Art. LIII.—Contact Metamorphism at the New Brockley Coal-mine (Malvern Hills).
[Read before the Philosophical Institute of Canterbury, 2nd November, 1898.]
The local metamorphism of carbonaceous beds, such as coal-seams, is always interesting. The literature of the subject is, however, somewhat scanty, and the conclusions arrived at by different writers, especially as regards the distance through
Which igneous action may be traced, very varying. It seemed, therefore, advisable to take the opportunity afforded by the new Brockley tunnel of studying the influence of the dolerite dyke, through which the tunnel cuts, upon the various strata in its immediate neighbourhood.
For position of mine and general geology of district, see “Broekley Coal-mine and Surrounding District,” by S. H. Cox, in the “New Zealand Geological Reports,” 1882, page 57 : “On Mount Somers and Malvern Hills District,” by S. H. Cox, in the “New Zealand Geological Reports,” 1883–84, page 33: and “Malvern Hills,” by Sir James Hector, in the “New Zealand Geological Reports,” 1870–71, page, 46; and Sir Julius von Haast, in the “New Zealand Geological Reports,” 1870–71, page 135, and 1871–72, page 46.
Conditions at Mine.
Entering the main tunnel, which cuts the dyke nearly at right angles, the following strata are passed in turn, the miners' names being used: (1.) Dolerite, about 300ft. (2.) Fireclay (so-called), 7ft. 10 in. (3.) Greystone, 5ft. 7 in. (4.) Plumbago, 4ft. 6 in. (5.) Anthracite, 2ft. 6 in. (6.) Ordinary coal-measures, about 130ft. (7.) Brown coal, 3ft. (8.) Measures, 12ft. (9.) Brown coal, 4 ft.
At the time of my visit, owing to a fall in the tunnel, no specimens could be taken from the inner end. The brown coal spoken of later was taken from the centre of the 3ft. seam, where it has been cut by an upper drive. For other specimens I take this opportunity of thanking Mr. Henry Lee, the working manager at the mine.
1. So-called Fireclay and Greystone.
These are undoubtedly products of contact-action. It is in the case of argillaceous rocks that we find the greatest variety of products resulting from such action. “When fine-grained siliceous clays are exposed to the action of heat by contact with igneous masses they pass into the hard compact materials often called hornstones, porcellanites, &c., and in some of these materials traces of fossils contained in the original rocks may still be detected.”*
The whitish outer band of porcellanite and the inner grey, or rather blue-grey, stone will probably prove to be but parts of the same stratum. At some distance from the dyke, fossil remains—charred imprints—of the flattened stems or leaves of old vegetation are often to be found. (It is, however, quite possible that the outer portion of the white-clay rock has been
[Footnote] * Judd's “Lyell” (1896), page.556.
derived from the dyke itself, as many instances are recorded pf such “white rocks.” Microscopic observation should in that case show traces of the original crystalline structure of the igneous mass, though the component minerals would, of course, be entirely altered. Chemically it is certainly quite within the range of possibility that the reducing action of the carbonaceous materials which must have abounded in the clays near the dyke should be exercised through a distance of several feet.)
This “white rock” should prove of economic value. It could well be used for road-metal, and when ground down (it pulverizes fairly easily), and mixed with some lighter fire-clay, should make excellent, firebricks, or a resisting lining for furnaces. Similar material, ground to a fine sand, is much used in England in the casting of braps.
2. Plumbago (so-called) and Anthracite.
Both of these seams fall really under the same heading. Anthracite is generally defined as “the densest, hardest, and most lustrous of all varieties of coal.” It burns with very little fame, but gives forth great heat—contains very little volatile matter; splinters when heated, and ignites with difficulty. Its colour is generally given as black. Its fracture is lamellar, parallel to the bedding, and conchoidal in other directions. Ure * includes three varieties—viz., anthracite proper (defined much as above); culm, an impure shaly kind; and fossil coke, an American form, more compact than artificial coke, and supposed to be produced by the action of trap rocks on anthracite (sic).
The plumbago of the Brockley Mine is such an anthracite shale, and will be reported on in detail (as also the outer white rock) in a subsequent paper.
