Page:EB1911 - Volume 13.djvu/487

Rh Although the northern limits of the Tsanpo basin are not sufficiently well known to locate the Indo-Tibetan watershed even approximately, there exists some scattered evidence of the nature of that strip of Northern Himalaya on the Tibeto-Nepalese border which lies between the line of greatest elevation and the trough of the Tsanpo. Recent investigations show that all the chief rivers of Nepal flowing southwards to the Tarai take their rise north of the line of highest crests, the “main range” of the Himalaya; and that some of them drain long lateral high-level valleys enclosed between minor ridges whose strike is parallel to the axis of the Himalaya and, occasionally, almost at right angles to the course of the main drainage channels breaking down to the plains. This formation brings the southern edge of the Tsanpo basin to the immediate neighbourhood of the banks of that river, which runs at its foot like a drain flanking a wall. It also affords material evidence of that wrinkling or folding action which accompanied the process of upheaval, when the Central Asian highlands were raised, which is more or less marked throughout the whole of the north-west Indian borderland. North of Bhutan, between the Himalayan crest and Lhasa, this formation is approximately maintained; farther east, although the same natural forces first resulted in the same effect of successive folds of the earth’s crust, forming extensive curves of ridge and furrow, the abundant rainfall and the totally distinct climatic conditions which govern the processes of denudation subsequently led to the erosion of deeper valleys enclosed between forest-covered ranges which rise steeply from the river banks.

Although suggestions have been made of the existence of higher peaks north of the Himalaya than that which dominates the Everest group, no evidence has been adduced to support such a contention. On the other hand the observations of Major Ryder and other surveyors who explored from Lhasa to the sources of the Brahmaputra and Indus, at the conclusion of the Tibetan mission in 1904, conclusively prove that Mount Everest, which appears from the Tibetan plateau as a single dominating peak, has no rival amongst Himalayan altitudes, whilst the very remarkable investigations made by permission of the Nepal durbar from peaks near Kathmandu in 1903, by Captain Wood, R.E., not only place the Everest group apart from other peaks with which they have been confused by scientists, isolating them in the topographical system of Nepal, but clearly show that there is no one dominating and continuous range indicating a main Himalayan chain which includes both Everest and Kinchinjunga. The main features of Nepalese topography are now fairly well defined. So much controversy has been aroused on the subject of Himalayan altitudes that the present position of scientific analysis in relation to them may be shortly stated. The heights of peaks determined by exact processes of trigonometrical observation are bound to be more or less in error for three reasons: (1) the extraordinary geoidal deformation of the level surface at the observing stations in submontane regions; (2) ignorance of the laws of refraction when rays traverse rarefied air in snow-covered regions; (3) ignorance of the variations in the actual height of peaks due to the increase, or decrease, of snow. The value of the heights attached to the three highest mountains in the world are, for these reasons, adjudged by Colonel S. G. Burrard, the Supt. Trigonometrical Surveys in India, to be in probable error to the following extent:

These determinations have the effect of placing Kinchinjunga second and K2 third on the list.

Geology.—The Himalaya have been formed by violent crumpling of the earth’s crust along the southern margin of the great tableland of Central Asia. Outside the arc of the mountain chain no sign of this crumpling is to be detected except in the Salt Range, and the Peninsula of India has been entirely free from folding of any importance since early Palaeozoic times, if not since the Archean period itself. But the contrast between the Himalaya and the Peninsula is not confined to their structure: the difference in the rocks themselves is equally striking. In the Himalaya the geological sequence, from the Ordovician to the Eocene, is almost entirely marine; there are indeed occasional breaks in the series, but during nearly the whole of this long period the Himalayan region, or at least its northern part, must have been beneath the sea—the Central Mediterranean Sea of Neumayr or Tethys of Suess. In the peninsula, however, no marine fossils have yet been found of earlier date than Jurassic and Cretaceous, and these are confined to the neighbourhood of the coasts; the principal fossiliferous deposits are the plant-bearing beds of the Gondwana series, and there can be no doubt that, at least since the Carboniferous period, nearly the whole of the Peninsula has been land. Between the folded marine beds of the Himalaya and the nearly horizontal strata of the peninsula lies the Indo-Gangetic plain, covered by an enormous thickness of alluvial and wind-blown deposits of recent date. The deep boring at Lucknow passed through 1336 ft. of sands—reaching nearly to 1000 ft. below sea-level—without any sign of approaching the base of the alluvial series. It is clear, then, that in front of the Himalaya there is a great depression, but as yet there is no indication that this depression was ever beneath the sea.

