Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note ii. § 6.

Origin of coal.

137. The vegetable origin of coal seems to be sufficiently proved by the reasoning in § 5. and 6.; and that reasoning will appear still more satisfactory, from what is said at § 28. and 29., concerning the consolidation of this fossil. Dr Hutton has treated both of the matter of coal and of its consolidation. Part. I. Chap. 8., of his Theory of the Earth [47].

[47] Vol. i. p. 558, &c.

The notion, however, that coal is of vegetable origin, is not peculiar to this theory, but has been for some time the prevailing opinion. Buffon supposes this mineral to be formed from vegetable and animal substances, the oil and fat of which have been converted into bitumen by the action of acids.[48] A fundamental mistake, however, is committed by this author, and by M. Gensanne, (author of the natural history of Languedoc,) on whose observations he greatly relies, in considering coal as consisting of bitumen united to earth, thus omitting the only ingredient essential to coal, namely the carbon or charcoal. This may truly be considered as the essential part, because coal may exist without bitumen, as in the instance of blind coal, but not without charcoal.

[48] Hist. Nat. des Mineraux. tom. i. p. 429, 4to edit.

Another theory of coal, very analogous to Dr Hutton's, is that of Arduino, professor of mineralogy at Venice, in which he supposes it formed from vegetable and animal remains from the land and sea, but chiefly from the latter.[49] This theory of coal is contained in Dr Hutton's, in which the animal and vegetable remains must be supposed to come both from the earth and the sea. It seems to be without any good reason that Arduino considers the sea as the chief source of these materials. His remarks, however, are very ingenious, and deserving of attention.

[49] Saggio Fisico-mineralogico del Sig. Giov. Arduino; Atti di Siena, tom. v. p. 228, 281, &c.

These accounts of the origin of coal are all nearly the same; it is in what relates to the distinction between the common coal, in which there is no ligneous structure, and those varieties of it in which that structure is apparent, and again in explaining the consolidation of both, that the theory laid down here is peculiar.

138. Some other mineralogists refer one of the ingredients of coal to the vegetable kingdom, but not the other. Unable to resist the conviction which arises from the fibrous structure of parts of strata, and even entire strata of coal, they have supposed, that wood, which had been somehow buried in the earth, or perhaps deposited at the bottom of the sea, had become impregnated with bitumen, which last, however, they consider as of mineral origin. This appears to be the opinion of Lehman; and also of some very late writers. There seems, however, to be hardly less reason for referring the origin of one part of coal to the vegetable or animal kingdom than another. The two last are certainly capable of furnishing both the carbonic and bituminous parts; and therefore, to derive these from different sources, is at least a very unnecessary complication of hypotheses.

139. Another explanation of coal, very different from any of the preceding, has lately been advanced and set up in opposition to the Huttonian Theory. Mr Kirwan,[50] the only mineralogist, I believe, who has attempted to derive both the carbonic and bituminous matter of coal from the mineral kingdom, distinguishes between wood coal and mineral coal, and gives a theory entirely new of the formation of the latter. Wood coal is that in which the ligneous structure is so apparent, as to leave no doubt of its vegetable origin; mineral coal is that in which no such structure can be discovered, and is the same which Dr Hutton derives from the vegetable juices, and other remains, comminuted, dispersed, carried into the sea, and there precipitated, so as to unite with different proportions of earth, and to become afterwards mineralized.

[50] Geol. Essays, essay vii. p. 290.

These two species of coal, which the Huttonian theory considers as gradations of the same substance, Mr Kirwan regards as perfectly distinct, constituting two minerals, of an origin and formation entirely different. He therefore endeavours to ascertain the distinguishing characters of each, considered geologically.

140. But here the leading distinction, implied in all the rest, that the two kinds of coal are never found in the same bed, but always in different situations, and with different laws of stratification, is expressly contradicted by matter of fact. Coal, as is said above, with its ligneous texture quite apparent, and coal with no such structure visible, are often found in the same seam, are brought up from the same mine, and united in the same specimen. I have a specimen from a bed of coal, in the Isle of Sky, found under a basaltic rock, consisting of a ligneous part, which graduates into one in which there is no vestige of a fibrous texture, and in which the surface is smooth and glossy, with a fracture almost vitreous. The upper part of the specimen is therefore perfect wood coal, and the under part perfect mineral-coal, in the language of Mr Kirwan; at the same time that the transition from the one to the other is made by insensible degrees. This specimen, were it perfectly solitary, is sufficient to prove the identity of the two species of coal we are now speaking of, and to show, that the difference between them is accidental, not essential. The specimen, however, is far from being solitary; the number of similar appearances is so great, as hardly to have escaped the observation of any mineralogist. Mr Kirwan admits, that wood coal is often found under basaltes;[51] but what is essential to be remarked is, that, in this instance, we have both the wood coal and the common mineral-coal, lying under that rock, and the one passing gradually into the other It appears, indeed, that many of the facts which Mr Kirwan produces, in treating of what he calls carboniferous soils, are quite inconsistent with the distinction he would make between wood-coal and mineral coal.[52]

[51] Geol. Essays, p. 310.

