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.

ALLUVIAL AND LACUSTRINE DEPOSIT.

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.