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field season I commenced a systematic survey of the hills, starting from the already described sections on the Sumesary, and working westwards along the southern base of the main range of gneiss, which runs in a west-north-west direction through the centre of the district.
The formations represented in the Garo Hills are the following:-Gneiss, cretaceous sandstones with coal, nummulitic limestone with associated sandstones, and upper tertiary sandstones. A still more recent formation, consisting of terraces of gravel and boulders at a considerable elevation above the existing river courses, is extensively exposed in the south-western portion of the district, but I have not had many opportunities as yet of examining it.
In the adjoining Khasia Hills other formations intervene between the gneiss and the cretaceous rocks, viz., the Shillong series, the Khasia trap and granite and the Sylhet bedded traps. Numerous dykes of a fine grained compact trap do occur intrusive in the gneiss of the Garo Hills, which may possibly represent some of the igneous rocks of the Khasia Hills, but in that case one would expect to find similar dykes intrusive in the Khasia Hill gneiss, and as far as I know they do not occur in it. The dykes in the Garo Hills are pretty generally distributed throughout the district, but are most numerous in the north-western portion of the hills, where it is rare to find a section of the gneiss without coming upon one or more of these dykes. They are generally narrow and coincident, or nearly so, with the strike of the gneiss, but sometimes appear to have been intruded horizontally along joint planes, and often show well defined columnar jointing perpendicular to the surfaces of contact. In many places where sections are not available these dykes betray their presence by the numerous rounded exfoliating blocks of greenstone, derived from the breaking up and weathering of the columns, which are scattered over the surface of the ground.
The cretaceous rocks, which rest immediately upon the gneiss, have hitherto received the greatest amount of attention in this district, because in some places they contain good seams of coal. The only workable seams, however, seem to be confined to the valley of the Sumesary and the country to the east of it, in which direction they extend to the Langrin coal-field in the south-west Khasia Hills. At the village of Aruak, only a few miles to the west of Siju on the Sumesary (where coal of good quality occurs), a considerable thickness of cretaceous rocks is exposed, resting nearly vertically against the gneiss, without a trace of coal. Still further to the west, on the head waters of the Nitai and Bogai rivers, the cretaceous rocks rest horizontally on a platform of gneiss, from 4 to 5 miles broad, which extends for about 14 miles along the base of the main range. Numerous sections are exposed in the river gorges, the total thickness of the sandstones being from 500 to 600 feet, but in no case did I find any trace of coal. At the southern edge of this platform of gneiss the cretaceous rocks bend over and quickly disappear beneath the nummulitic limestone and newer rocks. Further still to the west, below Tura, and on the Singmari plateau, the few exposures of coaly matter that do occur were long ago shown to be worthless. The question whether this coal improves towards the deep, which can only be ascertained by boring, still remains undecided; though from the fact mentioned above of its disappearance west of the Sumesary, it is not likely that such would be the case.
A white shaly indurated clay, or lithomarge, occurs in most places where the cretaceous rocks are exposed, in bands of 2 or 3 feet in thickness; when better communications through the hills are opened out, it may prove to be of some value as a pottery clay. There is a thick deposit of it in the station of Tura ; where it has been used for white-wash. At present the Garos obtain the material they require for their rude pottery from beds of a stiff yellow clay which occurs locally immediately above the nummulitic limestone.
Conformably overlying the cretaceous sandstones is a series consisting of nummulitic limestone and fossiliferous sandstones with sandy shales. The main band of limestone always occurs at the base of the series, thin bands of limestone also occurring in places in the sandstones above. The thinning out of the limestone between the Sumesary and Damalgiri Thana, where in 1868 Mr. Medlicott noticed the western extremity of the bed in a thin band of rusty concretionary limestone, is very marked, as well as its rapid deterioration in quality. About 150 feet of it are exposed in the Bogai at Ganchi, but the whole is sandy earthy and nodular. The sandstones which overlie the limestone are fine grained, ferruginous, and generally show a tendency to become nodular. In these rocks fossils had previously been found only in one locality, viz., on Nongkulang hill in the south-west corner of the Khasia Hills, where Colonel Godwin Austen collected a large number of specimens from sandstones immediately overlying the nummulitic limestone. These were examined by Dr. Stoliczka, who remarked that none of the species appeared to be identical with those known from the nummulitic rocks of the district. Unfortunately there are very few of Colonel Godwin Austen's specimens in the museum, nor do any of them appear to be those examined by Stoliczka, while no description of the fossils from the nummulitic limestone of these hills has been published; so that I have been obliged to compare my specimens with those described in D'Archiac and Haime's nummulitic fauna of Sind, and with European species. Among them I have been able to identify the following with more or less certainty :
From the cart road between Tura and Mankarchar, about 7 miles from Tura
From scarp west of Warimagiri village (marked Warogiri on map)
? Achatina fragilis.
