Fig. 339.Restored outline of a fish of the genusPalæoniscus, Agass.Palæothrissum, Blainville.
Fig. 339.
Restored outline of a fish of the genusPalæoniscus, Agass.Palæothrissum, Blainville.
ThePalæoniscusabove mentioned belongs to that division of fishes which M. Agassiz has called "Heterocercal," which have their tails unequally bilobate, like the recent shark and sturgeon, and the vertebral column running along the upper caudal lobe. (Seefig. 340.) The "Homocercal" fish, which comprise almost all the 8000 species at present known in the living creation, have the tail-fin either single or equally divided; and the vertebral column stops short, and is not prolonged into either lobe. (Seefig. 341.)
Fig. 340.Shark.Heterocercal.
Fig. 340.
Shark.
Heterocercal.
Fig. 341.Shad. (Clupea, Herring tribe.)Homocercal.
Fig. 341.
Shad. (Clupea, Herring tribe.)
Homocercal.
Now it is a singular fact, first pointed out by Agassiz, that the heterocercal form, which is confined to a small number of genera in the existing creation, is universal in the Magnesian limestone, and all the more ancient formations. It characterizes the earlier periods of the earth's history, when the organization of fishes made a greater approach to that of saurian reptiles than at later epochs. In all the strata above the Magnesian limestone the homocercal tail predominates.
A full description has been given by Sir Philip Egerton of the species of fish characteristic of the marl-slate in Mr. King's monograph before referred to, where figures of the ichthyolites which are very entire and well preserved, will be found. Even a single scale is usually so characteristically marked as to indicate the genus, and sometimes even the particular species. They are often scattered through the beds singly, and maybe useful to a geologist in determining the age of the rock.
Fig. 342.Palæoniscus comtus,Agassiz. Scalemagnified. Marl-slate.
Fig. 342.
Palæoniscus comtus,Agassiz. Scalemagnified. Marl-slate.
Fig. 343.Palæoniscus elegans,Sedg. Undersurface of scalemagnified. Marl-slate.
Fig. 343.
Palæoniscus elegans,Sedg. Undersurface of scalemagnified. Marl-slate.
Fig. 344.Palæoniscus glaphyrus,Ag. Undersurface of scalemagnified. Marl-slate.
Fig. 344.
Palæoniscus glaphyrus,Ag. Undersurface of scalemagnified. Marl-slate.
Fig. 345.Cœlacanthus caudalis,Egerton. Scaleshowing granulated surfacemagnified. Marl-slate.
Fig. 345.
Cœlacanthus caudalis,Egerton. Scaleshowing granulated surfacemagnified. Marl-slate.
Scales offish. Magnesianlimestone.Fig. 346.Pygopterus mandibularis,Ag. Marl-slate.a.Outside of scale magnified.b.Under surface of same.Fig. 347.Acrolepis Sedgwickii,Ag. Marl-slate.
Scales offish. Magnesianlimestone.
Fig. 346.Pygopterus mandibularis,Ag. Marl-slate.
Fig. 347.Acrolepis Sedgwickii,Ag. Marl-slate.
Theinferior sandstones(No. 6. Tab.p. 301.), which lie beneath the marl-slate, consist of sandstone and sand, separating the magnesian limestone from the coal, in Yorkshire and Durham. In some instances, red marl and gypsum have been found associated with these beds. They have been classed with the magnesian limestone by Professor Sedgwick, as being nearly co-extensive with it in geographical range, though their relations are very obscure. In some regions we find it stated that the imbedded plants are all specifically identical with those of the carboniferous series; and, if so, they probably belong to that epoch; for the true Permian flora appears, from the researches of MM. Murchison and de Verneuil in Russia, and of Colonel von Gutbier in Saxony, to be, with few exceptions, distinct from that of the coal (seep. 307.).
