ON PEAT-WOOD, LIGNITE, AND COAL.

Before entering upon the examination of the specific and generic characters of fossil plants, and the natural relations of the extinct forms with those of the existing Floras, it will be requisite to notice those vast beds of vegetable matter, in various states of carbonization, which occur in the palÆozoic, secondary, and tertiary formations.

Submerged Forests. Peat.—The phenomenon of extensive tracts of marsh-land, with layers of prostrate trees of all ages, lying but a few feet beneath the common alluvial soil, is of frequent occurrence, both inland, and in many places along the shores of our island. (Geol. S. E. p. 18). These submerged forests are generally situated below the level of the sea, and afford unquestionable proof of subsidences of the land. The trees are of the kinds indigenous to the districts in which they occur; and leaves and seeds of the hazel, beech, elm, &c. are often preserved in the silt in which the prostrate forests are imbedded. On the Sussex coast there are accumulations of this kind, at Bexhill, Pevensey levels, Felpham, &c.

The extensive subterranean forests exposed in the Fens of Lincolnshire by the operations carried on for draining that district, must be familiar to those who travel by the Great Northern Railway: the protruding upright stems, broken off at a short distance above the primitive soil, will remind the geological observer of the petrified forest of the Isle of Portland.

The wood in these cases has undergone no change but that of being dyed black, by an impregnation of solutions of iron; and many trunks are in so sound a state as to be employed in building. The oak timbers of the Royal George, lately raised up from off Portsmouth, after being immersed in silt about sixty years, closely resemble in colour and texture the wood of the submerged forests. Skeletons of deer, horse, swine, &c. are occasionally found imbedded in these subterranean accumulations of vegetable remains; and sometimes canoes, formed of the trunk of an oak, constructed by the aboriginal inhabitants of Britain, with stone implements called celts, are met with at considerable depths.

In the peat-bogs of Ireland (Wond. p. 66), large forest trees often occur, together with the skeletons of the elk, deer, and other animals of the chase; and in a few instances the bodies of the primitive hunters, wrapped in skins, have been discovered.

In Belfast Lough, a bed of peat is situated beneath the ordinary level of the waters, but is generally left bare at the ebb tides. Trunks and branches of trees, with vast quantities of hazel nuts, are imbedded in the peat; the whole being covered by layers of sand, and blue clay, or silt. In most cases the pericarps of the nuts are empty, the kernels having perished; but on the eastern side of the Lough, which is bounded by limestone rocks, they contain calc-spar, which in some examples forms a lining of delicate crystals (Plate V. fig. 6); while in others the kernel is transmuted into calcareous spar (see Plate III. fig. 7); but the pericarps are unchanged, and in the state of common dried nut-shells; the water which deposited the spar in their cavities not having left a particle of mineral matter in the ligneous substance through which it had filtrated.

In a subterranean forest at Ferry-bridge, Yorkshire, hazel nuts in a similar mineralized state occur, and the branches and stems of the trees have undergone a like change; the central ligneous axis is petrified, while the outer zones have undergone no lapidification, but remain in the state of dry rotten wood.[46]

Lignite, Brown Coal, or Cannel Coal; these are terms employed to designate certain varieties of carbonized wood, in which the ligneous structure is more or less distinctly preserved. Lignite may be regarded as an imperfect coal, for in its chemical properties it holds an intermediate place between peat and bituminous coal. It is for the most part found in tertiary formations, but is not unfrequent in ancient secondary deposits, and may occur in the earliest sedimentary rocks which contain vegetable remains.

The newer deposits of Brown or wood-coal, are commonly situated in depressions or basins, as if they had been produced by the submergence of woods and forests, in a swamp or morass; and in many instances the ligneous structure is distinct in one part of the bed, while in another the mass is a pure black coal, differing in no respect from true coal, except that it is less dense.

