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.