Dolomitic conglomerate of Bristol.—Near Bristol, in Somersetshire, and in other counties bordering the Severn, the unconformable beds of the Lower New Red, resting immediately upon the Coal, consist of a conglomerate called "dolomitic," because the pebbles of older rocks are cemented together by a red or yellow base of dolomite or magnesian limestone. This conglomerate or breccia, for the imbedded fragments are sometimes angular, occurs in patches over the whole of the downs near Bristol, filling up the hollows and irregularities in the mountain limestone, and being principally composed at every spot of the debris of those rocks on which it immediately rests. At one point we find pieces of coal shale, in another of mountain limestone, recognizable by its peculiar shells and zoophytes.Fractured bones, also, and teeth of saurians, are dispersed through some parts of the breccia.
These saurians (which until the discovery of theArchegosaurusin the coal were the most ancient examples of fossil reptiles) are all distinguished by having the teeth implanted deeply in the jaw-bone, and in distinct sockets, instead of being soldered, as in frogs, to a simple alveolar parapet. In the dolomitic conglomerate near Bristol the remains of species of two distinct genera have been found, calledThecodontosaurusandPalæosaurusby Dr. Riley and Mr. Stutchbury[306-A]; the teeth of which are conical, compressed, and with finely serrated edges (figs. 348and349.).
Fig. 348.Tooth ofPalæosaurusplatyodon, nat. size.
Fig. 348.
Tooth ofPalæosaurusplatyodon, nat. size.
Fig. 349.Tooth ofThecodontosaurus, 3 times magnified.
Fig. 349.
Tooth ofThecodontosaurus, 3 times magnified.
In Russia, also, Thecodont saurians occur, in beds of the Permian age, of several genera, while others namedProtorosaurusare met with in the Zechstein of Thuringia. This family of reptiles is allied to the living monitor, and its appearance in a primary or paleozoic formation, observes Mr. Owen, is opposed to the doctrine of the progressive development of reptiles from fish, or from simpler to more complex forms; for, if they existed at the present day, these monitors would take rank at the head of the Lacertian order.[306-B]
In Russia the Permian rocks are composed of white limestone, with gypsum and white salt; and of red and green grits, with occasionally copper ore; also magnesian limestones, marlstones, and conglomerates.
The country of Mansfeld, in Thuringia, may be called the classic ground of the Lower New Red, or Magnesian Limestone, or Permian formation, on the Continent. It consists there principally of, first, the Zechstein, corresponding to the upper portion of our English series; and, secondly, the marl-slate, with fish of species identical with those of the bed so called in Durham. This slaty marlstone is richly impregnated with copper pyrites, for which it is extensively worked. Magnesian limestone, gypsum, and rock-salt, occur among the superior strata of this group. At its base lies the Rothliegendes, supposed to correspond with the Inferior or Lower New Red Sandstoneabove mentioned, which occupies a similar place in England between the marl-slate and coal. Its local name of Rothliegendes,red-lyer, or "Roth-todt-liegendes,"red-dead-lyer, was given by the workmen in the German mines from its red colour, and because the copper hasdied outwhen they reach this rock, which is not metalliferous. It is, in fact, a great deposit of red sandstone and conglomerate, with associated porphyry, basaltic trap, and amygdaloid.
Permian Flora.—We learn from the recent investigation of Colonel von Gutbier, that in the Permian rocks of Saxony no less than sixty species of fossil plants have been met with, forty of which have not yet been found elsewhere. Two or three of these, asCalamites gigas,Sphenopteris erosa, andS. lobata, are also met with in the government of Perm in Russia. Seven others, and among themNeuropteris Loshii,Pecopteris arborescens, andP. similis, with several species ofWalchia(Lycopodites), are common to the coal-measures.
Among the genera also enumerated by Colonel Gutbier areAsterophyllitesandAnnularia, so characteristic of the carboniferous period; alsoLepidodendron, which is common to the Permian of Saxony, Thuringia, and Russia, although not abundant.Noeggerathia(seefig. 350.), supposed by A. Brongniart to be allied toCycas, is another link between the Permian and carboniferous vegetation. Coniferæ, of the Araucarian division, also occur; but these are likewise met with both in older and newer rocks. The plants calledSigillariaandStigmaria, so marked a feature in the carboniferous period, are as yet wanting.