The anthracite, or, as it might also apparently be named, “fossil coke,” is a really useful seam, and should prove of great value as a fuel, either alone or mixed with the hydrous coals of the inner seams. It is rather coky in appearance, is semi-lustrous, does not soil the fingers, is hard to powder in a mortar, but, owing to the marked development of the lamellar structure, and the existence in places of a columnar structure at right-angles to the bedding-planes, easy to break into lumps. It gives very little flame, contains but little over 1 per cent. of sulphur, and, except for the some- what high percentage of water, has every characteristic of a true “fossil coke,” or stone coal.†
[Footnote] * Dictionary of Arts, &c.,” vol. i., p. 744.
[Footnote] † For analytical details of these coals, see below Art. LIV., “Analyses of New Zealand Coal.”
3. Brown, Coal of the 3ft. Seam.
This is a compact fairly hard coal, which weathers well, and is in every way superior to the average coal of the district. Mr. Page, of the chemical department of Canterbury College, kindly placed at my disposal a sealed specimen of the altered brown coal from the old Brockley working, so that I was enabled to compare it directly with the coal now being got out. The comparison showed that the coal-substances of the two are very much alike. The present coal, being near the surface, contains, however, as would be expected, more ash and more water per cent. than the old. Already, as the drive gets further in, this proportion of ash is lessening. The coal burns well, and is almost entirely free from the fetid odour so noticeable with many of the Malvern “browns.”
Origin of the Anthracite.
The anthracites of Europe and America are almost universally held to represent the final stage of that natural process of destructive distillation which has given us the whole range of brown and bituminous coals. The reason for the change is in some instances fairly obvious; in many others, however, still a mystery.
Generally speaking, we may divide the main anthracite beds into three groups—(1) Those apparently due to the direct action of heat; (2) those apparently due to the direct action of pressure (and heat ?); and (3) those whose origin is still unaccounted for.
To the first class belong the smaller anthracite seams in the neighbourhood of igneous dykes and floes. That these dykes actually alter ordinary coals in the direction of anthracite is a well-established fact.
Woodward,* in speaking of the various characteristics of the South Staffordshire Coalfield, says, “The Rowley Rag basalt is well known in connection with the district; according to Jukes it forms part of the coal-measure series, having been poured out as a sheet of lava during this period. The coal beneath the basalt has been altered, and has lost its inflammability.”
Again, “The Cleveland, Cockfield, and Annathwaite dyke commences six miles south of Whitby, and extends …more than ninety miles…. It is probably of Tertiary age. In some localities where it does not reach the surface it has been, proved in colliery workings; but the coal in proximity to the eruptive rock becomes anthracitic, and ultimately worthless.”
Professor Hull mentions that at Whitwick a sheet of
[Footnote] * Geology of England and Wales,” by H. Woodward, p. 189, &c.
dolerite intervenes between the coal-measures and the new red sandstone. At Whit wick Colliery it is 60 ft. thick, and has turned to cinders a seam of coal with which it comes in contact.
“The Cornbrook Coalfield is to a large extent covered by basalt, from 60 ft. to 150 ft. thick, and in some places the coal is altered and ‘sooty.’ The known instances of this class are well summed up by Geikie in his ‘Outlines of Field Geology.’”* “Sometimes,” he says, “the coal has been entirely consumed, and a layer of igneous rock has taken its place. At other times a thin sheet of molten lava has been injected along the top, bottom, or centre of the coal-seam, converting it into a kind of anthracite or into a mere cinder. Examples may be found where the coal has been fused into a cellular mass, and has subsequently had its vesicles filled with infiltrated carbonate of lime. In Ayrshire numerous beautiful sections have been laid bare, when the coal has been rendered prismatic, the hexagonal or polygonal prisms, like so many bundles of pencils, diverging from the surface of the intruded igneous rock.”
To the second, or “pressure,” class belongs the great anthracite bed of the Appalachian system. “In Pennsylvania the strata of coal are horizontal to the westward of the. Appalachian Mountains, where Professor Rogers pointed out that they were most bituminous; but as we travel southeastward, where they ho longer remain level and unbroken, the same seams become progressively debituminized in proportion as the rocks become more bent and distorted. At first on the Ohio River the proportion of hydrogen, oxygen, and other volatile matters ranges from 40 to 50 per cent. Eastward of this line, on the Monongahela, it still approaches 40 per cent., when the strata begin to experience some gentle flexures. On entering the Appalachian Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in proportion of 18 or 20 per cent. At length, when we arrive at some isolated coalfields associated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over—as near Pottsville—we find the coal to contain only from 6 per cent, of volatile matter, thus becoming a genuine anthracite.”† Portions of the Pembrokeshire anthracite beds of the South Wales Coalfield belong to this class.
