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]. [Footnote 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. [Footnote 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. [Footnote 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.