Bovey Coal.—One of the most instructive deposits of brown coal in England, is that of Bovey Heathfield, near Chudleigh in Devonshire, which is of considerable thickness and extent, and presents all the characters of a true coal-field; namely, beds of carbonized vegetables, alternating with layers of clay and marl. The Bovey coal is in the state of bituminized wood, the vascular tissue (which is coniferous in the specimens that have come under my notice) being apparent. It is easily chipped or split, and leaves a considerable quantity of white ashes after combustion. The layers of coal vary in thickness from one foot to three feet; and there are eighteen or twenty in a depth of about 120 feet; this coal-field extends seven or eight miles. No leaves or fruits have been discovered; bitumen occurs both in the coal and in the intermediate clays. Calcareous spar, and iron pyrites, prevail in many of the strata. In some places this brown coal is covered by a bed of peat, in which trunks and cones of firs are imbedded. The whole series appears to have been a lacustrine deposit; probably formed in a lake, into whose basin rafts of pine forests were drifted by periodical land-floods. (Org. Rem. I. p. 327).

The brown-coal formations on the banks of the Rhine, present the same phenomena on a more extended scale, and complicated with changes induced by volcanic action. In Iceland, where at the present time forests are unknown, there are extensive deposits of lignite of a peculiar kind, termed surturbrand.

Jet.—The beautiful substance called Jet, is a compact lignite, and the vascular tissue may be detected even in the most solid masses; when prepared in very thin slices, it appears of a rich brown colour by transmitted light, and the woody texture is visible to the naked eye. Jet is found in great purity and abundance in the cliffs of alum-shale on the Yorkshire coast, which were celebrated in the early centuries for the production of this substance. At Whitby and Scarborough extensive manufactories of ornaments and trinkets of jet are established. The sandstone cliffs near Whitby contain masses of a very compact variety, locally termed stone-jet. In the front of the cliff, on the north-west side of Haiburn Wyke, the stump of a tree was observed in an erect position, about three feet high, and fifteen inches in diameter; the roots traversed a bed of shale, and were in the state of coarse jet, but the trunk, which extended into the sandstone, was in part silicified, while other portions were decayed and had a sooty aspect.[47]

Thin seams and layers, and nodular masses, as well as regular coal-fields of lignite, occur in the tertiary formations. At Castle Hill, near Newhaven, in Sussex (Wond. p. 239), a seam of lignite resembling the surturbrand of Iceland, a few inches thick, is interposed between strata of red marl in which are carbonized leaves of dicotyledonous trees.

At Alum Bay in the Isle of Wight, a layer of lignite occurs between the beds of vertical gravel and sand of that interesting locality.

Wealden Coal.—The Wealden formation, in some districts, contains layers of lignite, which alternate with finely laminated micaceous sandstones, marls, and clays, abounding in minute carbonized fragments of fern-leaves, with fresh-water shells, and entomostracous crustaceans. This series of strata so strikingly resembles in its general aspect the characters of a coal-field, that some years since extensive works were undertaken in Sussex, in the expectation that coal might be obtained of suitable quality for economical purposes. The search was unsuccessful, but the attempt deserves not the censure that was bestowed upon it, in the infancy of geological science;[48] for experience has since shown, that although the true coal-measures are only found beneath the Triassic and Permian formations, good combustible bituminous coal is not necessarily restricted to any period or series of strata, but may occur wherever the local conditions were favourable to the accumulation and bituminization of vegetable matter. In fact, the coal-fields of the north of Germany are of the Wealden epoch; and this coal more closely approaches in its chemical characters the black-coal of the ancient carboniferous formations, than any of the lignites and brown-coals of the tertiary strata. Some of the beds are highly bituminous, especially those of Schaumberg, and of the principality of BÜckeburg, which may rank with the best English Newcastle coal; but those layers which are derived from coniferous trees and plants are more laminated, and somewhat resemble the brown-coal. These deposits have originated for the most part from carbonized conifers and cycads, with a few ferns and lycopodiaceÆ, or club-mosses.

The brown-coal of Hohen-Warte by the Osterweld, is chiefly formed of the Abies Linkii, and Pterophyllum Lyellianum, whose leaves and twigs, closely impacted together, are generally of a brownish colour, have a glossy surface, and, when soaked in water, are perfectly flexible. The other modification of Wealden coal appears to have undergone a greater degree of pressure, and of exclusion from the atmosphere; no ligneous structure is apparent, but indistinct impressions of leaves are perceptible, and these are chiefly of ferns and club-mosses. This coal has probably resulted from an accumulation of plants of less firm texture, and more perishable, than those of which the former is composed.[49]

Many interesting facts relating to the carbonization of vegetables, came under my observation during my researches in the Wealden strata; and it is a subject of regret to me, that circumstances prevented my following up the investigation of those still imperfectly explored deposits. Small nodular portions of coal, in which no structure is apparent, often occur in the calciferous grit of Tilgate Forest; and sometimes large masses of lignite, fissured in every direction, and having the interstices filled with white calcareous spar.[50] Some of the sandstones are discoloured by the abundance of minute particles of lignite, produced by the disintegration of ferns peculiar to the country of the Iguanodon.

