Imagens das páginas
PDF
ePub

The western boundary is formed by a well-defined ridge known as the Singalela ridge from the principal pass in it. It is continued southward into the Darjiling district, and is traceable as far as Gum, south of Darjiling.

The principal peaks in this ridge, for which there are names, are Singalela (12,130 feet), Lampheram (Sagu, 12,130 feet), Negadachenphuk (14,770 feet), and Kanchanjinga (28,150 feet). It forms the watershed between Sikkim and Nepal, the streams on the Sikkim side being feeders of the Tistá and those on the Nepal side tributaries of the Ganges. The chief passes on the western frontier are Kanglanangma (16,740 feet), and Chia Bhanjan (10,320 feet).

The southern boundary of Protected Sikkim is formed by the Rummam, the Rangit, the Tista, the Rungpo and the Rhenock spur.

Sikkim is essentially a mountainous country without a flat piece of land of any extent anywhere. The mountains rise in elevation northward. The high serrated snow-capped spurs and peaks, culminating in the Kanchanjinga, which form such a characteristic and attractive feature in the scenery of Sikkim, are found in this direction. The northern portion of the country is deeply cut into steep escarpments, and, except in the glacial valleys (the. Lachen and the Lachung), up to a height of about 8,8co feet, is not populated. Southern Sikkim is lower, more open and fairly well cultivated.

This configuration of the country is partly due to the direction of the main drainage, which is southern. The Himalayas on the Indian side must have sloped to the south from the earliest geological times, when the gneiss which constitutes their main body was elevated. For, all the later rocks—the submetamorphic slate group, the coal-bearing Damudas, and the Tertiaries—which fringe the outer Himalaya are evidently formed of detritus carried by rivers like the Tistá from the north.

The physical configuration of Sikkim is also partly due to geological structure. The northern, eastern and western portions of the country are constituted of hard massive gneissose rocks, capable of resisting denudation to a considerable extent. The central and southern portion, on the other hand, is chiefly formed of comparatively soft, thin, slaty and half-schistose rocks, which are denuded with facility; and it is this area which is the least elevated and the most populated in Sikkim.

The trend of the mountain system, viewed as a whole and from a distance, is in a general east-west direction, nearly parallel to the predominant rock-strike. The chief ridges in Sikkim, however, with the exception of the Donkiá ridge, run in a more or less north-south direction, that is, at right angles to the strike of the rocks. The Singalela and the Cholá ridges have been mentioned above. Another northsouth ridge runs through the central portion of Sikkim separating the Rangit from the Tistá valley. Tendong (8,676 feet) and Moinám (10,637 feet) are two of its best known peaks. This north-south direction of the principal ridges is due no doubt to the original southern slope of the Himálaya. The Rangit and the Tistá, which form the main channels of drainage, run nearly north-south, that is, transversely to the rock-strike. The meridional ridges throw off lateral spurs, which run roughly parallel to the strike; and corresponding to these spurs there are streams running in the same direction.

The valleys cut by the rivers and their chief feeders are very deep. The valleys of the Rangit, the Tistá and of their chief tributaries are generally not less than

5,000 feet in depth. They are rather open towards the top, but usually attain a steep gorge-like character as we approach the beds of the rivers. As a consequence of this and also of the comparative insalubrity of the lower portion of the valleys, all the monasteries and principal villages are situated at an elevation ranging from 4,000 to 6,000 feet.

The snow-capped ridges in the northern portion of the country send down glaciers which at present usually come down to about 13,500 feet; those from the Kanchinjinga appear to descend about 1,000 feet lower. The perpetual snowline in Sikkim may be approximately put down at 16,000 feet, so that the glaciers descend 3,500 to 2,500 feet below that line. Formerly they used to descend much lower than at present. Láchung, for instance, of which the elevation is 8,790 feet, stands at the foot of an immense terminal moraine. The Bidangcho lake, on the road between Gnáthong and Jalep pass, at an elevation of 12,700 feet, is dam med at the southern end by a bank of boulders which are distinctly of glacial origin. Moraines occur also about Thangme in the Prágchu valley at an elevation of about 13,000 feet. The retreat of the glaciers backward towards the névé in these cases has been recent, and the ancient moraines evidencing their advance are still in situ. But the excessive rainfall of Sikkim amounting annually to probably no less than 200 inches makes the removal and rearrangement of the glacial boulders a question of very short time; and once brought within the action of the torrential streams the boulders soon lose all traces of their glacial origin. The peculiar configuration of the hills passed over by glaciers is also soon lost owing to pluvial denudation. The glacial valleys, as, for instance, the Prágchu, the Lachen, and the Láchung valleys, are open and shaped, and this shape is one of the most reliable evidences of their origin. But after the retreat of the glaciers, the streams taking their place soon cut the valleys down deeply into V-shaped gorges, and the striking distinction between glacier and river valleys is soon effaced. Thus, owing to excessive rainfall, traces of past glacial action are liable to extinction in Sikkim, and it is impossible to tell how far the glaciers extended in remote times. The lowest height of glacial extension for which I found unmistakeable evidence is that of Lachung (8,790 feet). Below Lachung also, down to a height of about 7,000 feet, the valley is open and has a glacial look about it.

