268. It has been objected to the supposition of coal having its bituminous part driven off by the heat of the whinstone, that this ought not, on Dr Hutton's principles, to happen in the mineral regions. But it may be replied, as has been done above, that the local application of heat might certainly produce this effect, and might drive off the volatile ports from a hotter to a colder part of the same stratum. The bitumen has not been so volatilized and expanded as entirely to escape from the mineral regions; but it has been expelled from some parts of a mass, only to be condensed and concentrated in others. This supposition coincides exactly with the appearances.
269. The native or fossil coke which accompanies whinstone, has been distinguished into two varieties. The first is the most common, in which, though the coal is perfectly charred, it is solid, and breaks with a smooth and shining surface. The second is also perfect charcoal, but is very porous and spongy. This substance is much rarer than the other. Dr Hutton mentions an instance of it at the mouth of the river Ayr, where there is a whinstone dike.[137] I had the satisfaction of visiting it along with him. It was in the bed of the river, below the high water mark; the specimens had the exact appearance of a cinder.
[137] Theory of the Earth, vol. i. p. 611.
In the banks of the same river, some miles higher up, he found a piece of coal, belonging to a regular stratum, involved in whinstone, and extremely incombustible. It consumed very slowly in the fire, and deflagrated with nitre like plumbago. This be considered as the same fossil which has been described under the name of plombagine. Near it, and connected with the same vein of whinstone, was a real and undoubted plumbago.
From these circumstances he also concluded, that plumbago is the extreme of a gradation, of which fossil-coal is the beginning, and is nothing else than this last reduced to perfect charcoal This agrees with the chemical analysis, which shows plumbago to be composed of carbon, combined with iron.
In confirmation of this theory, he mentions a specimen, in his possession, of steatical whinstone, from Cumberland, containing nodules of a very perfect and beautiful plumbago; and he also takes notice of a mine of this last, in Ayrshire, which, on the authority of Dr Kennedy, who has examined it with great care, I can state as being contained, or enveloped in whinstone; and I hope the public will soon be favoured with a particular description of this very interesting spot, by the same ingenious and accurate observer.
270. Thus the mineralogical and chemical discoveries agree in representing coal, blind coal, plombagine, plumbago, as all modifications of the same substance, and as exhibiting the same principle, carbon, in a state of greater or less combination. As the last and highest term of this series should be placed the diamond; but we are yet unacquainted with the matrix of this curious fossil, and its geological relation to other minerals. When known, they will probably give to this substance the same place in the geological, as in the chemical arrangement: in the mean time, it is hardly necessary to remark, how well all the preceding facts agree with the hypothesis of the igneous formation of whinstone, and how anomalous and unconnected they appear, according to every other theory.
271. Notwithstanding all this accumulated and unanswerable evidence for the igneous formation of basaltes, a great objection would still remain to our theory, were it not for the very accurate and conclusive experiments concerning the fusion of this fossil, referred to above, § 75. A strong prejudice against the production of any thing like a real stone by means of fusion, had arisen, even among those mineralogists, who were every day witnesses of the stony appearance assumed by volcanic lava. They still maintained, on the authority of their own imperfect experiments, that nothing but glass can ever be obtained by the melting of earths or of stones, in whatever manner they are combined.
An ingenious naturalist, after describing a block of basaltes, in which he discovered such appearances, as inclined him to admit its igneous consolidation, rejects that hypothesis, merely from the imaginary inability of fire to give to any substance a stony character: "Quelque mélange,"says he, "de terres que l'on suppose, quelque soit le degré de feu que l'on imagine, quelque soit le tems que l'on emploie, il est très certain que l'on n'obtiendra pas, par le seul fluide igné, ni basalte, ni rien qui lui ressemble."[138]
[138] Journal de Phys. tom. xlix. (1799,) p. 36.