Page images

resembling brecciated portions of a flow similar to that of the matrix, but of a lighter tint. Further south, near Suini, purer rhyolitic rocks set in; the whole series tending to become thicker in this direction. There are now displayed several varieties of a compact type, and others slightly vesicular. In the small stream mile N.E. of Marwara village, near Lobah, I found the rock mentioned above as intermediate between the ancient rhyolites and the gneissose granite of Dudatoli. The whole of these volcanic rocks, together with some subordinate beds of dark grit, ashy grit, brown and yellow non-schistose quartzite, and some steatitic slates, cover the massive limestone as a skin, down the face of the hills on the east side of the Ramganga river. Owing to this position, modified by faulting, it is sometimes difficult to trace their exact stratigraphical relations; but they seem to merge upwards, with sometimes an intervening thin band of cream-coloured limestone into slightly schistose slates and schistose ashes. The latter, in many places high up in the series, become very basic in character; whilst the whole of the series is entirely different from the schistose series west of the great fault. I may here mention that N.W. of the Dudatoli massif another set of acidic lavas are found along the ridge 3 miles west of Dobri trigonometrical station. There seems no doubt that they are of the same age as the Lobah flows.

At this point it will be convenient to close the description of these rocks, so far as their field relations go, and to describe them more minutely by means of a set of specimens. These and their accompanying microscope slides will be referred to by their registered number. Thus labelled, they are now in the Geological Survey Museum, Calcutta.

Specimen No. 5.-This rock was found near Sainji, about 4 mile on the pathway to Suini M., Sp. gr. 270. Contains 79 per cent. of silica. Before the blowpipe it fuses with difficulty on the edges of thin splinters. In the hand it is a compact rock of a pale grey-green colour, sometimes flesh-white, and altered much by weathering. It breaks with a flinty fracture. It appears dotted over in many place by white spots about the size of pin-heads. These, which I first thought were spherulites, are small gas-pores, filled with an alteration product in the form of amygdules.

Microscopical. Under the 1 inch objective the field of the microscope is filled by an almost structureless ground-mass, containing no porphyritic crystals, or included fragments of any kind. There is only a faint indication, here and there, of a gathering of the material of the ground-mass into hazy clusters. The gas-pores appear merely as round holes in the slice. Under the and inch objectives the ground-mass is seen to possess a clear base, which, with ordinary transmitted light, cannot be split up into grains, but appears uniformly amorphous. Through it are scattered in the wildest profusion and without any order a countless number of microlites, very small, and of irregular shape. They are sometimes more or less rounded, sometimes elongated needles; but more often of an irregular form longer one way than another. There is no spicular arrangement, as in the Arran pitchstones, no dendriform structure, and no trace of spherulites. The only thing noticeable is that in some places the microlitic groupings are denser than in others. The microlites are pale yellowish green in colour. Minute specks of opacite are only sparingly represented, except in connection with the gas-pores, which are more or less


lined by them. The microlites cannot be regarded as having any claim to a crystalline structure; for when the nicols are crossed, they have no influence on polarized light. Their outline can then only dimly be seen through the base, which becomes divided up into irregular polygonal patches of light, and dark blue-grey colour, whose boundaries are very uncertain. These outlines are quite independent of the position of the microlites, a fact indicating that it is the base alone in which the microlites lie, which has been altered molecularly, and which behaves under crossed nicols something like a crystalline aggregate. It is necessary to note, then, that the visible structure under ordinary light is replaced under crossed nicols by a previously invisible, structure; and the microlites, though not absolutely vanishing from view, can only be seen with difficulty. They have only a negative influence on the light. They seem to run in among the petrosiliceous material which now, in contradistinction to the microlites, shews its inherent differences of molecular constitution by its positive effects on the light.

The rock would seem from this to be a petrosiliceous rock, with a confused devitrified base, crowded with minute microlites. The mottled or mosaic-like appearance of the base under crossed nicols seems to indicate that quartzose and felspathic minerals are separating out, though they have not done so so entirely as to be visible under simple transmitted light. The question arises, what was the destiny of the microlites; would they have been absorbed, or would they, if progress had not been arrested by solidification (as seems indicated by their yellowish colour), have become some other mineral, such as mica or hornblende?


Specimen No. 6-This specimen is taken from the crags above Rheetheea Tea Factory, near Lobah. Sp. gr. 2:65. I at first overlooked this rock in the field, taking it for a quartzite, as it weathers on the surface exactly like one. A freshly broken specimen shows it to be entirely different, with a dark green ground-mass, vaguely showing flow-structure and full of angular fragments of compact, lighter tinted rocks, flesh-coloured, and sometimes pale greenish grey. It is a very beautiful rock, and undoubtedly forms a portion of the same flow as No. 7. It appears to pass into the Lobah conglomerate by the fragments becoming more and more rounded, and the petrosiliceous matrix becoming less pronounced, or replaced by fine clastic material. (See above.)

