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Some notes on the Doletite of the Chor, by COLONEL C. A. MCMAHON,
[Received December 23rd, 1886.]
No. 1. A compact dark-grey rock, very dense and very hard. Sp. G. 307. Batteori, Chor
No. 2. A compact dark-grey rock. Sp. G. 3'05. Barela, Chor Mountain.
None of the above named villages are entered in the Government of India Survey Map, Sheet No. 47. Batteori is on the south side of the Chor, on the road between Nhára (Nara of map) and Tálichoag, both of which places will be found marked on Mr. Medlicott's map (Vol. iii, Memoirs G. S. I.) near the edge of the gneissose granite ("granitoid rocks" of map).
Barela is on the western flank of the Chor, at the edge of the outcrop of gneissose granite, and is, I presume, the place entered on the above mentioned maps as Banallah.
In my field journal I have noted the Batteori rock as occurring in the gneissose granite above the village, and cutting down the hill side in the direction of Tálichoag. The Barela outcrop occurs as a dyke in mica schists, within a few yards of a garnetiferous hornblende rock. This dyke is on the crest of the ridge just above the village, but another outcrop of it is to be found 500 or 600 feet below, just above the bed of the stream. It appears there between beds of felspathic and mica schists, but the outcrop is cut off abruptly at both ends, and does not extend for any distance as a continuous sheet.
Sohana is entered on both the above mentioned maps, in a line between Barela and Tálichoag. I gather from my journals that the outcrop between Barela and Sohana is in mica schist. This rock is evidently the dense trap alluded to in the Memoirs, Geological Survey, III, 42.
I have examined nine thin slices of the above rocks under the microscope, and the result shows that the trap retains essentially the same character throughout Following the nomenclature adopted by Professor Judd in his recent papers (Q.J.G.S., xlii, 61, 62) it may be classed as a dolerite verging towards a gabbro.
The examination of thin slices under the microscope shows that the Chor dolerite is a perfectly holocrystalline rock. It is composed of triclinic felspar in the form of lath-shaped crystals (namely, small elongated prisms) imbedded in a ground mass of augite which plays the rôle of a magma, or base. Olivine, magnetite, red mica, and apatite, are also component crystals.
The felspar is all twinned on the albite plan; but pericline macles are not unfrequent and baveno twins also occur. Judging from its optical characters the felspar belongs to the labradorite-anorthite group, and more than one species would seem to be present. In one of the Batteori specimens a few comparatively large felspars occur, otherwise they are all in the form of elongated prisms matted together.
Even in the thinnest slices, the felspars are tinted a pale, reddish-buff colour. This seems to be a special characteristic of the Chor dolerite; it is common to all the
specimens described in this paper, and I have not met with any thing resembling this tint in the felspar of other rocks. It is doubtless connected with the presence of a large amount of iron in this dolerite, but the colouring matter is so finely disseminated that the highest magnifying powers applicable are insufficient to determine its
"Schillerization" has been set up in all the felspars, and amongst other microscopic inclusions opacite is abundant. These products of alteration, however, are very minute, showing that the process of schillerization had not extended far.
The augite is quite colourless in transmitted light, and is very fresh. It is not unfrequently twinned, but it never shows any trace of external crystallographic form. Club-shaped grains of this mineral are not uncommon.
The most interesting mineral in the rock is olivine, for it is in almost precisely the condition of the black olivines described by Professor Judd, in his paper on the Peridotites of Scotland (Q.J.G.S., xli, 382), the development of opacite, magnetite, or other iron oxide, within the mineral, being due to the process called schillerization. I may note, in passing, that when I showed one or two of these slices to Professor Judd some years ago, he at once confirmed my supposition that the mineral is olivine. In all these slices this mineral presents a uniform character. It is very dark, owing to the development of a fine dust of opacite along certain plains, which dust occasionally arranges itself in very fine lines; at other times magnetite has been developed to such an extent that only small eyes of olivine have here and there been left undecomposed. In some specimens a still further change has set in; namely, the magnetite, or titaniferous iron, has been removed, and a whitish, opaque substance resembling leucoxene has been left behind.
In rather thick slices the olivine is feebly, but distinctly, dichroic; and with the aid of converging polarised light, the double refraction of the mineral is seen to be positive. The olivine is in shapeless grains; and, as is usual with this mineral, though the grains are much cracked, the cracks give no clue to the crystallographic shape. of the mineral. These cracks are apparently due to the strain caused by the cooling of the rock at the time of its consolidation, for the schillerization process has been extensively set up along them, the substance developed being a dusty-looking iron oxide, and not serpentine. The latter mineral would doubtless have been formed had the alteration been caused by sub-aerial agencies. Some of the iron in these olivines is certainly magnetite, for it has occasionally segregated in crystals that on being sliced have yielded square and triangular forms.