The original structure and composition of a plant doubtless affected its carbonization; for in the same layer of stone, the stems of Endogenites, hereafter described, invariably possess a thick, outer crust, of coal; while those of ClathrariÆ, plants allied to the Cycads, have not a particle of carbonaceous matter, but are surrounded by a reddish brown earthy substance. The nature of the stratum in which the plants were imbedded, must also have influenced the process of bituminization. Masses of vegetables buried beneath beds of tenacious clay, by which the escape of the gaseous elements set free by decomposition was prevented, must have been placed under the most favourable conditions for their conversion into lignite and coal.

That the production of lignite is still going on there can be no doubt; and the following instance of a bed of recent origin, affords an instructive illustration of the subject . Near Limerick, in the district of Maine, one of the States of North America, there are peat-bogs of considerable extent, in which a substance similar to cannel-coal is found at the depth of three or four feet from the surface, amidst the remains of rotten logs of wood, and beaver-sticks:[51] the peat is twenty feet thick, and rests upon white sand. This coal was discovered on digging a ditch to drain a portion of the bog, for the purpose of obtaining peat for manure. The substance is a true bituminous coal, containing more bitumen than is found in any other variety,[52] Polished sections of the compact masses exhibit the peculiar structure of coniferous trees, and prove that the coal was derived from a species allied to the American fir.

Coal.—We proceed to the examination of that remarkable substance which has resulted from the perfect bituminization of the vegetables of the most ancient Flora which geological researches have brought to light, and to which the term Coal is commonly restricted.

Although Balthazar Klein in the sixteenth century affirmed that coal owed its formation to wood and other vegetable substances,[53] yet I can well remember when many eminent geologists were sceptical on this point; and the truth in this, as in most other questions of natural philosophy, was established with difficulty. The experiments and observations of the late Dr. Macculloch, mainly contributed to solve the problem as to the vegetable nature of this substance; and that eminent chemist and geologist successfully traced the transition of vegetable matter from peat-wood, brown-coal, lignite, and jet, to coal, anthracite, graphite, and plumbago. Nor must the meritorious labours of that accomplished naturalist, and excellent man, the late Mr. Parkinson, author of the "Organic Remains of a Former World," in this field of research, be forgotten.[54] The first volume of that work, which treats on fossil plants, contains much original information on the transmutation of vegetables into the various mineral substances in which the nature and original structure of the originals are altogether changed and obliterated; it may still be consulted by the student with advantage.

Although the vegetable origin of all coal will not admit of question, yet evidence of the internal organization of the plants of which it is composed, is not always attainable; for the most perfect coal has undergone a complete liquefaction, and if any portions of the structure remain, they appear under the microscope as if imbedded in a pure bituminous mass. The slaty coal generally preserves traces of cellular or vascular tissue, and the spiral vessels, and the dotted cells of coniferous trees, may readily be detected in chips or slices, prepared in the manner previously pointed out (ante, p. 66.). In many examples the cells are filled with an amber-coloured resinous substance; in others the organization is so well preserved, that on the exposed surface of a piece of coal cracked by exposure to heat, the vascular tissue, spiral vessels, and cells studded with glands, may be detected. Even in the white ashes left after combustion, traces of the spiral vessels are often discernible under a highly magnifying powder. Some beds of coal are wholly composed of minute leaves and disintegrated foliage; and if a mass recently extracted from the mine be split asunder, the surface is seen to be covered with flexible pellicles of carbonized leaves and fibres, matted together; and flake after flake may be peeled off through a thickness of many inches, and the same structure be apparent. Rarely are any large trunks or branches observable in the coal; the appearance of many beds being that of a deposit of foliage, shed and accumulated in a forest, (as may be observable in existing pine-districts,) and consolidated by pressure, while undergoing that peculiar change by which vegetable matter is converted into a carbonaceous mass.

