THE STORY OF A PIECE OF COAL
WHAT IT IS, WHENCE IT COMES, AND WHITHER IT GOES
BY EDWARD A. MARTIN, F.G.S.
The knowledge of the marvels which a piece of coal possesses within itself, and which in obedience to processes of man's invention it is always willing to exhibit to an observant enquirer, is not so widespread, perhaps, as it should be, and the aim of this little book, this record of one page of geological history, has been to bring together the principal facts and wonders connected with it into the focus of a few pages, where, side by side, would be found the record of its vegetable and mineral history, its discovery and early use, its bearings on the great fog-problem, its useful illuminating gas and oils, the question of the possible exhaustion of British supplies, and other important and interesting bearings of coal or its products.
In the whole realm of natural history, in the widest sense of the term, there is nothing which could be cited which has so benefited, so interested, I might almost say, so excited mankind, as have the wonderful discoveries of the various products distilled from gas-tar, itself a distillate of coal.
Coal touches the interests of the botanist, the geologist, and the physicist; the chemist, the sanitarian, and the merchant.
In the little work now before the reader I have endeavoured to recount, without going into unnecessary detail, the wonderful story of a piece of coal.
I. THE ORIGIN OF COAL AND THE PLANTS OF WHICH IT IS COMPOSED
II. A GENERAL VIEW OF THE COAL-BEARING STRATA
III. VARIOUS FORMS OF COAL AND CARBON
IV. THE COAL-MINE AND ITS DANGERS
V. EARLY HISTORY—ITS USE AND ITS ABUSE
VI. HOW GAS IS MADE—ILLUMINATING OILS AND BYE-PRODUCTS
VII. THE COAL SUPPLIES OF THE WORLD
VIII. THE COAL-TAR COLOURS
CHART SHEWING THE PRODUCTS OF COAL
LIST OF ILLUSTRATIONS.
FIG. 1. Stigmaria " 2. Annularia radiata " 3. Rhacopteris inaequilatera " 4. Frond of Pecopteris " 5. Pecopteris Serlii " 6. Sphenopteris affinis " 7. Catamites Suckowii " 8. Calamocladus grandis " 9. Asterophyllites foliosa " 10. Spenophyllum cuneifolium " 11. Cast of Lepidodendron " 12. Lepidodendron longifolium " 13. Lepidodendron aculeatum " 14. Lepidostrobus " 15. Lycopodites " 16. Stigmaria ficoides " 17. Section of Stigmaria " 18. Sigillarian trunks in sandstone " 19. Productus " 20. Encrinite " 21. Encrinital limestone " 22. Various encrinites " 23. Cyathophyllum " 24. Archegosaurus minor " 25. Psammodus porosus " 26. Orthoceras " 27. Fenestella retepora " 28. Goniatites " 29. Aviculopecten papyraceus " 30. Fragment of Lepidodendron " 31. Engine-house at head of a Coal-Pit " 32. Gas Jet and Davy Lamp " 33. Part of a Sigillarian trunk " 34. Inside a Gas-holder " 35. Filling Retorts by Machinery " 36. "Condensers" " 37. "Washers" " 38. "Purifiers"
THE ORIGIN OF COAL AND THE PLANTS OF WHICH IT IS COMPOSED.
From the homely scuttle of coal at the side of the hearth to the gorgeously verdant vegetation of a forest of mammoth trees, might have appeared a somewhat far cry in the eyes of those who lived some fifty years ago. But there are few now who do not know what was the origin of the coal which they use so freely, and which in obedience to their demand has been brought up more than a thousand feet from the bowels of the earth; and, although familiarity has in a sense bred contempt for that which a few shillings will always purchase, in all probability a stray thought does occasionally cross one's mind, giving birth to feelings of a more or less thankful nature that such a store of heat and light was long ago laid up in this earth of ours for our use, when as yet man was not destined to put in an appearance for many, many ages to come. We can scarcely imagine the industrial condition of our country in the absence of so fortunate a supply of coal; and the many good things which are obtained from it, and the uses to which, as we shall see, it can be put, do indeed demand recognition.
Were our present forests uprooted and overthrown, to be covered by sedimentary deposits such as those which cover our coal-seams, the amount of coal which would be thereby formed for use in some future age, would amount to a thickness of perhaps two or three inches at most, and yet, in one coal-field alone, that of Westphalia, the 117 most important seams, if placed one above the other in immediate succession, would amount to no less than 294 feet of coal. From this it is possible to form a faint idea of the enormous growths of vegetation required to form some of our representative coal beds. But the coal is not found in one continuous bed. These numerous seams of coal are interspersed between many thousands of feet of sedimentary deposits, the whole of which form the "coal-measures." Now, each of these seams represents the growth of a forest, and to explain the whole series it is necessary to suppose that between each deposit the land became overwhelmed by the waters of the sea or lake, and after a long subaqueous period, was again raised into dry land, ready to become the birth-place of another forest, which would again beget, under similarly repeated conditions, another seam of coal. Of the conditions necessary to bring these changes about we will speak later on, but this instance is sufficient to show how inadequate the quantity of fuel would be, were we dependent entirely on our own existing forest growths.
However, we will leave for the present the fascinating pursuit of theorising as to the how and wherefore of these vast beds of coal, relegating the geological part of the study of the carboniferous system to a future chapter, where will be found some more detailed account of the position of the coal-seams in the strata which contain them. At present the actual details of the coal itself will demand our attention.
Coal is the mineral which has resulted, after the lapse of thousands of thousands of years, from the accumulations of vegetable material, caused by the steady yearly shedding of leaves, fronds and spores, from forests which existed in an early age; these accumulated where the trees grew that bore them, and formed in the first place, perhaps, beds of peat; the beds have since been subjected to an ever-increasing pressure of accumulating strata above them, compressing the sheddings of a whole forest into a thickness in some cases of a few inches of coal, and have been acted upon by the internal heat of the earth, which has caused them to part, to a varying degree, with some of their component gases. If we reason from analogy, we are compelled to admit that the origin of coal is due to the accumulation of vegetation, of which more scattered, but more distinct, representative specimens occur in the shales and clays above and below the coal-seams. But we are also able to examine the texture itself of the various coals by submitting extremely thin slices to a strong light under the microscope, and are thus enabled to decide whether the particular coal we are examining is formed of conifers, horse-tails, club-mosses, or ferns, or whether it consists simply of the accumulated sheddings of all, or perhaps, as in some instances, of innumerable spores.
In this way the structure of coal can be accurately determined. Were we artificially to prepare a mass of vegetable substance, and covering it up entirely, subject it to great pressure, so that but little of the volatile gases which would be formed could escape, we might in the course of time produce something approaching coal, but whether we obtained lignite, jet, common bituminous coal, or anthracite, would depend upon the possibilities of escape for the gases contained in the mass.
Everybody has doubtless noticed that, when a stagnant pool which contains a good deal of decaying vegetation is stirred, bubbles of gas rise to the surface from the mud below. This gas is known as marsh-gas, or light carburetted hydrogen, and gives rise to the ignis fatuus which hovers about marshy land, and which is said to lure the weary traveller to his doom. The vegetable mud is here undergoing rapid decomposition, as there is nothing to stay its progress, and no superposed load of strata confining its resulting products within itself. The gases therefore escape, and the breaking-up of the tissues of the vegetation goes on rapidly.
The chemical changes which have taken place in the beds of vegetation of the carboniferous epoch, and which have transformed it into coal, are even now but imperfectly understood. All we know is that, under certain circumstances, one kind of coal is formed, whilst under other conditions, other kinds have resulted; whilst in some cases the processes have resulted in the preparation of large quantities of mineral oils, such as naphtha and petroleum. Oils are also artificially produced from the so-called waste-products of the gas-works, but in some parts of the world the process of their manufacture has gone on naturally, and a yearly increasing quantity is being utilised. In England oil has been pumped up from the carboniferous strata of Coalbrook Dale, whilst in Sussex it has been found in smaller quantities, where, in all probability, it has had its origin in the lignitic beds of the Wealden strata. Immense quantities are used for fuel by the Russian steamers on the Caspian Sea, the Baku petroleum wells being a most valuable possession. In Sicily, Persia, and, far more important, in the United States, mineral oils are found in great quantity.
In all probability coniferous trees, similar to the living firs, pines, larches, &c., gave rise for the most part to the mineral oils. The class of living coniferae is well known for the various oils which it furnishes naturally, and for others which its representatives yield on being subjected to distillation. The gradually increasing amount of heat which we meet the deeper we go beneath the surface, has been the cause of a slow and continuous distillation, whilst the oil so distilled has found its way to the surface in the shape of mineral-oil springs, or has accumulated in troughs in the strata, ready for use, to be drawn up when a well has been sunk into it.
