Fig. 35Fig. 35.—Cupressocrinus crassus.
Fig. 35.—Cupressocrinus crassus.
In Herefordshire, Worcestershire, Shropshire, Gloucestershire, and South Wales, the Old Red Sandstone is largely developed, and sometimes attains the thickness of from 8,000 to 10,000 feet, dividedinto: 1. Conglomerate; 2. Brown stone, withEurypterus; 3. Marl and cornstones, with irregular courses of concrete limestone, in which are spines of Fishes and remains ofCephalaspisandPteraspis; 4. Thin olive-coloured shales and sandstone, intercalated with beds of red marl, containingCephalaspisandAuchenaspis. In Scotland, south of the Grampians, a yellow sandstone occupies the base of the system; conglomerate, red shales, sandstone and cornstones, containingHoloptychiusandCephalaspis, and the Arbroath paving-stone, containing what Agassiz recognised as a huge Crustacean.
Fig. 36Fig. 36.—Trinucleus Lloydii. (Llandeilo Flags.)
Fig. 36.—Trinucleus Lloydii. (Llandeilo Flags.)
Some of the phenomena connected with the older rocks of Devonshire are difficult to unravel. The Devonian, it is now understood, is the equivalent, in another area, of the Old Red Sandstone, and in Cornwall and Devonshire lie directly on the Silurian strata, while elsewhere the fossils of the Upper Silurian are almost identical with those in the Devonian beds. The late Professor Jukes, with some other geologists, was of opinion that the Devonian rocks of Devonshire only represented the Old Red Sandstone of Scotland and South Wales in part; the Upper Devonian rocks lying between the acknowledged Old Red Sandstone and the Culm-measures being the representatives of the lower carboniferous rocks of Ireland.
Mr. Etheridge, on the other hand, in an elaborate memoir upon the same subject, has endeavoured to prove that the Devonian and Old Red Sandstone, though contemporaneous in point of time, were deposited in different areas and under widely different conditions—the one strictly marine, the other altogether fresh-water—or, perhaps, partly fresh-water and partly estuarine. This supposition is strongly supported by his researches into the mollusca of the Devonian system, and also by the fish-remains of the Devonian and Old Red Sandstone of Scotland and the West of England and Wales.[42]The difficulty of drawing a sharply-defined line of demarcation between different systems is sufficient to dispel the idea which has sometimes been entertained that specialfaunæwere created and annihilated in the mass at the close of each epoch. There was no close: each epoch disappears or merges into that which succeeds it, and with it the animals belonging to it, much as we have seen them disappear from our own fauna almost within recent times.
In the history of our globe the Carboniferous period succeeds to the Devonian. It is in the formations of this latter epoch that we find the fossil fuel which has done so much to enrich and civilise the world in our own age. This period divides itself into two great sub-periods: 1. TheCoal-measures; and 2. TheCarboniferous Limestone. The first, a period which gave rise to the great deposits of coal; the second, to most important marine deposits, most frequently underlying the coal-fields in England, Belgium, France, and America.
The limestone-mountains which form the base of the whole system, attain in places, according to Professor Phillips, a thickness of 2,500 feet. They are of marine origin, as is apparent by the multitude of fossils they contain of Zoophytes, Radiata, Cephalopoda, and Fishes. But the chief characteristic of this epoch is its strictly terrestrial flora—remains of plants now become as common as they were rare in all previous formations, announcing a great increase of dry land. In older geological times the present site of our island was covered by a sea of unlimited extent; we now approach a time when it was a forest, or, rather, an innumerable group of islands, and marshes covered with forests, which spread over the surface of the clusters of islands which thickly studded the sea of the period.
Fig. 37Fig. 37.—Ferns restored. 1 and 2. Arborescent Ferns. 3 and 4. Herbaceous Ferns.
Fig. 37.—Ferns restored. 1 and 2. Arborescent Ferns. 3 and 4. Herbaceous Ferns.