Valleys to which glaciers come down or whence these have but recently retired abound in small lakes or tarns, which are dammed in at the outlet by moraines. Some of these tarns have been described by me in my “ Journal of a trip to the glaciers of Kabru, Pandim, &c." (Records, Vol. XXIV, pt. 1, p. 46). The Bidangcho lake, 3 miles north-east of Gnáthong, is the best instance I came across of a glacial lake in a valley whence the glacier has recently retired. It is 143 mile in length, and its greatest breadth is 1/2 mile.

The following hot springs are known in Sikkim :i. Phut Sachu.-On the east side of the Rangit river, 2 miles north-east of

Rinchinpong monastery. Situated amongst dark-coloured, massive, siliceous limestones; hot fetid water bubbles up at several spots. Temperature at one spring 100*4° F. The springs are situated in the bed of the river, which at the time I visited them (March) was dry. These springs are referred to in Dr. Oldham's “ Thermal springs of India" (Mem., Vol. XIX, pt. 2, p. 32) as Phug Sachu.

2. Ralong Sachu.-On the west bank of the Rangit river, about 2 miles

N.N.W. of Rálong monastery. Elevation about 3,100 feet. Situated
amongst finely laminated phyllites with abundance of veinquartz, at
a height of about 100 feet above the bed of the river. Hot-water
flows out through fissures at several places. The temperature of the
hottest spring close to where it comes out is 131° F.; in a reservoir
constructed for bathing purposes it is 118.4° F. The temperature of
another spring close to where the water flows out is 114.8°; in the
reservoir it is 107.6o. (The temperature of an adjacent stream was
found to be 53.6°). A whitish deposit which effervesces strongly on the
application of hydrochloric acid is formed at the mouths of the springs.
It is stained green at places with carbonate of copper, due, no doubt,
to the springs/passing through cupriferous strata. It is very likely these
springs that are referred to in Dr. Oldham's list as “Puklaz-Sachu,"
"about one day's journey from the monastery of Pemlong” (op.cit.,
p. 32). Probably “Pemlong" is meant for Ralong. I heard of a hot
spring about half a mile north of Rálong Sachu, which I had not time to

visit.
3. Veumtong.-On the east bank of the Láchung river, half a mile below

Yeumtong river. Though I passed the springs I could not get at them owing to the bridge over the Láchung not having been constructed at the time of my visit (May). They are described in Dr. Oldham's list (op. cit., p. 32). “The discharge amounts to a few gallons per minute. The temperature at the source is 112%2°, and in the bath 106°. The water has a slightly saline taste. It is colourless, but emits bubbles of sulphuretted hydrogen gas, blackening silver.” (Hooker, Him.

Journ., 1855, Vol. II, p. 126.) 4. Momay.-"Hot springs burst from the ground near some granite rocks

about 16,000 feet above the sea, and only a mile below the glacier [of Kinchinjhow), and the water collects in pools; its temperature is 110°, and in places 116o." (Hooker, “ Himalayan Journal," 1855, Vol. II, p. 140 ; see also Dr. Oldham, op. cit., p. 33.)

II.-GEOLOGY. The following publications contain scattered notes on the rocks of the country :1854.-" Himalayan Journals,” by Sir Joseph Hooker.

1854.- Journal, Asiatic Society, Bengal, Vol. XXII, pp. 540, 611:-“Notes upon a tour in the Sikkim Himalayas along a portion of the western, or Nepal frontier,” by Captain W. S. Sherwill.

1862.- Journal, Asiatic Society, Bengal, Vol. XXXI, p. 457. “ Journal of a trip undertaken to explore the glaciers of the Kanchanjinga group,” by Major J. L. Sherwill.

1871.- Journal, Asiatic Society, Bengal, for 1871, p. 367 :—"Account of a visit to the eastern and northern frontiers of Sikkim,” by W. T. Blanford.

1891.-Records, Geological Survey of India, Vol. XXIV, pt. I, p. 46:—"Extracts from the Journal of a trip to the glaciers of the Kabru, Pandim, &c., undertaken in September 1889,” by P. N. Bose.

1.- The Gneissic Group. The rocks belonging to this group are the oldest, and constitute the main body of the Himalayas. From near Kurseong, south of Darjiling,.to the northern frontier of Sikkim, it is uninterruptedly traced over a distance of some 75 miles in a straight line; whereas all the later rocks—the submetamorphic slate group, the Damudas, and the Tertiaries—together cover an area in the outer Himalayas nowhere more than 6 miles in width. Two forms of the gneiss are met with(a) In Southern Sikkim, approximately south of the parallel of Jongri and

Boktolá (about Lat. 27° 25') the gneiss is highly micaceous and frequently passes into mica schists. Both muscovite and biotite occur, the former predominating. Hornblende, garnet, and schorl are the chief accessory minerals. Bands of quartzite are common. Veins of calcite occur at places, as near Lingtu by the road to Gnáthong. The gneiss is well foliated, and exhibits strongly marked features of disturbance, in that it is much folded and crumpled, especially in the extreme south

about Darjiling. The prevailing strike is W.N.W.-E.S.E. (6) In Northern Sikkim, as north and south-west of Jongri, about Lachung,

