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On the boring assays alone, these coal-fields must be generally condemned as being unlikely to yield good workable and useful coal within the areas tried ; and when I write "areas tried" I am practically condemning a very large extent of country indeed, in which, however, there is plenty of room for chance local developments that might pay private enterprise. Now, this, though a deliberate and sweeping condemnation, based on what has always hitherto been considered conclusive evidence, namely, the poorness of boring assays, does, on the face of it, seem too strong for so very great an area of coal-measures, carrying generally two very thick seams of shales and coal besides many other thin ones. Indeed, particularly so, when even the shadow of a doubt has been cast upon the reliability of the boring samples, and while such a promising local improvement shows itself at Ghordewa.
The chances that the boring samples have in some, if not many, cases been spoiled by admixture with foreign and deleterious débris of shales, and perhaps a little sand, are considerably reduced by our close and severe examinations of the Pasana outcrop and borings in the Mand valley; but they are not altogether eliminated, and, when the interests involved in the existence of coal in this part of the country are so great, it would, I think, be folly to leave the exploration in so unsatisfactory a state.
There is little use, however, in having more borings put down unless some more perfect apparatus and more skilled and reliable boring experts can be employed than have hitherto been available. The more immediately feasible plan is to have a small pit sunk on or near the site of some of the old borings in order that the seams they touched or ierced should be tried in bulk.
The difficulty is to settle on a site, for I cannot point to one place as being preferable to another for its outcrops and bore samples. Such being the case, I fall back on a position as conveniently situated for railway purposes as possible. The Rampur coal-field has this advantage, and it is next nearest for possible supply to the Bengal fields, on the eastern side of the Peninsula. The boring assays from here give as much promise, bad as it is, as from anywhere else; the seams are of fair thickness, the depths at which they were encountered not being excessive; while the shortest distance from the proposed line of railway is only about 4 miles. By the journals or sections of No. 1 bore hole near Chowdibahal, a 7-ft. seam occurs at 77 feet from the surface, and there is another seam, 8 feet in thickness at 142 feet. This bore hole is at a good distance from the outcrop, so that there is no fear of any deterioration of the coal, such as, it may be, from proximity to weathering influences; and, as the depth is not excessive, a small pit might be run down on it. As the seams are reached, they should be well cut into, when the con ditions of their coal ought to settle the fate of the Rampur coal-field beyond dispute. The next best situated place, though it be about 20 miles in a direct N.-S. line from the railway trace, is Korba: and it has these points about it, namely, that there is a great seam manifest to the eye of doubting and practical men of what looks like what ought to be coal, though it may have any amount of shale associated with it; and above all that so excellent an authority as Mr. W. T. Blanford did in the first instance report very favourably on it, and pointed out bands the better portions of which, from the evidence before him, he considered "equal in quality to
1 Records, G. S. I., XIX, p. 226, and plate, fig. 1.
any coal found in the Ranigunj field." Of course, this opinion of Mr. Blanford was somewhat qualified by the assays received by him at a later date. A small trial pit could be easily run down here on the site of our boring to a total depth of 94 feet, which would pass through the entire seam and show in fair bulk the constitution of the many coal bands.
Local improvement.-That places of better promise may eventually be found is shown by the case near Ghordewa on the Aharan river, which is only about 5 miles west-north-west of Korba; but, as stated in the previous paper, to go on searching for such would be very much like looking for a needle in a bundle of hay. The Ghordewa case remains, however, and it certainly is promising enough to demand further exploration as to the extent and quality of its coal. The place is about 40 miles north-west of Bilaspur, and its shortest distance due south to the Bilaspur-Champa section of the Nagpore-Bengal Railway trace is about 25 miles : it is thus further from railway development than any other spot I have thought worthy of examination,' the Baisandar valley excepted. The great feature about it is that the outcrop gives the best 4 feet of coal known to exist in Chhattisgarh, and that the bore assays prove that it does not deteriorate to the deep. Hitherto, we have found such assays giving a decided and damnatory deterioration at only a few yards from the outcrop, but the assays in this case were made on material procured at 143 yards distance. The fear in my mind arising from what I have seen in many areas of these coal measures, in the case of a clean seam of coal-that is without much shale,— between thick beds of fine and uniform sandstones, is that the seam may not be very extensive but this is no fair ground for deterring exploration. I think, on the whole, there is really more chance of finding by boring that the coal of the Ghordewa area shall continue as good and as thick over an area, say, of a couple of square miles, than that a pit shall show in any of the other localities that the coal is so much superior to the stuff brought up from the borings as to be extensively useful for railway consumption.
Note.-The Chhattisgarh coal-fields have, previous to this paper, been referred to in the following published accounts:
W. T. BLANFORD.-Report on the Coal at Korba in the Bilaspur District. Records, G. S. I.,
III, p. 54.
Note on the Occurrence of Coal, east of Chhattisgarh in the country
V. BALL.-The Raigur and Hingir (Gangpur) Coal-field. Records, G. S. I., IV, p. 101.
On the Coal-bearing rocks of the
valleys of the upper Rer and the Mand Rivers Records, G. S. I., XV, p. 108.
in Western Chutia Nagpur.