Fig. 350.Noeggerathia cuneifolia.Ad. Brongniart.[307-A]
Fig. 350.
Noeggerathia cuneifolia.Ad. Brongniart.[307-A]
Among the remarkable fossils of the rothliegendes, or lowest part of the Permian in Saxony and Bohemia, are the silicified trunks of tree-ferns called genericallyPsaronius. Their bark was surrounded by a dense mass of air-roots, which often constituted a great addition to the original stem, so as to double or quadruple its diameter. The same remark holds good in regard to certain living extra-tropical arborescent ferns, particularly those of New Zealand.
Psaronites are also found in the uppermost coal of Autun in France, and in the upper coal-measures of the State of Ohio in the United States, but specifically different from those of the rothliegendes. They serve to connect the Permian flora with the more modern portion of the preceding or carboniferous group. Upon the whole, it isevident that the Permian plants approach nearer to the carboniferous ones than to the triassic; and the same may be said of the Permian fauna.
Carboniferous strata in the south-west of England — Superposition of Coal-measures to Mountain limestone — Departure from this type in North of England and Scotland — Section in South Wales — Underclays with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariæ,Stigmariæ — Coniferæ — Endogens — Absence of Exogens — Coal, how formed — Erect fossil trees — Parkfield Colliery — St. Etienne, Coal-field — Oblique trees or snags — Fossil forests in Nova Scotia — Brackish water and marine strata — Origin of Clay-iron-stone.
Carboniferous strata in the south-west of England — Superposition of Coal-measures to Mountain limestone — Departure from this type in North of England and Scotland — Section in South Wales — Underclays with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariæ,Stigmariæ — Coniferæ — Endogens — Absence of Exogens — Coal, how formed — Erect fossil trees — Parkfield Colliery — St. Etienne, Coal-field — Oblique trees or snags — Fossil forests in Nova Scotia — Brackish water and marine strata — Origin of Clay-iron-stone.
Thenext group which we meet with in the descending order is the Carboniferous, commonly called "The Coal;" because it contains many beds of that mineral, in a more or less pure state, interstratified with sandstones, shales, and limestones. The coal itself, even in Great Britain and Belgium, where it is most abundant, constitutes but an insignificant portion of the whole mass. In the north of England, for example, the thickness of the coal-bearing strata has been estimated at 3000 feet, while the various coal-seams, 20 or 30 in number, do not in the aggregate exceed 60 feet.[308-A]
The carboniferous formation comprises two very distinct members: 1st, that usually called the Coal-measures, of mixed freshwater, terrestrial, and marine origin, often including seams of coal; 2dly, that named in England the Mountain or Carboniferous limestone, of purely marine origin, and containing corals, shells, and encrinites.
In the south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of by English geologists are:
The millstone grit may be considered as one of the coal sandstonesof coarser texture than usual, with some accompanying shales, in which coal plants are occasionally found. In the north of England some bands of limestone, with pectens, oysters, and other marine shells, occur in this grit, just as in the regular coal-measures, and even a few seams of coal. I shall treat, therefore, of the whole group, as consisting of two divisions only, the Coal-measures and Mountain Limestone. The latter is found in the southern British coal-fields, at the base of the system, or immediately in contact with the subjacent Old Red Sandstone; but as we proceed northwards to Yorkshire and Northumberland it begins to alternate with true coal-measures, the two deposits forming together a series of strata about 1000 feet in thickness. To this mixed formation succeeds the great mass of genuine mountain limestone.[309-A]Farther north, in the Fifeshire coal-field in Scotland, we observe a still wider departure from the type of the south of England, or a more complete intercalation of dense masses of marine limestones with sandstones, and shales containing coal.
