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ironstones, and coals, which has hitherto yielded the main supplies of mineral fuel, and to which Britain owes so much of her mechanical superiority and commercial greatness. As this system will form the subject of a separate Sketch, we need only here observe that its coals occur in many seams, of every thickness, from a few inches to forty feet; of all degrees of purity, from earthy masses that can scarcely be ignited, to clear bituminous seams that burn leaving scarcely a trace of ashes; and fitted for every economical purpose — household fuel, gas-making, oil-distillation, steam - raising, smelting, and metal-working. Like other coals, the thickest, the purest, and most continuous seams have evidently grown and accumulated in situ; those embedding stony and earthy layers have been interrupted in their accumulation; and others, again, less regular and continuous in thickness, and mingled more with extraneous impurities, have apparently been formed of drift and water-logged vegetation. In lakes, in estuaries, and along great shallow sea-reaches, the flora of the Carboniferous era flourished for ages, the land now sinking, now rising, but on the whole subsiding, to receive the vast thickness of sediments which compose the system. Eead in the light of what is now taking place at the present day, there is nothing abnormal or preternatural in the Coal-formation, and we behold in its various coals merely the peat - growths, swamp - growths, jungle - growths, and vegetable-drifts of the period, compressed and mineralised during the lapse of ages. The caking-coals, splint-coals, cannelcoals, and anthracites or stone-coals of the miner are merely different expressions of this mineralisation or metamorphism—different conditions of deposit, as rapid covering up, exposure to decay, nature of vegetation, and compactness of overlying strata, al\ affecting the ultimate quality of the coal. If much earthy matter has mingled with the vegetable mass during its aggregation, the coal will be stony and impure; if the vegetable mass has been rapidly covered up by retentive muds and clays (now converted into shales and fire-clays), the coal will likely be soft and highly bituminous; if the superincumbent stratum be open and porous, so as to admit the escape of volatile matters, the coal will in all likelihood be hard, dry, and less bituminous; and if the vegetable mass has undergone extreme chemical change, or has, as a coal, been subjected to the heat of igneous rocks, it will less or more be deprived of its gaseous elements and converted into an anthracite. And thus, and thus only, can the great variety of coals occurring in the palaeozoic coal-fields of Europe and North America be satisfactorily accounted for.
Beyond the Carboniferous system coals become rare and comparatively unimportant. It is true that in some districts we cannot fix any very sharp line of demarcation between the Coal-formation and the Old Eed Sandstone, but, generally speaking, the two systems are sufficiently distinct, and it is curious that up to the present time no coal-seams of any thickness have been detected in the latter. Indeed, with the exception of some insignificant bands described by Principal Dawson as occurring at Gaspe in Canada, the Old Eed Sandstone is altogether barren of coal, though vegetable fragments are scattered in some abundance throughout its shales and flagstones. In the Silurian and more highly metamorphosed Cambrian and Laurentian strata we have thin bands and irregular patches of anthracite and graphite; but though these are generally ranked with the coal family, their vegetable structure has been so obliterated that we cannot say whether they have been formed from terrestrial or marine vegetation, or indeed whether graphite is always certainly of organic origin.
Here, then, we perceive that Coals, or minerals of the Coal Family, occur in all formations, from the accumulations now going forward on the earth's surface down through every stratified system, whether belonging to tertiary, secondary, or primary epochs. From peat we pass to lignite, from lignite to true coal, from coal to anthracite, and from anthracite to graphite. All are but compressed and chemically altered masses of vegetation, the slow fermentation or distillation of which results in the gradual expulsion of the gaseous or volatile portions, and in the retention of the carbonaceous or coaly residue.* The following tabulation exhibits, proximately, this gradation of chemical change by which wood is converted into peat, peat into lignite, lignite into coal, coal into anthracite, and anthracite into graphite:
(At 212'.) Carbon. Hydrogen. Oxygen. Nitrogen. lMj^<C
Wood 48—54 6—10 35-45
Peat 56-66 5— 9 18—33 2—4 1— 6
Lignite 56—70 3— 7 13—27 1—0 1—13
Coal 70-92 2—6 1—8 0—2 3-14
Anthracite.... 74—94 1-4 0—3 trace 1— 7
Graphite 80—98 ... ... ... 1— 7
Here it will be observed that the gaseous substances, hydrogen and oxygen, so abundant in recent wood and
* According to M. Fremy, the following are the degrees of alteration of woody tissue: 1. Turf and Peat.—Characterised by the presence of ulmic acid, and also by the woody fibres or the cellules of the medullary rays, which may be purified or extracted in notable quantities by means of nitric acid or hydrochlorites, in which they are insoluble. 2. Fossil Wood or Woody Lignite.—This, like the preceding, is partially soluble in alkalies, but its alteration is more advanced, for it is nearly wholly dissolved by nitric acid and hydrochlorites. 3. Compact or Perfect Lignite.—This substance is characterised by its complete solubility in hydrochlorites and in nitric acid. Alkaline solutions do not in general act on perfect lignites, Reagents in this variety show a passage of the organic matter into coal. 4. Coal.—Insoluble in alkaline solutions and hydrochlorites. 5. Anthracite.— An approximation to graphite; resists the reagents which act on the above-mentioned combustibles, and is only acted on by nitric acid with extreme slowness.