The monuments of this era of profuse vegetation reveal themselves in the precious Coal-measures of England and Scotland. These give us some idea of the rich verdure which covered the surface of the earth, newly risen from the bosom of its parent waves. It was the paradise of terrestrial vegetation. The grandSigillaria, theStigmaria, and other fern-like plants, were especially typical of this age, and formed the woods, which were left to grow undisturbed; for as yet no living Mammals seem to have appeared; everything indicates a uniformly warm, humid temperature, the only climate in which the gigantic ferns of the Coal-measures could have attained their magnitude. InFig. 37the reader has a restoration of the arborescent and herbaceous Fernsof the period. Conifers have been found of this period with concentric rings, but these rings are more slightly marked than in existing trees of the same family, from which it is reasonable to assume that the seasonal changes were less marked than they are with us.
Everything announces that the time occupied in the deposition of the Carboniferous Limestone was one of vast duration. Professor Phillips calculates that, at the ordinary rate of progress, it would require 122,400 years to produce only sixty feet of coal. Geologists believe, moreover, that the upper coal-measures, where bed has been deposited upon bed, for ages upon ages, were accumulated under conditions of comparative tranquillity, but that the end of this period was marked by violent convulsions—by ruptures of the terrestrial crust, when the carboniferous rocks were upturned, contorted, dislocated by faults, and subsequently partially denuded, and thus appear now in depressions or basin-shaped concavities; and that upon this deranged and disturbed foundation a fourth geological system, called Permian, was constructed.
The fundamental character of the period we are about to study is the immense development of a vegetation which then covered much of the globe. The great thickness of the rocks which now represent the period in question, the variety of changes which are observed in these rocks wherever they are met with, lead to the conclusion that this phase in the Earth’s history involved a long succession of time.
Coal, as we shall find, is composed of the mineralised remains of the vegetation which flourished in remote ages of the world. Buried under an enormous thickness of rocks, it has been preserved to our days, after being modified in its inward nature and external aspect. Having lost a portion of its elementary constituents, it has become transformed into a species of carbon, impregnated with those bituminous substances which are the ordinary products of the slow decomposition of vegetable matter.
Thus, coal, which supplies our manufactures and our furnaces, which is the fundamental agent of our productive and economic industry—the coal which warms our houses and furnishes the gas which lights our streets and dwellings—is the substance of the plants which formed the forests, the vegetation, and the marshes of the ancient world, at a period too distant for human chronology to calculate with anything like precision. We shall not say—with some persons, who believe that all in Nature was made with reference to man, and who thus form a very imperfect idea of the vast immensity of creation—that the vegetables of the ancient world have lived and multiplied only, some day, to prepare for man the agents of his economic and industrial occupations. We shall rather direct the attention of our young readers to the powers of modern science, which can thus, aftersuch a prodigious interval of time, trace the precise origin, and state with the utmost exactness, the genera and species of plants, of which there are now no identical representatives existing on the face of the earth.
Let us pause for a moment, and consider the general characters which belonged to our planet during the Carboniferous period. Heat—though not necessarily excessive heat—and extreme humidity were then the attributes of its atmosphere. The modern allies of the species which formed its vegetation are now only found under the burning latitudes of the tropics; and the enormous dimensions in which we find them in the fossil state prove, on the other hand, that the atmosphere was saturated with moisture. Dr. Livingstone tells us that continual rains, added to intense heat, are the climatic characteristic of Equatorial Africa, where the vigorous and tufted vegetation flourishes which is so delightful to the eye.
It is a remarkable circumstance that conditions of equable and warm climate, combined with humidity, do not seem to have been limited to any one part of the globe, but the temperature of the whole globe seems to have been nearly the same in very different latitudes. From the Equatorial regions up to Melville Island, in the Arctic Ocean, where in our days eternal frost prevails—from Spitzbergen to the centre of Africa, the carboniferous flora is identically the same. When nearly the same plants are found in Greenland and Guinea; when the same species, now extinct, are met with of equal development at the equator as at the pole, we cannot but admit that at this epoch the temperature of the globe was nearly alike everywhere. What we now callclimatewas unknown in these geological times. There seems to have been then only one climate over the whole globe. It was at a subsequent period, that is, in later Tertiary times, that the cold began to make itself felt at the terrestrial poles. Whence, then, proceeded this general superficial warmth, which we now regard with so much surprise? It was a consequence of the greater or nearer influence of the interior heat of the globe. The earth was still so hot in itself, that the heat which reached it from the sun may have been inappreciable.
