Part 6 (2/2)
In favourable specimens, again, almost the whole ground substance appears to be made up of similar bodies--more or less carbonized or blackened--and, in these, there can be no doubt that, with the exception of patches of mineral charcoal, here and there, the whole ma.s.s of the coal is made up of an acc.u.mulation of the larger and of the smaller sacs.
But, in one and the same slice, every transition can be observed from this structure to that which has been described as characteristic of ordinary coal. The latter appears to rise out of the former, by the breaking-up and increasing carbonization of the larger and the smaller sacs. And, in the anthracitic coals, this process appears to have gone to such a length, as to destroy the original structure altogether, and to replace it by a completely carbonized substance.
Thus coal may be said, speaking broadly, to be composed of two const.i.tuents: firstly, mineral charcoal; and, secondly, coal proper.
The nature of the mineral charcoal has long since been determined. Its structure shows it to consist of the remains of the stems and leaves of plants, reduced to little more than their carbon. Again, some of the coal is made up of the crushed and flattened bark, or outer coat, of the stems of plants, the inner wood of which has completely decayed away. But what I may term the ”saccular matter” of the coal, which, either in its primary or in its degraded form, const.i.tutes by far the greater part of all the bituminous coals I have examined, is certainly not mineral charcoal; nor is its structure that of any stem or leaf.
Hence its real nature is, at first, by no means apparent, and has been the subject of much discussion.
The first person who threw any light upon the problem, as far as I have been able to discover, was the well-known geologist, Professor Morris. It is now thirty-four years since he carefully described and figured the coin-shaped bodies, or larger sacs, as I have called them, in a note appended to the famous paper ”On the Coal-brookdale Coal-Field,” published at that time, by the present President of the Geological Society, Mr. Prestwich. With much sagacity, Professor Morris divined the real nature of these bodies, and boldly affirmed them to be the spore-cases of a plant allied to the living club-mosses.
But discovery sometimes makes a long halt; and it is only a few years since Mr. Carruthers determined the plant (or rather one of the plants) which produces these spore-cases, by finding the discoidal sacs still adherent to the leaves of the fossilized cone which produced them. He gave the name of _Flemingites gracilis_ to the plant of which the cones form a part. The branches and stem of this plant are not yet certainly known, but there is no sort of doubt that it was closely allied to the _Lepidodendron_, the remains of which abound in the coal formation. The _Lepidodendra_ were shrubs and trees which put one more in mind of an _Araucaria_ than of any other familiar plant; and the ends of the fruiting branches were terminated by cones, or catkins, somewhat like the bodies so named in a fir, or a willow.
These conical fruits, however, did not produce seeds; but the leaves of which they were composed bore upon their surfaces sacs full of spores or sporangia, such as those one sees on the under surface of a bracken leaf. Now, it is these sporangia of the Lepidodendroid plant _Flemingites_ which were identified by Mr. Carruthers with the free sporangia described by Professor Morris, which are the same as the large sacs of which I have spoken. And, more than this, there is no doubt that the small sacs are the spores, which were originally contained in the sporangia.
The living club-mosses are, for the most part, insignificant and creeping herbs, which, superficially, very closely resemble true mosses, and none of them reach more than two or three feet in height.
But, in their essential structure, they very closely resemble the earliest Lepidodendroid trees of the coal: their stems and leaves are similar; so are their cones; and no less like are the sporangia and spores; while even in their size, the spores of the _Lepidodendron_ and those of the existing _Lycopodium_, or club-moss, very closely approach one another.
Thus, the singular conclusion is forced upon us, that the greater and the smaller sacs of the ”Better-Bed” and other coals, in which the primitive structure is well preserved, are simply the sporangia and spores of certain plants, many of which were closely allied to the existing club-mosses. And if, as I believe, it can be demonstrated that ordinary coal is nothing but ”saccular” coal which has undergone a certain amount of that alteration which, if continued, would convert it into anthracite; then, the conclusion is obvious, that the great ma.s.s of the coal we burn is the result of the acc.u.mulation of the spores and spore-cases of plants, other parts of which have furnished the carbonized stems and the mineral charcoal, or have left their impressions on the surfaces of the layer.
Of the mult.i.tudinous speculations which, at various times, have been entertained respecting the origin and mode of formation of coal, several appear to be negatived, and put out of court, by the structural facts the significance of which I have endeavoured to explain. These facts, for example, do not permit us to suppose that coal is an acc.u.mulation of peaty matter, as some have held.
Again, the late Professor Quekett was one of the first observers who gave a correct description of what I have termed the ”saccular”
structure of coal; and, rightly perceiving that this structure was something quite different from that of any known plant, he imagined that it proceeded from some extinct vegetable organism which was peculiarly abundant amongst the coal-forming plants. But this explanation is at once shown to be untenable when the smaller and the larger sacs are proved to be spores or sporangia.
Some, once more, have imagined that coal was of submarine origin; and though the notion is amply and easily refuted by other considerations, it may be worth while to remark, that it is impossible to comprehend how a ma.s.s of light and resinous spores should have reached the bottom of the sea, or should have stopped in that position if they had got there.