In South Wales the coal-measures have been ascertained by actual measurement to attain the extraordinary thickness of 12,000 feet, the beds throughout, with the exception of the coal itself, appearing to have been formed in water of moderate depth, during a slow but perhaps intermittent depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.
In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten principal masses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by masses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of them, which has two or three layers of clay interposed, attaining 9 feet.[309-B]At other points in the same coal-field the shales predominate over the sandstones. The horizontal extent of some seams of coal is much greater than that of others, but they all present one characteristic feature, in having, each of them, what is called itsunderclay. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called firestone, because it can be made into bricks which stand the fire ofa furnace. They vary in thickness from 6 inches to more than 10 feet; and Mr. Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the one hundred seams of coal met with in their coal-field. They are said to form theflooron which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.
All of them, as Mr. Logan pointed out, are characterized by inclosing a peculiar species of fossil vegetable calledStigmaria, to the exclusion of other plants. It was also observed that, while in the overlying shales or "roof" of the coal, ferns and trunks of trees abound without anyStigmariæ, and are flattened and compressed, those singular plants in the underclays always retain their natural forms, branching freely, and sending out their slender leaves, as they were formerly styled, through the mud in all directions. Several species ofStigmariahad long been known to botanists, and described by them, before their position under each seam of coal was pointed out. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, and which were finally enveloped in the same mud.
These statements will suffice to convince the reader that we cannot arrive at a satisfactory theory of the origin of coal till we understand the true nature ofStigmaria; and in order to explain what is now known of this plant, and of others which have contributed by their decay to produce coal, it will be necessary to offer a brief preliminary sketch of the whole carboniferous flora, an assemblage of fossil plants, with which we are better acquainted than with any other which flourished antecedently to the tertiary epoch. It should also be remarked that Göppert has ascertained that the remains of every family of plants scattered through the coal-measures are sometimes met with in the pure coal itself, a fact which adds greatly to the geological interest attached to this flora.
Ferns.—The number of species of carboniferous plants hitherto described amounts, according to M. Ad. Brongniart, to about 500. These may perhaps be a fragment only of the entire flora, but they are enough to show that the state of the vegetable world was then extremely different from that now established. We are struck at the first glance with the similarity of many of the ferns to those now living, and the dissimilarity of almost all the other fossils except the coniferæ. Among the ferns, as in the case ofPecopterisfor example (fig. 351.), it is not always easy to decide whether they should be referred to different genera from those established for the classification of living species; whereas, in regard to most of the other contemporary tribes, with the exception of the coniferæ, it is often difficultto guess the family, or even the class, to which they belong. The ferns of the carboniferous period are generally without organs of fructification, but in some specimens these are well preserved. In the general absence of such characters, they have been divided into genera, distinguished chiefly by the branching of the fronds, and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group calledCaulopteris, by Lindley, and thePsaroniusof the upper or newest coal-measures, before alluded to (p. 307.).
Fig. 351.Pecopteris lonchitica.(Foss. Flo. 153.)
Fig. 351.
Pecopteris lonchitica.(Foss. Flo. 153.)
Fig. 352.a.Sphenopteris crenata.b.The same, magnified.(Foss. Flo. 101.)
Fig. 352.
(Foss. Flo. 101.)
Fig. 353.Caulopteris primæva, Lindley.
Fig. 353.
Caulopteris primæva, Lindley.
All the recent tree-ferns belong to one tribe (Polypodiaceæ), and to a small number only of genera in that tribe, in which the surface of the trunk is marked with scars, or cicatrices, left after the fall of the fronds. These scars resemble those ofCaulopteris(seefig. 353.). No less than 250 ferns have already been obtained from the coal strata; and even if we make some reduction on the ground of varieties which have been mistaken, in the absence of their fructification, for species, still the result is singular, because the whole of Europe affords at present no more than 50 indigenous species.
Living tree-ferns of differentgenera. (Ad.Brong.)Fig. 354. Tree-fern from Isle of Bourbon.Fig. 355.Cyathea glauca, Mauritius.Fig. 356. Tree fern from Brazil.