Another hypothesis, which has been advanced with much less certainty than the preceding, relates to the chemical composition of the air during the Carboniferous period. Seeing the enormous mass of vegetation which then covered the globe, and extended from one pole to the other; considering, also, the great proportion of carbon and hydrogen which exists in the bituminous matter of coal, it has been thought, and not without reason, that the atmosphere of the periodmight be richer in carbonic acid than the atmosphere of the present day. It has even been thought that the small number of (especially air-breathing) animals, which then lived, might be accounted for by the presence of a greater proportion of carbonic acid gas in the atmosphere than is the case in our own times. This, however, is pure assumption, totally deficient in proof. Nothing proves that the atmosphere of the period in question was richer in carbonic acid than is the case now. Since we are only able, then, to offer vague conjectures on this subject, we cannot profess with any confidence to entertain the opinion that the atmospheric air of the Carboniferous period contained more carbonic acid gas than that which we now breathe. What we can remark, with certainty, as a striking characteristic of the vegetation of the globe during this phase of its history, was the prodigious development which it assumed. The Ferns, which in our days and in our climate, are most commonly only small perennial plants, in the Carboniferous age sometimes presented themselves under lofty and even magnificent forms.
Fig. 38Fig. 38.—Calamite restored. Thirty to forty feet high.
Fig. 38.—Calamite restored. Thirty to forty feet high.
Every one knows those marsh-plants with hollow, channelled, and articulated cylindrical stems; whose joints are furnished with a membranous, denticulated sheath, and which bear the vulgar name of “mare’s-tail;” their fructification forming a sort of catkin composed of many rings of scales, carrying on their lower surface sacs full ofsporesor seeds. These humbleEquisetawere represented during the Coal-period by herbaceous trees from twenty to thirty feet high and four to six inches in diameter. Their trunks, channelled longitudinally, and divided transversely by lines of articulation, have been preserved to us: they bear the name ofCalamites. The engraving (Fig. 38) represents one of these gigantic mare’s-tails, or Calamites, of the Coal-period, restored under the directions of M. Eugene Deslongchamps. It is represented with its fronds of leaves, and its organs of fructification. They seem to have grown by means of an underground stem, while new buds issued from the ground at intervals, as represented in the engraving.
TheLycopodsof our age are humble plants, scarcely a yard in height, and most commonly creepers; but the Lycopodiaceæ of the ancient world were trees of eighty or ninety feet in height. It was theLepidodendronswhich filled the forests. Their leaves were sometimes twenty inches long, and their trunks a yard in diameter. Such are the dimensions of some specimens ofLepidodendron carinatumwhich have been found. Another Lycopod of this period, theLomatophloyos crassicaule, attained dimensions still more colossal. TheSigillariassometimes exceeded 100 feet in height. Herbaceous Ferns werealso exceedingly abundant, and grew beneath the shade of these gigantic trees. It was the combination of these lofty trees with such shrubs (if we may so call them), which formed the forests of the Carboniferous period. The trunks of two of the gigantic trees, which flourished in the forests of the Carboniferous period, are represented inFigs. 39and40, reduced respectively to one-fifth and one-tenth the natural size.
What could be more surprising than the aspect of this exuberant vegetation!—these immense Sigillarias, which reigned over the forest! these Lepidodendrons, with flexible and slender stems! these Lomatophloyos, which present themselves asherbaceoustrees of gigantic height, furnished with verdant leaflets! these Calamites, forty feet high!these elegant arborescent Ferns, with airy foliage, as finely cut as the most delicate lace! Nothing at the present day can convey to us an idea of the prodigious and immense extent of never-changing verdure which clothed the earth, from pole to pole, under the high temperature which everywhere prevailed over the whole terrestrial globe. In the depths of these inextricable forests parasitic plants were suspended from the trunks of the great trees, in tufts or garlands, like the wild vines of our tropical forests. They were nearly all pretty, fern-like plants—Sphenopteris,Hymenophyllites, &c.; they attached themselves to the stems of the great trees, like the orchids andBromeliaceæof our times.