The plants which have gone to make up the coal are not at once apparent to the naked eye. We have to search among the shales and clays and sandstones which enclose the coal-seams, and in these we find petrified specimens which enable us to build up in our mind pictures of the vegetable creation which formed the jungles and forests of these immensely remote ages, and which, densely packed together on the old forest floor of those days, is now apparent to us as coal.
A very large proportion of the plants which have been found in the coal-bearing strata consists of numerous species of ferns, the number of actual species which have been preserved for us in our English coal, being double the number now existing in Europe. The greater part of these do not seem to have been very much larger than our own living ferns, and, indeed, many of them bear a close resemblance to some of our own living species. The impressions they have left on the shales of the coal-measures are most striking, and point to a time when the sandy clay which imbedded them was borne by water in a very tranquil manner, to be deposited where the ferns had grown, enveloping them gradually, and consolidating them into their mass of future shale. In one species known as the neuropteris, the nerves of the leaves are as clear and as apparent as in a newly-grown fern, the name being derived from two Greek words meaning "nerve-fern." It is interesting to consider the history of such a leaf, throughout the ages that have elapsed since it was part of a living fern. First it grew up as a new frond, then gradually unfolded itself, and developed into the perfect fern. Then it became cut off by the rising waters, and buried beneath an accumulation of sediment, and while momentous changes have gone on in connection with the surface of the earth, it has lain dormant in its hiding-place exactly as we see it, until now excavated, with its contemporaneous vegetation, to form fuel for our winter fires.
Although many of the ferns greatly resembled existing species, yet there were others in these ancient days utterly unlike anything indigenous to England now. There were undoubted tree-ferns, similar to those which thrive now so luxuriously in the tropics, and which throw out their graceful crowns of ferns at the head of a naked stem, whilst on the bark are the marks at different levels of the points of attachment of former leaves. These have left in their places cicatrices or scars, showing the places from which they formerly grew. Amongst the tree-ferns found are megaphyton, paloeopteris, and caulopteris, all of which have these marks upon them, thus proving that at one time even tree-ferns had a habitat in England.
One form of tree-fern is known by the name of Psaronius, and this was peculiar in the possession of masses of aerial roots grouped round the stem. Some of the smaller species exhibit forms of leaves which are utterly unknown in the nomenclature of living ferns. Most have had names assigned to them in accordance with certain characteristics which they possess. This was the more possible since the fossilised impressions had been retained in so distinct a manner. Here before us is a specimen in a shale of pecopteris, as it is called, (pekos, a comb). The leaf in some species is not altogether unlike the well-known living fern osmunda. The position of the pinnules on both sides of the central stalk are seen in the fossil to be shaped something like a comb, or a saw, whilst up the centre of each pinnule the vein is as prominent and noticeable as if the fern were but yesterday waving gracefully in the air, and but to-day imbedded in its shaly bed.
Sphenopteris, or "wedge-fern," is the name applied to another coal-fern; glossopteris, or "tongue-leaf"; cyclopteris, or "round-leaf"; odonlopteris, or "tooth-leaf," and many others, show their chief characteristics in the names which they individually bear. Alethopteris appears to have been the common brake of the coal-period, and in some respects resembles pecopteris.
In some species of ferns so exact are the representations which they have impressed on the shale which contains them, that not only are the veins and nerves distinctly visible, but even the fructification still remains in the shape of the marks left by the so-called seeds on the backs of the leaves. Something more than a passing look at the coal specimens in a good museum will well repay the time so spent.
What are known as septarian nodules, or snake-stones, are, at certain places, common in the carboniferous strata. They are composed of layers of ironstone and sandstone which have segregated around some central object, such as a fern-leaf or a shell. When the leaf of a fern has been found to be the central object, it has been noticed that the leaf can sometimes be separated from the stone in the form of a carbonaceous film.
Experiments were made many years ago by M. Goppert to illustrate the process of fossilisation of ferns. Having placed some living ferns in a mass of clay and dried them, he exposed them to a red heat, and obtained thereby striking resemblances to fossil plants. According to the degree of heat to which they were subjected, the plants were found to be either brown, a shining black, or entirely lost. In the last mentioned case, only the impression remained, but the carbonaceous matter had gone to stain the surrounding clay black, thus indicating that the dark colour of the coal-shales is due to the carbon derived from the plants which they included.
Another very prominent member of the vegetation of the coal period, was that order of plants known as the Calamites. The generic distinctions between fossil and living ferns were so slight in many cases as to be almost indistinguishable. This resemblance between the ancient and the modern is not found so apparent in other plants. The Calamites of the coal-measures bore indeed a very striking resemblance, and were closely related, to our modern horse-tails, as the equiseta are popularly called; but in some respects they differed considerably.
Most people are acquainted with the horse-tail (equisetum fluviatile) of our marshes and ditches. It is a somewhat graceful plant, and stands erect with a jointed stem. The foliage is arranged in whorls around the joints, and, unlike its fossil representatives, its joints are protected by striated sheaths. The stem of the largest living species rarely exceeds half-an-inch in diameter, whilst that of the calamite attained a thickness of five inches. But the great point which is noticeable in the fossil calamites and equisetites is that they grew to a far greater height than any similar plant now living, sometimes being as much as eight feet high. In the nature of their stems, too, they exhibited a more highly organised arrangement than their living representatives, having, according to Dr Williamson, a "fistular pith, an exogenous woody stem, and a thick smooth bark." The bark having almost al ways disappeared has left the fluted stem known to us as the calamite. The foliage consisted of whorls of long narrow leaves, which differed only from the fern asterophyllites in the fact that they were single-nerved. Sir William Dawson assigns the calamites to four sub-types: calamite proper, calamopitus, calamodendron, and eucalamodendron.
[Image: FIG. 7.—Root of Catamites Suckowii. Coal-shale.]
[Image: FIG 8.—Calamocladus grandis. Carboniferous sandstone.]
Having used the word "exogenous," it might be as well to pay a little attention, in passing, to the nomenclature and broad classification of the various kinds of plants. We shall then doubtless find it far easier thoroughly to understand the position in the scale of organisation to which the coal plants are referable.
The plants which are lowest in organisation are known as Cellular. They are almost entirely composed of numerous cells built up one above the other, and possess none of the higher forms of tissue and organisation which are met with elsewhere. This division includes the lichens, sea-weeds, confervae (green aquatic scum), fungi (mushrooms, dry-rot), &c.
The division of Vascular plants includes the far larger proportion of vegetation, both living and fossil, and these plants are built up of vessels and tissues of various shapes and character.
All plants are divided into (1) Cryptogams, or Flowerless, such as mosses, ferns, equisetums, and (2) Phanerogams, or Flowering. Flowering plants are again divided into those with naked seeds, as the conifers and cycads (gymnosperms), and those whose seeds are enclosed in vessels, or ovaries (angiosperms).
Angiosperms are again divided into the monocotyledons, as the palms, and dicotyledons, which include most European trees.
- (M.A. Brongniart). (Lindley). CELLULAR Cryptogams (Flowerless) Fungi, seaweeds, Thallogens lichens VASCULAR Cryptogams (Flowerless) Ferns, equisetums, Acrogens mosses, lycopodiums Phanerogams (Flowering) Gymnosperms (having Conifers and Gymnogens naked seeds) cycads Two or more Cotyledons Angiosperms (having enclosed seeds) Monocotyledons Palms, lilies, Endogens grasses Dicotyledons Most European Exogens trees and shrubs -
Adolphe Brongniart termed the coal era the "Age of Acrogens," because, as we shall see, of the great predominance in those times of vascular cryptogamic plants, known in Dr Lindley's nomenclature as "Acrogens."
Two of these families have already been dealt with, viz., the ferns (felices), and the equisetums, (calamites and equisetites), and we now have to pass on to another family. This is that which includes the fossil representatives of the Lycopodiums, or Club-mosses, and which goes to make up in some coals as much as two-thirds of the whole mass. Everyone is more or less familiar with some of the living Lycopodiums, those delicate little fern-like mosses which are to be found in many a home. They are but lowly members of our British flora, and it may seem somewhat astounding at first sight that their remote ancestors occupied so important a position in the forests of the ancient period of which we are speaking. Some two hundred living species are known, most of them being confined to tropical climates. They are as a rule, low creeping plants, although some few stand erect. There is room for astonishment when we consider the fact that the fossil representatives of the family, known as Lepidodendra, attained a height of no less than fifty feet, and, there is good ground for believing, in many cases, a far greater magnitude. They consist of long straight stems, or trunks which branch considerably near the top. These stems are covered with scars or scales, which have been caused by the separation of the petioles or leaf-stalks, and this gives rise to the name which the genus bears. The scars are arranged in a spiral manner the whole of the way up the stem, and the stems often remain perfectly upright in the coal-mines, and reach into the strata which have accumulated above the coal-seam.
