In the structure of Scotland at the present day we have a state of circumstances which appears to me capable of throwing much light on the nature of these deposits. We find there a series of narrow arms of the sea, stretching far into the land, and separated by rugged and generally steep ranges of metamorphic or plutonic rocks. They are all more or less silted up by sedimentary matter, and near their mouths, especially where, as is often the case, they are much contracted, we generally find a bar, shallower than the remainder. At various elevations above the sea-level again there is a series of fresh-water lakes, differing little in aspect from the arms of the sea. We find also in many parts of the Highlands of Scotland long valleys, nearly level, which are filled with incoherent sedimentary deposits, and bounded like the lochs by steep mountains. If these were formerly arms of the sea, which by the elevation of the land have been converted into dry land, then the fresh-water lakes probably occupy those parts of the narrow channels which were originally deepest, or which, being wider than the rest, have remained unoccupied by sedimentary matter at the time of the elevation. In conformity with this view we find that at the lower end of these lakes the mountains generally approach very close to one another.

If we were to suppose the gradual elevation of Scotland to continue till the mountains attained an elevation equal to that of the Himalaya, it is evident that a continued series of marine sedimentary deposits would extend from the summit to the sea-level, unless removed by the action of streams or other ordinary causes. Some of the valleys would be of considerable width, and would contain marine fossils in great abundance; but in the narrower mountain valleys the gravel and boulders would be quite destitute of fossils. Here and there fresh-water formations of partial extent would occur, but they would be separated from one another by large tracts filled with marine beds. The gradual elevation of the land would bring to bear upon these incoherent strata the powerful action of running water, which would remove portion after portion, till at last deep valleys would be excavated, and small patches only of the gravel and clay would remain where the action of the streams was least powerful. Such I conceive to be the present state of Tibet, but a much more detailed investigation of that remarkable country would be necessary, before this view can be regarded in any other light than an hypothesis.

The causes by which the metamorphic rocks, which must have been brought into their present remarkable state at a great depth in the interior of the globe, acquired their present configuration of mountain and valley, form a question on which I am not now prepared to enter. One continued process of elevation seems inadequate to produce the observed effects; but however numerous the alternations of elevation and depression may have been, it is evident that the alluvial deposits at present existing must all be referable to the last period of elevation, as such incoherent strata could not withstand the continued action of the sea.

INDEX.