To the third class belongs much of the South Wales anthracite. Speaking of the gradual change in character of
[Footnote] * Outlines of Field Geology,” by Sit A. Geikie, p. 160.
[Footnote] † Judd's “Lyell,” page 33.
the coal, Hull* says, “Nor was this alteration …accompanied by outburst of igneous rocks, or by violent crumplings and contortions of the beds; on the contrary, the strata are usually but slightly thrown out of the horizontal position. Other causes must therefore be sought for…. We may offer conjectural solutions of it, such as the greater increase of temperature over the western or anthracitic region as compared with that over the eastern; or that owing to fissures exceptionally numerous in the western area greater facility was afforded for the escape of the gaseous products. But none of these reasons are quite satisfactory, and this remains one of the problems in physical geology which yet awaits solution.”
Both the causes directly mentioned above—rise of temperature and increase of pressure—seem competent to bring about the chemical action necessary for that gradual elimination of oxygen and hydrogen which produces an anthracite from an ordinary hard or even a soft hydrous variety of coal. In the case under consideration—the Brockley—there is abundant evidence also of both. The dyke is large, and, even if, as seems at first sight most probable, it is the result of a single effort of injection,† must have elevated the temperature of the surrounding strata considerably during a long interval of time.
That enormous local pressure must also have been brought into play is evident from the fact that the measures have been thrown into a vertical position by the dyke, and in part actually overturned.
The two causes are also almost inseparably connected. Increase of pressure would certainly result in increase of temperature‡ and chemical action dependent thereon, even if the rise of temperature were not evident, and the increase of temperature due to the injection of the dyke would in many instances be followed by a great increase of pressure as the mineral masses composing it took solid form. §
The Brockley dolerite dyke has certainly come close enough
[Footnote] * E. Hull, “The Coalfields of Great Britain,” 3rd ed., p. 259.
[Footnote] † Such a dyke is usually fairly homogeneous, and therefore probably fills a fissure opened by the pressure accompanying its injection. (See O. Fisher, “Physics of the Earth's Crust,” p. 283.).
[Footnote] ‡ Mr. Sorby has suggested that under the conditions which exist within the earth's crust there may be even a direct conversion of mechanical into chemical energy, and that many familiar geological phenomena will have to be explained in this way. (Proc. Roy. Soc., 12, p. 538.).
[Footnote] § The result of recent experiments appears to show that such substances as whinstone and granite are less dense in the solid than in the liquid state at the melting temperature, and must therefore expand on solidifying.” (Fisher, op. cit., p. 291.)
to the coal-measures to account for the production of the anthracitic shales and anthracitic seam by rise of temperature only, if such a limitation were necessary. Personally, I do not think there is any necessity to demand a great rise of temperature. A moderate rise of temperature, combined with a considerable, increase of pressure, would, I believe, completely determine the required elimination of volatile matter, especially as that elimination would be aided by the fissures sure to be formed by increase of pressure.
Alteration of Brown Coal at Some Distance from the Dyke.
It has at times been urged that the action of the dykes must be limited to a few feet or yards at the most, and that the alteration of brown coals at a distance of some 50 yards cannot therefore be attributed to such action. The researches of Professor Spring,* of Liège, upon the effects of pressure, apart from those of elevation of temperature, seem to answer this objection.
Amongst other, interesting conclusions, Spring states that the rubbing or sliding of the particles of solid bodies over one another under intense pressure powerfully promotes chemical action between them; and that, when the particles of solid bodies have been brought into contact by intense pressure, the chemical action between them goes on even when the pressure is removed.
If, therefore, during the injection of a dyke into or past coal-measures the coal-seams are subjected, as we may surely suppose, to intense pressure, there is reason to believe that the natural processes of oxidation would receive a great acceleration, and that even after the pressure had been equalised again those oxidizing processes would, at any rate for a considerable period, continue at the rate induced by pressure.
It is, I believe, somewhat in this direction that we must look for an explanation of the altered brown coals of the Brockley.
It should—at least, indirectly—prove of value if experiments were carried out with the object of determining the effect of (1) pressure, (2) elevation of temperature, on the ordinary (so-called unaltered) brown coals of the district. The question as to whether the anthracites had ever been
[Footnote] * Journ. Chem., Soc. (London), 1890, p. 404.
subjected to a high temperature would be rendered easier by a determination of the electric conductivity of various specimens.