Part 16

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 grand Sigillaria, the Stigmaria, 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. In Fig. 37 the reader has a restoration of the arborescent and herbaceous Ferns of 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, after such 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 call climate was 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 period might 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.

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 of spores or seeds. These humble Equiseta were 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 of Calamites. 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.

The Lycopods of 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 the Lepidodendrons which filled the forests. Their leaves were sometimes twenty inches long, and their trunks a yard in diameter. Such are the dimensions of some specimens of Lepidodendron carinatum which have been found. Another Lycopod of this period, the Lomatophloyos crassicaule, attained dimensions still more colossal. The Sigillarias sometimes exceeded 100 feet in height. Herbaceous Ferns were also 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 in Figs. 39 and 40, 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 as herbaceous trees 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 and Bromeliaceæ of our times.

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, and Asterophyllites.

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.

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.

The Annulariæ were 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. The Sigillariæ were, 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. 41 represents the bark of one of these Sigillariæ, which is often met with in coal-mines.

The Stigmariæ (Fig. 42), according to palæontologists, were roots of Sigillariæ, with a subterranean fructification; all that is known of them 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 erect Sigillariæ in the sea-cliffs of the South Joggins in Nova Scotia, and found that from the lower extremities of the trunk they sent out Stigmariæ as 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 of Sigillariæ as merely provisional.

Quart. Jour. Geol. Soc., vol. xxii., p. 129.

Two other gigantic trees grew in the forests of this period: these were Lepidodendron carinatum and Lomatophloyos 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.

The Lepidodendrons, of which there are about forty known species, have cylindrical bifurcated branches; that is, the branches were evolved in pairs, or were dichotomous to 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 of Lepidostrobus (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 of L. 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. 46 represents a portion of a branch of L. elegans furnished with leaves. M. Eugene Deslongchamps has drawn the restoration of the Lepidodendron Sternbergii, represented in Fig. 47, which is shown entire in Fig. 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.

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 was Sphenopteris 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.

CARBONIFEROUS LIMESTONE. (SUB-PERIOD.)

The seas of this epoch included an immense number of Zoophytes, nearly 400 species of Mollusca, and a few Crustaceans and Fishes. Among the Fishes, Psammodus and Coccosteus, whose massive teeth inserted in the palate were suitable for grinding; and the Holoptychius and Megalichthys, are the most important. The Mollusca are chiefly Brachiopods of great size. The Productæ attained here exceptional development, Producta Martini (Fig. 53), P. semi-reticulata and P. gigantea, being the most remarkable. Spirifers, also, were equally abundant, as Spirifera trigonalis and S. glabra. In Terebratula hastata the coloured bands, which adorned the shell of the living animal, have been preserved to us. The Bellerophon, 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 by Bellerophon costatus (Fig. 54), and B. hiulcus (Fig. 56). Again, among the Cephalopods, we find the Orthoceras (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.

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 genera Platycrinus and Cyathocrinus. We also have in these rocks many Polyzoa.

Among the corals of the period, we may include the genera Lithostrotion and Lonsdalea, of which Lithostrotion basaltiforme (Fig. 58), and Lonsdalea floriformis (Fig. 59), are respectively the representatives, with Amplexus coralloïdes. Among the Polyzoa are the genera Fenestrella and Polypora. 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 the Foraminifera (Fig. 60), microscopic animals, which clustered either in one body, or divided into segments, and covered with a calcareous, many-chambered shell, as in Fig. 60, Fusulina cylindrica. These little creatures, which, during the Jurassic and Cretaceous periods, formed enormous banks and entire masses of rock, began to make their appearance in the period which now engages our attention.

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, the Lasmocyathus, the Chætetes, and the Ptylopora. The Mollusc which occupies the extremity of the elongated and conical tube in the shape of a sabre is an Aploceras. 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 have Bellerophon hiulcus (Fig. 56), the Nautilus Koninckii, and a Producta, with the numerous spines which surround the shell. (See Fig. 62.)

On the left are other corals: the Cyathophyllum with straight cylindrical stems; some Encrinites (Cyathocrinus and Platycrinus) 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, this engraving shows us a series of islets, rising out of a 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.