&c., the gneiss is not quite so micaceous. Muscovite is either rare, or is entirely absent, but biotite is abundant. Schorl and hornblende are the chief accessory minerals. Intrusive granitic rocks occur as dykes and sheets ; in some of them muscovite is well developed. The northern gneiss is apparently not so highly disturbed as the southern; the dips being, as a rule, rather easy, indeed, sometimes as low as 10°. The prevailing strike is the same as that of the southern gneiss, the general

direction of dip being north-eastern. The northern gneiss agrees in some of its petrological characters with the “Centrai Gneiss" of Stoliczka. As the southern gneiss, however, was uninterruptedly traced into it, and as no physical break was perceptible anywhere, they are very likely of the saine age. Mr. Medlicott takes the same view in the Manual of Indian Geology.

The relation between the Gneissic group and the next group (the Dalings), which includes submetamorphic slates, phyllites, &c., is far from clear. At the eastern boundary between the two groups, which passes by Gántok, the present capital of Sikkim, the Dalings apparently underlie the gneiss, the dip of both being northeastern. So do they also at the western boundary, which passes by Pemionchi, the first monastery in Sikkim, the dip there being north-western. At the southern boundary, which is in the Darjiling district, and which appears to be faulted, the dips of both the groups are southern. Wherever the junction between the two groups is observed, the Dalings appear to underlie the gneiss; and the fact that the former pass into mica schists at places near the junction makes it appear as if there was a passage from the one to the other group. Indeed, Mr. Mallett considered the gneiss as more recent than the Dalings. But the former being presumably the older rock, it would be preferable to find some other explanation for the apparent underlie of the Dalings; and an explanation will be suggested later on.

1 Vol. ii, 597, 614. 2 Mem. vol. xi, pt. I, p. 42.

estone is just south of Nariably occ

The fact that near the junction everywhere the gneiss dips in the same direction as the Dalings shows that the former was largely affected by the disturbing forces which tilted up the latter. The low north-eastern dip in the northern gneiss is due to disturbance at some previous period; possibly it accompanied the elevation of the gneiss.

2.The Dalings. This name was given by Mr. Mallet to a group of submetamorphic rocks, after a place called Daling in the Darjiling district. Phyllites form the predominant rock in this group. At the boundary between it and the gneissose rocks they pass into silvery mica schists; in fact, in this position the passage is sometimes so gradual that it is difficult to say where the one group begins and the other ends. Dark clay slates with thick quartzite bands prevail near Chákang, Páchikháni (south of Pakyang), &c.; the workable copper ores invariably occur amongst these rocks. Gritstone is sometimes met with just south of Namchi: conglomerate is not known. Impure siliceous limestone is found north-east and north-west of Namchi; and highly carbonaceous shales occur in the Mangpurjhora just south of Namchi, by the Rangit, east of Chákang, and by the Rum mám near Gok (south of Chákang).

Igneous rocks are rare. A rather thick band of gneissose granite was met with between Murtám and Rámthek, which continues for some distance northward, as well as southward, forming the serrated peaks D. 5 and Maphila. It is probably of intrusive origin, and of great interest in connection with the age of the intrusive rocks in the gneissie group in northern Sikkim. An unquestionably intrusive dioriticlooking rock was encountered penetrating through slates by the road between Song and Tikobu. All these rocks will be described in detail hereafter.

In Sikkim, Dalings occur somewhat in the form of a dome-shaped anticlinal. On the south side the dip is southern; east of the Rangit it is chiefly E.N.E. to N.E.; west of that river the inclination is north-western; and on the north side, as near Ralong, the dip is mainly northern. The southern boundary between the Dalings and the gneissose rocks, which passes a little north of Darjiling, was shown by Mr. Mallet to be faulted. The eastern boundary passes by Gántok, and the western by Pemionchi. As in the case of the Damuda-Tertiary and the Daling-Damuda boundaries in the Sub-Himalayas, both of these boundaries may represent “lines of original contact, possibly modified by subsequent faulting."? The Daling rocks would in this case have to be supposed as deposited in a lake, of which steep gneiss-escarpments formed the sides : the lower gorge of the Tista below its junction with the Rangit which, except close to its debouchure, is composed of Daling rocks, forming the outlet of such a lake. The Dalings, it should be noted in this connection, unquestionably bear the impress of lacustrine, rather shallow-water deposits, falsebedding being noticeable at places. In fact, they recall to one's mind the micaceous clays and sandstones of Tertiary age in the outermost fringe of the Sub-Himalayas. By subsequent tangential pressure, which caused their disturbance, the Dalings would be tilted up against the original gneiss-escarpments in such a manner as to present an appearance of conformable underlie and of faulting. The greater metamorphism of the Dalings at the boundary between them and the gneissose rocks—a fact which has been noted before—may be account

[ocr errors]
« AnteriorContinuar »