Microscopical. Under the 1 inch objective the rock is seen to be as much clastic as volcanic. The ground mass is of an olive green colour, showing very distinct flow structure. The fragments of other petrosiliceous rocks included are angular and of all shapes. Their intimate structure is very like that of the previously described specimen; and they probably represent caught up dust and fragments of older flows which have become brecciated. No doubt both the included fragments and the enclosing rock were practically of the same age. Quartz is also present in small grains, all of which are angular and the remains of more or less perfect crystals. There is a certain amount of black and dark brown opacite. Small cracks in the rock are lined by secondary quartz, chalcedony or " quartz grenu." Under the inch objective the ground-mass can be discerned as having a clear base like the last described rock; but it is more thickly crowded with pale green microlites, which are clustered together in irregular patches, as well as being generally disseminated through the base. These green microlites have apparently but little effect

optically, except that their density prevents so much light passing through under crossed nicols as there would otherwise be. The mosaic appearance of the base is consequently to a large extent shrouded. I could not see any, or very little polarization colour due to the microlites, as the stage was revolved, such as can be made out in the rock next to be mentioned. Doubtless the microlites in this case were just too small. The flow-structure, which is very prominent, is manifested by differences in the amount of colour and in the fineness of the bands of microlites.

Altogether, the rock exactly resembles a brecciated, devitrified rhyolite, in which a considerable quantity of fragments of other flows have become mixed or caught up.


Specimen No. 737.-Loose block in the stream east of Saliana near Lobah and found in situ 1⁄2 mile N.E. of Marwara near the road to the Dewalikhal pass. Sp. gr. 273. This rock links the gneissose granite of Dudatoli with the ancient rhyolites and brecciated rhyolites just described in detail. It seems to have a matrix very similar to that of the rhyolites themselves, and dispersed through it are large eyes and more or less rounded crystals of felspar, which give it a porphyritic augen structure. It has also plenty of free quartz in large irregular grains. I take the following extract from my field note-book :-"Matrix, a dull pale greenish grey, perfectly compact so far as the eye can see, save for a flow structure and faint banded appearance, dividing it into sinuous lines of paler and darker colour. The contents distributed in the matrix are a smaller set of granules of somewhat dark quartz, and small rounded eyes of felspar. Its porphyritic nature is given to it however by the presence of large eyes of felspar, rounded and oval, between which, as well as between the smaller particles, the matrix appears to have flowed. In some cases the eyes of felspar, especially the larger ones, have become cracked and partially displaced, and the matrix seems to have flowed in between the adjoining portions and so encompassed them."

Microscopical.-This rock, so far as its ground-mass is concerned, is very slightly coarser than No. 7, the green mineral being rather more strongly represented by somewhat larger microlites. Like it, however, it is of an olive green colour, and by small differences in tint and in the density of the microlites, flowstructure is manifest among the petrosiliceous material which has found its way between the porphyritic crystals and included fragments scattered through it. Under the higher powers of the microscope the evidence of its having flowed is added to by a vaguely linear arrangement of the microlites, indicating that they had become turned more nearly parallel to the direction of flow, as they were swept on in the molten current. Of the porphyritic elements in the rock the large orthoclase crystals are the most conspicuous. They are of an opaque porcellaneous appearance under a low power, and have a very ancient look, disfigured by a corroded outline, and by the ramifying of innumerable veins of secondary quartz through them, and also by portions of the ground-mass, full of microlites, being similarly thrust in between widely open cleavage cracks at right angles to one another. The secondary quartz was introduced last of all, for it cuts through the ground-mass in the matrix and through the tongues of the same which penetrate the orthoclase. The free quartz is usually arranged in hexagonal groupings of three or more crystals, with some of their outlines fairly intact and with others corroded away and jagged, indicating that, though the quartz

had crystallized out originally, it had suffered considerably during its transit in the molten flow. In one place a portion of the compound crystal was very nearly separated from the rest by inclusions of the ground-mass along parallel cracks; which gave the nearly separated portion the aspect of being connected by threads with the remainder, and ready to part at any instant. Secondary quartz along lines of infiltration invades the crystalline quartz also. Innumerable minute cavities throng the quartz, but they are unresolved under the inch objective. Besides porphyritic orthoclase and quartz, there are also portions of other petrosiliceous flows in rounded fragments, a few sharply marked off from the groundmass, but mostly appearing as half fused up and amalgamated with the ground-mass. Distinct irregular clumps of opacite are present in fairly large quantities.