In fine, a gradual transition may be traced from the peat-wood and submerged forests of modern times, in which leaves, fruits, and trunks of indigenous trees and plants are preserved, to those vast deposits of mineral coal, formed by the bituminization of the extinct Floras which flourished in the palÆozoic ages.

The geological position of the ancient coal, the manner in which it is interstratified with layers of clay, shale, micaceous sandstone, grit, and ironstone—in some districts associated with beds of fresh-water shells (Sil. Syst. p. 84)—in others alternating with strata containing marine remains,—are fully treated of in Wond. pp. 729-733, and Bd. p. 525; and it is not within the scope of the present work to dwell in detail upon what may be termed the physical geology of the carboniferous deposits. But a few observations on the phenomena presented by these accumulations of bituminized vegetables and their associated strata, are necessary to render the subsequent remarks on the habits and affinities of the plants composing the palÆozoic Flora intelligible to the general reader.

While the essential conditions for the conversion of vegetable substances into coal appear to be the imbedding of large quantities of recent trees and plants in a deposit which shall exclude the air, and prevent the escape of the gaseous elements when released by decomposition from their organic combination, so, according to the more or less perfect manner in which these conditions are fulfilled, will result coal, jet, lignite, brown-coal, or peat-wood; or a mass of partially carbonized vegetables, like that observable when new-mown hay undergoes spontaneous combustion, from bituminous fermentation in the atmosphere (Wond. p. 701. Org. Rem. I. p. 181).

The manner in which the carboniferous strata have been deposited, has been a subject of much discussion. Some contend that the coal-measures were originally in the state of peat-bogs, and that the successive layers were formed by the subsidences of forests which grew on the sites now occupied by their carbonized remains; others suppose that the vegetable matter originated from rafts, like those of the Mississippi, which floated out to sea, and became engulfed; while many affirm that the coal-measures were accumulated in inland seas or lakes, the successive beds of vegetable matter being supplied by periodical land-floods; and the supporters of each hypothesis bring numerous facts in corroboration of their respective opinions. There can, I think, be no doubt that the production of coal has taken place under each of these conditions, and that at different periods, and in various localities, all these causes have been in operation; in some instances singly, in others in combination. Coal may have been formed at the bottom of fresh-water lakes, as in those instances where it is associated with fresh-water shells and crustaceans, as at Burdie House (Wond. p. 693), and in some of the Derbyshire and Yorkshire deposits; in the beds of rivers and estuaries, as in the Wealden, and in the Shrewsbury coal-field;[55] and from drifted forests, like the rafts of the American rivers, transported into the sea, and engulfed in the abyss of the ocean;[56] and the remains of terrestrial, lacustrine, and marine animals, may accordingly be found associated with it.[57] But though many coal-fields (or basins, as they are termed, because they occupy depressions) have evidently been produced by different, and local agencies, the sedimentary deposits and coal-beds comprised in the carboniferous formations, setting aside unimportant variations, present a remarkable uniformity of character in their nature and arrangement, not only throughout Great Britain and Europe, but in every other part of the known world.

Stratification of a Coal-field.—The group of strata constituting a coal-field consists of an alternation of layers of coal and of clay, of variable thickness, resting, very generally, on grit, or marine limestone abounding in shells, corals, and crinoidea.

My late excellent friend, Mr. Bakewell, used to exemplify the manner in which the beds of coal are interstratified with layers of clay and shale, by the following apt illustration; let a series of mussel-shells be placed one within the other, and a layer of clay be interposed between each; the shells will represent the beds of coal, and the partitions of clay the earthy strata intercalated between the carboniferous layers; now, if one side of the series of shells be raised to indicate the general rise of the strata in that direction, and the whole be dislocated by partial cracks and fissures, the general arrangement and subsequent displacement of the beds will be represented.