W. KING. On the selection of Sites for Borings in the Raigarh-Hingir Coal-field. Re
cords, G. S. I., XVII, p. 123.
Sketch of the Progress of Geological Work in the Chhattisgarh Division of the
Boring Exploration in the Chhattisgarh Coal-fields. Records, G. S. I., XIX,
'It may be mentioned that there is a very promising field in this same great area of coalbearing rocks in the Uprora-Lakhanpur country, but this is too out of the way and difficult of approach for consideration in the present railway system. It is on the eastern extension of the Southern Rewa coal-fields.
Some remarks on Pressure Metamorphism with reference to the Foliation of the Himalayan Gneissose-Granite; by COLONEL C. A. MCMAHON, F.G.S.
A few years ago the theory found favour with some geologists that granite was the product of the extreme metamorphism of slate, sandstone, and other rocks of sedimentary origin, and that this metamorphism was the result of the heat developed by the compression of strata in the course of mountain formation. As there were serious chemical and other difficulties, however, in the way of the acceptance of this hypothesis, it gradually lost its hold on the geological mind, and another one, the converse of that above alluded to, now holds the field and seems likely to acquire especial prominence in the future in connection with the geology of the Scotch highlands.
The application of the first theory to the Dalhousie rocks I considered in my paper on the geology of that region (Records, XV, 39, 45, 46), and I came to the conclusion that the granitic structure of the gneissose-granite was not due to heat produced by pressure. The principal argument on which I relied was, shortly stated, as follows; if the heat which produced the granite structure were the product of local pressure, one would expect to find the greatest thickness of granite, and the most perfect granitic structure, developed at the point of greatest pressure; whereas, in the Dalhousie region, the contrary is the case, and at the point of greatest strain the gneissose-granite exhibits its minimum thickness and maximum amount of foliation.
In my previous papers, however, I did not consider whether the ascertained facts could be explained by the pressure metamorphism theory, and I now propose to offer a few remarks on this subject.
The pressure metamorphism hypothesis, shortly stated, may be said to consist of two parts. It is known as a fact that solid bodies such as lead, limestone, ice, &c., can, under the influence of great pressure, be made to flow; and from this fact the advocates of the pressure metamorphism hypothesis infer that many square miles of solid crystalline rocks have flowed with the plasticity of treacle under the influence of enormous pressure. This theory further supposes that when rocks are set in motion in this way the shear, and friction, develope sufficient heat to fuse, or dissolve, the minerals of which the rock is composed and that recombinations of the chemical constituents, and recrystallization, take place on cooling. This hypothesis, it will be observed, is a very comprehensive one and likely to be very useful to the puzzled geologist in the field. The conversion of a granite into a mica-schist can be accounted for by the application of the first part of the theory; whilst the conversion of a mica-schist into good granite can be explained by the second branch of the hypothesis.
I have no present wish to enter the lists against this theory, as a theory; indeed I have myself called in the aid of pressure to account for the foliation observed in the Sutlej diorites and lavas and the production of hornblende schists in that locality (Records, XIX, 80-83); all that I propose to do is to consider whether the facts ascertained regarding the gneissose-granite of the N. W. Himalayas, and recorded
in my previous papers, harmonize with the theory that the foliation of that rock was produced by pressure applied after its consolidation as a granite.
As I have discussed this question in an article on the gneissose-granite of the Himalayas recently published in the Geological Magazine (p. 212, May 1887), I think it will suffice to give an extract from this publication. It may be convenient to the future student of the Himalayan gneissose-granite to have his attention directed to this branch of the enquiry in connection with these papers
"The cause, or causes, which result in the foliation of igneous rocks is a subject which at present occupies the attention of many geologists, and seems likely, in the near future, to lead to some discussion. In view of this, a short account of the foliated granite of the Himalayas may be of interest. It may be as well, however, to preface my remarks by saying that I believe that foliation may be produced in several distinct ways, and the explanation which I offer of the mode in which the foliation of the Himalayan granite has been brought about is only intended to apply to the case of that granite.
"Cause of the Foliation of the Gneissose Granite.
"A realization of the eruptive character of the rock described in the above pages removes many difficulties from the way of the Himalayan geologist. Despite the wonders performed by flexure of strata in mountain regions," wrote Mr. Medlicott, the Director of the Geological Survey of India, in his Annual Report for 1883, "the structural features presented by this rock in certain cases were impossible of satisfactory explanation on the supposition of its being a really stratified gneiss." But if the eruptive origin of the gneissose-granite be admitted, the further question arises whether the foliation observed in it was produced prior, or posterior, to the consolidation of the rock. In considering this question, I leave out of sight altogether evidence of fluxion structure as being really irrelevant to the question at issue, though I think it material to state that the rock does show very decided evidence of fluxion. Without laying any stress on this fact, however, I think the following considerations prove that the foliation was not produced by pressure acting on the rock after its consolidation.