Living tree-ferns of differentgenera. (Ad.Brong.)
Lepidodendron Sternbergii. Coal-measures,near Newcastle.Fig. 357. Branching trunk, 49 feet long, supposed to have belonged toL. Sternbergii. (Foss. Flo. 203.)Fig. 358. Branching stem with bark and leaves ofL. Sternbergii. (Foss. Flo. 4.)Fig. 359. Portion of same nearer the root; natural size. (Ibid.)
Lepidodendron Sternbergii. Coal-measures,near Newcastle.
Lepidodendra.—These fossils belong to the family ofLycopodiums, yet most of them grew to the size of large trees. The annexed figures represent a large fossilLepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification. Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude. The living club-mosses, of which there are about 200 species, are abundant in tropical climates, where one species is sometimes met with attaining a height of 3 feet. They usually creep onthe ground, but some stand erect, as theL. densum, from New Zealand (fig. 360.).
Fig. 360.a.Lycopodium densum; banks of R. Thames, New Zealand.b.branch, natural size.c.part of same, magnified.
Fig. 360.
In the carboniferous strata of Coalbrook Dale, and in many other coal-fields, elongated cylindrical bodies, called fossil cones, named by M. Adolphe BrongniartLepidostrobus, are met with. (Seefig. 361.) They often form the nucleus of concretionary balls of clay-iron-stone, and are well preserved, exhibiting a conical axis, around which a great quantity of scales were compactly imbricated. The opinion of M. Brongniart is now generally adopted, that theLepidostrobusis the fruit ofLepidodendron.
Fig. 361.Lepidostrobus ornatus, Brong.; half nat.size. Shropshire.
Fig. 361.
Lepidostrobus ornatus, Brong.; half nat.size. Shropshire.
Fig. 362.Calamites cannæformis, Schlot. (Foss. Flo.79.) Lowerend with rootlets.
Fig. 362.
Calamites cannæformis, Schlot. (Foss. Flo.79.) Lowerend with rootlets.
Fig. 363.Calamites Suckowii, Brong.; naturalsize. Commonin coal throughout Europe.
Fig. 363.
Calamites Suckowii, Brong.; naturalsize. Commonin coal throughout Europe.
Equisetaceæ.—To this family belong two species of the genusEquisetites, allied to the living "horse-tail" which now grows in marshy grounds. Other species, which have jointed stems, depart more widely fromEquisetum, but are yet of analogous organization.They differed from it principally in being furnished with a thin bark, which is represented in the stem ofC. Suckowii(fig. 363.), in which it will be seen that the striped external pattern does not agree with that left on the stone where the bark is stripped off; so that if the two impressions were seen separately, they might be mistaken for two distinct species.
The tallest living "horse-tails" are only 2 or 3 feet high in Europe, and even in tropical climates only attain, as in the case ofEquisetum giganteum, discovered by Humboldt and Bonpland, in South America, a height of about 5 feet, the stem being an inch in diameter. Several of the Calamites of the coal acquired the height and dimensions of small trees.
Fig. 364.Asterophyllites foliosa. (Foss.Flo.25.) Coal-measures,Newcastle.
Fig. 364.
Asterophyllites foliosa. (Foss.Flo.25.) Coal-measures,Newcastle.
Asterophyllites.—In this family, M. Brongniart includes several genera, and among themCalamodendron,Asterophyllites, andAnnularia. The graceful plant, represented in the annexed figure, is supposed to be the branch of a shrub calledCalamodendron, a new genus, divided off by Brongniart from theCalamitesof former authors. Its pith and medullary rays seem to show that it was dicotyledonous, and it appears to have been allied, by the nature of its tissue, to the gymnogens, or, still more, to theSigillaria, which will next be mentioned.
