Note ix. § 81.
Petroleum, &c.
173. According to the theory of coal laid down above, its two chief materials, charcoal and bitumen, being furnished by the vegetable and animal kingdoms, both of the land and of the sea, have formed with one another a new combination, by the action of subterraneous heat; but have also, in some cases, been separated by that same action, where the degree of compression necessary for their union, happened to be wanting. The carbonic part, when thus separated from the bituminous, forms an infusible coal, which burns without flame: the bituminous part, when separated from the carbonic, is found in the various states of naphtha, petroleum, asphaltes, and jet.
The great resemblance of infusible or blind coal, to the residuum obtained by the distillation of bituminous coal; and again, the coincidence of the bitumens just named, with the volatile part, or the matter brought over by such distillation, are strong arguments in favour of this theory. The other facts in the natural history of coal, serve to confirm the same conclusion; but it must be confessed, that what we know of the pure bitumens, except the circumstance just mentioned, is of a more ambiguous nature, and may be reconciled with different theories. The drops of petroleum contained within the cavities of the limestone, mentioned at § 31, are however strong facts in confirmation of Dr Hutton's opinions, and they are furnished by the substances purely bituminous. A careful examination would probably make us acquainted with others of the same kind, for limestone is very often the matrix in which petroleum and asphaltes are contained. The greatest mine of asphaltes in Europe, that in the Val de Travers in the territory of Neufchâtel, is in limestone, from which, though it in some places exudes, it is in general extracted by the application of heat. The strata for several leagues are impregnated with bitumen; and, if examined with attention, would probably afford specimens similar to those which have just been mentioned.
174. It is a general remark, that, where petroleum is found, on digging deeper, they come to asphaltes; and, at a depth still greater, they discover coal. This probably does not hold invariably; but it is certain, that most of the fountains of petroleum are in the neighbourhood of coal strata. Petroleum and asphaltes are found in great abundance in Alsace, in a bed of sand, between two beds of clay or argillaceous schistus, and the same country also affords coal.[81] This is true likewise of the fossil pitch of Coal-Brookdale; and of the petroleum found in St Catharine's Well, near Edinburgh. Auvergne[82] contains abundance of fossil pitch, which exudes, in the warm season, from a rock impregnated with it through its whole mass. There are also coal strata in the same country, not far distant.
[81] Encyclopédia, mot, Asphalte.
[82] Voyage en Auvergne, par Lagrand, tom. i. p. 351.
A very satisfactory observation relating to this subject, has lately been communicated from a country, with whose natural history we were till of late entirely unacquainted. In the Burmha empire, petroleum is dug up in an argillaceous earth, from the depth of seventy cubits. This argillaceous earth, or schistus, lies under a bed of freestone; and under all, about one hundred and thirty cubits from the surface, is a bed of coal.[83]
[83] Asiatic Researches, vol. vi, art. 6. p. 130.
175. In the petroleum lake of the Island of Trinidad, described Phil Trans. 1789, the petroleum evidently exudes from the rock, and is collected in a variety of springs in the bottom, after which it hardens, and acquires the consistency of pitch. The manner, therefore, in which petroleum exists in the strata, is very consistent with the idea of its having been introduced in the form of a hot vapour.