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_. [Footnote 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.