Origin of coal—Evident traces of plants and trees in coal-mines—Coal made up of the same elements as wood—Beds of coal found resting upon clay in which are preserved the roots of trees—Insensible transition from wood to coal—Forest-covered swamps—Accumulations of drift-wood in lakes and estuaries—Peat bogs—Beds of Lignite—Seams of pure coal with half-carbonized trees, some lying prostrate, some standing erect—Summary of the argument hitherto pursued—Objection to this argument from the Omnipotence of God—Answer to the objection.

As animals, by organic action, extract lime from the waters of the ocean they inhabit, which, being converted in the first instance into minute shells, or stony skeletons, afterward passes into a compact and solid rock, so in like manner do plants and trees extract carbon from the atmosphere in which they vegetate, and convert it into coal. No reasonable doubt can now be entertained that coal derives its existence, almost entirely, from the woody tissue of sunken swamps and forests. Though the nature of the process by which this transformation takes place, is yet but imperfectly understood, and is, indeed, at the present moment a subject of much discussion and controversy, nevertheless the fact that the change has taken place is fully accepted by all as an established truth, and is supported by an accumulation of evidence which it is not easy to resist.

The first circumstance to which we shall call attention, is the wonderful profusion of vegetable life that is always associated with coal. Every one who has descended at any time into a coal mine, or who has examined the specimens usually exhibited in a well-furnished museum, must have been struck by the countless forms of trees and plants, which still remain vividly impressed on this black and unsightly mineral. Dr. Buckland has described this phenomenon with much vigor and beauty in his celebrated Bridgewater Treatise: “The finest example I have ever witnessed is that of the coal mines of Bohemia just mentioned. The most elaborate imitations of living foliage upon the painted ceilings of Italian palaces, bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung in wild irregular profusion over every portion of its surface. The effect is heightened by the contrast of the coal-black color of these vegetables with the light ground-work of the rock to which they are attached. The spectator feels himself transported, as if by enchantment, into the forests of another world; he beholds trees of forms and characters now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigor of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation, which began and terminated in times of which these relics are the infallible historians.”

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.

First, then, we have the great forest-covered swamps, like those which now occupy the valley and delta of the Mississippi. They are composed in many cases of pure vegetable matter without any intermixture of earthy sediment. A dense growth of reeds, and shrubs, and herbage of every kind, covers the whole surface of the land, mixed up with the decaying leaves and prostrate trunks of forest-trees. Sir Charles Lyell mentions a very remarkable fact observed in the swamps of Louisiana. During an unusually hot season, when any part of a swamp is dried up, if the surface be set on fire, a pit is burned into the ground many feet deep, in fact, as far down as the fire can descend without meeting water; and it is then found that scarcely any residuum or earthy matter is left.62

Vegetable strata of this kind are produced, not only upon dry land by the growth and decay of forests, but also beneath the waters of lakes and estuaries, by the accumulation of Drift-timber borne along in the current of swollen rivers. The Mackenzie River, which drains a great part of Northwestern America, affords many admirable illustrations. Flowing as it does from south to north, it is subject to annual inundations when the snows begin to melt in the higher parts of its course, while the channel lower down, situated in colder latitudes, is still blocked up with ice. At this season then it overflows its banks, and sweeping through vast forests, carries away thousands of uprooted trees in its impetuous torrent.

“As the trees,” says Dr. Richardson, “retain their roots, which are often loaded with earth and stones, they readily sink, especially when water-soaked; and accumulating in the eddies, form shoals, which ultimately augment into islands. A thicket of small willows covers the new-formed island as soon as it appears above water, and their fibrous roots serve to bind the whole firmly together. Sections of these islands are annually made by the river; and it is interesting to study the diversities of appearances they present according to their different ages. The trunks of the trees gradually decay until they are converted into a blackish-brown substance resembling peat, but still retaining more or less of the fibrous structure of the wood; and layers of this often alternate with layers of clay and sand, the whole being penetrated, to a depth of four or five yards or more, by the long fibrous roots of the willows. A deposition of this kind, with the aid of a little infiltration of bituminous matter, would produce an excellent imitation of Coal, with vegetable impressions of the willow roots.

“It was in the rivers only that we could observe sections of these deposits; but the same operation goes on, on a much more magnificent scale, in the lakes. A shoal of many miles in extent is formed on the south side of Athabasca Lake by the Drift-timber and vegetable dÉbris brought down by the Elk River; and the Slave Lake itself must in process of time be filled up by the matters daily conveyed into it from Slave River. Vast quantities of Drift-timber are buried under the sand at the mouth of the river, and enormous piles of it are accumulated on the shores of every part of the lake.”