At the same time, it is proper to remark that I do not presume to suggest that all coal must needs have the same structure; or that there may not be coals in which the proportions of wood and spores, or spore-cases, are very different from those which I have examined. All I repeat is, that none of the coals which have come under my notice have enabled me to observe such a difference. But, according to Princ.i.p.al Dawson, who has so sedulously examined the fossil remains of plants in North America, it is otherwise with the vast acc.u.mulations of coal in that country.
”The true coal,” says Dr. Dawson, ”consists princ.i.p.ally of the flattened bark of Sigillarioid and other trees, intermixed with leaves of Ferns and _Cordaites_, and other herbaceous _debris_, and with fragments of decayed wood, const.i.tuting 'mineral charcoal,' all these materials having manifestly alike grown and acc.u.mulated where we find them.”[1]
[Footnote 1: ”Acadian Geology,” 2nd edition, p. 138.]
When I had the pleasure of seeing Princ.i.p.al Dawson in London last summer, I showed him my sections of coal, and begged him to re-examine some of the American coals on his return to Canada, with an eye to the presence of spores and sporangia, such as I was able to show him in our English and Scotch coals. He has been good enough to do so; and in a letter dated September 26th, 1870, he informs me that--
”Indications of spore-cases are rare, except in certain coa.r.s.e shaly coals and portions of coals, and in the roofs of the seams. The most marked case I have yet met with is the shaly coal referred to as containing _Sporangites_ in my paper on the conditions of acc.u.mulation of coal (_Journal of the Geological Society_, vol. xxii. pp. 115, 139, and 165). The purer coals certainly consist princ.i.p.ally of cubical tissues with some true woody matter, and the spore-cases, &c., are chiefly in the coa.r.s.e and shaly layers. This is my old doctrine in my two papers in the _Journal of the Geological Society_, and I see nothing to modify it. Your observations, however, make it probable that the frequent _clear spots_ in the cannels are spore-cases.”
Dr. Dawson's results are the more remarkable, as the numerous specimens of British coal, from various localities, which I have examined, tell one tale as to the predominance of the spore and sporangium element in their composition; and as it is exactly in the finest and purest coals, such as the ”Better-Bed” coal of Lowmoor, that the spores and sporangia obviously const.i.tute almost the entire ma.s.s of the deposit.
Coal, such as that which has been described, is always found in sheets, or ”seams,” varying from a fraction of an inch to many feet in thickness, enclosed in the substance of the earth at very various depths, between beds of rock of different kinds. As a rule, every seam of coal rests upon a thicker, or thinner, bed of clay, which is known as ”under-clay.” These alternations of beds of coal, clay, and rock may be repeated many times, and are known as the ”coal-measures;”
and in some regions, as in South Wales and in Nova Scotia, the coal-measures attain a thickness of twelve or fourteen thousand feet, and enclose eighty or a hundred seams of coal, each with its under-clay, and separated from those above and below by beds of sandstone and shale.
The position of the beds which const.i.tute the coal-measures is infinitely diverse. Sometimes they are tilted up vertically, sometimes they are horizontal, sometimes curved into great basins; sometimes they come to the surface, sometimes they are covered up by thousands of feet of rock. But, whatever their present position, there is abundant and conclusive evidence that every under-clay was once a surface soil. Not only do carbonized root-fibres frequently abound in these under-clays; but the stools of trees, the trunks of which are broken off and confounded with the bed of coal, have been repeatedly found pa.s.sing into radiating roots, still embedded in the under-clay.
On many parts of the coast of England, what are commonly known as ”submarine forests” are to be seen at low water. They consist, for the most part, of short stools of oak, beech, and fir trees, still fixed by their long roots in the bed of blue clay in which they originally grew. If one of these submarine forest beds should be gradually depressed and covered up by new deposits, it would present just the same characters as an under-clay of the coal, if the _Sigillaria_ and _Lepidodendron_ of the ancient world were subst.i.tuted for the oak, or the beech, of our own times.
In a tropical forest, at the present day, the trunks of fallen trees, and the stools of such trees as may have been broken by the violence of storms, remain entire for but a short time. Contrary to what might be expected, the dense wood of the tree decays, and suffers from the ravages of insects, more swiftly than the bark. And the traveller, setting his foot on a prostrate trunk, finds that it is a mere sh.e.l.l, which breaks under his weight, and lands his foot amidst the insects, or the reptiles, which have sought food or refuge within.
The trees of the coal forests present parallel conditions. When the fallen trunks which have entered into the composition of the bed of coal are identifiable, they are mere double sh.e.l.ls of bark, flattened together in consequence of the destruction of the woody core; and Sir Charles Lyell and Princ.i.p.al Dawson discovered, in the hollow stools of coal trees of Nova Scotia, the remains of snails, millipedes, and salamander-like creatures, embedded in a deposit of a different character from that which surrounded the exterior of the trees. Thus, in endeavouring to comprehend the formation of a seam of coal, we must try to picture to ourselves a thick forest, formed for the most part of trees like gigantic club-mosses, mares-tails, and tree ferns, with here and there some that had more resemblance to our existing yews and fir-trees. We must suppose that, as the seasons rolled by, the plants grew and developed their spores and seeds; that they shed these in enormous quant.i.ties, which acc.u.mulated on the ground beneath; and that, every now and then, they added a dead frond or leaf; or, at longer intervals, a rotten branch, or a dead trunk, to the ma.s.s.
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