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No. 14-19 was collected by Mr. V. Ball, F.R.S., and it is believed to be a sample of the rock near the Simra Bungalow, described at p. 66 of his memoir on the Geology of the Rájmahál Hills (Memoirs, G. S., Vol. XIII).
Viewed macroscopically this specimen has all the aspect of a trachyte; but as the predominant felspar is certainly not orthoclase (sanidine), it must be classed with the andesites.
To prevent any mistake I had a second slice of this hand specimen prepared, but I have been unable to detect in either of the slices a single crystal of sanidine. The felspar, judged by these slices, is all triclinic; and belongs, like that of the augite-andesites, described above, to the labradorite-anorthite group. It is frequently zoned and presents examples of every form of twinning-albite twins being combined with those of carlsbad, pericline, and baveno.
The rock is composed of a mass of felspar crystals of various sizes (some being comparatively large) imbedded in a devitrified glassy base. The base is much more prominent in one slice than in the other.
The Simra andesite must originally have contained a large amount of magnetite. Some of it has been left in an unaltered condition; but the greater part has been converted into a red oxide; and it is to the presence of the latter in large quantities that the reddish-pink colour of the rock is due.
Olivine is totally absent, and augite is very sparse; being limited to the presence of a few stray, rounded, and corroded crystals. These have apparently not been formed in situ, as they are externally very much corroded, as if by an acid magma.
I thought it would be interesting to compare the Rájmahál specimens with the samples of Deccan traps in my possession; and accordingly sliced and examined the following, which are good typical examples from five different, and widely separated, localities.
No. 1-A sub-vitrious lava. Antop Hill. Bombay.
A detailed description of the geology of Antop Hill and of this rock, the "black rock of Antop hill, (?) felstone" of the map which accompanies Mr. A. B. Wynne's paper, will be found at p. 36-40, Memoirs, Geological Survey, Vol. V. It is described as a "black splintery rock." "The whole rock of this hill," Mr. Wynne informs us is "of a curious compact and almost flinty kind," which "differs greatly in appearance from any of the trap-rocks commonly found in the neighbourhood;" but it "reappears at Seoree," and forms "the rock called Cross Island, in the harbour, at a distance of several miles." "Some previous observers thought it resembled an altered argillaceous deposit," but Mr. Wynne successfully combated that notion in his memoir.
Under the microscope this is seen to be a true volcanic rock. It is singularly, fine-grained-finer grained than any lava I have ever before seen; but thin slices. under the microscope have none of the aspects of a basalt-glass on the one hand, or of a felsite on the other.
The rock is composed of microscopic prisms of triclinic felspar, and microscopic grains of augite, and magnetite. The microliths of felspar are well formed though minute, and they exhibit distinct fluxion structure in their arrangement.
None of the lavas of Bombay described in my paper in Records XVI, 42, come any where near this specimen in fineness of grain.
The rock does not contain any glassy base-the base being composed of extremely minute granular crystals, possessing low double refraction.
There are some porphyritic crystals of triclinic felspar, and augite, and the slice contains round holes stopped with opal, and a red and greenish substance which may possibly be analcime, or an allied mineral. The slice is also stained red here and there, in streaks, with oxide of iron. We thus see that even an extremely finegrained, compact rock like this, is unable to resist the penetrating power of water. No. 2.-Amygdaloid of Pukarni quarries near Harda. A favourite building stone. This is not a very interesting rock under the microscope owing to the amount of alteration it has undergone. The remains of the magnetite, which was apparently very abundant, may still be seen dotted about through the matrix; but the principal part of it has been converted into the red oxide of iron which has made the matrix very opaque, and has given it a red colour.
The felspar prisms imbedded in the ground-mass are more or less decomposed, and replaced by white products of decomposition; the amygdules are stopped with a zeolite, chalcedony, and opal, and the substance of the matrix is invaded by free quartz and zeolitic substances. The original porous character of the rock has evidently greatly facilitated the alteration of its component minerals.
The general appearance presented by the slice in the field of the microscope is that of a net-work of felspar prisms, imbedded in an opaque brick-coloured matrix, but owing to the progress of decomposition the outlines of the felspars are not sharp or regular. No augite is to be seen.
No. 3.-Ball trap. A very common type all over the Deccan.
This consists of a net-work of triclinic felspar prisms, granular augite, and magnetite, in which ground-mass large crystals of plagioclase are porphyritically imbedded.