Count Sternberg remarked that we are unacquainted with any existing species of plant, which like the Lepidodendron, preserves at all ages, and throughout the whole extent of the trunk, the scars formed by the attachment of the petioles, or leaf-stalks, or the markings of the leaves themselves. The yucca, dracaena, and palm, entirely shed their scales when they are dried up, and there only remain circles, or rings, arranged round the trunk in different directions. The flabelliform palms preserve their scales at the inferior extremity of the trunk only, but lose them as they increase in age; and the stem is entirely bare, from the middle to the superior extremity. In the ancient Lepidodendron, on the other hand, the more ancient the scale of the leaf-stalk, the more apparent it still remains. Portions of stems have been discovered which contain leaf-scars far larger than those referred to above, and we deduce from these fragments the fact that those individuals which have been found whole, are not by any means the largest of those which went to form so large a proportion of the ancient coal-forests. The lepidodendra bore linear one-nerved leaves, and the stems always branched dichotomously and possessed a central pith. Specimens variously named knorria, lepidophloios, halonia, and ulodendron are all referable to this family.
In some strata, as for instance that of the Shropshire coalfield, quantities of elongated cylindrical bodies known as lepidostrobi have been found, which, it was early conjectured, were the fruit of the giant club-mosses about which we have just been speaking. Their appearance can be called to mind by imagining the cylindrical fruit of the maize or Indian corn to be reduced to some three or four inches in length. The sporangia or cases which contained the microscopic spores or seeds were arranged around a central axis in a somewhat similar manner to that in which maize is found. These bodies have since been found actually situated at the end of branches of lepidodendron, thus placing their true nature beyond a doubt. The fossil seeds (spores) do not appear to have exceeded in volume those of recent club-mosses, and this although the actual trees themselves grew to a size very many times greater than the living species. This minuteness of the seed-germs goes to explain the reason why, as Sir Charles Lyell remarked, the same species of lepidodendra are so widely distributed in the coal measures of Europe and America, their spores being capable of an easy transportation by the wind.
One striking feature in connection with the fruit of the lepidodendron and other ancient representatives of the club-moss tribe, is that the bituminous coals in many, if not in most, instances, are made up almost entirely of their spores and spore-cases. Under a microscope, a piece of such coal is seen to be thronged with the minute rounded bodies of the spores interlacing one another and forming almost the whole mass, whilst larger than these, and often indeed enclosing them, are flattened bag-like bodies which are none other than the compressed sporangia which contained the former.
Now, the little Scottish or Alpine club-moss which is so familiar, produces its own little cones, each with its series of outside scales or leaves; these are attached to the bags or spore-cases, which are crowded with spores. Although in miniature, yet it produces its fruit in just the same way, at the terminations of its little branches, and the spores, the actual germs of life, when examined microscopically, are scarcely distinguishable from those which are contained in certain bituminous coals. And, although ancient club-mosses have been found in a fossilised condition at least forty-nine feet high, the spores are no larger than those of our miniature club-mosses of the present day.
The spores are more or less composed of pure bitumen, and the bituminous nature of the coal depends largely on the presence or absence of these microscopic bodies in it. The spores of the living club-mosses contain so much resinous matter that they are now largely used in the making of fireworks, and upon the presence of this altered resinous matter in coal depends its capability of providing a good blazing coal.
At first sight it seems almost impossible that such a minute cause should result in the formation of huge masses of coal, such an inconceivable number of spores being necessary to make even the smallest fragment of coal. But if we look at the cloud of spores that can be shaken from a single spike of a club-moss, then imagine this to be repeated a thousand times from each branch of a fairly tall tree, and then finally picture a whole forest of such trees shedding in due season their copious showers of spores to earth, we shall perhaps be less amazed than we were at first thought, at the stupendous result wrought out by so minute an object.
Another well-known form of carboniferous vegetation is that known as the Sigillaria, and, connected with this form is one, which was long familiar under the name of Stigmaria, but which has since been satisfactorily proved to have formed the branching root of the sigillaria. The older geologists were in the habit of placing these plants among the tree-ferns, principally on account of the cicatrices which were left at the junctions of the leaf-stalks with the stem, after the former had fallen off. No foliage had, however, been met with which was actually attached to the plants, and hence, when it was discovered that some of them had long attenuated leaves not at all like those possessed by ferns, geologists were compelled to abandon this classification of them, and even now no satisfactory reference to existing orders of them has been made, owing to their anomalous structure. The stems are fluted from base to stem, although this is not so apparent near the base, whilst the raised prominences which now form the cicatrices, are arranged at regular distances within the vertical grooves.
When they have remained standing for some length of time, and the strata have been allowed quietly to accumulate around the trunks, they have escaped compression. They were evidently, to a great extent, hollow like a reed, so that in those trees which still remain vertical, the interior has become filled up by a coat of sandstone, whilst the bark has become transformed into an envelope of an inch, or half an inch of coal. But many are found lying in the strata in a horizontal plane. These have been cast down and covered up by an ever-increasing load of strata, so that the weight has, in the course of time, compressed the tree into simply the thickness of the double bark, that is, of the two opposite sides of the envelope which covered it when living.
Sigillarae grew to a very great height without branching, some specimens having measured from 60 to 70 feet long. In accordance with their outside markings, certain types are known as syringodendron, favularia, and clathraria. Diploxylon is a term applied to an interior stem referable to this family.
But the most interesting point about the sigillariae is the root. This was for a long time regarded as an entirely distinct individual, and the older geologists explained it in their writings as a species of succulent aquatic plant, giving it the name of stigmaria. They realized the fact that it was almost universally found in those beds which occur immediately beneath the coal seams, but for a long time it did not strike them that it might possibly be the root of a tree. In an old edition of Lyell's "Elements of Geology," utterly unlike existing editions in quality, quantity, or comprehensiveness, after describing it as an extinct species of water-plant, the author hazarded the conjecture that it might ultimately be found to have a connection with some other well-known plant or tree. It was noticed that above the coal, in the roof, stigmariae were absent, and that the stems of trees which occurred there, had become flattened by the weight of the overlying strata. The stigmariae on the other hand, abounded in the underclay, as it is called, and were not in any way compressed but retained what appeared to be their natural shape and position. Hence to explain their appearance, it was thought that they were water-plants, ramifying the mud in every direction, and finally becoming overwhelmed and covered by the mud itself. On botanical grounds, Brongniart and Lyell conjectured that they formed the roots of other trees, and this became the more apparent as it came to be acknowledged that the underclays were really ancient soils. All doubt was, however, finally dispelled by the discovery by Mr Binney, of a sigillaria and a stigmaria in actual connection with each other, in the Lancashire coal-field.
Stigmariae have since been found in the Cape Breton coal-field, attached to Lepidodendra, about which we have already spoken, and a similar discovery has since been made in the British coal-fields. This, therefore, would seem to shew the affinity of the sigillaria to the lepidodendron, and through it to the living lycopods, or club-mosses.
Some few species of stigmarian roots had been discovered, and various specific names had been given to them before their actual nature was made out. What for some time were thought to be long cylindrical leaves, have now been found to be simply rootlets, and in specimens where these have been removed, the surface of the stigmaria has been noticed to be covered with large numbers of protuberant tubercles, which have formed the bases of the rootlets. There appears to have also been some special kind of arrangement in their growth, since, unlike the roots of most living plants, the tubercles to which these rootlets were attached, were arranged spirally around the main root. Each of these tubercles was pitted in the centre, and into these the almost pointed ends of the rootlets fitted, as by a ball and socket joint.
"A single trunk of sigillaria in an erect forest presents an epitome of a coal-seam. Its roots represent the stigmaria underclay; its bark the compact coal; its woody axis, the mineral charcoal; its fallen leaves and fruits, with remains of herbaceous plants growing in its shade, mixed with a little earthy matter, the layers of coarse coal. The condition of the durable outer bark of erect trees, concurs with the chemical theory of coal, in showing the especial suitableness of this kind of tissue for the production of the purer compact coals."—(Dawson, "Structures in Coal.")