[52] Ibid. p. 311.

141. It is, however, true, that there are instances in which the wood coal, or fossil wood, as it is usually called, forms entire beds, quite unconnected with the ordinary coal, and stratified in some respects differently. Such is the Bovey coal in Devonshire, the wood-coal in the north of Ireland, and perhaps the Surturbrandt of Iceland. With respect to the Bovey coal, it does by no means answer to one of Mr Kirwan's remarks, viz. that late observations have ascertained, that no such parallelism of the beds, as in mineral coal, nor even any distinct number of strata is found. In the Bovey coal, the number of strata is very well defined, by beds of clay regularly interposed; but as to the extent of these beds, the coal having been worked only at one place, and by an open pit, without any extensive subterraneous excavation, nothing is known with certainty.

In the Bovey coal too, I must observe, though its beds have the ligneous structure very distinct, the clay interposed between these beds, which is but little indurated, contains a great deal of coaly matter, in the form of thin flakes, interspersed through it. So far as I know, there are no mineral reins nor shifts, nor any bed of indurated stone, that accompany this coal; so that, though one can not doubt of its vegetable origin, some doubt may be entertained concerning the nature of the mineralizing operations, to which it has been subjected The consideration of these, however, does not belong to the present argument; and the peculiarities of this semi-mineralized coal, as it may be called, have nothing to do with the general question, whether wood coal and mineral coal are the same substance; about which question, if the gradations are properly considered, I think, no reasonable doubt can remain.

142. One of Mr Kirwan's objections to the vegetable origin of coal, is founded on this fact, that there is, in the museum at Florence, a cellular sandstone, the cells of which are filled with genuine mineral coal. "Could this (adds he) have been originally wood?"[53] The answer to the interrogatory proposed here as a reductio ad absurdum, is, that most undoubtedly it may have been wood. Sandstone with charred wood, that is, with wood coal in it, is not an uncommon phenomenon in coal countries. I have seen a specimen of this kind from the Hales Quarry, near Edinburgh, consisting of a piece of charred wood, imbedded in sandstone; the wood was much altered, but the remains of its fibrous structure were distinctly visible. This affords a perfect commentary on the specimen in the Florence cabinet.

[53] Geol. Essays, p. 321.

143. If then it be granted, as I think it must, that the two kinds of coal we have been speaking of are of the same origin, it is not very necessary to enter on a refutation of Mr Kirwan's theory with respect to either of them. His account of the formation of mineral coal, however, is so singular, that it cannot be passed over without remark.

Mr Kirwan supposes, 1mo, That natural carbon was originally contained in many mountains of the granite and porphyritic order, and also in siliceous schistus; and might, by disintegration and decomposition, be separated from the stony particles. 2do, That both petrol and carbon are often contained in trap, since hornblende, which has lately been found to contain carbon, very frequently enters into its composition.

"My opinion (adds he) is, that coal mines, or strata of coal, as well as the mountains in which they are found, owe their origin to the disintegration of primeval mountains, either now totally destroyed, or whose height and bulk, in consequence of such disintegration, are considerably lessened; and that these rocks, anciently destroyed, contained most probably a far larger proportion of carbon and petrol than those of the same denomination now contain, since their disintegration took place at so early a period.[54]

[54] Geol. Essays, p. 328, &c.

"By the decomposition of these mountains, the feldspar and hornblende were converted into clay; the bituminous particles, thus set free, reunited, and were absorbed, partly by the argil, but chiefly by the carbonaceous matter, with which they have the greatest affinity. The carbonic and bituminous particles, thus united, being difficultly miscible with water, and specifically heavier, sunk through the moist, pulpy, incoherent argillaceous masses, and formed the lowest stratum," &c.

Such is Mr Kirwan's theory of the formation of coal, and nobody I think will dispute the originality of it.

144. To enter on a formal refutation of an opinion so loaded with objections, would be a task as irksome as unnecessary. A few observations will suffice.