From the Kilburn stream near Khenigiri village
'Jour. As. Soc. Bengal, Vol. XXXVIII, Pt. 2.
From about 1 mile east of Khenigiri village
From scarp east of Bogai river near Ganchi
Rostrum of Balanus.
From ridge between Damakchi and Rongpha streams near Duching village
Rostrum of Balanus.
The above species are nearly all found in the eocene deposits of Europe. All the fossils occur as casts in fine sandstone, the original shell being in no case preserved, so that it is often difficult to make out the specific characters. The specimens found at Nongkulang hill by Colonel Godwin Austen belong to the genera Conus, Dolium, Dentalium, Cardita, Cardium, Tellina, Nucula, Leda, Cuculloa, &c., but the names of none of the species are given. There is little doubt however that these fossiliferous sandstones should be classed as nummulitic owing to the thin bands of limestone containing minute nummulites, which are intercalated with them. One of these beds (in the Mandar stream, about 3 miles west of Emangiri) which is well up from the bottom limestone band, contains numerous specimens of Operculina canalifera, a species very common in the nummulitic limestone of Cherra.
The fossiliferous sandstones pass up into a mass of very similar sandstones, which attain a thickness of about 1,200 feet in Joksongram hill near Shushung Durgapur. In these no fossils have as yet been found, partly, I believe, owing to the coarseness of the sandstones, some of them being almost conglomerates and not suited to the preservation of casts. I am hopeful, however, that further search will reveal the presence of fossils in these rocks, without which their separation from the underlying nummulitics will be a matter of great difficulty.
Note on some Indian image-stones, by COLONEL C. A. MCMAHON, F.G.S. Gaya (Gya): Nos. 7-200. 7-201. 7-202. 7-203. 7-205. The specimens were sent to me by Mr. Medlicott, Director of the Geological Survey, and I have given the Survey register numbers for facility of reference. Gaya is a very sacred place both for Hindus and Buddhists, and this stone is extensively worked into images and utensils of various kinds for traffic with the pil'Mem. G. S. I., Vol. VII, Pt. 3, p. 9 note.
grims. A reference to its mode of occurrence in the field will be found in Vol. II of the Records of the Geological Survey, page 42.
M.-These samples are all so similar one to another under the microscope, that a general description of them is all that is requisite. The matrix of the rock appears to be quartz, though free quartz is not prominent in all the slices. It is abundant in No. 201, but is almost absent in No. 205. The quartz is very pure and limpid, and contains no liquid cavities.
The most prominent feature in the rock, perhaps, is the abundance of chlorite and sericite which it contains; the two minerals being intimately mixed up with each other. Under the microscope sericite in several respects much resembles talc; but some of the colourless, foliated, mineral seen in the field of the microscope is certainly a mica, and from the amount of water small pieces of the rock yield in a glass tube under the blow pipe, I think it is safe to assume that the whole of it is sericite. Indeed, the flame of a spirit lamp, without a blow-pipe, supplies sufficient heat to make the rock give up water abundantly. No. 205 is highly micaceous in its macroscopic aspect.
Felspar, all of which belongs to the triclinic system, is also a prominent feature in the rock taken as a whole. It is entirely wanting in my slice of No. 205 (in which free quartz is also extremely sparse), and it is very subordinate to the quartz in No. 201; but it is present rather abundantly in the other slices. In No. 203 it occurs in sufficiently large, and sufficiently numerous crystals, to impart a speckled appearance to the hand-specimen. It is beautifully twinned in single crystals, or in groups of crystals; but in no case does it exhibit any external crystallographic outline, being, for the most part, in more or less rounded nodules. Magnetite or ilmenite, apparently the latter, is present in all the slices, except, No. 205, where it has been converted into a substance resembling leucoxene. The iron contains numerous inclusions, or intergrowths, of chlorite.
Some of the slices contain a few micro-garnets; No. 201 contains a very little schorl, and No. 203 a minute sphene.