There is yet one other family of plants which must be mentioned, and which forms a very important portion of the constituent flora of the coal period. This is the great family of the coniferae, which although differing in many respects from the highly organised dicotyledons of the present day, yet resembled them in some respects, especially in the formation of an annual ring of woody growth.
The conifers are those trees which, as the name would imply, bear their fruit in the form of cones, such as the fir, larch, cedar, and others. The order is one which is familiar to all, not only on account of the cones they bear, and their sheddings, which in the autumn strew the ground with a soft carpet of long needle-like leaves, but also because of the gum-like secretion of resin which is contained in their tissues. Only a few species have been found in the coal-beds, and these, on examination under the microscope, have been discovered to be closely related to the araucarian division of pines, rather than to any of our common firs. The living species of this tree is a native of Norfolk Island, in the Pacific, and here it attains a height of 200 feet, with a girth of 30 feet. From the peculiar arrangement of the ducts in the elongated cellular tissue of the tree, as seen under the microscope, the fossil conifers, which exhibit this structure, have been placed in the same division.
The familiar fossil known to geologists as Sternbergia has now been shown to be the cast of the central pith of these conifers, amongst which may be mentioned cordaites, araucarites, and dadoxylon.. The central cores had become replaced with inorganic matter after the pith had shrunk and left the space empty. This shrinkage of the pith is a process which takes place in many plants even when living, and instances will at once occur, in which the stems of various species of shrubs when broken open exhibit the remains of the shrunken pith, in the shape of thin discs across the interval cavity.
We might reasonably expect that where we find the remains of fossil coniferous trees, we should also meet with the cones or fruit which they bear. And such is the case. In some coal-districts fossil fruits, named cardiocarpum and trigonocarpum, have been found in great quantities, and these have now been decided by botanists to be the fruits of certain conifers, allied, not to those which bear hard cones, but to those which bear solitary fleshy fruits. Sir Charles Lyell referred them to a Chinese genus of the yew tribe called salisburia. Dawson states that they are very similar to both taxus and salisburia.. They are abundant in some coal-measures, and are contained, not only in the coal itself, but also in the sandstones and shales. The under-clays appear to be devoid of them, and this is, of course, exactly what might have been expected, since the seeds would remain upon the soil until covered up by vegetable matter, but would never form part of the clay soil itself.
In connection with the varieties which have been distinguished in the families of the conifers, calamites, and sigillariae, Sir William Dawson makes the following observations: "I believe that there was a considerably wide range of organisation in cordaitinae as well as in calamites and sigillariae, and that it will eventually be found that there were three lines of connection between the higher cryptogams (flowerless) and the phaenogams (flowering), one leading from the lycopodes by the sigillariae, another leading by the cordaites, and the third leading from the equisetums by the calamites. Still further back the characters, afterwards separated in the club-mosses, mare's-tails, and ferns, were united in the rhizocarps, or, as some prefer to call them, the heterosporous filicinae."
In concluding this chapter dealing with the various kinds of plants which have been discovered as contributing to the formation of coal-measures, it would be as well to say a word or two concerning the climate which must have been necessary to permit of the growth of such an abundance of vegetation. It is at once admitted by all botanists that a moist, humid, and warm atmosphere was necessary to account for the existence of such an abundance of ferns. The gorgeous waving tree-ferns which were doubtless an important feature of the landscape, would have required a moist heat such as does not now exist in this country, although not necessarily a tropical heat. The magnificent giant lycopodiums cast into the shade all our living members of that class, the largest of which perhaps are those that flourish in New Zealand. In New Zealand, too, are found many species of ferns, both those which are arborescent and those which are of more humble stature. Add to these the numerous conifers which are there found, and we shall find that a forest in that country may represent to a certain extent the appearance presented by a forest of carboniferous vegetation. The ferns, lycopods, and pines, however, which appear there, it is but fair to add, are mixed with other types allied to more recent forms of vegetation.
There are many reasons for believing that the amount of carbonic acid gas then existing in the atmosphere was larger than the quantity which we now find, and Professor Tyndall has shown that the effect of this would be to prevent radiation of heat from the earth. The resulting forms of vegetation would be such as would be comparable with those which are now reared in the green-house or conservatory in these latitudes. The gas would, in fact, act as a glass roof, extending over the whole world.
A GENERAL VIEW OF THE COAL-BEARING STRATA.
In considering the source whence coal is derived, we must be careful to remember that coal itself is but a minor portion of the whole formation in which it occurs. The presence of coal has indeed given the name to the formation, the word "carboniferous" meaning "coal-bearing," but in taking a comprehensive view of the position which it occupies in the bowels of the earth, it will be necessary to take into consideration the strata in which it is found, and the conditions, so far as are known, under which these were deposited.
Geologically speaking, the Carboniferous formation occurs near the close of that group of systems which have been classed as "palaeozoic," younger in point of age than the well known Devonian and Old Red Sandstone strata, but older by far than the Oolites, the Wealden, or the Cretaceous strata.
In South Wales the coal-bearing strata have been estimated at between 11,000 and 12,000 feet, yet amongst this enormous thickness of strata, the whole of the various coal-seams, if taken together, probably does not amount to more than 120 feet. This great disproportion between the total thickness and the thickness of coal itself shows itself in every coal-field that has been worked, and when a single seam of coal is discovered attaining a thickness of 9 or 10 feet, it is so unusual a thing in Great Britain as to cause it to be known as the "nine" or "ten-foot seam," as the case may be. Although abroad many seams are found which are of greater thicknesses, yet similarly the other portions of the formation are proportionately greater.
It is not possible therefore to realise completely the significance of the coal-beds themselves unless there is also a knowledge of the remaining constituents of the whole formation. The strata found in the various coal-fields differ considerably amongst themselves in character. There are, however, certain well-defined characteristics which find representation in most of the principal coal-fields, whether British or European. Professor Hull classifies these carboniferous beds as follows:—
UPPER CARBONIFEROUS. Upper coal-measures. Reddish and purple sandstones, red and grey clays and shales, thin bands of coal, ironstone and limestone, with spirorbis and fish.
Middle coal-measures. Yellow and gray sandstones, blue and black clays and shales, bands of coal and ironstone, fossil plants, bivalves and fish, occasional marine bands.
MIDDLE CARBONIFEROUS. Gannister beds or Lower coal-measures. Millstone grit. Flagstone series in Ireland. Yoredale beds. Upper shale series of Ireland.
LOWER CARBONIFEROUS. Mountain limestone. Limestone shale.
Each of the three principal divisions has its representative in Scotland, Belgium, and Ireland, but, unfortunately for the last-named country, the whole of the upper coal-measures are there absent. It is from these measures that almost all our commercial coals are obtained.
This list of beds might be further curtailed for all practical purposes of the geologist, and the three great divisions of the system would thus stand:—
Upper Carboniferous, or Coal-measures proper.
Lower Carboniferous, or Mountain limestone.
In short, the formation consists of masses of sandstone, shale, limestone and coal, these also enclosing clays and ironstones, and, in the limestone, marbles and veins of the ores of lead, zinc, and antimony, and occasionally silver.
As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these were deposited in the waters of the carboniferous ages, and this we can best do by considering the laws under which strata of a similar nature are now being deposited as sedimentary beds.
A great proportion consists of sandstone. Now sandstone is the result of sand which has been deposited in large quantities, having become indurated or hardened by various processes brought to bear upon it. It is necessary, therefore, first to ascertain whence came the sand, and whether there are any peculiarities in its method of deposition which will explain its stratification. It will be noticed at once that it bears a considerable amount of evidence of what is called "current-bedding," that is to say, that the strata, instead of being regularly deposited, exhibit series of wedge-shaped masses, which are constantly thinning out.
Sand and quartz are of the same chemical composition, and in all probability the sand of which every sandstone in existence is composed, appeared on this earth in its first solid form in the shape of quartz. Now quartz is a comparatively heavy mineral, so also, therefore, will sand be. It is also very hard, and in these two respects it differs entirely from another product of sedimentary deposition, namely, mud or clay, with which we shall have presently to deal when coming to the shales. Since quartz is a hard mineral it necessarily follows that it will suffer, without being greatly affected, a far greater amount of wearing and knocking about when being transported by the agency of currents and rivers, than will a softer substance, such as clay. An equal amount of this wearing action upon clay will reduce it to a fine impalpable silt. The grains of sand, however, will still remain of an appreciable average size, and where both sand and clay are being transported to the sea in one and the same stream, the clay will be transported to long distances, whilst the sand, being heavier, bulk for bulk, and also consisting of grains larger in size than grains of clay, will be rapidly deposited, and form beds of sand. Of course, if the current be a violent one, the sand is transported, not by being held in suspension, but rather by being pushed along the bed of the river; such an action will then tend to cause the sand to become powdered into still finer sand.
When a river enters the sea it soon loses its individuality; it becomes merged in the body of the ocean, where it loses its current, and where therefore it has no power to keep in suspension the sediment which it had brought down from the higher lands. When this is the case, the sand borne in suspension is the first to be deposited, and this accumulates in banks near the entrance of the river into the sea. We will suppose, for illustration, that a small river has become charged with a supply of sand. As it gradually approaches the sea, and the current loses its force, the sand is the more sluggishly carried along, until finally it falls to the bottom, and forms a layer of sand there. This layer increases in thickness until it causes the depth of water above it to become comparatively shallow. On the shallowing process taking place, the current will still have a certain, though slighter, hold on the sand in suspension, and will transport it yet a little further seaward, when it will be thrown down, at the edge of the bank or layer already formed, thus tending to extend the bank, and to shallow a wider space of river-bed.
As a result of this action, strata would be formed, shewing stratification diagonally as well as horizontally, represented in section as a number of banks which had seemingly been thrown down one above the other, ending in thin wedge-shaped terminations where the particular supply of sediment to which each owed its formation had failed.
The masses of sandstone which are found in the carboniferous formation, exhibit in a large degree these wedge-shaped strata, and we have therefore a clue at once, both as to their propinquity to sea and land, and also as to the manner in which they were formed.
There is one thing more, too, about them. Just as, in the case we were considering, we could observe that the wedge-shaped strata always pointed away from the source of the material which formed them, so we can similarly judge that in the carboniferous strata the same deduction holds good, that the diagonally-pointing strata were formed in the same way, and that their thinning out was simply owing to temporary failure of sediment, made good, however, by a further deposition of strata when the next supply was borne down.
It is scarcely likely, however, that sand in a pure state was always carried down by the currents to the sea. Sometimes there would be some silt mixed with it. Just as in many parts large masses of almost pure sandstone have been formed, so in other places shales, or, as they are popularly known by miners, "bind," have been formed. Shales are formed from the clays which have been carried down by the rivers in the shape of silt, but which have since become hardened, and now split up easily into thin parallel layers. The reader has no doubt often handled a piece of hard clay when fresh from the quarry, and has remembered how that, when he has been breaking it up, in order, perhaps, to excavate a partially-hidden fossil, it has readily split up in thin flakes or layers of shaly substance. This exhibits, on a small scale, the chief peculiarity of the coal shales.
The formation of shales will now demand our attention. When a river is carrying down with it a quantity of mud or clay, it is transported as a fine, dusty silt, and when present in quantities, gives the muddy tint to the water which is so noticeable. We can very well see how that silt will be carried down in greater quantities than sand, since nearly all rivers in some part of their course will travel through a clayey district, and finely-divided clay, being of a very light nature, will be carried forward whenever a river passes over such a district. And a very slight current being sufficient to carry it in a state of suspension, it follows that it will have little opportunity of falling to the bottom, until, by some means or other, the current, which is the means of its conveyance, becomes stopped or hindered considerably in its flow.
When the river enters a large body of water, such as the ocean or a lake, in losing its individuality, it loses also the velocity of its current, and the silt tends to sink down to the bottom. But being less heavy than the sand, about which we have previously spoken, it does not sink all at once, but partly with the impetus it has gained, and partly on account of the very slight velocity which the current still retains, even after having entered the sea, it will be carried out some distance, and will the more gradually sink to the bottom. The deeper the water in which it falls the greater the possibility of its drifting farther still, since in sinking, it would fall, not vertically, but rather as the drops of rain in a shower when being driven before a gale of wind. Thus we should notice that clays and shales would exhibit a regularity and uniformity of deposition over a wide area. Currents and tides in the sea or lake would tend still further to retard deposition, whilst any stoppages in the supply of silt which took place would give the former layer time to consolidate and harden, and this would assist in giving it that bedded structure which is so noticeable in the shales, and which causes it to split up into fine laminae. This uniformity of structure in the shales over wide areas is a well ascertained characteristic of the coal-shales, and we may therefore regard the method of their deposition as given here with a degree of certainty.
There is a class of deposit found among the coal-beds, which is known as the "underclay," and this is the most regular of all as to the position in which it is found. The underclays are found beneath every bed of coal. "Warrant," "spavin," and "gannister" are local names which are sometimes applied to it, the last being a term used when the clay contains such a large proportion of silicious matter as to become almost like a hard flinty rock. Sometimes, however, it is a soft clay, at others it is mixed with sand, but whatever the composition of the underclays may be, they always agree in being unstratified. They also agree in this respect that the peculiar fossils known as stigmariae abound in them, and in some cases to such an extent that the clay is one thickly-matted mass of the filamentous rootlets of these fossils. We have seen how these gradually came to be recognised as the roots of trees which grew in this age, and whose remains have subsequently become metamorphosed into coal, and it is but one step farther to come to the conclusion that these underclays are the ancient soils in which the plants grew.
No sketch of the various beds which go to form the coal-measures would be complete which did not take into account the enormous beds of mountain limestone which form the basis of the whole system, and which in thinner bands are intercalated amongst the upper portion of the system, or the true coal-measures.
Now, limestones are not formed in the same way in which we have seen that sandstones and shales are formed. The last two mentioned owe their origin to their deposition as sediment in seas, estuaries or lakes, but the masses of limestone which are found in the various geological formations owe their origin to causes other than that of sedimentary deposition.
In carboniferous times there lived numberless creatures which we know nowadays as encrinites. These, when growing, were fixed to the bed of the ocean, and extended upward in the shape of pliant stems composed of limestone joints or plates; the stem of each encrinite then expanded at the top in the shape of a gorgeous and graceful starfish, possessed of numberless and lengthy arms. These encrinites grew in such profusion that after death, when the plates of which their stems consisted, became loosened and scattered over the bed of the sea, they accumulated and formed solid beds of limestone. Besides the encrinites, there were of course other creatures which were able to create the hard parts of their structures by withdrawing lime from the sea, such as foraminifera, shell-fish, and especially corals, so that all these assisted after death in the accumulation of beds of limestone where they had grown and lived.
There is one peculiarity in connection with the habitats of the encrinites and corals which goes some distance in supplying us with a useful clue as to the conditions under which this portion of the carboniferous formation was formed. These creatures find it a difficult matter, as a rule, to live and secrete their calcareous skeleton in any water but that which is clear, and free from muddy or sandy sediment. They are therefore not found, generally speaking, where the other deposits which we have considered, are forming, and, as these are always found near the coasts, it follows that the habitats of the creatures referred to must be far out at sea where no muddy sediments, borne by rivers, can reach them. We can therefore safely come to the conclusion that the large masses of encrinital limestone, which attain such an enormous thickness in some places, especially in Ireland, have been formed far away from the land of the period; we can at the same time draw the conclusion that if we find the encrinites broken and snapped asunder, and the limestone deposits becoming impure through being mingled with a proportion of clayey or sandy deposits, that we are approaching a coast-line where perhaps a river opened out, and where it destroyed the growth of encrinites, mixing with their dead remains the sedimentary debris of the land.
We have lightly glanced at the circumstances attending the deposition of each of the principal rocks which form the beds amongst which coal is found, and have now to deal with the formation of the coal itself. We have already considered the various kinds of plants and trees which have been discovered as contributing their remains to the formation of coal, and have now to attempt an explanation of how it came to be formed in so regular a manner over so wide an area.
Each of the British coal-fields is fairly extensive. The Yorkshire and Derbyshire coal-fields, together with the Lancashire coal-field, with which they were at one time in geological connection, give us an area of nearly 1000 square miles, and other British coal-fields show at least some hundreds of square miles. And yet, spread over them, we find a series of beds of coal which in many cases extend throughout the whole area with apparent regularity. If we take it, as there seems every reason to believe was the case, that almost all these coal-fields were not only being formed at the same time, but were in most instances in continuation with one another, this regularity of deposition of comparatively narrow beds of coal, appears all the more remarkable.
The question at once suggests itself, Which of two things is probable? Are we to believe that all this vegetable matter was brought down by some mighty river and deposited in its delta, or that the coal-plants grew just where we now find the coal?
Formerly it was supposed that coal was formed out of dead leaves and trees, the refuse of the vegetation of the land, which had been carried down by rivers into the sea and deposited at their mouths, in the same way that sand and mud, as we have seen, are swept down and deposited. If this were so, the extent of the deposits would require a river with an enormous embouchure, and we should be scarcely warranted in believing that such peaceful conditions would there prevail as to allow of the layers of coal to be laid down with so little disturbance and with such regularity over these wide areas. But the great objection to this theory is, that not only do the remains still retain their perfection of structure, but they are comparatively pure,—i.e., unmixed with sedimentary depositions of clay or sand. Now, rivers would not bring down the dead vegetation alone; their usual burden of sediment would also be deposited at their mouths, and thus dead plants, sand, and clay would be mixed up together in one black shaly or sandy mass, a mixture which would be useless for purposes of combustion. The only theory which explained all the recognised phenomena of the coal-measures was that the plants forming the coal actually grew where the coal was formed, and where, indeed, we now find it. When the plants and trees died, their remains fell to the ground of the forest, and these soon turned to a black, pasty, vegetable mass, the layer thus formed being regularly increased year by year by the continual accumulation of fresh carbonaceous matter. By this means a bed would be formed with regularity over a wide area; the coal would be almost free from an admixture of sandy or clayey sediment, and probably the rate of formation would be no more rapid in one part of the forest than another. Thus there would be everywhere uniformity of thickness. The warm and humid atmosphere, which it is probable then existed, would not only have tended towards the production of an abnormal vegetation, but would have assisted in the decaying and disintegrating processes which went on amongst the shed leaves and trees.
When at last it was announced as a patent fact that every bed of coal possessed its underclay, and that trees had been discovered actually standing upon their own roots in the clay, there was no room at all for doubt that the correct theory had been hit upon—viz., that coal is now found just where the trees composing it had grown in the past.
But we have more than one coal-seam to account for. We have to explain the existence of several layers of coal which have been formed over one another on the same spot at successive periods, divided by other periods when shale and sandstones only have been formed.
A careful estimate of the Lancashire coal-field has been made by Professor Hull for the Geological Survey. Of the 7000 feet of carboniferous strata here found, spread out over an area of 217 square miles, there are on the average eighteen seams of coal.
This is only an instance of what is to be found elsewhere. Eighteen coal-seams! what does this mean? It means that, during carboniferous times, on no less than eighteen occasions, separate and distinct forests have grown on this self-same spot, and that between each of these occasions changes have taken place which have brought it beneath the waters of the ocean, where the sandstones and shales have been formed which divide the coal-seams from each other. We are met here by a wonderful demonstration of the instability of the surface of the earth, and we have to do our best to show how the changes of level have been brought about, which have allowed of this game of geological see-saw to take place between sea and land. Changes of level! Many a hard geological nut has only been overcome by the application of the principle of changes of level in the surface of the earth, and in this we shall find a sure explanation of the phenomena of the coal-measures.
Great changes of the level of the land are undoubtedly taking place even now on the earth's surface, and in assuming that similar changes took place in carboniferous times, we shall not be assuming the former existence of an agent with which we are now unfamiliar. And when we consider the thicknesses of sandstone and shale which intervene beneath the coal-seams, we can realise to a certain extent the vast lapses of years which must have taken place between the existence of each forest; so that although now an individual passing up a coal-mine shaft may rapidly pass through the remains of one forest after another, the rise of the strata above each forest-bed then was tremendously slow, and the period between the growth of each forest must represent the passing away of countless ages. Perhaps it would not be too much to say that the strata between some of the coal-seams would represent a period not less than that between the formation of the few tertiary coals with which we are acquainted, and a time which is still to us in the far-away future.
The actual seams of coal themselves will not yield much information, from which it will be possible to judge of the contour of the landmasses at this ancient period. Of one thing we are sure, namely, that at the time each seam was formed, the spot where it accumulated was dry land. If, therefore, the seams which appear one above the other coincide fairly well as to their superficial extent, we can conclude that each time the land was raised above the sea and the forest again grew, the contour of the land was very similar. This conclusion will be very useful to go upon, since whatever decision may be come to as an explanation of one successive land-period and sea-period on the same spot, will be applicable to the eighteen or more periods necessary for the completion of some of the coal-fields.
We will therefore look at one of the sandstone masses which occur between the coal-seams, and learn what lessons these have to teach us. In considering the formation of strata of sand in the seas around our river-mouths, it was seen that, owing to the greater weight of the particles of the sand over those of clay, the former the more readily sank to the bottom, and formed banks not very far away from the land. It was seen, too, that each successive deposition of sand formed a wedge-shaped layer, with the point of the wedge pointing away from the source of origin of the sediment, and therefore of the current which conveyed the sediment. Therefore, if in the coal-measure sandstones the layers were found with their wedges all pointing in one direction, we should be able to judge that the currents were all from one direction, and that, therefore, they were formed by a single river. But this is just what we do not find, for instead of it the direction of the wedge-shaped strata varies in almost every layer, and the current-bedding has been brought about by currents travelling in every direction. Such diverse current-bedding could only result from the fact that the spot where the sand was laid down was subject to currents from every direction, and the inference is that it was well within the sphere of influence of numerous streams and rivers, which flowed from every direction. The only condition of things which would explain this is that the sandstone was originally formed in a closed sea or large lake, into which numerous rivers flowing from every direction poured their contents.
Now, in the sandstones, the remains of numerous plants have been found, but they do not present the perfect appearance that they do when found in the shales; in fact they appear to have suffered a certain amount of damage through having drifted some distance. This, together with the fact that sandstones are not formed far out at sea, justify the safe conclusion that the land could not have been far off. Wherever the current-bedding shows itself in this manner we may be sure we are examining a spot from which the land in every direction could not have been at a very great distance, and also that, since the heavy materials of which sandstone is composed could only be transported by being impelled along by currents at the bed of the sea, and that in deep water such currents could not exist, therefore we may safely decide that the sea into which the rivers fell was a comparatively shallow one.
Although the present coal-fields of England are divided from one another by patches of other beds, it is probable that some of them were formerly connected with others, and a very wide sheet of coal on each occasion was laid down. The question arises as to what was the extent of the inland sea or lake, and did it include the area covered by the coal basins of Scotland and Ireland, of France and Belgium? And if these, why not those of America and other parts? The deposition of the coal, according to the theory here advanced, may as well have been brought about in a series of large inland seas and lakes, as by one large comprehensive sea, and probably the former is the more satisfactory explanation of the two. But the astonishing part of it is that the changes in the level of the land must have been taking place simultaneously over these large areas, although, of course, while one quarter may have been depressed beneath the sea, another may have been raised above it.
In connection with the question of the contour of the land during the existence of the large lakes or inland seas, Professor Hull has prepared, in his series of maps illustrative of the Palaeo-Geography of the British Islands, a map showing on incontestible grounds the existence during the coal-ages of a great central barrier or ridge of high land stretching across from Anglesea, south of Flint, Staffordshire, and Shropshire coal-fields, to the eastern coast of Norfolk. He regards the British coal-measures as having been laid down in two, or at most three, areas of deposition—one south of this ridge, the remainder to the north of it. In regard to the extent of the former deposits of coal in Ireland, there is every probability that the sister island was just as favourably treated in this respect as Great Britain. Most unfortunately, Ireland has since suffered extreme denudation, notably from the great convulsions of nature at the close of the very period of their deposition, as well as in more recent times, resulting in the removal of nearly all the valuable upper carboniferous beds, and leaving only the few unimportant coal-beds to which reference has been made.
We are unable to believe in the continuity of our coal-beds with those of America, for the great source of sediment in those times was a continent situated on the site of the Atlantic Ocean, and it is owing to this extensive continent that the forms of flora found in the coal-beds in each country bear so close a resemblance to one another, and also that the encrinital limestone which was formed in the purer depths of the ocean on the east, became mixed with silt, and formed masses of shaly impure limestone in the south-western parts of Ireland.
It must be noted that, although we may attribute to upheaval from beneath the fact that the bed of the sea became temporarily raised at each period into dry land, the deposits of sand or shale would at the same time be tending to shallow the bed, and this alone would assist the process of upheaval by bringing the land at least very near to the surface of the water.
Each upheaval, however, could have been but a temporary arrest of the great movement of crust subsidence which was going on throughout the coal period, so that, at its close, when the last coal forest grew upon the surface of the land, there had disappeared, in the case of South Wales, a thickness of 11,000 feet of material.
Of the many remarkable things in connection with coal-beds, not the least is the state of purity in which coal is found. On the floor of each forest there would be many a streamlet or even small river which would wend its way to meet the not very distant sea, and it is surprising at first that so little sediment found its way into the coal itself. But this was cleverly explained by Sir Charles Lyell, who noticed, on one of his visits to America, that the water of the Mississippi, around the rank growths of cypress which form the "cypress swamps" at the mouths of that river, was highly charged with sediment, but that, having passed through the close undergrowth of the swamps, it issued in almost a pure state, the sediment which it bore having been filtered out of it and precipitated. This very satisfactorily explained how in some places carbonaceous matter might be deposited in a perfectly pure state, whilst in others, where sandstone or shale was actually forming, it might be impregnated by coaly matter in such a way as to cause it to be stained black. In times of flood sediment would be brought in, even where pure coal had been forming, and then we should have a thin "parting" of sandstone or shale, which was formed when the flood was at its height. Or a slight sinking of the land might occur, in which case also the formation of coal would temporarily cease, and a parting of foreign matter would be formed, which, on further upheaval taking place, would again give way to another forest growth. Some of the thicker beds have been found presenting this aspect, such as the South Staffordshire ten-yard coal, which in some parts splits up into a dozen or so smaller beds, with partings of sediment between them.
In the face of the stupendous movements which must have happened in order to bring about the successive growth of forests one above another on the same spot, the question at once arises as to how these movements of the solid earth came about, and what was the cause which operated in such a manner. We can only judge that, in some way or other, heat, or the withdrawal of heat, has been the prime motive power. We can perceive, from what is now going on in some parts of the earth, how great an influence it has had in shaping the land, for volcanoes owe their activity to the hidden heat in the earth's interior, and afford us an idea of the power of which heat is capable in the matter of building up and destroying continents. No less certain is it that heat is the prime factor in those more gradual vertical movements of the land to which we have referred elsewhere, but in regard to the exact manner in which it acts we are very much in the dark. Everybody knows that, in the majority of instances, material substances of all kinds expand under the influence of heat, and contract when the source of heat is withdrawn. If we can imagine movements in the quantity of heat contained in the solid crust, the explanation is easy, for if a certain tract of land receive an accession of heat beneath it, it is certain that the principal effect will be an elevation of the land, consequent on the expansion of its materials, with a subsequent depression when the heat beneath the tract in question becomes gradually lessened. Should the heat be retained for a long period, the strata would be so uplifted as to form an anticlinal, or saddle-back, and then, should subsequent denudation take place, more ancient strata would be brought to view. It was thus in the instance of the tract bounded by the North and South Downs, which were formerly entirely covered by chalk, and in the instance of the uprising of the carboniferous limestone between the coal-fields of Lancashire, Staffordshire, and Derbyshire.
How the heat-waves act, and the laws, if any, which they obey in their subterranean movements, we are unable to judge. From the properties which heat possesses we know that its presence or absence produces marked differences in the positions of the strata of the earth, and from observations made in connection with the closing of some volcanoes, and the opening up of fresh earth-vents, we have gone a long way towards establishing the probability that there are even now slow and ponderous movements taking place in the heat stored in the earth's crust, whose effects are appreciably communicated to the outside of the thin rind of solid earth upon which we live.
Owing to the great igneous and volcanic activity at the close of the deposition of the carboniferous system of strata, the coal-measures exhibit what are known as faults in abundance. The mountain limestone, where it outcrops at the surface, is observed to be much jointed, so much so that the work of quarrying the limestone is greatly assisted by the jointed structure of the rock. Faults differ from joints in that, whilst the strata in the latter are still in relative position on each side of the joint, they have in the former slipped out of place. In such a case the continuation of a stratum on the opposite side of a fault will be found to be depressed, perhaps a thousand feet or more. It will be seen at once how that, in sinking a new shaft into a coal-seam, the possibility of an unknown fault has to be brought into consideration, since the position of the seam may prove to have been depressed to such an extent as to cause it to be beyond workable depth. Many seams, on the other hand, which would have remained altogether out of reach of mining operations, have been brought within workable depth by a series of step-faults, this being a term applied to a series of parallel faults, in none of which the amount of down-throw is great.
The amount of the down-throw, or the slipping-down of the beds, is measured, vertically, from the point of disappearance of a layer to an imaginary continuation of the same layer from where it again appears beyond the fault. The plane of a fault is usually more or less inclined, the amount of the inclination being known as the hade of the fault, and it is a remarkable characteristic of faults that, as a general rule, they hade to the down-throw. This will be more clearly understood when it is explained that, by its action, a seam of coal, which is subject to numerous faults, can never be pierced more than once by one and the same boring. In mountainous districts, however, there are occasions when the hade is to the up-throw, and this kind of fault is known as an inverted fault.
Lines of faults extend sometimes for hundreds of miles. The great Pennine Fault of England is 130 miles long, and others extend for much greater distances. The surfaces on both sides of a fault are often smooth and highly polished by the movement which has taken place in the strata. They then show the phenomenon known as slicken-sides. Many faults have become filled with crystalline minerals in the form of veins of ore, deposited by infiltrating waters percolating through the natural fissures.
In considering the formation and structure of the better-known coal-bearing beds of the carboniferous age, we must not lose sight of the fact that important beds of coal also occur in strata of much more recent date. There are important coal-beds in India of Permian age. There are coal-beds of Liassic age in South Hungary and in Texas, and of Jurassic age in Virginia, as well as at Brora in Sutherlandshire; there are coals of Cretaceous age in Moravia, and valuable Miocene Tertiary coals in Hungary and the Austrian Alps.
Again, older than the true carboniferous age, are the Silurian anthracites of Co. Cavan, and certain Norwegian coals, whilst in New South Wales we are confronted with an assemblage of coal-bearing strata which extend apparently from the Devonian into Mesozoic times.
Still, the age we have considered more closely has an unrivalled right to the title, coal appearing there not merely as an occasional bed, but as a marked characteristic of the formation.
The types of animal life which are found in this formation are varied, and although naturally enough they do not excel in number, there are yet sufficient varieties to show probabilities of the existence of many with which we are unfamiliar. The highest forms yet found, show an advance as compared with those from earlier formations, and exhibit amphibian characteristics intermediate between the two great classes of fishes and reptiles. Numerous specimens proper to the extinct order of labyrinthodontia have been arranged into at least a score of genera, these having been drawn from the coal-measures of Newcastle, Edinburgh, Kilkenny, Saaerbruck, Bavaria, Pennsylvania, and elsewhere. The Archegosaurus, which we have figured, and the Anthracosaurus, are forms which appear to have existed in great numbers in the swamps and lakes of the age. The fish of the period belong almost entirely to the ancient orders of the ganoids and placoids. Of the ganoids, the great megalichthys Hibberti ranges throughout the whole of the system. Wonderful accumulations of fish remains are found at the base of the system, in the bone-bed of the Bristol coal-field, as well as in a similar bed at Armagh. Many fishes were armed with powerful conical teeth, but the majority, like the existing Port Jackson shark, were possessed of massive palates, suited in some cases for crushing, and in others for cutting.
In the mountain limestone we see, of course, the predominance of marine types, encrinital remains forming the greater proportion of the mass. There are occasional plant remains which bear evidence of having drifted for some distance from the shore. But next to the encrinites, the corals are the most important and persistent. Corals of most beautiful forms and capable of giving polished marble-like sections, are in abundance. Polyzoa are well represented, of which the lace-coral (fenestella) and screw-coral (archimedopora) are instances. Cephalopoda are represented by the orthoceras, sometimes five or six feet long, and goniatites, the forerunner of the familiar ammonite. Many species of brachiopods and lammellibranchs are met with. Lingula, most persistent throughout all geological time, is abundant in the coal-shales, but not in the limestones. Aviculopecten is there abundant also. In the mountain limestone the last of the trilobites (Phillipsia) is found.
We have evidence of the existence in the forests of a variety of centipede, specimens having been found in the erect stump of a hollow tree, although the fossil is an extremely rare one. The same may be said of the only two species of land-snail which have been found connected with the coal forests, viz., pupa vetusta and zonites priscus, both discovered in the cliffs of Nova Scotia. These are sufficient to demonstrate that the fauna of the period had already reached a high stage of development. In the estuaries of the day, masses of a species of freshwater mussel (anthracosia) were in existence, and these have left their remains in the shape of extensive beds of shells. They are familiar to the miner as mussel-binds, and are as noticeable a feature of this long ago period, as are the aggregations of mussels on every coast at the present day.
VARIOUS FORMS OF COAL AND CARBON.
