The Botanic Garden, a Poem in Two Parts. Part 1: the Economy of Vegetation

by Erasmus Darwin

The mountain, into which this canal enters, consists of siliceous sand, in which however a few marine productions, apparently in their recent state, have been found, and are now in the possession of Mr. WILLIAM REYNOLDS of Ketly Bank. About three hundred yards from the entrance into the mountain, and about twenty-eight yards below the surface of it, the tar is found oozing from the sand-rock above into the top and sides of the canal.

Beneath the level of this canal a shaft has been sunk through a grey argillaceous substance, called in this country clunch, which is said to be a pretty certain indication of coal; beneath this lies a stratum of coal, about two or three inches thick, of an inferior kind, yielding little flame in burning, and leaving much ashes; below this is a rock of a harder texture; and beneath this are found coals of an excellent quality; for the purpose of procuring which with greater facility the canal, or horizontal aperture, is now making into the mountain. July, 1788.

Beneath these coals in some places is found salt water, in other parts of the adjacent country there are beds of iron-stone, which also contain some bitumen in a less fluid state, and which are about on a level with the new canal, into which the fossil tar oozes, as above described.

There are many interesting circumstances attending the situation and accompaniments of this fountain of fossil tar, tending to develop the manner of its production. 1. As the canal passing into the mountain runs over the beds of coals, and under the reservoir of petroleum, it appears that a natural distillation of this fossil in the bowels of the earth must have taken place at some early period of the world, similar to the artificial distillation of coal, which has many years been carried on in this place on a smaller scale above ground. When this reservoir of petroleum was cut into, the slowness of its exsudation into the canal was not only owing to its viscidity, but to the pressure of the atmosphere, or to the necessity there was that air should at the same time insinuate itself into the small cavities from which the petroleum descended. The existence of such a distillation at some antient time is confirmed by the thin stratum of coal beneath the canal, (which covers the hard rock,) having been deprived of its fossil oil, so as to burn without flame, and thus to have become a natural coak, or fossil charcoal, while the petroleum distilled from it is found in the cavities of the rock above it.

There are appearances in other places, which favour this idea of the natural distillation of petroleum, thus at Matlock in Derbyshire a hard bitumen is found adhering to the spar in the clefts of the lime-rocks in the form of round drops about the size of peas; which could perhaps only be deposited there in that form by sublimation.

2. The second deduction, which offers itself, is, that these beds of coal have been exposed to a considerable degree of heat, since the petroleum above could not be separated, as far as we know, by any other means, and that the good quality of the coals beneath the hard rock was owing to the impermeability of this rock to the bituminous vapour, and to its pressure being too great to permit its being removed by the elasticity of that vapour. Thus from the degree of heat, the degree of pressure, and the permeability of the superincumbent strata, many of the phenomena attending coal-beds receive an easy explanation, which much accords with the ingenious theory of the earth by Dr. Hutton, Trans. of Edinb. Vol. I.

In some coal works the fusion of the strata of coal has been so slight, that there remains the appearance of ligneus fibres, and the impression of leaves, as at Bovey near Exeter, and even seeds of vegetables, of which I have had specimens from the collieries near Polesworth in Warwickshire. In some, where the heat was not very intense and the incumbent stratum not permeable to vapour, the fossil oil has only risen to the upper part of the coal-bed, and has rendered that much more inflammable than the lower parts of it, as in the collieries near Beaudesert, the seat of the EARL OF UXBRIDGE in Staffordshire, where the upper stratum is a perfect cannel, or candle-coal, and the lower one of an inferior quality. Over the coal-beds near Sir H. HARPUR'S house in Derbyshire a thin lamina of asphaltum is found in some places near the surface of the earth, which would seem to be from a distillation of petroleum from the coals below, the more fluid part of which had in process of time exhaled, or been consolidated by its absorption of air. In other coal-works the upper part of the stratum is of a worse kind than the lower one, as at Alfreton and Denbigh in Derbyshire, owing to the supercumbent stratum having permitted the exhalation of a great part of the petroleum; whilst at Widdrington in Northumberland there is first a seam of coal about six inches thick of no value, which lies under about four fathom of clay, beneath this is a white freestone, then a hard stone, which the workmen there call a whin, then two fathoms of clay, then another white stone, and under that a vein of coals three feet nine inches thick, of a similar nature to the Newcastle coal. Phil. Trans. Abridg. Vol. VI. plate II. p. 192. The similitude between the circumstances of this colliery, and of the coal beneath the fountain of tar above described, renders it highly probable that this upper thin seam of coal has suffered a similar distillation, and that the inflammable part of it had either been received into the clay above in the form of sulphur, which when burnt in the open air would produce alum; or had been dissipated for want of a receiver, where it could be condensed. The former opinion is perhaps in this case more probable as in some other coal-beds, of which I have procured accounts, the surface of the coal beneath clunch or clay is of an inferior quality, as at West Hallum in Nottinghamshire. The clunch probably from hence acquires its inflammable part, which on calcination becomes vitriolic acid. I gathered pieces of clunch converted partially into alum at a colliery near Bilston, where the ground was still on fire a few years ago.

The heat, which has thus pervaded the beds of morass, seems to have been the effect of the fermentation of their vegetable materials; as new hay sometimes takes fire even in such very small masses from the sugar it contains, and seems hence not to have been attended with any expulsion of lava, like the deeper craters of volcanos situated in beds of granite.

3. The marine shells found in the loose sand-rock above this reservoir of petroleum, and the coal-beds beneath it, together with the existence of sea-salt beneath these coals, prove that these coal beds have been at the bottom of the sea, during some remote period of time, and were afterwards raised into their present situation by subterraneous expansions of vapour. This doctrine is further supported by the marks of violence, which some coal-beds received at the time they were raised out of the sea, as in the collieries at Mendip in Somersetshire. In these there are seven strata of coals, equitant upon each other, with beds of clay and stone intervening; amongst which clay are found shells and fern branches. In one part of this hill the strata are disjoined, and a quantity of heterogeneous substances fill up the chasm which disjoins them, on one side of this chasm the seven strata of coal are seen corresponding in respect to their reciprocal thickness and goodness with the seven strata on the other side of the cavity, except that they have been elevated several yards higher. Phil. Trans. No. 360. abridg. Vol. V. p. 237.

The cracks in the coal-bed near Ticknall in Derbyshire, and in the sand- stone rock over it, in both of which specimens of lead-ore and spar are found, confirm this opinion of their having been forcibly raised up by subterraneous fires. Over the colliery at Brown-hills near Lichfield, there is a stratum of gravel on the surface of the ground; which may be adduced as another proof to shew that those coals had some time been beneath the sea, or the bed of a river. Nevertheless, these arguments only apply to the collieries above mentioned, which are few compared with those which bear no marks of having been immersed in the sea.