Sigillaria.—A large portion of the trees of the carboniferous period belonged to this genus, of which about thirty-five species are known. The structure, both internal and external, was very peculiar, and, with reference to existing types, very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, which they resemble in the scalariform texture of their vessels, and, in some degree, in the form of the cicatrices left by the base of the leafstalks which have fallen off (seefig. 365.). But with these points of analogy to cryptogamia, they combine an internal organization much resembling that of cycads, and some of them are ascertained to have had long linear leaves, quite unlike those of ferns. They grew to a great height, from 30 to 60, or even 70 feet, with regular cylindrical stems, and without branches, although some species weredichotomous towards the top. Their fluted trunks, from 1 to 5 feet in diameter, appear to have decayed rapidly in the interior, so as to become hollow, when standing; when, therefore, they were thrown prostrate on the mud, they were squeezed down and flattened. Hence, we find the bark of the two opposite sides (now converted into bright shining coal) to constitute two horizontal layers, one upon the other, half an inch, or an inch, in thickness. These same trunks, when they are placed obliquely or vertically to the planes of stratification, retain their original rounded form, and are uncompressed, the cylinder of bark having been filled with sand, which now affords a cast of the interior.
Fig. 365.Sigillaria lævigata, Brong.
Fig. 365.
Sigillaria lævigata, Brong.
Stigmaria.—This fossil, the importance of which has already been pointed out, was formerly conjectured to be an aquatic plant. It is now ascertained to be the root ofSigillaria. The connection of the roots with the stem, previously suspected, on botanical grounds, by Brongniart, was first proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been shown, even more distinctly, by Mr. Richard Brown, in his description of theStigmariæoccurring in the underclays of the coal-seams of the Island of Cape Breton, in Nova Scotia.
Fig. 366.Stigmaria attached to a trunk ofSigillaria.[315-A]
Fig. 366.
Stigmaria attached to a trunk ofSigillaria.[315-A]
In a specimen of one of these, represented in the annexed figure (fig. 366.), the spread of the roots was 16 feet, and some of them sent out rootlets, in all directions, into the surrounding clay.
The manner of attachment of the fibres to the stem resembles that of a ball and socket joint, the base of each rootlet being concave, and fitting on to a tubercle (seefigs. 367and368.). Rows of these tubercles are arranged spirally round each root, which have always a medullary cavity and woody texture, much resembling thatofSigillaria, the structure of the vessels being, like it, scalariform.
Fig. 367.Surface of another individual of same species, showing form oftubercles. (Foss.Flo. 34.)
Fig. 367.
Surface of another individual of same species, showing form oftubercles. (Foss.Flo. 34.)
Fig. 368.Stigmaria ficoides,Brong. Onefourth of nat.size. (Foss.Flo. 32.)
Fig. 368.
Stigmaria ficoides,Brong. Onefourth of nat.size. (Foss.Flo. 32.)
Conifers.—The coniferous trees of this period are referred to five genera; the woody structure of some of them showing that they were allied to the Araucarian division of pines, more than to any of our common European firs. Some of their trunks exceeded 44 feet in height.
Endogens.—Hitherto but few monocotyledonous plants have been discovered in the coal-strata. Most of these consist of fruits referred by some botanists to palms. The three-sided nuts, calledTrigonocarpum, seven species of which are known, appear to have the best claim to rank as palms, although M. Ad. Brongniart entertains some doubt even as to their being monocotyledons.
The entire absence, so far as our paleontological investigations have hitherto gone, of ordinary dicotyledons or exogens in the coal measures, is most remarkable. Hence, M. Adolphe Brongniart has called this period the age of acrogens, in consequence of the vast preponderance of ferns andLepidodendra.[316-A]Nevertheless, a forest of the period, now under consideration, may have borne a considerable resemblance to those woody regions of New Zealand, in which ferns, arborescent and herbaceous, and lycopodiums, with many coniferæ, abound.
The comparative proportion of living ferns andAraucariæ, in Norfolk Island, to all the other plants, appears to be very similar to that formerly borne by these tribes respectively in a forest of the coal-period.