Fig. 39Fig. 39.—Trunk of Calamites. One-fifth natural size.
Fig. 39.—Trunk of Calamites. One-fifth natural size.
The margin of the waters would also be covered with various plants with light and whorled leaves, belonging, perhaps, to the Dicotyledons;Annularia fertilis,Sphenophyllites, andAsterophyllites.
How this vegetation, so imposing, both on account of the dimensions of the individual trees and the immense space which they occupied,so splendid in its aspect, and yet so simple in its organisation, must have differed from that which now embellishes the earth and charms our eyes! It certainly possessed the advantage of size and rapid growth; but how poor it was in species—how uniform in appearance! No flowers yet adorned the foliage or varied the tints of the forests. Eternal verdure clothed the branches of the Ferns, the Lycopods, and Equiseta, which composed to a great extent the vegetation of the age. The forests presented an innumerable collection of individuals, but very few species, and all belonging to the lower types of vegetation. No fruit appeared fit for nourishment; none would seem to have been on the branches. Suffice it to say that few terrestrial animals seem to have existed yet; animal life was apparently almost wholly confined to the sea, while the vegetable kingdom occupied the land, which at a later period was more thickly inhabited by air-breathing animals. Probably a few winged insects (some coleoptera, orthoptera, and neuroptera) gave animation to the air while exhibiting their variegated colours; and it was not impossible but that many pulmoniferous mollusca (such as land-snails) lived at the same time.
Fig. 40Fig. 40.—Trunk of Sigillaria. One-tenth natural size.
Fig. 40.—Trunk of Sigillaria. One-tenth natural size.
But, we might ask, for what eyes, for whose thoughts, for whose wants, did the solitary forests grow? For whom these majestic and extensive shades? For whom these sublime sights? What mysterious beings contemplated these marvels? A question which cannot be solved, and one before which we are overwhelmed, and our powerless reason is silent; its solution rests with Him who said, “Before the world was, I am!”
The vegetation which covered the numerous islands of the Carboniferous sea consisted, then, of Ferns, of Equisetaceæ, of Lycopodiaceæ, and dicotyledonous Gymnosperms. The Annularia and Sigillariæ belong to families of the last-named class, which are now completely extinct.
Fig. 41Fig. 41.—Sigillaria lævigata. One-third natural size.
Fig. 41.—Sigillaria lævigata. One-third natural size.
TheAnnulariæwere small plants which floated on the surface of fresh-water lakes and ponds; their leaves were verticillate, that is, arranged in a great number of whorls, at each articulation of the stem with the branches. TheSigillariæwere, on the contrary, great trees, consisting of a simple trunk, surmounted with a bunch or panicle of slender drooping leaves, with the bark often channelled, and displaying impressions or scars of the old leaves, which, from their resemblance to a seal,sigillum, gave origin to their name.Fig. 41represents the bark of one of these Sigillariæ, which is often met with in coal-mines.
Fig. 42Fig. 42.—Stigmaria. One-tenth natural size.
Fig. 42.—Stigmaria. One-tenth natural size.
TheStigmariæ(Fig. 42), according to palæontologists, were roots of Sigillariæ, with a subterranean fructification; all that is known ofthem is the long roots which carry the reproductive organs, and in some cases are as much as sixteen feet long. These were suspected by Brongniart, on botanical grounds, to be the roots of Sigillaria, and recent discoveries have confirmed this impression. Sir Charles Lyell, in company with Dr. Dawson, examined several erectSigillariæin the sea-cliffs of the South Joggins in Nova Scotia, and found that from the lower extremities of the trunk they sent outStigmariæas roots, which divided into four parts, and these again threw out eight continuations, each of which again divided into pairs. Twenty-one specimens of Sigillaria have been described by Dr. Dawson from the Coal-measures of Nova Scotia; but the differences in the markings in different parts of the same tree are so great, that Dr. Dawson regards the greater part of the recognised species ofSigillariæas merely provisional.[43]
Two other gigantic trees grew in the forests of this period: these wereLepidodendron carinatumandLomatophloyos crassicaule, both belonging to the family of Lycopodiaceæ, which now includes only very small species. The trunk of the Lomatophloyos threw out numerous branches, which terminated in thick tufts of linear and fleshy leaves.