"First, the granite is not always foliated at its contact with the rocks into which it has been intruded; on the contrary, though still porphyritic, it is not unfrequently decidedly granitic along its margin. This fact presents no difficulty to the acceptance of the hypothesis advocated below, but I think it offers an insuperable barrier to the acceptance of the view that the folia. tion was produced by pressure. Simple pressure will not do: that would not explain the crumpled micas and the very decided evidence of flow or fluxion. Pressure resulting in shear motion, the development of heat and concomitant chemical and mineralogical action, might possibly account for the fluxion structure; but if shear and motion were established on the grand scale required after the consolidation of the rock, the granitic portions along the margins could not possibly have escaped the effects of this action.
Secondly, the apparently capricious passage from a granitic to a foliated structure in the main mass of the granite is another serious impediment in the way of the acceptance of the theory of dry pressure.'
"Thirdly, the conjunction of the outer band of gneissose granite at Dalhousie with the carboniferous series, presents another almost insuperable difficulty. The outer band is the most intensely foliated of all the Dalhousie granite. Parts of it look as if it had been rolled under a gigantic steam roller. Unquestionably it has been subjected to very great pressure, and to either traction or shearing; and yet this rock is chock and block with little altered black carboniferous rocks. He who would apply the dry-pressure theory to explain the intense foliation of the outer band of granite, would have to invent a new set of conditions out of his inner consciousness and bring some other rock into position next the granite before he applied the squeeze.
1 I use this expression as a short term to indicate pressure appled after the consolidation of a rock, though of course, I am aware that pressure so applied may produce heat and even fusion
"Fourthly, the condition of the long tent-peg-like splinter of schist included in the granite, alluded to above, shows conclusively that the granite at the point where the inclusion was found was not subjected to extreme pressure of the character under consideration after its consolidation. Had it been, the splinter of schist would have been flattened to a wafer. A mere glance at the plate, a photograph reproduced by the heliogravure process, will show this
"Fifthly, neither the crypto-crystalline mica, nor the fish-roe quartz, described ante, can possibly have been produced by the grinding down of the mica and the quartz in the consoli dated rock, or by any analogous process; for, besides the crypto-crystalline mica, and the fishroe quartz, we have very numerous large crystals of muscovite, biotite, and quartz. The muscovite and biotite are large and beautiful specimens of these minerals, and they orient in all directions and at every angle up to a right angle to the strings of crypto-crystalline mica. Mechanical action potent enough to have reduced mica to the pulpy condition of the crypto. crystalline mica would not have left the larger micas untouched. Similarly, the fish-roe quartz not only fills cracks in felspars, and forms a sort of setting to quartz grains, but it meanders about in the interior of large quartz grains, and terminates abruptly inside them, in a way that does not suggest to the observer that he is looking at cracks stopped with micro-crystalline quartz, but rather that the crystallization of the quartz was brought to a comparatively rapid termination towards its close.
"Indeed, properly considered, I think the crypto-crystalline mica, and the fish-roe quartz, furnish a clue to the riddle. I may mention in passing that I have observed in a felsite patches of material closely resembling the crypto-crystalline mica mixed up with the quartz and the ordinary felsitic base; but I desire more particularly to refer to a series of rocks which occur in the peninsula of India about eighty-five miles nearly due west of Delhi. We have there a very interesting group ranging from felsites, quartz-porphyries, and granite-porphy ries to almost true granites. The felsites appear to be true lavas; and the others, though merging gradually into rocks of plutonic character, are probably more or less directly connected with them. These rocks never show any trace of foliation, or give any indication of crushing. But what is important to note is, that the gradual genesis, so to speak, of the fish-roe quartz may be observed in these rocks. The quartz gradually becomes more and more developed in the felsitic base; it begins to crystallize out in grains of microscopic size, and the grains increase in number, until at last the whole base, or ground-mass, of the granite-porphyries partakes closely of the characters of the fish-roe quartz of the Dalhousie granite. The true explanation of the foliation of the latter rock I believe to be briefly as follows:-The rock had partially consolidated before it was moved into place; large porphyritic crystals of felspar, and numerous micas and quartz grains had formed; it was very much in the condition of a fels parporphyry, or a granite-porphyry; when, in the course of the earth-movements that were contorting, crumpling, and folding the strata of the Himalayas, this imperfectly consolidated granite-porphyry was forced through the faults that had been formed along the axes of over thrust-folds; the semi-plastic mass was subjected to enormous pressure; the mica was crumpled; the crystals of felspar were cracked and ruptured; and so much of the micaceous siliceous materials as remained unconsolidated were forced into the rents made in the already formed minerals. The final consolidation took place under conditions of continued strain; but before it was actually accomplished minor and subsidiary eruptions took place which forced new supplies of the granitic material into fissures formed in the previously injected rock, and this fresh material consolidated under conditions somewhat different from those of the first eruptions.
"I think this view meets all the difficulties of the case, and that the intelligent reader will with its aid be able to harmonize all the facts stated above without detailed exposition on my part."
1 See Records G. S. XVII, p. 168.