Some Salient Points in the Science of the Earth

by Sir John William Dawson

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My early boyhood was spent on the Coal formation rocks and in the vicinity of collieries; and among my first natural history collections, in a childish museum of many kinds of objects, were some impressions of fern leaves from the shales of the coal series. It came to pass in this way that the Carboniferous rocks were those which I first studied as an embryo geologist, and much of my later work has consisted in collecting and determining the plants of that ancient period, and in studying microscopic sections of coals and fossil woods accompanying them. For this reason, and because I have published so much on this subject, my first decision was to leave it out of these Salient Points: but on second thoughts it seemed that this might be regarded as a dereliction of duty; more especially as some of the conclusions supposed to be the best established on this subject have recently been called in question.

Had I been writing a few years ago, I might have referred to the mode of formation of coal as one of the things most surely settled and understood. The labours of many eminent geologists, microscopists and chemists in the old and the new worlds had shown that coal nearly always rests upon old soil-surfaces penetrated with roots, and that coal beds have in their roofs erect trees, the remains of the last forests that grew upon them. Logan and the writer have illustrated this in the case of the series of more than eighty successive coal beds exposed at the South Joggins, and of the great thirty feet seam of the Picton coal series, whose innumerable laminæ have all been subjected to careful scrutiny, and have shown unequivocal evidence of land surfaces accompanying the deposition of the coal. Microscopical examination has proved that these coals are composed of the materials of the same trees whose roots are found in the underclays, and their stems and leaves in the roof shales; that much of the material of the coal has been partially subjected to subaërial decay at the time of its accumulation; and that in this, ordinary coal differs from bituminous shale, earthy bitumen and some kinds of cannel, which have been formed under water; that the matter remaining as coal consists almost entirely of epidermal tissues, which being suberose or corky in character are highly carbonaceous, very durable and impermeable by water, and are, hence, the best fitted for the production of pure coal; and finally, that the vegetation and the climatal and geographical features of the coal period were eminently fitted to produce in the vast swamps of that period precisely the effects observed. All these points and many others have been thoroughly worked out for both European and American coal fields, and seemed to leave no doubt on the subject. But several years ago certain microscopists observed in slices of coal, thin layers full of spore cases, a not unusual circumstance, since these were shed in vast abundance by the trees of the coal forests, and because they contain suberose matter of the same character with epidermal tissues generally. Immediately we were informed that all coal consists of spores, and this being at once accepted by the unthinking, the results of the labours of many years are thrown aside in favour of this crude and partial theory. A little later, a German microscopist has thought proper to describe coal as made up of minute algæ, and tries to reconcile this view with the appearances, devising at the same time a new and formidable nomenclature of generic and specific names, which would seem largely to represent mere fragments of tissues. Still later, some local facts in a French coal field have induced an eminent observer of that country to revive the drift theory of coal, in opposition to that of growth in situ. Views of this kind have also recently been advanced in England by some of those younger men who would earn distinction rather by overthrowing the work of their seniors than by building on it. These writers base their conclusions on a few exceptional facts, as the occasional occurrence of seams of coal without distinct underlays, and the occurrence of clay partings showing aquatic conditions in the substance of thick coals; and they fail to discern the broader facts which these exceptions confirm. Let us consider shortly the essential nature of coal, and some of the conditions necessary to its formation.

A block of the useful mineral which is so important an element in national wealth, and so essential to the comfort of our winter homes, may tell us much as to its history if properly interrogated, and what we cannot learn from it alone we may be taught by studying it in the mine whence it is obtained, and in the cliffs and cuttings where the edges of the coaly beds and their accompaniments are exposed.

Our block of coal, if anthracite, is almost pure carbon. If bituminous coal, it contains also a certain amount of hydrogen, which in combination with carbon enables it to yield gas and coal tar, and which causes it to burn with flame. If, again, we examine some of the more imperfect and more recent coals, the brown coals, so called, we shall find that in composition and texture they are intermediate between coal proper and hardened or compressed peat. Now such coaly rocks can, under the present constitution of nature, be produced only in one way, namely, by the accumulation of vegetable matter, for vegetation alone has the power of decomposing the carbonic acid of the atmosphere, and accumulating it as carbon. This we see in modern times in the vegetable soil, in peaty beds, and in vegetable muck accumulated in ponds and similar places. Such vegetable matter, once accumulated, requires only pressure and the changes which come of its own slow putrefaction to be converted into coal.

But in order that it may accumulate at all, certain conditions are necessary. The first of these includes the climatal and organic arrangements necessary for abundant vegetable growth. The second is the facility for the preservation of the vegetable matter, without decay or intermixture with earthy substances; and this, for a long time, till a great thickness of it accumulates. The third is its covering up by other deposits, so as to be compressed and excluded from air. It is evident that when we have to consider the formation of a bed of coal several feet in thickness, and spread, perhaps, over hundreds of square miles, many things must conduce to such a result, and the wonder is perhaps rather that such conditions should ever have been effectively combined. Yet this has occurred at different periods of geological history and in many places, and in some localities it has been so repeated as to produce many beds of coal in succession.