Fig. 43Fig. 43.—Lepidodendron Sternbergii.
Fig. 43.—Lepidodendron Sternbergii.
TheLepidodendrons, of which there are about forty known species, have cylindrical bifurcated branches; that is, the brancheswere evolved in pairs, or weredichotomousto the top. The extremities of the branches were terminated by a fructification in the form of a cone, formed of linear scales, to which the name ofLepidostrobus(Fig. 45) has been given. Nevertheless, many of these branches were sterile, and terminated simply in fronds (elongated leaves). In many of the coal-fields fossil cones have been found, to which this name has been given by earlier palæontologists. They sometimes form the nucleus of nodular, concretionary balls of clay-ironstone, and are well preserved, having a conical axis, surrounded by scales compactly imbricated. The opinion of Brongniart is now generally adopted, that they are the fruit of the Lepidodendron. At Coalbrookdale, and elsewhere, these have been found as terminal tips of a branch of a well-characterised Lepidodendron. Both Hooker and Brongniart place them with the Lycopods, having cones with similar spores and sporangia, like that family. Most of them were large trees. One tree ofL. Sternbergii, nearly fifty feet long, was found in the Jarrow Colliery, near Newcastle, lying in the shale parallel to the plane of stratification. Fragments of others found in the same shale indicated, by the size of the rhomboidal scars which covered them, a still greater size. Lepidodendron Sternbergii (Fig. 43) is represented as it is found beneath the shales in the collieries of Swina, in Bohemia.Fig. 46represents a portion of a branch ofL. elegansfurnished with leaves. M. Eugene Deslongchamps has drawn the restoration of the Lepidodendron Sternbergii, represented inFig. 47, which is shown entire inFig. 44, with its stem, its branches, fronds, and organs of fructification. The Ferns composed a great part of the vegetation of the Coal-measure period.
Fig. 44Fig. 44.—Lepidodendron Sternbergii restored. Forty feet high.
Fig. 44.—Lepidodendron Sternbergii restored. Forty feet high.
The Ferns differ chiefly in some of the details of the leaf.Pecopteris, for instance (Fig. 48), have the leaves once, twice, or thrice pinnatifid with the leaflets adhering either by their whole base or by the centre only; the midrib running through to the point.Neuropteris(Fig. 49) has leaves divided like Pecopteris, but the midrib does not reach the apex of the leaflets, but divides right and left into veins.Odontopteris(Fig. 51) has pinnatifid leaves, like the last, but its leaflets adhere by their whole base to the stalk.Lonchopteris(Fig. 50) has the leaves several times pinnatifid, the leaflets more or less united to one another, and the veins reticulated. Among the most numerous species of forms of the Coal-measure period wasSphenopteris artemisiæfolia(Fig. 52), of which a magnified leaf is represented. Sphenopteris has twice or thrice pinnatifid leaves, the leaflets narrow at the base, and the veins generally arranged as if they radiated from the base; the leaflets are frequently wedge-shaped.
Fig. 45Fig. 45.—Lepidostrobus variabilis.
Fig. 45.—Lepidostrobus variabilis.
Fig. 46Fig. 46.—Lepidodendron elegans.
Fig. 46.—Lepidodendron elegans.