Under crossed nicols, as in the previous cases, no undifferentiated glassy base can be satisfactorily made out. The grey-blue mosaic indicates at once devitrification structure, which is not quite so much obscured as in the previous rock slice. The general effect is not quite so dark. The embryonic quartz and felspar appear to have gone a little further towards separating out into distinct granules. Without the crossed nicols, however, this separation is quite invisible. Under the and inch objectives the microlites of the green mineral are seen to be rather larger than in the last rock, and to have a decided effect on polarized light, so that when the nicols are crossed, the somewhat parallel microlites light up the field of the microscope with multiform coloured brush-like aggregates.

From all points of view this rock must be considered akin to No. 7, save that it contains crystals of the "first consolidation," viz., porphyritic quartz and felspar. It may therefore be called a devitrified porphyritic rhyolite. Had the rock solidified under a pressure of super-incumbent rock and cooled slowly, it seems probable that it would have resulted as a microgranulitic rock or elvan, with the porphyritic addition of quartz and orthoclase. It would be difficult, therefore, to deny it an intermediate position between the gneissose granite of Dudatoli and the purer rhyolitic lavas.

Specimen No. 8.-This is from the same locality as No. 7. Sp. gr.=3'03. It is undoubtedly a vesicular variety of the purer rhyolite. In the hand it is pale whitey green in colour, splitting with some difficulty along the direction of original flow. Under the microscope its only peculiarity, as distinguished from No. 7, is the large number of gas-pores, which are filled with reddish iron oxide. These account for its high specific gravity; for another fragment taken from the same piece of rock, but less full of amygdules, has a specific gravity of only 2·66.

Specimen No. 79.—Sp. gr.=263. This rock is not from the locality of Lobah, but from the stream-head running west to Peera from the Dobri ridge. It is mentioned here because it links the thoroughly brecciated and clastic forms of the rhyolite with the amorphous compact varieties. It is pale grey or whitey green in colour, showing a distinctly banded structure, which has in places become interrupted by one of the bands becoming distorted and brecciated, and by the interbanding of minute rounded quartz grains, suggestive of a fragmental origin. This structure clearly evinces that partial cooling had been followed by further flowing of the semi-solidified mass.

These few examples are sufficiently typical of the forms, as developed in the neighbourhood of Lobah.

acid lavas and their allied Others from localities N.W.

167 of the Dudatoli massif will be described in due course, and also some allied compact red porphyrites, or ancient rocks, of a less acidic character, found near Charmarguri trigonometrical station. There also remain to be considered the basic lavas which seem to replace or overlie the acidic lavas at some parts of the margin of the Dudatoli area. Though far more abundant than the ancient rhyolites, I have deferred their examination until later, because their connection with the gneissose granite seems to be inadmissible. It is indeed difficult to account for the plutonic representatives of these lavas; nevertheless, until the district is completely mapped, it is unsafe to say that they have no such representatives.

I will conclude these petrological notes by a reference to the differences in metamorphism between the old schistose series west of the great fault near Lobah, and the very slightly schistose rocks which lie to the east of it. The garnets, and well developed plates of mica, which characterize the old schistose series end abruptly at the fault. Now, as was shewn in my last paper, the production of garnets in the schist and the appearance of the Dudatoli gneissose granite must have been contemporaneous, and probably inter-dependent. It therefore follows that the argument for the great age of the gneissose granite previously founded on other and more general grounds, is now buttressed by this additional fact concerning the distribution of the contemporary metamorphism.

The Iron Industry of the Western Portion of the District of Raipur, by PRAMATHA NATH BOSE, B.Sc. (Lond.), F.G.S, Deputy Superintendent, Geological Survey of India.

[ocr errors]

Literature. The notice taken in the article on Raipur" in the Central Provinces Gazetteer (published 1870) of the iron-ores of the district is very poor. Only two localities are mentioned, and not a word is said about the mode of occurrence, the extent, or the working of the ore. It is no wonder, therefore, that Mr. Ball, the accomplished writer of the "Economic Geology of India," should have summarily disposed of the iron-ores of the Raipur district by saying, “Little or nothing is recorded as to the iron-ores of this district."

In Appendix III, G. (Mines and Quarries) of the last number of the Central Provinces Administration Report (1885-86), half-a-dozen iron-ore localities are mentioned as occurring in the district of Raipur, viz., "Kondkasar, Bhindo, Lahora, Dalli, Sambarsingha, and Magarkund." The history of several of these names is not without interest. In the Administration Report for 1868-69 the names given are "Condkusar, Bhindo and Lohara, Dallee, and Muggurkund." Two years later we find "Lohara" separated from "Bhindo" and joined on to Dallee as "LoharaDullee;" it was subsequently again disjoined from Dallee, and transformed into "Lahora." But I know of no place of that name in the Raipur district where ironores occur.1 "Lohara-Dalli" would be more intelligible, for the hill of Dalli,

1 There is a Lohara hill in the Chanda district where iron-ores of good quality are known to


« PreviousContinue »