The notion of the great degradation of mountains, involved in this hypothesis, is the part of it to which I am least disposed to object. But I cannot help reminding Mr Kirwan, that the effects of waste are not supposed less in this, than in Dr Hutton's theory; and that he has assumed the very principle, of which that theory makes so much use, though he has reserved to himself, as it should seem, the right of denying it, when it does not accord with his system. It is indeed worth while to compare what is said concerning the degradation of mountains, in the above quotations, and still more fully in the book itself, with what is advanced concerning their indestructibility, in another passage of the same volume:[55]

[55] Page 436.

"All mountains are not subject to decay; for instance, scarce any of those that consist of red granite. The stone of which the Runic rocks are formed, have withstood decomposition for two thousand years, as their characters evince," &c.

"Basaltic pillars, in general, bid defiance to decay," &c. He goes on to deny every step of the degradation of land, by which it is wasted, carried into the sea, and spread out over its bottom, though all these are necessary postulata in his theory of the formation of coal. One can be at no loss about estimating the value of a system, in which such gross inconsistencies make a necessary part.

145. The quantity of hornblende and siliceous schistus, necessary to be decomposed, in order to produce the coal strata presently existing, is enormous, and would lead to an estimate of what is worn away from the primeval mountains, far exceeding any thing that Dr Hutton has supposed. It is true, that Mr Kirwan, never at all embarrassed about preserving a similitude between nature as she is now, and as she was heretofore, lays it down, that the part of the primeval mountains which is worn away, contained much more carbon than the part which is left behind. This, however, is an arbitrary supposition; and since, in this system, such suppositions are so easily admitted, why may we not conceive, in the primeval mountains, a more copious source of carbonic matter than hornblende or siliceous schistus? We have but to imagine, that the diamond existed among these mountains in such abundance, as to constitute large rocks. This stone being made up of pure, or highly concentrated carbon, the adamantine summits of a single ridge, by their decomposition, might afford a carbonic basis, sufficient for the coal beds of all the surrounding plains.

146. We may also object to Mr Kirwan, that the siliceous part of the mountains has not been chemically dissolved; it has been only abraded and worn away. Mechanical action has reduced the quartz to gravel and sand, but has not produced on it any chemical change. The carbon, therefore, could not be let loose. Experiment, indeed, might be employed, to determine whether the siliceous matter of the secondary, and of the primary strata contains this substance in the same proportion.

Again, a more fatal symptom can hardly be imagined in any theory, than that, when the circumstances of the phenomena to be explained are a little changed, the theory is under the necessity of changing a great deal. Now, this is what happens to Mr Kirwan's theory, in the attempt made to explain by it the stratum of coal described in the Annales de Chimie,[56] as cutting a mountain of argillaceous strata in two, at about three-fourths of its height. This stratum, Mr Kirwan says, must have been formed by transudation from the superior part of the mountain,[57] Besides that this is a gratuitous supposition of a thing, without example, it involves in it an absurdity, which becomes evident the moment the question is asked, What occupied the place of the coal-bed before the transudation from the upper part of the mountain? Has the liquid coal, as it percolated through the upper strata, expelled any substance from the place it now occupies? or has it been powerful enough to raise up, or to float, as it were, the upper part of the mountain?

[56] Tom. xi. p. 272.

[57] Geol. Essays, p. 338.

The situation of this bed of coal is not singular, and its formation is easily explained on Dr Hutton's theory. It is part of a stratum of coal, which has been deposited, like all others, at the bottom of the sea; from whence certain causes, of very general operation, have raised it up, together with the attending strata: these strata have since been all cut down, and worn away by the operations of the surface; and the mountain, with the coal stratum in the middle of it, is a part of them which has been left behind. There is no wonder, that a coal stratum should be found alternating with others, in a mountain, any more than in the bowels of the earth, and no more need of a separate explanation.[58]

[58] This stratum of coal, which is described by Hassenfratz, is remarkable for being in a mountain which rests immediately on primary schistus and granite.

147. After all, it may be asked, for what purpose is it that so many incongruous and ill supported hypotheses are thus piled on one another? is it only to avoid ascribing the carbonic and bituminous matter of coal to a substance in which we know with certainty that such matter resides in great abundance, in order to derive it from other substances, in which a subtle analysis has shown, that it exists in a very small proportion? Such reasoning is so great a trespass on every principle of common sense, not to say of sound philosophy, that, to bestow any time on the refutation of it, is, in some degree, to fall under the same censure.