Not unfrequently it happens that these strata of vegetable matter, with the roots and trunks of trees, their branches, fruits, and leaves, more or less perfectly preserved, are covered over by subsequent deposits. Such accumulations, we are assured by Doctor Mantell, have been found deep in the soil on the coast of England, in places that are still subject to periodical inundations. “The trees are chiefly of the oak, hazel, fir, birch, yew, willow, and ash; in short, almost every kind that is indigenous to this island occasionally occurs. The trunks and branches are dyed throughout of a deep ebony color by iron; and the wood is firm and heavy, and occasionally fit for domestic use; in Yorkshire and elsewhere, timber of this kind is sometimes employed in the construction of houses.”63 Here, then, is the first stage of the conversion of wood into Coal,—a stratum more or less compacted together of vegetable matter, spread out sometimes over the surface of the dry land, sometimes on the floor of lakes and estuaries, and often buried beneath an accumulation of subsequent deposits.

The next stage in the process of transformation may be represented by those Peat Bogs which constitute one of the most remarkable physical characteristics of Ireland, covering as they do an area equal to one-tenth of the whole island. In these the vegetable matter is more closely condensed, but the structure of the plants from which the Peat is derived is still preserved, and may be distinctly recognized by the naked eye. Nay, we have still the prostrate trunks of trees lying around on every side as they fell to the ground in their ancient forests. The researches recently pursued upon this subject have brought to light a fact which is very much to our present purpose; for it seems to prove our thesis by direct evidence. “In Limerick, in the district of Maine, one of the States of North America, there are Peat Bogs of considerable extent, in which a substance exactly similar to cannel coal is found at the depth of three or four feet from the surface amidst the remains of rotten logs of wood and beaver sticks: the peat is twenty feet thick, and rests upon white sand. This coal was discovered on digging a ditch to drain a portion of the bog, for the purpose of obtaining peat for manure. The substance is a true bituminous coal, containing more bitumen than is found in any other variety. Polished sections of the compact masses exhibit the peculiar structure of coniferous trees, and prove that the coal was derived from a species allied to the American Fir.”64 A similar phenomenon was observed by Doctor Dieffenbach in the Chathain Islands. In the same bed of peat he was able distinctly to trace a gradual transition from pure vegetable matter to a mineral substantially identical with common coal.65

But though Peat may thus, as it should seem, pass directly into pure Coal, there are many cases in which it first assumes a more imperfect form, known under the name of Lignite. This substance is described as of a brownish color, “soft and mellow in consistence when freshly quarried, but becoming brittle by exposure, the fracture following the direction of the fibre of the wood.”66 It clearly occupies an intermediate position between Peat and Coal. Like the former, it still exhibits the stems and woody fibre of the plants from which it is derived, very little altered in their structure; while on the other hand it is already beginning to acquire some of the consistency and density of Coal; to which also it approaches much more closely in its chemical composition. It should be remembered, moreover, that Lignite does not designate a substance of a fixed, invariable character. On the contrary, under the one general name are comprised a definite number of varieties, leading from one extreme to the other by a series of almost insensible gradations; the extreme variety on one side being scarcely distinguishable from Peat, while the extreme variety on the other is practically identical with ordinary Coal. It can hardly be doubted, therefore, that Coal must have the same origin as Lignite, while it is at least equally certain that Lignite has been derived from Peat; and we have already seen what overwhelming evidence may be adduced to show that the origin of Peat is to be sought for in the sunken swamps and forests of a long past age.

Lastly, when we come to examine the texture of Coal itself, we find much to confirm the conclusion at which we have thus arrived. In beds of pure Coal the remains of many species of plants have been detected, and sometimes in such abundance as to constitute visibly the bulk of the Coal. Even large trees are sometimes found standing erect in the Coal fields, with their bark actually converted into this mineral. The annexed Figure represents a portion of the stem, together with the roots of a tall forest tree, Sigillaria, discovered not long ago in a Coal mine at Saint Helens, near Liverpool. The stem, which was nine feet high, was found erect in the seam of Coal, while the roots, ten in number, stretched away into the vegetable soil beneath.