The rock, under the microscope, has a very fresh appearance; the felspar and augite, in particular, looking remarkably so; nevertheless it appears to have suffered considerably from the agents of decomposition, for calcite is present in considerable abundance, whilst limonite, and hæmatite, have not unfrequently replaced the magnetite.
An orange-coloured substance is rather abundant, and forms a striking object in these slices, regarding which it is difficult to speak very positively. At first sight it looks like a mineral, but it plays the rôle of a glassy base; it is absolutely without external form or cleavage, and nearly all of it is inert between crossed nicols. Occasionally it polarises feebly in its own natural colour. Closely allied to this is a dark-green substance, which in polarised light is perfectly isotropic. Soaking in hot hydrochloric acid sufficiently long to completely remove the hæmatite, and magnetite, makes scarcely any impression on the orange substance, and little on the green, that is to say, it removes the green colour, and converts the orange into a dull red, but leaves a glass behind absolutely inert in polarised light.
On the whole, then, I have come to the conclusion that both the bright-orange and the dull-green substances, represent the original magma, or glassy base of the rock, and that they owe their mineral appearance to iron colouration.
The larger felspar crystals contain glass inclusions with fixed bubbles. The felspar appears to belong to the labradorite-anorthite group. No. 4. This specimen was labelled 'average basalt: quarries. Belgaum.'
It is a perfectly compact trap, of dark-grey colour.
One Tree Hill1
Under the microscope this is seen to be a fine-grained rock, composed of prisms of plagioclase felspar and granular augite, with some magnetite; in which ground-mass comparatively large crystals of triclinic felspar are porphyritically imbedded. Some of the grains of augite are of considerable size, but none of them show any approach to external crystallographic form.
The remains of the original glassy base, in part of green colour, and in part reddish-brown, is visible here and there. What I take to be the glassy base exhibits undulating marginal lines of colour that follow the borders of the bounding minerals. This appearance has been described by Zirkel in his Microscopical Petrography, p. 234, and depicted in Plate XI, fig. 1, of that work. Zirkel described this substance as the globulitic base of an altered basalt, metamorphosed into amygdaloidal nests. In the case of the Belgaum rock, however, the metamorphism can hardly have proceeded beyond the colouration of the glassy base, for boiling in hydrochloric acid makes little or no impression on it. It has no action on polarised light.
The felspar belongs to the labradorite-anorthite group.
No. 5.2-From a dyke more than 100 yards wide, in the Satpura basin of Gondwana rocks; believed to be Deccan trap, which appears to have once covered the Gondwánas of the Satpura region (Manual, Geology of India, Part I, pp. 213, 214).
This is a large-grained, perfectly holocrystalline dolerite, exhibiting no trace of a glassy base. Indeed in structure this rock approximates to a gabbro. (Judd, Q. J. G. S. xlii, 62.)
The whole of the felspar is triclinic, and the prisms of which it is composed are massed together solidly, rather than in lath-shaped forms. The augite ́is granular, but massive; and its mutual relation to the felspar is more granitic than in a true lava. Apatite, magnetite, or ilmenite, and some iron pyrites occur in the rock. The felspar is much altered in places, and the augite is changed along cracks into a fibrous substance.
For the whole of the specimens described in this paper I am indebted to Mr. Medlicott, Director of the Geological Survey of India. The Deccan Trap samples were received some years back; the Rájmahál ones recently.
1 Foote: Mem. Geol. Sur., India, XII, p. 182.
2 Survey number 43-2. The specimen was taken from the bottom of the deep gorge about a mile west of Korángla Hill (2,221 feet, Lon. 78° 42'. Lat. 22°, 34′), on the same dyke, may be 900 feet higher than the gorge. This great dyke, and several others in this field, are many miles in length, and suggest fissure-eruption, rather than volcanic vents. From Korángla a great loop-dyke emerges to the north, and joins again some 5 miles to the west, forming a bow-shaped ring round the Mirakota plateau, of Mahadeva sandstone.-H. B. M.
