« PreviousContinue »
THE GEOLOGICAL SURVEY OF INDIA.
Note on some points in Himalayan Geology, by R. D. OLDHAM, A.R.S.M., F.G.S., Deputy Superintendent, Geological Survey of India.
The observations on which the following remarks are based were made partly in company with my colleague Mr. C. S. Middlemiss, partly by myself alone during the spring of 1884. During a brief tour in the lower Himalayas my attention was directed to an attempt to elucidate some doubtful points in, and as far as possible to complete, the sequence of sedimentary formations in the lower Himalayas. As a consequence very little detailed mapping was done, and that only where the structure of the hills appeared to be simple enough to allow of some definite conclusion being arrived at as to the superposition and relation of the rock groups. Nothing was published at the time, as the conclusions arrived at were for the most part provisional and liable to alteration. Since then there has been no opportunity of correcting or amplifying them, but on consideration there seem to be some points of sufficient interest and importance to warrant publication.
The limestone of the Naira valley-In the valley of the Naira river (Neweli of map) in eastern Sirmúr a massive limestone is exposed, some of the beds having a strong sulphurous odour, while near the base are some oolitic bands. As regards lithological appearance, it does not differ markedly from either the Krol or Deoban limestones, it extends on either side of the valley and forms the Juin peak on the north, and the hill which rises above Dugána on the south. At the Juin station it is cut off by a combination of a uniclinal fold and faulting; to the south it is cut off by denudation. On the summit of the peak above Dugána there is a limestone conglomerate, cemented by calcareous mud, which resembles some of the conglomerates in the Mandháli series on the Deoban ridge. This fact, combined with the occurrence of carbonaceous slates in the upper part of the Naira valley, where they appear to overlie the limestone, led me at the time to regard the latter as of Deoban, rather than Krol age. Subsequent consideration has, however, led me to doubt this
1 I shall show later on that the two limestones are distinct in age.
conclusion, the superposition of the carbonaceous slates may well be due to disturbance, for the beds are very much disturbed in the upper Naira valley, while the limestone conglomerate is a rock which might naturally be expected to occur as the bottom bed of a formation resting unconformably on massive limestone.
The Blaini group in the Naira and Bangál valleys.-Below the limestone there come grey slates in which the Blaini limestone and boulder bed occur; it was traced from the crest of the ridge south of the Naira valley into the Bangál valley, being much cut up by faults; it is carried up one side and down the other of the ridge which separates these two valleys by a series of step faults, which are especially marked on the south side of the ridge. These faults run in the direction of the boundary of the massive limestone on the Juin hill, and in that rock seem to die out as faults, and become converted into a uniclinal fold.
The group is of the same type as that first described in the Blaini valley, consisting of a single band of pink limestone and conglomerate, the latter containing many fragments of volcanic rock derived from the volcanic series which will presently be mentioned. In the bed of the stream which flows east of the village of Bombhil, close to its junction with the Naira there is an exposure of the boulder bed which distinctly marks its mode of origin. The matrix is laminated and the laminæ are seen to be bent down under, and to arch over, the larger included fragments, showing that these must have, been dropped into a tranquil sea in which the fine silt of the matrix was being deposited.
It is not necessary to recapitulate here the reasons why I regard the boulders as having been dropped by floating ice, but I may mention that where the boulder bed crosses the Naira valley we discovered a pebble (now in the museum) showing striations similar to those usually ascribed to glacier action. It is, however, necessary to notice the extraordinary similitude to a volcanic breccia exhibited by the Blaini "conglomerate" on the Juin ridge: not only are the included fragments. mostly of volcanic rock, but the matrix itself very strongly resembles an impure volcanic ash. It is not natural to suppose that this rock should here be of volcanic origin, while elsewhere, and in general, such an origin can be shown to be untenable, so we must look to some other cause for the resemblance. This I imagine to be the same as explains the other features presented by the rock, viz., a severe climate. Under the climatic conditions which now prevail even at an altitude of six or seven thousand feet, the volcanic beds disintegrate, principally by the decomposition of their constituent minerals, but it is conceivable that, under a climate severe enough to produce floating ice at the sea level, the disintegration of the rock would be more rapid than the decomposition of its constituent minerals; in this case it would be difficult to distinguish the sandy material so produced from the ash directly produced by a volcanic eruption. I know of no observations confirmatory of this hypothesis or the reverse, but it seems a possible one, and at any rate does not conflict with conclusions arrived at on independent grounds.
The volcanic beds and "lower Chakrata" quartzites of the Bangál and Naira valleys.-Under the Blaini beds, but separated from them by slates, comes a series of volcanic beds, consisting of felsitic lavas and ashes, underlaid conformably by purplish red and mottled quartzites, with interbedded schistose slates. This series of beds I have no hesitation in correlating with what I have described as the Lower Chak
ratas, and overlying volcanic beds of Jaunsar, as they resemble each other, not only in lithological character, but in the order of superposition of the beds.
Regarding this, the most important fact to be noticed is that part, at least, of the volcanic beds is of subaërial origin. In proof of this statement, I appeal to a specimen, preserved in the Imperial Museum at Calcutta, where portions of two distinct lava flows are seen to include between them a string of well-rounded, water-worn pebbles. Were these only of lava, it would not indicate more than an isolated volcanic island, which need not have been raised more than enough to bring its summit within reach of the breakers, but the majority of the pebbles are of vein quartz, which must have been derived from some land surface of non-volcanic rocks.
On the north side of the Bangál valley below the village of Lána there occurs, far below the volcanics, a bed which, like the Blaini "conglomerate," can hardly but be of glacial origin. It consists of rounded, waterworn boulders of quartzite, imbedded in a fine grained, red coloured, subschistose matrix. This rock was not seen in situ, so there is some doubt attaching to its geological position, but it occurs in large fragments just where a continuously descending section of the red quartzites and volcanic beds is faulted against the Deoban limestone. It is important to note that this is not a volcanic bed; it is separated from the volcanics by the "Lower Chakratas" and a considerable thickness of schistose slates that underlie them; and the well-rounded condition of the boulders, which range to over a foot in diameter, shows that they have been waterworn.
In northern Jaunsar I have seen a very similar bed at about the same horizon; its position is represented by the southernmost of the two blue patches in Jaunsar on the map illustrating Col. McMahon's paper on the Blaini groups and central gneiss of the Simla district.1
Relation of the Blaini beds to underlying rocks.-On the north side of the Bangál valley the volcanic series is overlaid by about 1,000 ft. of subschistose grey slates. On the south side there are at most a few hundred feet of slates between the Blaini and the volcanic beds, and these appear to belong rather to the Blaini than the volcanics. On the crest of the Juin ridge the volcanics do not seem to have thinned out appreciably, but down the slope into the Naira the thickness of them lying between the purple quartzites and the Blaini slates diminishes, till in the Naira valley the volcanics have disappeared entirely.
It is possible that this may be due to faulting, but the recognizable faults are almost all dip-faults, which would not affect the apparent thickness of strata between the purple quartzites and the Blaini group, while the beds lie too flat for their appearance to be due to a squeezing out of the rocks in consequence of contortion; nor, were such to take place, would one expect the volcanics to be squeezed out rather than the overlying slates.
It might be supposed that the thinning out of the volcanics represented their original limit of extension, but that is hardly probable, for a band of limestone is inter-stratified with them throughout Jaunsar, while, as I have already shown, they are of subaerial origin in Bangál valley. It is reasonable therefore to suppose that this locality marks the neighbourhood of a focus of eruption, and a sudden thinning out in its neighbourhood is not what would be expected.
1 Records, G. S. I., X, p. 222.