The seas of this epoch included an immense number ofZoophytes, nearly 400 species of Mollusca, and a few Crustaceans and Fishes. Among the Fishes,PsammodusandCoccosteus, whose massive teeth inserted in the palate were suitable for grinding; and theHoloptychiusandMegalichthys, are the most important. The Mollusca are chiefly Brachiopods of great size. The Productæ attained here exceptional development,Producta Martini(Fig. 53),P. semi-reticulataandP. gigantea, being the most remarkable. Spirifers, also, were equally abundant, asSpirifera trigonalisandS. glabra. InTerebratula hastatathe coloured bands, which adorned the shell of the living animal, have been preserved to us. TheBellerophon, whose convoluted shell in some respects resembles the Nautilus of our present seas, but without its chambered shell, were then represented by many species, among others byBellerophon costatus(Fig. 54), andB. hiulcus(Fig. 56). Again, among the Cephalopods, we find theOrthoceras(Fig. 57), which resembled a straight Nautilus; and Goniatites (Goniatites evolutus,Fig. 55), a chambered shell allied to the Ammonite, which appeared in great numbers during the Secondary epoch.
Fig. 47Fig. 47.—Lepidodendron Sternbergii.
Fig. 47.—Lepidodendron Sternbergii.
Crustaceans are rare in the Carboniferous Limestone strata; the genus Phillipsia is the last of the Trilobites, all of which became extinct at the close of this period. As to the Zoophytes, they consist chiefly of Crinoids and Corals. The Crinoids were represented by the generaPlatycrinusandCyathocrinus. We also have in these rocks many Polyzoa.
Fig. 48Fig. 48.—Pecopteris lonchitica, a little magnified.
Fig. 48.—Pecopteris lonchitica, a little magnified.
Fig. 49Fig. 49.—Neuropteris gigantea.
Fig. 49.—Neuropteris gigantea.
Fig. 50Fig. 50.—Lonchopteris Bricii.
Fig. 50.—Lonchopteris Bricii.
Fig. 51Fig. 51.—Odontopteris Brardii.
Fig. 51.—Odontopteris Brardii.
Fig. 52Fig. 52.—Sphenopteris artemisiæfolia, magnified.
Fig. 52.—Sphenopteris artemisiæfolia, magnified.
Among the corals of the period, we may include the generaLithostrotionandLonsdalea, of whichLithostrotion basaltiforme(Fig. 58), andLonsdalea floriformis(Fig. 59), are respectively the representatives, withAmplexus coralloïdes. Among the Polyzoa are the generaFenestrellaandPolypora. Lastly, to these we may add a group of animals which will play a very important part and become abundantly represented in the beds of later geological periods, but which already abounded in the seas of the Carboniferous period. We speak of theForaminifera(Fig. 60), microscopic animals, which clustered either in one body, or divided into segments, and covered with a calcareous, many-chambered shell, as inFig. 60,Fusulina cylindrica. These little creatures, which, during the Jurassic and Cretaceous periods, formed enormous banks and entire masses ofrock, began to make their appearance in the period which now engages our attention.
Fig. 53Fig. 53.—Producta Martini. One-third nat. size.
Fig. 53.—Producta Martini. One-third nat. size.
Fig. 54Fig. 54.—Bellerophon costatus. Half nat. size.
Fig. 54.—Bellerophon costatus. Half nat. size.
Fig. 55Fig. 55.—Goniatites evolutus. Nat. size.
Fig. 55.—Goniatites evolutus. Nat. size.
Fig. 56Fig. 56.—Bellerophon hiulcus.
Fig. 56.—Bellerophon hiulcus.
Fig. 57Fig. 57.—Orthoceras laterale.
Fig. 57.—Orthoceras laterale.
Fig. 58Fig. 58.—Lithostrotion basaltiforme.
Fig. 58.—Lithostrotion basaltiforme.
Fig. 59Fig. 59.—Lonsdalea floriformis.
Fig. 59.—Lonsdalea floriformis.
The plate opposite (Plate X.) is a representation of an ideal aquarium, in which some of the more prominent species, which inhabited the seas during the period of the Carboniferous Limestone, are represented. On the right is a tribe of corals, with reflections of dazzling white: the species represented are, nearest the edge, theLasmocyathus, theChætetes, and thePtylopora. The Mollusc which occupies the extremity of the elongated and conical tube in the shape of a sabre is anAploceras. It seems to prepare the way for the Ammonite; for if this elongated shell were coiled round itself it would resemble the Ammonite and Nautilus. In the centre of the foreground we haveBellerophon hiulcus(Fig. 56), theNautilus Koninckii, and aProducta, with the numerous spines which surround the shell. (SeeFig. 62.)