In the light thrown by recent researches on the structure and origin of mountain chains the explanation of these facts is no longer difficult. From early Palaeozoic times the peninsula of India has been dry land, a part, indeed, of a great continent which in Mesozoic times extended across the Indian Ocean towards South Africa. Its northern shores were washed by the Sea of Tethys, which, at least in Jurassic and Cretaceous times, stretched across the Old World from west to east, and in this sea were laid down the marine deposits of the Himalaya. The tangential pressures which are known to be set up in the earth’s crust—either by the contraction of the interior or in some other way—caused the deposits of this sea to be crushed up against the rigid granites and other old rocks of the peninsula and finally led to the whole mass being pushed forward over the edge of the part which did not crumple. The Indo-Gangetic depression was formed by the weight of the over-riding mass bending down the edge over which it rode, or else it is the lower limb of the S-shaped fold which would necessarily result if there were no fracture—the Himalaya representing the upper limb of the S.

Geologically, the Himalaya may be divided into three zones which correspond more or less with orographical divisions. The northern zone is the Tibetan, in which fossiliferous beds of Palaeozoic and Mesozoic age are largely developed—excepting in the north-west no such rocks are known on the southern flanks. The second is the zone of the snowy peaks and of the lower Himalaya, and is composed chiefly of crystalline and metamorphic rocks together with unfossiliferous sedimentary beds supposed to be of Palaeozoic age. The southern zone comprises the Sub-Himalaya and consists entirely of Tertiary beds, and especially of the upper Tertiaries. The oldest beds which have hitherto yielded fossils, belong to the Ordovician system, but it is highly probable that the underlying “Haimantas” of the central Himalaya are of Cambrian age. From these beds up to the top of the Carboniferous there appears to be no break; but the Carboniferous beds were in some places eroded before the deposition of the Productus shales, which belong to the Permian period. It is, however, possible that this erosion was merely local, for in other places there seems to be a complete passage from the Carboniferous to the Permian. From the Permian to the Lias the sequence in the central Himalaya shows no sign of a break, nor has any unconformity been proved between the Liassic beds and the overlying Spiti shales, which contain fossils of Middle and Upper Jurassic age. The Spiti shales are succeeded conformably by Cretaceous beds (Gieumal sandstone below and Chikkim limestone above), and these are followed without a break by Nummulitic beds of Eocene age, much disturbed and altered by intrusions of gabbro and syenite. Thus, in the Spiti area at least, there appears to have been continuous deposition of marine beds from the Permian Productus shales to the Eocene Nummulitic formation. The next succeeding deposit is a sandstone, often highly inclined, which rests unconformably upon the Nummulitic beds and resembles the Lower Siwaliks of the Sub-Himalaya (Pliocene) but which as yet has yielded no fossils of any kind. The whole is overlaid unconformably by the younger Tertiaries of Hundes, which are perfectly horizontal and have been quite unaffected by any of the folds.

From the absence of any well-marked unconformity it is evident that in the northern part of the Himalayan belt, at least in the Spiti area, there can have been no post-Archaean folding of any magnitude until after the deposition of the Nummulitic beds, and that the folding was completed before the later Tertiaries of Hundes were laid down. It was, therefore, during the Miocene period that the elevation of this part of the chain began, while the disturbance of the Siwalik-like sandstone indicates that the folding continued into the Pliocene period. Along the southern flanks of the Himalaya the history of the chain is still more clearly shown. The sub-Himalaya are formed of Tertiary beds, chiefly Siwalik or upper Tertiary, while the lower Himalaya proper consist mainly of pre-Tertiary rocks