The rock appears to be of metamorphic origin. Nos. 201 and 203 have, macroscopically considered, a specially trappoidal appearance; but the rock presents nothing under the microscope to suggest an igneous origin. The quartz is not like the quartz of granite, or its allies; and the rounded character of the felspar crystals appears to me to be due to segregation having taken place in an imperfectly plastic mass, and not to have had its origin in the solvent action of an igneous matrix. The felspar contains no inclusions of the matrix.
Dhalbhum E. 30.-A light grey coloured rock, slightly soapy to the feel, and of more "cheesy" type, than the previously described specimens. Under the microscope it is seen to be composed of chlorite, a micaceous-talcose mineral that may be either a hydro-mica (sericite) or talc, magnetite, sphene, and calcite; the latter is not inconsiderable in amount. This is a more orthodox potstone. In his Geology of Manbhum and Singhbhum1 (of which latter district Dhalbhum is a sub-division) Mr. Ball mentions eight localities where this rock has been extensively worked. It occurs in thick beds in the 'sub-metamorphic series.'
Gaur: Nos. 7-244 and 7-246.-Principally used for carved work in the ruined 'Memoirs Geol. Sur. Ind., Vol. XVIII, p. 148.
buildings of Gaur, in the Maldah District, the ancient capital of Bengal; but the actual specimens are from the famous Adina mosque, at Panduah, 20 miles northeast of Gaur. It is not known where the stone used for these buildings was quarried. The ruins are in the delta of the Ganges, and the nearest (25 miles) rocks are the Rajmahal traps, beyond the Ganges. In appearance Nos. 244, 246 look like argillites, but they do not greatly differ in macroscopic aspect from some of the Gya potstones. They might pass for very fine grained varieties of No. 7--202.
Nos. 7-244 and 7-246 consist of very minute grains of quartz averaging about one thousandth part of an inch in diameter (the largest is under three thousandths of an inch in diameter), and minute fibres of mica (probably a hydro-mica) set in a structureless base that represents, I apprehend, the original setting of fine mud. In this ground mass, there are starred about skeleton prisms-ghost-like forms, without bones or substance-of what may possibly be the spirits of hornblende in the hades of metamorphism. They have no action on polarised light; their ends are unfinished; and in neither of my slices are cross-sections of the prisms to be obtained. Indeed, they have so little substance about them that they have not displaced, or assimilated, the quartz grains of the matrix which show sharp and clear within them, and have been wholly undisturbed by the formation of the prisms. Evidently their genesis has been due to segregative action. These crystals are of dark greenish colour in transmitted light; but they are without internal structure and exhibit no dichroism. It is impossible to give them any definite mineralogical name.
On applying high powers these slices are seen to contain very numerous microsphenes from one to one-and-a-half thousandths of an inch in length. The quartz grains are sub-angular-none are distinctly rounded. The fibres of mica all point in the same general direction, but the rock does not present any distinct lamination.
This argillite may possibly come from some sedimentary beds intercalated with the Rajmahal traps (I am personally ignorant of the locality), but there is nothing in the structure of the argillite to indicate any connection with it and the trap. In composition, too, it is seen under the microscope to be different from the Gaya potstones. The latter are essentially micaceous-chloritic-schistose rocks, or talcosemicaceous-chloritic rocks. The Gaur stones are fine grained sandy argillites. Both are metamorphic sedimentary rocks. The Gaur rock is an interesting one to the student of metamorphism.1
In the Gaya potstones I have felt considerable difficulty in discriminating between hydro-micas and talc. Some of these rocks undoubtedly contain mica; a silvery mica, for instance, is very prominent in No. 7-205, even when viewed. macroscopically; but it is, I think, in the present state of our knowledge, impossible to say from optical evidence whether the micaceous-talcose mineral present in these rocks is a hydro-mica (presumably sericite) or talc. M. M. Fouqué and Michel Lévy in their Minéralogie Micrographique give some useful hints for discriminating between sericite and talc, but I have not found them sufficient to enable me to satisfy my mind in this case, though I have bestowed considerable labour on the subject.
1 Colonel McMahon's decision that this Gaur stone is not trappean, increases the difficulty as to whence it can have been derived. The known intertrappean beds of the Rajmahal hills are utterly different from this stone, and always exhibit distinctly their sedimentary origin; so, if not igneous, the Gaur stone can hardly have come from that ground, and all other rocks in situ are very distant.-H. B. M.