Plate XX.—Ideal view of marine life in the Carboniferous Period.
X.—Ideal view of marine life in the Carboniferous Period.
On the left are other corals: theCyathophyllumwith straight cylindrical stems; some Encrinites (CyathocrinusandPlatycrinus) wound round the trunk of a tree, or with their flexible stem floating in the water. Some Fishes,Amblypterus, move about amongst these creatures, the greater number of which are immovably attached, like plants, to the rock on which they grow.
In addition, thisengravingshows us a series of islets, rising out ofa tranquil sea. One of these is occupied by a forest, in which a distant view is presented of the general forms of the grand vegetation of the period.
Fig. 60Fig. 60.—Foraminifera of the Mountain Limestone, forming the centre of an oolitic grain. Power 120.
Fig. 60.—Foraminifera of the Mountain Limestone, forming the centre of an oolitic grain. Power 120.
Fig. 61Fig. 61.—Foraminifera of the Chalk, obtained by brushing it in water. Power 120.
Fig. 61.—Foraminifera of the Chalk, obtained by brushing it in water. Power 120.
Fig. 62Fig. 62.—Producta horrida. Half natural size.
Fig. 62.—Producta horrida. Half natural size.
It is of importance to know the rocks formed by marine deposits during the era of the Carboniferous Limestone, inasmuch as they include coal, though in much smaller quantities than in the succeeding sub-period of the true coal-deposit. They consist essentially of a compact limestone, of a greyish-blue, and even black colour. The blow of the hammer causes them to exhale a somewhat fetid odour, which is owing to decomposed organic matter—the modified substance of the molluscs and zoophytes—of which it is to so great an extent composed, and whose remains are still easily recognised.
In the north of England, and many other parts of the British Islands, the Carboniferous Limestone forms, as we have seen, lofty mountain-masses, to which the termMountain Limestoneis sometimes applied.
In Derbyshire the formation constitutes rugged, lofty, and fantastically-shaped mountains, whose summits mingle with the clouds, while its picturesque character appears here, as well as farther north, in thedalesor valleys, where rich meadows, through which the mountain streams force their way, seem to be closed abruptly by masses of rock, rising above them like the grey ruins of some ancient tower; while the mountain bases are pierced with caverns, and their sides covered with mosses and ferns, for the growth of which the limestone is particularly favourable.
The formation ismetalliferous, and yields rich veins of lead-ore inDerbyshire, Cumberland, and other counties of Great Britain. The rock is found in Russia, in the north of France, and in Belgium, where it furnishes the common marbles, known as Flanders marble (Marbre de FlandresandM. de petit granit). These marbles are also quarried in other localities, such as Regneville (La Manche), either for the manufacture of lime or for ornamental stonework; one of the varieties quarried at Regneville, being black, with large yellow veins, is very pretty.
In France, theCarboniferous Limestone, with its sandstones and conglomerates, schists and limestones, is largely developed in the Vosges, in the Lyonnais, and in Languedoc, often in contact with syenites and porphyries, and other igneous rocks, by which it has been penetrated and disturbed, and evenmetamorphosedin many ways, by reason of the various kinds of rocks of which it is composed. In the United States the Carboniferous Limestone formation occupies a somewhat grand position in the rear of the Alleghanies. It is also found forming considerable ranges in our Australian colonies.
In consequence of their age, as compared with the Secondary and Tertiary limestones, the Carboniferous rocks are generally more marked and varied in character. The valley of the Meuse, from Namur to Chockier, above Liège, is cut out of this formation; and many of our readers will remember with delight the picturesque character of the scenery, especially that of the left bank of the celebrated river in question.