The principal feature which arrests attention on the examination of the section of a coal-pit, is the uniform presence of a thick bed of clay beneath every layer of coal; but a still more extraordinary fact remains to be mentioned, namely, that a common plant of the coal strata, called Stigmaria, (hereafter described, see Lign. 36, 38,) invariably occurs, more or less abundantly, in this bed of under-day, although very rarely to be met with in the coal or shale above. This phenomenon, long since noticed by Martin, Macculloch, and other authors, but whose value was not duly estimated till the recent observations of Mr. Logan, (Geol. Proc. vol. iii. p. 275,) is also found to prevail throughout the Welsh coal formation, which is upwards of twelve thousand feet in thickness, and contains more than sixty beds of coal, and as many of clay with stigmariÆ; the Appalachian coal-measures of the United States present the same characters.[58] To place this fact before the student in a clear point of view, I will describe one of the triple series of beds which compose a coal-field.

1. Under-clay; the lowermost stratum. A tough argillaceous substance, which upon drying becomes a grey friable earth: it is occasionally black, from the presence of carbonaceous matter. It contains innumerable stems of stigmariÆ, which are generally of considerable length, and have their rootlets or fibres (see Lign. 38) attached, and extending in every direction through the clay: these stems commonly lie parallel with the planes of the bed, and nearer to the top than to the bottom.

2. Coal. A carbonized mass, in which the external forms of the plants and trees composing it are obliterated, but the internal structure remains; large trunks or stems, and leaves, are rarely distinguishable in it, but the presence of coniferous wood in many beds of coal, proves that this arises, not from the absence of trees, but from their external forms having been obliterated.

3. The Roof, or upper bed. This generally consists of slaty clay, abounding in leaves, trunks, stems, branches, and fruits, and contains layers and nodules of ironstone, inclosing leaves, insects, crustaceans, &c.

In some localities beds of fresh-water mussels, and in others of marine shells, are intercalated; layers of shale, finely laminated clay, micaceous sand and grit, and pebbles of limestone, granite, sandstone, and other rocks, are often present. The most illustrative examples of the foliage of the carboniferous flora are found in this deposit, which appears to be an accumulation of drifted materials derived from other rocks, and promiscuously intermingled with the dense foliage and stems of a prostrate forest; the whole having been transported from a distance by a powerful current or flood.

Thus we have, in the first place, spread uniformly over the bottom, and constituting the bed on which the coal reposes, a stratum of clay (Under-clay), composed of fine pulverulent materials, which may have once constituted the soil of a vast plain or savannah; the only remains found in it are the roots of gigantic trees (see Lign. 36); for such the stigmariÆ are now proved to have been, and not aquatic plants, as was formerly supposed (Bd. p. 476).

Secondly, a bituminous mass (Coal), composed of coniferous wood, gigantic ferns, club-mosses, &c.; occasionally with trunks of trees penetrating vertically through it.

Thirdly, a deposit of drift or water-worn materials (the Roof), mixed with the foliage and stems of numerous species of terrestrial plants; the whole appearing to have been subjected to the action of currents. The first, or Under-clay, may have been the natural soil, in which the stigmariÆ grew; the next,—the Coal,—the carbonized stems, and other remains of the trees to which the roots belonged: and the last, or uppermost, forming the roof of the coal, may have resulted from the foliage and branches of a prostrate forest, overwhelmed and buried beneath the transported detritus of distant rocks.

These phenomena may be explained by supposing the inundation of a thickly-wooded plain from an irruption of the sea; or of a vast inland lake, occasioned by the sudden removal of some barrier; or by a subsidence of the tract of country on which the forest grew. But when we find an accumulation of strata, in which triple deposits of this kind are repeated some thirty or forty times through a thickness of many thousand feet, this solution of the problem is not satisfactory. Not only subsidence after subsidence must have taken place, but the first submergence have been followed by an elevation of the land—another soil, fit for the growth of forest trees, must have been produced—another generation of vegetables, of precisely the same species and genera, have sprung up, and arrived at maturity—and then another subsidence, and another accumulation of drift. And these periodical oscillations in the relative level of the land and water must have gone on uninterruptedly through a long period of time, not in one district or country only, but in various parts of the world, during the same geological epoch. At present I do not think we have data sufficient to explain these phenomena; what has been advanced may, perhaps, serve to elicit further information, by pointing out the difficulties in which the question is involved, and showing what interesting fields of discovery are still unexplored, and how comprehensive and important are the objects that come within the scope of geological investigation.[59]

[59] I would refer the student for a fuller consideration of the phenomena thus briefly noticed, to the 6th edition of my Wonders of Geology, pp. 669, 718, 731.