Fig. 9.—Pecopteris Adiantoides.
Fig. 10.—Sphenopteris Affinis.
Fossil Ferns found in the Coal Measures of Europe and America.
The next important fact that points to the vegetable origin of Coal is, that wood and Coal are both composed of the same ultimate elements—carbon, hydrogen, and oxygen. This analogy is the more remarkable when we are told that no other rock except Coal exhibits anything approaching to this composition. It is true that the elements just enumerated do not exist in the same proportions in wood and in Coal. But the difference, when rightly understood, rather tends to confirm our theory that the one is derived from the other. There is more Carbon in Coal than in wood; while there is less oxygen and less hydrogen. To explain how this may have come to pass during the process of transition, we must call in the assistance of the chemist. It appears from the researches of Liebig that, when vegetable matter is buried in the earth, exposed to moisture, and partially or entirely excluded from the air, the process of decomposition sets in, and that under this process carbonic acid gas and carburetted hydrogen gas are slowly evolved. At the same time a portion of the oxygen when set free would naturally enter into a new combination with a portion of the hydrogen, and form water. The result of these several changes would necessarily be, that the accumulation of vegetable matter buried in the earth would part, in course of time, with no small share of its carbon, its hydrogen, and its oxygen, but not with all in the same proportions: for the new combinations would use up more of the oxygen than of the hydrogen, and more of the hydrogen than of the carbon.[58] In other words, if the process should have gone on for a sufficient lapse of ages, these elements would no longer exist together in the proportions which are necessary to constitute wood, but would rather exist in the proportions which are found to constitute coal.[59]
This explanation is confirmed by a fact with which our readers are no doubt familiar. According to the explanation, carbonic acid and carburetted hydrogen are evolved during the process by which coal is produced from wood. We should therefore expect to find these gases closely associated with Coal. If they are not so associated, their absence is a serious objection against our theory; but if they are so associated, their presence is a strong evidence in its favor. Now on this point, as every one knows, practical miners bear testimony that the fact corresponds exactly with our theory. They tell us that reservoirs of Choke-damp, which is carbonic acid, and of Fire-damp, which is carburetted hydrogen, are found very commonly pent up in the crevices and cavities of coal beds, and are the cause, when tapped, of many of the accidents which take place. They even assure us that some beds of coal are so saturated with gas that, when cut into, it may be heard oozing from every pore of the rock, and the coal is called singing coal by the colliers.[60]
To sum up, then, what we have said on this point: it appears, first, that the same constituent elements are found in wood and Coal; secondly, though they do not exist in the same proportions in the two substances, the difference is fully accounted for by the changes which we should naturally expect to take place when large accumulations of vegetable matter are buried in the earth; thirdly, in the hypothesis of these changes, carbonic acid and carburetted hydrogen would certainly be developed; and in point of fact, these gases are found intimately associated with Coal all over the world.
There is another remarkable fact which fits in most admirably with our theory. Coal is found at the present day in the Crust of the Earth, disposed in thin seams or beds, and each bed is almost uniformly found to rest upon a stratum of fine clay, sometimes several feet in thickness. This is just what our theory would lead us to expect. If coal is produced from plants and trees, these plants and trees must have grown upon some suitable soil; and, therefore, in this hypothesis we should expect, ordinarily speaking at least, to find a bed of clay beneath every bed of coal. But this is not all. When we examine more closely the stratum on which the coal reposes, we find the roots and stems of trees mingled with the clay in the greatest profusion. In the Welsh coal field, in a depth of twelve thousand feet, there are from fifty to a hundred beds of coal, each lying on a stratum of clay abounding in these remains.[61]
We now come to an argument of a practical kind which appeals to common sense and common experience. Let us suppose that a person wholly unacquainted with the art of manufacturing paper, were to enter a paper-mill when the workmen are away, and the process of manufacture for a time suspended. At first sight he would probably find it difficult to persuade himself, that the piles of clean white paper, which attract his notice at one end of the building, are produced from the heaps of filthy rags which he sees accumulated at the other. But if he be a sagacious observer, he will soon find evidence to convince him that this is really the case. For he will perceive, upon close examination, that the self-same material is exhibited in every intermediate state of progress from one extreme to the other. First, there is the great chest with its numerous compartments, in which the rags are seen carefully sorted, according to their various degrees of quality and texture. Next comes the fulling-mill, where they are washed and bleached. Then the revolving cylinder, furnished on the exterior surface with sharp blades or cutters; and the vat in which it moves is filled with the rags, which now assume the form of a thin liquid pulp. Advancing still further he will see this pulp evenly spread out upon a wire-gauze frame, and now at last it is beginning to exhibit some likeness to the form and substance of paper. Further on it is seen pressed and dried; and last of all cut into sheets and laid aside in lofty piles.
Now it seems to us that we are placed in somewhat of the same position, as regards the manufacture of Coal. We cannot observe the process actually going on; for though, in this process, the work is never suspended, the workmen never at rest, yet extending as it does over a space of many centuries, it is too slow to be sensible; and besides it is conducted in great part beneath the surface of the Earth. Nevertheless, we can trace the progress of change through each intermediate stage of the transition, from one extreme to the other,—from the primeval swamps and forests through the numerous varieties of the Peat and Lignite to the richest beds of pure Coal.