Let us now question our block of coal as to its origin, supposing it to be a piece of ordinary bituminous coal, or still better, a specimen of one of the impure somewhat shaly coals which one sometimes finds accidentally in the coal bin. In looking at the edge of our specimen we observe that it has a "reed" or grain, which corresponds with the lamination or bedding of the seam of coal from which it came. Looking at this carefully, we shall see that there are many thin layers of bright shining coal, and the more of these usually the better the coal. These layers, in tracing them along, we observe often to thin out and disappear. They are not very continuous. If our specimen is an impure coal, we will find that it readily splits along the surfaces of these layers, and that when so split, we can see that each layer of shining coal has certain markings, perhaps the flattened ribs and scars of Sigillaria or other coal-formation trees on its surface. In other words, the layers of fine coal are usually flattened trunks and branches of trees, or perhaps rather of the imperishable and impermeable bark of such trees, the wood having perished. A few very thin layers of shining coal we may also find to consist of the large-ribbed leaves of the plant known as Cordaites. This kind of coaly matter then usually represents trunks of trees which in a prostrate and flattened state may constitute more than half of the bulk of ordinary coal-formation coal. Under the microscope this variety of coal shows little structure, and this usually the thickened cells of cortical tissue. Intervening between these layers we perceive lamina?, more or less thick and continuous, of what we may call dull coal, black but not shining; resembling, in fact, the appearance of cannel coal. If we split the coal along one side of these layers, and examine it in a strong light, we may see shreds of leaf stalks and occasionally even of fern leaves, or skeletons of these, showing the veins, and many flattened disc-like bodies, spore cases and macrospores, shed by the plants which make up the coal. These layers represent what may be called compressed vegetable mould or muck, and this is by no means a small constituent of many coals. This portion of the coal is the most curious and interesting in microscopic slices, showing a great variety of tissues and many spores and spore cases. Lastly, we find on the surface of the coal, when split parallel to the bedding, a quantity of soft shining fibrous material, known as mineral charcoal or mother coal, which in some varieties of the mineral is very abundant, in others much more rare. This is usually too soft and incoherent to be polished in thin slices for the microscope; but if boiled for a length of time in nitric acid, so as to separate all the mineral matter contained in it, the fibres sometimes become beautifully translucent and reveal the tissues of the wood of various kinds of Carboniferous trees, more especially of Calamites, Cordaites and Sigillariæ. Fibres of mineral charcoal prepared in this way are often very beautiful microscopic objects under high powers; and this material of the coal is nothing else than little blocks of rotten wood and fibrous bark, broken up and scattered over the surface of the forming coal bed. All these materials, it must be observed, have been so compressed that the fragments of decayed wood have been flattened into films, the vegetable mould consolidated into a stony mass, and trunks of great trees converted by enormous pressure into laminæ of shining coal, a tenth of an inch in thickness, so that the whole material has been reduced to perhaps one-hundredth of its original volume.

Restoring the mass in imagination to its original state, what do we find? A congeries of prostate trunks with their interstices filled with vegetable muck or mould, and occasional surfaces where rotten wood, disintegrated into fragments, was washed about in local floods or rain storms, and thus thrown over the surface. Lyell seems very nearly to have hit the mark when he regarded the conditions of the great dismal swamp of Virginia as representing those of a nascent coal field. We have only to realize in the coal period the existence of a dense vegetation very different from that of modern Virginia, of a humid and mild climate, and of a vast extension of low swampy plains, to restore the exact conditions of the coal swamps.

But how does this correspond with the facts observed in mines and sections? To the late Sir William Logan is due the merit of observing that in South Wales the underclays or beds of indurated clay and earth underlying the coal seams are usually filled with the long cylindrical rootlets and branching roots of a curious plant, very common in the coal formation, the Stigmaria. He afterwards showed that the same fact occurs in the very numerous coal beds exposed in the fine section cut by the tides of the Bay of Fundy, in the coal rocks of Nova Scotia. In that district I have myself followed up his observations, examining in detail every one of eighty-one Coal Groups, as I have called them, each consisting of at least one bed of coal, large or small, with its accompaniments, and in many cases of several small seams with intervening clays or shales.[113] In nearly every case the Stigmaria "underclay" is distinctly recognisable, and often in a single coal group there are several small seams separated by underclays with roots and rootlets. These underclays are veritable fossil soils; sometimes bleached clays or sands, like the subsoils of modern swamps; sometimes loamy or sandy, or of the nature of hardened vegetable mould. They rarely contain any remains of aquatic animals, or of animals of any kind, but are filled with stigmaria roots and rootlets, and sometimes hold a few prostrate stems of trees.[114] While the underclay is thus a fossil soil, the roof or bed above the coal, usually of a shaly character, is full of remains of leaves and stems and fruits, and often holds erect stumps, the remains of the last trees that grew in the swamp before it was finally covered up.

[113] For details see Journal Geol. Society of London, 1865; and "Acadian Geology," last edition, 1891.