Apart from the olivine, magnetite is also abundantly developed in the rock. It is frequently surrounded by red mica, which in transmitted light, varies in colour from greenish-yellow to a rich orange-brown. The mica is powerfully dichroic, and belongs to the group that used to be referred to the hexagonal system.
Apatite is abundant in the Batteori specimens.
No. 4. My next specimen comes from Nhára (Nara) on the south flank of the Chor-the village alluded to above. A little to the south of Nhára, the road from Chaita passes over a ridge running east and west. This ridge is capped with 50 or 60 feet of massive, white, crystalline limestone, weathering grey, which rests on thin bedded mica schists, dipping south, at an angle that varies much within a few yards. The trap is intrusive in the schists and in the limestone.
The microscopic examination of thin slices of this rock shows that it is the same as that previously described. In structure it is perfectly holocrystalline. The augite is more abundant relatively to the felspar than in the other specimens, and both the augite and the felspar are deeper in tint. Apatite, red mica, and magnetite, or ilmenite, are abundant. Olivine is present as usual, but it does not present the dark, dusty appearance of the other specimens. It has been more completely converted, however, into magnetite, or ilmenite, and the white decomposition product of the latter, though characteristic eyes of undecayed olivine, may still be seen in it here and there.
No. 5.-This specimen was collected at Serai, a village on the north-east flank of the Chor, close to the margin of the gneissose-granite. The proper name of the place is, I think, Seran (silent n), the name of one of the local demons of the Chor, and converted, through piety or ignorance, by a Mahommadan scribe into the Serai of the map. The outcrop occurs as a dyke in thin-bedded mica schists, and is well exposed near a waterfall in a valley to the west of the village.
The very peculiar colour of the felspar, as seen in thin slices under the microscope, shows that this dolerite is the same rock as that which occurs on the south of the Chor. The felspar is disposed to be more massive than in the other specimens ; but between crossed nicols it breaks up into well-twinned crystals of triclinic felspar; and, as in the other samples from the Chor, characteristic lath-shaped prisms of this mineral are imbedded in the augite.
Apatite, brown mica, and magnetite are, as usual, abundant; and some epidote is present as a secondary product. Indeed, this specimen is a good deal decomposed. The augite has been in part converted into hornblende and in part into micaceous chlorite. Similarly, the felspar has here and there been changed into a saussuritic mineral, and is invaded, more or less, by micaceous chlorite.
This slice contains no olivine; but I have only one slice, and I cannot lay my hands on the hand specimen. Of the outcrop itself, however, I have a distinct recollection; and I have not only the entry in my journal but a sketch of one portion of the dyke, showing its relation to the schists.
No. 6. My last specimen comes from Roru, a village 24 miles, as the bird flies, from the Chor in a north-easterly direction. It is situated at the confluence of the Pabar and Sikni rivers, on the road to the Shatul and Borenda passes. The outcrop of the dolerite occurs a little south of Roru, about a quarter of a mile from Serai,1 as a dyke in mica schists.
The felspar of this rock is all of reddish-dun colour, of exactly the same tint as the mineral of the other specimens. It is a very peculiar colour, and I think it is sufficient to identify the Roru rock as an offshoot of the Chor dolerites.
The structure is more granitic than in the other specimens ; the felspar is more massed together; but under crossed nicols it all breaks up-even the more massive looking portion-into long prisms (lath-shaped crystals) of triclinic felspar. The latter mineral belongs to the labradorite-anorthite group.
Augite is abundant, and, as compared with the other specimens, it is grouped more massively. It has been, to a considerable extent, converted into secondary hornblende; and this slice affords numerous and instructive instances of the con
1 Not the village alluded to ante.
version of the one mineral into the other by processes set up after the first consolidation of the rock. Clear, colourless augite, often presenting its characteristic cross cleavage lines to view, frequently constitutes the kernel of grains, the whole of the outside being formed hornblende. At other times nearly the whole has been converted into hornblende, small patches of clear augite, with well-marked cross cleavage lines, having been left, here and there, in the midst of the hornblende.
It is interesting to note that the metamorphism of the augite is not a mere alteration in the colour of that mineral; but that it involves a complete change in its internal molecular structure and optical properties; thus the converted mineral, as may be seen in this slice, has not only acquired colour and strong dichroism; but a sensible modification has resulted in its cleavage and in the angle of extinction; the angle at which the cleavage lines intersect each other, and the angle at which extinction takes place, differing considerably in the amphibole and in the parent pyroxene that forms the kernel of the crystalline mass. In one case in which the augite is twinned, the external margin of secondary hornblende is also twinned, but the twinning plane has been shifted a little on one side, and is not in exact continuation, or in quite a straight line with the twinning plane of the augite.