Not less than thirty such trees, some of them four or five feet in diameter, and all incrusted with Coal, were laid bare a short time since, in a Colliery near Newcastle, within an area of fifty yards square. “In 1830,” writes Sir Charles Lyell, “a slanting trunk was exposed in Craigleith quarry, near Edinburgh, the total length of which exceeded sixty feet. Its diameter at the top was about seven inches, and near the base, it measured five feet in its greater, and two feet in its lesser, width. The bark was converted into a thin coating of the purest and finest Coal.” Again, “in South Staffordshire, a seam of Coal was laid bare in the year 1844, in what is called an open work at Parkfield Colliery, near Wolverhampton. In the space of about a quarter of an acre, the stumps of no less than seventy-three trees, with their roots attached, appeared, some of them more than eight feet in circumference. The trunks, broken off close to the root, were lying prostrate in every direction, often crossing each other. One of them measured fifteen, another thirty feet in length, and others less. They were invariably flattened to the thickness of one or two inches, and converted into Coal. Their roots formed part of a stratum of Coal ten inches thick, which rested on a layer of clay two inches thick, below which was a second forest resting on a two-foot seam of Coal. Five feet below this again was a third forest, with large stumps of Lepidodendra, Calamites, and other trees.”67

We have now brought to a close a very important line of argument in the Science of Geology. We have pointed out that, in the strata which compose the Crust of the Earth, there are rocks of various kinds, distinguished from one another as well by the nature of the materials which compose them, as by the manner in which these materials are arranged together; and we have shown that rocks presenting the same general appearances, and composed of exactly the same materials, are being produced in the present age upon the Surface of the Earth, through the agency of natural causes. Moreover, we have closely examined, in certain cases, the nature of the process by which the formation of these rocks is accomplished at the present day; and we have seen how difficult it is, when the facts of the case are once clearly before us, to resist the conclusion that the rocks which we now find buried in the Earth, were produced in some former age, by the same causes which are still at work. We shall next proceed to inquire how far this conclusion is confirmed by the independent evidence of Fossil Remains.

But before entering on a new line of argument, it is fit we should take notice of an objection which has sometimes been urged against the reasoning we have hitherto pursued, and which has done much to create and to keep alive a prejudice unfavorable to the Science of Geology. Religious writers have not unfrequently insinuated, and sometimes have plainly asserted, that, in ascribing the present structure of the Earth’s Crust to the operation of natural causes, Geologists would seem to make no account of God’s Omnipotence. A moment’s reflection will convince the reader that this charge is utterly unphilosophical. Is it not plain that the more fully we appreciate and acknowledge the wonderful works of Nature, the more deeply must we become impressed with the power and wisdom of Him who is the Author and Ruler of Nature? To say that secondary causes exist, and to point out the monuments that bear witness to their operation in long passed ages, is not to deny, but rather to affirm the existence of a Great First Cause, upon whom they all depend for their existence, their preservation, and their guidance.

We are everywhere reminded by abundant evidence, that it has pleased the Great Creator to employ the agency of His creatures in the fashioning and the adorning of this material universe. He does not create at once, as He well might do, the great oak of the forest; but He allows the seed to sink into the earth, where it is watered by the gentle dews of Heaven, and fructified by the genial warmth of the sun; soon it puts forth a tender germ; the germ, in time, imbibing the elements of its support from the air and the earth, becomes a sappling, and the sappling a tree, which spreads its huge branches on every side, and serves for many purposes of ornament and of use. Or let us take the case of the honeycomb, that most curious and ingenious work, at once the palace and the storehouse of a vast and busy community. It is not produced in a moment by a simple act of creation. God has not made it Himself, but He has taught the bee to make it. In like manner He has provided for the little birds, not by building their nests, but by infusing into their nature that mysterious instinct which prompts them to build, and guides them in their work.

Geologists, therefore, when they undertake to explain the existence of Stratified Rocks, not by the immediate action of the Creator, but by the intervention of natural causes, are not on that account to be accused of impiety. They do not disparage, but rather magnify His glory, when they expatiate upon the endless variety of agents which, according to their theory, He has employed in the structure of the material world. If the honeycomb, as a work of contrivance and design, excites the wonder and admiration of the philosopher, what must we think of the contrivance and design exhibited by Him who has made, not the honeycomb only, but the bee that builds the honeycomb? And so, too, we get novel and unexpected views of God’s Omnipotence, when, through the science of Geology, we come to understand the vast and harmonious series of secondary causes by which he has brought the Crust of the Earth into its present form and shape. The impress of His hand is stamped upon His works; and all that is wonderful and attractive in Nature is but the token of His power and the shadow of His beauty. And so our national poet has sung: