The story of a piece of coal; what it is, whence it comes, and whither it goes

Preface

When the first edition of this book was published, Great Britain was the leading coal-production country of the world. In 1899, the United States supplanted Great Britain in the position of leadership, and since then the relative importance of this country has steadily advanced. In the 14 years from 1899 to 1913, the coal production of the United States increased from 254 million tons to 570 million tons, a gain of nearly 125 per cent. The production in Great Britain in the same period has increased a little over 30 per cent., from 247 million tons to 322 million tons. Germany, which ranks third among the coal-producing countries, has increased her production by 88 per cent., a ratio nearly three times that of Great Britain, from 150 million tons in 1899 to 282 million tons in 1913. The combined production of Great Britain and Germany exceeds that of the United States by less than 6 per cent. The total production of the world in 1913 was approximately 1,444 million tons, of which the United States contributed nearly 40 per cent.

6 PREFACE.

The development of the coal-mining industry in the United States to its present vast proportions, considered with the increase in the population, presents some interesting comparisons. The use of mineral fuel became an important factor in the manufacture of iron about the middle of the last century. In 1850, the production of coal amounted to 7,018,000 tons, or at the rate of three-tenths of a ton for each of the 23,192,000 inhabitants. In i860, the production was still less than one-half ton per person, but in 1880, when the population had increased to 50,189,000, the production of coal was 71,482,- 000 tons, an average of 1.42 tons per capita. In 1900, the production averaged 3.53 tons for each inhabitant, and in 1913 the average production rose to 5.87 tons.

A material of such immense importance to the industrial and commercial prosperity of the country is its own recommendation to careful study. The story of coal as told in the following pages gives the main facts of its origin, occurrence, distribution, mining, and application, in a form which has proved attractive to many thousands of readers.

Contents

CHAPTER

I. The Origin of Coal and the Plants of

WHICH IT IS COMPOSED .

Chapter I. 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 England 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.

IO THE STORY OF A PIECE OF COAL.

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 sub-aqueous 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

THE ORIGIN AND COMPOSITION OF COAL. II

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, such as the living firs, pines, larches, etc., 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

Fig. 2. — Annularia radiata. iferous sandstone.

Carbonwhilst the

*4

THE STORY OF A PIECE OF COAL.

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 found preserved in the English coals 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 our country 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,

Fig. 4.— Frond of Pecopteris. Coal-shale.

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 tnegaphyton, fialceopteris, and caulopteris, all of which have these marks upon them, thus proving that

Fig. 5. — Pecopteris Serin. Coal-shale.

at one time even tree-ferns had a habitat in our latitude.

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 "wedgefern," is the name applied to another coalfern ; gloss op teris, or "tongue-leaf " ; cyclopteris, or " round - leaf " ; odontopteris, or " toothleaf/' 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 spects resembles pecopteris.

Fig.

6. — Sphenopteris affinis* Coal-shale.

in some re-

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 Calamttes. The generic distinctions between fossil and living ferns were so slight in many cases as to be almost indistin-

FiG. 7. — Root of Catamites Suckowii. Coal-shale.

guishable. 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

THE STORY OF A PIECE OF COAL.

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 horsetail (equiselum 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

FiG. %.—Calamocladus grandts.

Carboniferous sandstone.

bark." The bark having almost always 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 aster op hy Hit es in the fact that they were single-nerved. Sir William Dawson assigns the calamites to four sub-types: calamite proper, calamopitus, calamodendron, and enc alamo dendr on.

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, since they are almost entirely composed of numerous cells built up one

Fig. 9. — Asterophyllites foltosa. Coal-measures.

THE STORY OF A PIECE pF COAL.

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 (i) Cryptogams, or Flowerless, such as mosses, ferns, equisetums, (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 trees of temperate regions. Thus : —

(M. A. Brongniart).

(Lindley).

Cellular

Cryptogams (Flowerless)

Fungi, seaweeds,

li phpnc

Thallogens

Vascular

IlL-llCIlb

Cryptogams (Flowerless)

Ferns, equisetums, mosses, lycopodiums

Acrogens

Phanerogams (Flowering)

Gymnosperms (having

Conifers and cy-

Gymnogens naked seeds)

cads i. Dicotyledons

Angiosperms (having enclosed seeds)

i. Monocotyledons

Palms, lilies, grasses

Endogens ii. Dicotyledons

Most European trees and shrubs

Exogens

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 (filices), and the equisetums^ {catamites and eguisetites), 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 as twoof the mass.

much thirds whole

Sphenophyllum cuneifolium* Coal- shale.

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 present 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

THE STORY OF A PIECE OF COAL.

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

Fig. t i.— Cast of Lepidodendron in sandstone.

Fig.

12.— Lepidodendron longifolium. Coal-shale.

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 telodendron are all referable to this family.

In some strata, as for instance that of the Shropshire coal-field, 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

Fig. 13. — Lepidodendron aculeatum in sandstone.

THE STORY OF A PIECE OF COAL.

been found actually 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

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 seedgerms 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

Fig. 14. — Lepidostrobus.

Coal-shale.

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,.

Fig. 15. — Lycopodites, Coal sandstone.

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 nolarger than those of the miniature ciub-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.

Sigillarice 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 dathraria. Diploxylon is a term applied to an interior stem referable to the family.

THE STORY OF A PIECE OF COAL.

But the most interesting point about the sigillaricE is the root. This was for a long time regarded as an entirely distinct individual, and the older geologists explained it in their writings

Fig. 16. — Stigmaria ficoides. Coal-shale.

as a species of succulent aquatic plant, giving it the name of stig??iaria. 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 " Element's 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 waterplants, 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.