I have already stated that Professor Göppert, after examining the fossil vegetables of the coal-fields of Germany, has detected, in beds of pure coal, remains of plants of every family hitherto known to occur fossil in the coal. Many seams, he remarks, are rich inSigillaria,Lepidodendron, andStigmaria, the latter in such abundance, as to appear to form the bulk of the coal. In some places, almost all the plants are calamites, in others ferns.[316-B]
Coal, how formed—Erect trees.—I shall now consider the manner in which the above-mentioned plants are imbedded in the strata, and how they may have contributed to produce coal. "Some of the plants of our coal," says Dr. Buckland, "grew on the identical banks of sand, silt, and mud, which, being now indurated to stone and shale, form the strata that accompany the coal; whilst other portions of these plants have been drifted to various distances from the swamps, savannahs, and forests that gave them birth, particularly those that are dispersed through the sandstones, or mixed with fishes in the shale beds." "At Balgray, three miles north of Glasgow," says the same author, "I saw in the year 1824, as there still may be seen, an unequivocal example of the stumps of several stems of large trees, standing close together in their native place, in a quarry of sandstone of the coal formation."[317-A]
Between the years 1837 and 1840, six fossil trees were discovered in the coal-field of Lancashire, where it is intersected by the Bolton railway. They were all in a vertical position, with respect to the plane of the bed, which dips about 15° to the south. The distance between the first and the last was more than 100 feet, and the roots of all were imbedded in a soft argillaceous shale. In the same plane with the roots is a bed of coal, eight or ten inches thick, which has been ascertained to extend across the railway, or to the distance of at least ten yards. Just above the covering of the roots, yet beneath the coal seam, so large a quantity of theLepidostrobus variabiliswas discovered inclosed in nodules of hard clay, that more than a bushel was collected from the small openings around the base of the trees (see figure of this genus,p. 313.). The exterior trunk of each was marked by a coating of friable coal, varying from one quarter to three quarters of an inch in thickness; but it crumbled away on removing the matrix. The dimensions of one of the trees is 151/2feet in circumference at the base, 71/2feet at the top, its height being 11 feet. All the trees have large spreading roots, solid and strong, sometimes branching, and traced to a distance of several feet, and presumed to extend much farther. Mr. Hawkshaw, who has described these fossils, thinks that, although they were hollow when submerged, they may have consisted originally of hard wood throughout; for solid dicotyledonous trees, when prostrated in tropical forests, as in Venezuela, on the shore of the Caribbean Sea, were observed by him to be destroyed in the interior, so that little more is left than an outer shell, consisting chiefly of the bark. This decay, he says, goes on most rapidly in low and flat tracks, in which there is a deep rich soil and excessive moisture, supporting tall forest-trees and large palms, below which bamboos, canes, and minor palms flourish luxuriantly. Such tracts, from their lowness, would be most easily submerged, and their dense vegetation might then give rise to a seam of coal.[317-B]
In a deep valley near Capel-Coelbren, branching from the higher part of the Swansea valley, four stems of uprightSigillariæwereseen, in 1838, piercing through the coal-measures of S. Wales; one of them was 2 feet in diameter, and one 13 feet and a half high, and they were all found to terminate downwards in a bed of coal. "They appear," says Sir H. De la Beche, "to have constituted a portion of a subterranean forest at the epoch when the lower carboniferous strata were formed.[318-A]
In a colliery near Newcastle, say the authors of the Fossil Flora, a great number ofSigillariæwere placed in the rock as if they had retained the position in which they grew. Not less than thirty, some of them 4 or 5 feet in diameter, were visible within an area of 50 yards square, the interior being sandstone, and the bark having been converted into coal. The roots of one individual were found imbedded in shale; and the trunk, after maintaining a perpendicular course and circular form for the height of about 10 feet, was then bent over so as to become horizontal. Here it was distended laterally, and flattened so as to be only one inch thick, the flutings being comparatively distinct.[318-B]Such vertical stems are familiar to our miners, under the name of coal-pipes. One of them, 72 feet in length, was discovered, in 1829, near Gosforth, about five miles from Newcastle, in coal-grit, the strata of which it penetrated. The exterior of the trunk was marked at intervals with knots, indicating the points at which branches had shot off. The wood of the interior had been converted into carbonate of lime; and its structure was beautifully shown by cutting transverse slices, so thin as to be transparent. (Seep. 40.)