This terrestrial period is characterised, in a remarkable manner, by the abundance and strangeness of the vegetation which then covered the islands and continents of the whole globe. Upon all points of the earth, as we have said, this flora presented a striking uniformity. In comparing it with the vegetation of the present day, the learned French botanist, M. Brongniart, who has given particular attention to the flora of the Coal-measures, has arrived at the conclusion that it presented considerable analogy with that of the islands of the equatorial and torrid zone, in which a maritime climate and elevated temperature exist in the highest degree. It is believed that islands were very numerous at this period; that, in short, the dry land formed a sort of vast archipelago upon the general ocean, of no great depth, the islands being connected together and formed into continents as they gradually emerged from the ocean.
This flora, then, consists of great trees, and also of many smaller plants, which would form a close, thick turf, or sod, when partially buried in marshes of almost unlimited extent. M. Brongniart indicates, as characterising the period, 500 species of plants belonging to families which we have already seen making their first appearance in the Devonian period, but which now attain a prodigious development. The ordinary dicotyledons and monocotyledons—that is, plants having seeds with two lobes in germinating, and plants having one seed-lobe—are almost entirely absent; the cryptogamic, or flowerless plants, predominate; especially Ferns, Lycopodiaceæ and Equisetaceæ—but of forms insulated and actually extinct in these same families. A few dicotyledonous gymnosperms, or naked-seed plants forming genera of Conifers, have completely disappeared, not only from the present flora, but since the close of the period under consideration, there being no trace of them in the succeeding Permian flora. Such is a general view of the features most characteristic of the Coal period, and of the Primary epoch in general. It differs, altogether and absolutely, from that of the present day; the climatic condition of these remote ages of the globe, however, enables us to comprehend the characteristics which distinguish its vegetation. A damp atmosphere, of an equable rather than an intense heat like that of the tropics, a soft light veiled by permanent fogs, were favourable to the growth of this peculiar vegetation, of which we search in vain for anything strictly analogous in our own days. The nearest approach to the climate and vegetation proper to the geological period which now occupies our attention, would probably be found in certain islands, or on the littoral of the Pacific Ocean—the island of Chloë, for example, where it rains during 300 days in the year, and where the light of the sun is shut out by perpetual fogs; where arborescent Ferns form forests, beneath whose shade grow herbaceous Ferns, which rise three feet and upwards above a marshy soil; which gives shelter also to a mass of cryptogamic plants, greatly resembling, in its main features, the flora of the Coal-measures. This flora was, as we have said, uniform and poor in its botanic genera, compared to the abundance and variety of the flora of the present time; but the few families of plants, which existed then, included many more species than are now produced in the same countries. The fossil Ferns of the coal-series in Europe, for instance, comprehend about 300 species, while all Europe now only produces fifty. The gymnosperms, which now muster only twenty-five species in Europe, then numbered more than 120.
It will simplify the classification of the flora of the Carboniferous epoch if we give a tabular arrangement adopted by the best authorities:—
Calamites are among the most abundant fossil plants of the Carboniferous period, and occur also in the Devonian. They are preserved as striated, jointed, cylindrical, or compressed stems, with fluted channels or furrows at their sides, and sometimes surrounded by a bituminous coating, the remains of a cortical integument. They were originally hollow, but the cavity is usually filled up with a substance into which they themselves have been converted. They were divided into joints or segments, and when broken across at their articulations they show a number of striæ, originating in the furrows of the sides, and turning inwards towards the centre of the stem. It is not known whether this structure was connected with an imperfect diaphragm stretched across the hollow of the stem at each joint, or merely represented the ends of woody plates of which the solid part of the stem is composed. Their extremities have been discovered to taper gradually to a point, as represented inC. cannæformis(Fig. 64), or to end abruptly, the intervals becoming shorter and smaller. The obtuse point is now found to be the root. Calamites are regarded as Equisetaceous plants; later botanists consider that they belong to an extinct family of plants.Sigillariæare the most abundant of all plants in the coal formation, and were those principally concerned in the accumulation of the mineral fuel of the Coal-measures. Not a mine is opened, nor a heap of shale thrown out, but there occur fragments of its stem, marked externally with small rounded impressions, and in the centre slight tubercles, with a quincuncial arrangement. From the tubercles arise long ribbon-shaped bodies, which have been traced in some instances to the length of twenty feet.