peat, gradually diminish as the mass becomes more and more mineralised, till at length they disappear, and leave in consequence a gradually-increasing residue of carbon in the true coals, anthracites, and graphites. Like all mixed rocks, however, coal presents itself in many varieties. We cannot conceive of vegetable matter (whether drifted or grown in situ) being associated with sedimentary strata without its being mingled more or less with the earthy impurities of these sediments. These impurities, according to their amount, must necessarily confer on different coals different structures, different aspects, and different qualities. Besides, varieties will also arise from the conditions of the vegetable mass itself, according as it may have been imbedded while fresh or been long exposed to atmospheric decay, according as it may have been suddenly covered up or long exposed to maceration and comminution in water, and notably also according to the nature of the plants composing the mass. These varieties, according to their structure, texture, and qualities, are generally known as caking-coal, which is soft and tender in the mass, like that of Newcastle, and swells and cakes together in burning; splint or slate coal, which is hard and slaty in texture, like most Scotch coals, and burns free and open; cannel or parrot coal, which is compact and jet-like in texture, spirts and crackles when thrown suddenly on the fire, but when ignited burns with a clear candle-like flame, and from its composition is chiefly used in gas-manufacture; and coarse, foliated, or cubic coal, which is more or less soft, breaks up into large square blocks, and contains in general a large percentage of earthy impurities. Between these varieties there is, of course, every gradation—coals so pure as to leave only one or two per cent of ash, others so mixed as to yield from ten to thirty per cent, and many so impure as to be unfit for fuel, and so to pass into shales more or less bituminous.* Eesides these varieties, founded chiefly on mineral characters, it is also customary to distinguish coals according to the purposes for which they seem best suited, or to which they are most frequently applied; hence we hear of household coals, furnace coals, smithy coals, steam coals, gas coals, oil coals, and similar distinctions.
We have thus occurring in the crust of the earth not only a great variety of coaly substances, but also coals of different aspects and qualities occurring in the same geological formation. The causes of these differences are, in general, sufficiently obvious:—age, and the amount of chemical change to which they have been subjected; the amount of earthy impurities commingled with them during their aggregation and deposition; the nature of the plants composing the bulk of the mass; the amount of decay which
* As bituminous shales are now so extensively mined for the distillation of paraffin, it may be of use to advert to some distinctions that subsist between them and the coals properly so called. A coal, though often containing a considerable amount of earthy impurity, consists chiefly of vegetable matter, or, in other words, carbon is its prevailing ingredient. Where the earthy or mineral ingredient greatly exceeds the organic, it becomes unfitted for combustion, and is regarded merely as a carboniferous stone, of which clay, sand, and the like form the main proportion. The term sliale, on the other hand, refers to structure rather than to composition, and is something that splits up or peels off in thin layers or lamiuie. Most consolidated muds are characterised by this quality of splitting or breaking up in thin leafy layers parallel to their bedding; hence shales may be regarded as consolidated muds, and may be distinguished as calcareous, arenaceous, or bituminous according to their predominating ingredient. Bituminous shales, therefore, have been mere vegetable muds—their richness, like those of the coals, depending upon the amount of organic matter and the conditions under which it was preserved. Some shales may be as bituminous as some poor varieties of coal, but this does not entitle them to be ranked as coals, any more than an excess of earthy matter in a hard stony coal would entitle it to be called a shale. The terms refer to structure rather than to composition; and though it is true that the shaly or leafy structure is almost invariably characteristic of the earthier ingredient, yet it must ever be borne in mind that both shales and coals are mixed rocks, and that not unfrequently the one may pass into the other by insensible gradations.