These "coal-pipes" are much dreaded by our miners, for almost every year in the Bristol, Newcastle, and other coal-fields, they are the cause of fatal accidents. Each cylindrical cast of a tree, formed of solid sandstone, and increasing gradually in size towards the base, and being without branches, has its whole weight thrown downwards, and receives no support from the coating of friable coal which has replaced the bark. As soon, therefore, as the cohesion of this external layer is overcome, the heavy column falls suddenly in a perpendicular or oblique direction from the roof of the gallery whence coal has been extracted, wounding or killing the workman who stands below. It is strange to reflect how many thousands of these trees fell originally in their native forests in obedience to the law of gravity; and how the few which continued to stand erect, obeying, after myriads of ages, the same force, are cast down to immolate their human victims.
It has been remarked, that if, instead of working in the dark, the miner was accustomed to remove the upper covering of rock from each seam of coal, and to expose to the day the soils on which ancient forests grew, the evidence of their former growth would be obvious. Thus in South Staffordshire a seam of coal was laid bare in the year 1844, in what is called an open work at Parkfield Colliery, nearWolverhampton. In the space of about a quarter of an acre the stumps of no less than 73 trees with their roots attached appeared, as shown in the annexed plan (fig. 369.), some of them more than 8 feet in circumference. The trunks, broken off close to the root, were lying prostrate in every direction, often crossing each other. One of them measured 15, another 30 feet in length, and others less. They were invariably flattened to the thickness of one or two inches, and converted into coal. Their roots formed part of a stratum of coal 10 inches thick, which rested on a layer of clay 2 inches thick, below which was a second forest, resting on a 2-foot seam of coal. Five feet below this again was a third forest with large stumps ofLepidodendra,Calamites, and other trees.
Fig. 369.Ground-plan of a fossil forest, Parkfield Colliery, near Wolverhampton, showing the position of 73 trees in a quarter of an acre.[319-A]
Fig. 369.
Ground-plan of a fossil forest, Parkfield Colliery, near Wolverhampton, showing the position of 73 trees in a quarter of an acre.[319-A]
In the account given, in 1821, by M. Alex. Brongniart of the coal-mine of Treuil, at St. Etienne, near Lyons, he states, that distinct horizontal strata of micaceous sandstone are traversed by vertical trunks of monocotyledonous vegetables, resembling bamboos or largeEquiseta.[319-B]Since the consolidation of the stone, there has been here and there a sliding movement, which has broken the continuity of the stems, throwing the upper parts of them on one side, so that they are often not continuous with the lower.
From these appearances it was inferred that we have here the monuments of a submerged forest. I formerly objected to this conclusion, suggesting that, in that case, all the roots ought to have been found at one and the same level, and not scattered irregularly through the mass. I also imagined that the soil to which the roots were attached should have been different from the sandstone in which the trunks are enclosed. Having, however, seen calamites near Pictou, in Nova Scotia, buried at various heights in sandstone and in similar erect attitudes, I have now little doubt that M. Brongniart's viewwas correct. These plants seem to have grown on a sandy soil, liable to be flooded from time to time, and raised by new accessions of sediment, as may happen in swamps near the banks of a large river in its delta. Trees which delight in marshy grounds are not injured by being buried several feet deep at their base; and other trees are continually rising up from new soils, several feet above the level of the original foundation of the morass. In the banks of the Mississippi, when the water has fallen, I have seen sections of a similar deposit in which portions of the stumps of trees with their rootsin situappeared at many different heights.[320-A]