All the Rájmahál rocks described in the preceding pages, except No. 446, are true lavas. No. 446 (Betia Hill), on the contrary belongs to the plutonic class of igneous rocks; and its perfectly granitic structure shows that it was consolidated at some distance from the surface. Whether it formed part of a dyke; or whether the mass from which this sample was taken constituted the root of one of the Rájmahál craters, it is impossible to say from the examination of this hand specimen. I have not been able to trace any notice of Betia Hill in the Manual of the Geology of India, or in Mr. Ball's memoir on the Rájmahal Hills. I infer from the observations at page 170, Manual, Part I, that Betia Hill was not suspected to be the root of an old volcano; but as the specimen was labelled "basalt" by whoever collected it, it might be worth while for some future observer to take another look at Betia Hill. The trap is certainly not a lava. It is a diorite of granitic structure, and very probably indicates the site of one of the old Rájmáhál volcanoes.
A similar remark applies to the Deccan trap specimen from the Sátpura Hills. Though its structure is not so decidedly granitic that the rock can be classed as a plutonic one; nevertheless, it is perfectly holocrystalline and appoximates in structure to a gabbro. Clearly the consolidation of the rock, and the crystallization of its component minerals, took place under considerable pressure. The sample was taken from a dyke of great width, shooting up through the Gondwana rocks, which seem nearly surrounded by, and to have been formerly overlain (Manual pp. 213, 214) by Deccan trap; and we have here, I should think, the site of one of the missing volcanic foci. Doubtless when the microscopic examination of the Deccan traps in the laboratory goes hand in hand with their close and detailed examination in the field, the general absence of traces of the volcanic vents from which the Deccan and Rájmahál traps were poured forth will no longer be complained of.
No connection between the Deccan and Rájmahál traps has been traced (Manual, xli); and the suggestion of some geologists that the "Rájmahál traps of the upper-Gondwána period, and the Deccan traps are portions of one continuous series of outbursts," is not favoured in the Manual: on the contrary, the conclusion is arrived at, that, "in the absence of any direct evidence, it is premature to suggest that there is any connection between the two formations, or to class them as portions of one great igneous series."
That being the state of the case, it may not be uninteresting to enquire whether the microscope throws any light on the subject. Without intending to imply any intimate connection between the two series in point of time, I think the study of the above described samples, though few in number, suggests the probability of their having a deep-seated common origin. The Rájmahál lavas seem to belong to precisely the same type of rock as the Deccan traps. They do not differ, for instance, from each other as the lavas of Aden do from those of Bombay; on the contrary I do not think that any one examining unnamed specimens of the Rájmahál or Deccan traps could possibly say, from any evidence revealed by the microscope, which series he was dealing with.
In all the samples described in this paper, as well as those in my previous one on the "Basalts of Bombay," the entire absence of olivine is a noticeable feature. Olivine is certainly present in considerable abundance in some Deccan traps,
and in some Rájmahál traps (Manual, Geology of India, Part I., pp. 170, 302), though it is specially mentioned in connection with the coarsely crystalline varieties; nevertheless, the entire absence of this mineral in every one of the samples from widely different localities, that I have examined,1 is a circumstance that seems to possess considerable significance. In the face of it I may, I think, fairly infer that olivine is generally absent from the Deccan and the Rájmahál traps; and that when present, it is an exceptional rather than a regular, and characteristic constituent.
In view of this fact, the question arises what are these lavas to be called? They have always, heretofore, from their macroscopic aspect been termed dolerites and basalts. In my paper on the Bombay traps, I said that "a good case might be made out for classing the Bombay rocks with augite-andesites rather than with basalts" (Records, xvi.-49), though I preferred, for reasons stated therein, to retain the name that had hitherto been given to them.
Professor Judd, speaking of the rocks of Fiji (Q. J. G. S., xlii, 427), writes "although both of these rocks have the general aspect of basalts, yet, as olivine is absent from them, I follow the great majority of continental petrographers in classing them with the pyroxene-andesites. I believe this course is practically more convenient than that of extending the groups of basalt and dolerite by including in them the larger part of the pyroxene-andesites." No doubt the presence or absence of olivine is a most important fact; but alas! for the working microscopist, it is the first mineral to decay; and although the original presence of olivine may often be detected when it is going, or just gone; it cannot, I think, be detected after decay has advanced beyond a certain stage without drawing on the scientific imagination more liberally than it is safe to do.
However, as it would be useless to set up a nomenclature of my own, it would seem to follow that the rocks described in this paper, excepting only the enstatitediorite of Betia Hill, must be called augite-andesites. The Sendurgusi Hill rock, with the aspect of a trachyte, will have to be called an andesite as it contains no sanidine.
1 I omit from this generalization two or three slides lent me by Mr. Medlicott some years ago. I did not see the hand specimens from which they were taken, or study the slices in detail.