In considering the various forms and combinations into which coal enters, it is necessary that we should obtain a clear conception of what the substance called "carbon" is, and its nature and properties generally, since this it is which forms such a large percentage of all kinds of coal, and which indeed forms the actual basis of it. In the shape of coke, of course, we have a fairly pure form of carbon, and this being produced, as we shall see presently, by the driving off of the volatile or vaporous constituents of coal, we are able to perceive by the residue how great a proportion of coal consists of carbon. In fact, the two have almost an identical meaning in the popular mind, and the fact that the great masses of strata, in which are contained our principal and most valuable seams of coal, are termed "carboniferous," from the Latin carbo, coal, and fero, I bear, tends to perpetuate the existence of the idea.
There is always a certain, though slight, quantity of carbon in the air, and this remains fairly constant in the open country. Small though it may be in proportion to the quantity of pure air in which it is found, it is yet sufficient to provide the carbon which is necessary to the growth of vegetable life. Just as some of the animals known popularly as the zoophytes, which are attached during life to rocks beneath the sea, are fed by means of currents of water which bring their food to them, so the leaves, which inhale carbon-food during the day through their under-surfaces, are provided with it by means of the currents of air which are always circulating around them; and while the fuel is being taken in beneath, the heat and light are being received from above, and the sun supplies the motive power to digestion.
It is assumed that it is, within the knowledge of all that, for the origin of the various seams and beds of coaly combinations which exist in the earth's crust, we must look to the vegetable world. If, however, we could go so far back in the world's history as the period when our incandescent orb had only just severed connection with a gradually-diminishing sun, we should probably find the carbon there, but locked up in the bonds of chemical affinities with other elements, and existing therewith in a gaseous condition. But, as the solidifying process went on, and as the vegetable world afterwards made its appearance, the carbon became, so to speak, wrenched from its combinations, and being absorbed by trees and plants, finally became deposited amongst the ruins of a former vegetable world, and is now presented to us in the form of coal.
We are able to trace the gradual changes through which the pasty mass of decaying vegetation passed, in consequence of the fact that we have this material locked up in various stages of carbonisation, in the strata beneath our feet. These we propose to deal with individually, in as unscientific and untechnical a manner as possible.
First of all, when a mass of vegetable matter commences to decay, it soon loses its colour. There is no more noticeable proof of this, than that when vitality is withdrawn from the leaves of autumn, they at once commence to assume a rusty or an ashen colour. Let the leaves but fall to the ground, and be exposed to the early frosts of October, the damp mists and rains of November, and the rapid change of colour is at once apparent. Trodden under foot, they soon assume a dirty blackish hue, and even when removed they leave a carbonaceous trace of themselves behind them, where they had rested. Another proof of the rapid acquisition of their coaly hue is noticeable in the spring of the year. When the trees have burst forth and the buds are rapidly opening, the cases in which the buds of such trees as the horse-chestnut have been enclosed will be found cast off, and strewing the path beneath. Moistened by the rains and the damp night-mists, and trodden under foot, these cases assume a jet black hue, and are to all appearance like coal in the very first stages of formation.
But of course coal is not made up wholly and only of leaves. The branches of trees, twigs of all sizes, and sometimes whole trunks of trees are found, the last often remaining in their upright position, and piercing the strata which have been formed above them. At other times they lie horizontally on the bed of coal, having been thrown down previously to the formation of the shale or sandstone, which now rests upon them. They are often petrified into solid sandstone themselves, whilst leaving a rind of coal where formerly was the bark. Although the trunk of a tree looks so very different to the leaves which it bears upon its branches, it is only naturally to be supposed that, as they are both built up after the same manner from the juices of the earth and the nourishment in the atmosphere, they would have a similar chemical composition. One very palpable proof of the carbonaceous character of tree-trunks suggests itself. Take in your hand a few dead twigs or sticks from which the leaves have long since dropped; pull away the dead parts of the ivy which has been creeping over the summer-house; or clasp a gnarled old monster of the forest in your arms, and you will quickly find your hand covered with a black smut, which is nothing but the result of the first stage which the living plant has made, in its progress towards its condition as dead coal. But an easy, though rough, chemical proof of the constituents of wood, can be made by placing a few pieces of wood in a medium-sized test-tube, and holding it over a flame. In a short time a certain quantity of steam will be driven off, next the gaseous constituents of wood, and finally nothing will be left but a few pieces of black brittle charcoal. The process is of course the same in a fire-grate, only that here more complete combustion of the wood takes place, owing to its being intimately exposed to the action of the flames. If we adopt the same experiment with some pieces of coal, the action is similar, only that in this case the quantity of gases given off is not so great, coal containing a greater proportion of carbon than wood, owing to the fact that, during its long burial in the bowels of the earth, it has been acted upon in such a way as to lose a great part of its volatile constituents.
From processes, therefore, which are to be seen going on around us, it is easily possible to satisfy ourselves that vegetation will in the long run undergo such changes as will result in the formation of coal.
There are certain parts in most countries, and particularly in Ireland, where masses of vegetation have undergone a still further stage in metamorphism, namely, in the well-known and famous peat-bogs. Ireland is par excellence the land of bogs, some three millions of acres being said to be covered by them, and they yield an almost inexhaustible supply of peat. One of the peat-bogs near the Shannon is between two and three miles in breadth and no less than fifty in length, whilst its depth varies from 13 feet to as much as 47 feet. Peat-bogs have in no way ceased to be formed, for at their surfaces the peat-moss grows afresh every year; and rushes, horse-tails, and reeds of all descriptions grow and thrive each year upon the ruins of their ancestors. The formation of such accumulations of decaying vegetation would only be possible where the physical conditions of the country allowed of an abundant rainfall, and depressions in the surface of the land to retain the moisture. Where extensive deforesting operations have taken place, peat-bogs have often been formed, and many of those in existence in Europe undoubtedly owe their formation to that destruction of forests which went on under the sway of the Romans. Natural drainage would soon be obstructed by fallen trees, and the formation of marsh-land would follow; then with the growth of marsh-plants and their successive annual decay, a peaty mass would collect, which would quickly grow in thickness without let or hindrance.
In considering the existence of inland peat-bogs, we must not lose sight of the fact that there are subterranean forest-beds on various parts of our coasts, which also rest upon their own beds of peaty matter, and very possibly, when in the future they are covered up by marine deposits, they will have fairly started on their way towards becoming coal.
Peat-bogs do not wholly consist of peat, and nothing else. The trunks of such trees as the oak, yew, and fir, are often found mingled with the remains of mosses and reeds, and these often assume a decidedly coaly aspect. From the famous Bog of Allen in Ireland, pieces of oak, generally known as "bog-oak," which have been buried for generations in peat, have been excavated. These are as black as any coal can well be, and are sufficiently hard to allow of their being used in the manufacture of brooches and other ornamental objects. Another use to which peat of some kinds has been put is in the manufacture of yarn, the result being a material which is said to resemble brown worsted. On digging a ditch to drain a part of a bog in Maine, U.S., in which peat to a depth of twenty feet had accumulated, a substance similar to cannel coal itself was found. As we shall see presently, cannel coal is one of the earliest stages of true coal, and the discovery proved that under certain conditions as to heat and pressure, which in this case happened to be present, the materials which form peat may also be metamorphosed into true coal.
Darwin, in his well-known "Voyage in the Beagle" gives a peculiarly interesting description of the condition of the peat-beds in the Chonos Archipelago, off the Chilian coast, and of their mode of formation. "In these islands," he says, "cryptogamic plants find a most congenial climate, and within the forest the number of species and great abundance of mosses, lichens, and small ferns, is quite extraordinary. In Tierra del Fuego every level piece of land is invariably covered by a thick bed of peat. In the Chonos Archipelago where the nature of the climate more closely approaches that of Tierra del Fuego, every patch of level ground is covered by two species of plants (Astelia pumila and Donatia megellanica), which by their joint decay compose a thick bed of elastic peat.
"In Tierra del Fuego, above the region of wood-land, the former of these eminently sociable plants is the chief agent in the production of peat. Fresh leaves are always succeeding one to the other round the central tap-root; the lower ones soon decay, and in tracing a root downwards in the peat, the leaves, yet holding their places, can be observed passing through every stage of decomposition, till the whole becomes blended in one confused mass. The Astelia is assisted by a few other plants,—here and there a small creeping Myrtus (M. nummularia), with a woody stem like our cranberry and with a sweet berry,—an Empetrum (E. rubrum), like our heath,—a rush (Juncus grandiflorus), are nearly the only ones that grow on the swampy surface. These plants, though possessing a very close general resemblance to the English species of the same genera, are different. In the more level parts of the country the surface of the peat is broken up into little pools of water, which stand at different heights, and appear as if artificially excavated. Small streams of water, flowing underground, complete the disorganisation of the vegetable matter, and consolidate the whole.