Apatite is abundant, whilst ilmenite and mica (part of the latter being of red and part of clear green colour) are present as secondary minerals.
The ilmenite exhibits its characteristic rhomboidal lines; and in some cases the deposit of iron along these crystallographic lines has been so small that the section of the mineral in the thin slice appears as a mere skeleton; the external shape, however, and the internal spaces being very sharply defined. As these spaces are filled with other minerals, the effect produced is clearly not due to the cutting and grinding of the slice; the absence of leucoxene, on the other hand, is opposed to the supposition that the internal spaces are due to decay. The conclusion at which I have arrived therefore is that, in this particular case, the ilmenite is a secondary mineral, and was deposited along with the secondary mica. The secondary character of the latter mineral is clearly seen from the way in which it invades the substance of the felspar.
The felspar and the hornblende in this slice have been to a considerable extent schillerized, though it is possible that the process may have taken place prior to the conversion of the augite into hornblende. The presence of the schiller inclusions has, in every case, either interfered with the development, or the alteration set up has obliterated all trace of the characteristic hornblende cleavage. When the hornblende is clear, it is highly coloured and exhibits characteristic cleavage lines; where the schiller inclusions are abundant, there is no trace of cleavage; in the latter case, double refraction is not so strong; the colour is less marked, and under crossed nicols the polarisation is confused and indeterminate.
The most prominent of the schiller material is an oxide of iron, which when in large grains is red and translucent. It is invariably devoid of crystalline form.
I have not observed any trace of olivine in this slice; but as I have had an opportunity of examining one slice only I do not think it is desirable to lay much stress on this point. Even if a more extensive examination of the Roru outcrop failed to reveal the presence of this mineral, all that I should be disposed to infer from the fact is that it is locally absent: that the Roru rock forms a portion of the
same eruptive mass as the highly basic rock of the Chor I see no reason to doubt.
The mode of occurrence of the Chor dolerite shows that it is an intrusive and not a contemporaneous eruptive rock, and the examination of thin slices under the microscope bears out the result of field observations. In structure the rock is com. pletely holocrystalline, with a tendency towards being granitic. That is to say, it is more nearly allied to a gabbro than to a lava.
The peculiar colour of the felspar, and the extent to which the olivine is impregnated with iron, are points which may be useful in leading to the identification of the rock in neighbouring localities.
It is now ten years since I visited Batteori, and I have no recollection of the particular outcrop at that village, but I see no reason to doubt the correctness of the entry in my field journal that the outcrop is in what we now call the gneissose granite. The exact wording of the entry is as follows-" At Batteori a very dense heavy trap occurs in the granitoid gneiss. I traced it for some distance above the village. It finally cut down the hill side across the road in the direction of Tálichoag." The outcrops at Barela, Batte ori, and Nhára, therefore would appear, roughly speaking, to be in a line with each other. Near Barela it occurs in the mica schists, near the boundary of the gneissose-granite; at Batteori it occurs in the gneissose-granite itself; it then edges away from the latter rock, and appears in mica schists at Nhára, and in the crystalline white limestones resting on them.
Corresponding outcrops also appear on the opposite side of the Chor, at Serai (near the margin of the gneissose-granite) and at Roru, distant 24 miles from the Chor. When the neighbourhood of the Chor comes to be explored it will doubtless be found in other places also.,
Going hand in hand with the determination of the eruptive character of the gneissose-granite, discussed in previous papers, was the question of age, and there seemed to be good reasons for referring it to the tertiary period. If this conclusion be sound, the dolerite of the Chor would seem to be of tertiary age also, for it appears to be intrusive in the gneissose-granite at Batteori.
The occurrence of highly basic eruptive rocks of undoubtedly tertiary age in the Central Himalayas has already been noted (Records XIX, 115, and references quoted therein); and I think it probable, therefore, that the dole-ite of the Chor may be referred to the period of the final building up of the Himalayas.
At many of the places noted in this paper, and at numerous others in the neighbourhood of the Chor, hornblende schists, and amphibolites, occur, which present a strong resemblance to those of the Satlej valley.
I have examined a good many thin slices of hornblende rocks from the following localities; namely, from Nhára, and Barela on the Chor (two of the localities where the dolerite occurs); from Júbal (Jabal of map) about 18 miles from the Chor in a north-north-east direction (the outcrop occurs near the top of the pass leading over into Kotkai); from between Kadara and Sungri (on the mountains that overhang the Satlej to the east, near Nirth); and form the waterparting between the Satlej and the Júbal valleys.
Under the microscope these Chor hornblende rocks and those which form con