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.
The _Coal-measures_; and 2. The _Carboniferous 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.
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.
[Illustration: Fig. 37.--Ferns restored. 1 and 2. Arborescent Ferns. 3
and 4. Herbaceous Ferns.]
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.
[Illustration: Fig. 38.--Calamite restored. Thirty to forty feet high.]
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.
[Illustration: Fig. 39.--Trunk of Calamites. One-fifth natural size.]
[Illustration: Fig. 40.--Trunk of Sigillaria. One-tenth 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_, 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.[43]
[43] _Quart. Jour. Geol. Soc._, vol. xxii., p. 129.
[Illustration: Fig. 41.--Sigillaria lævigata. One-third natural size.]
[Illustration: Fig. 42.--Stigmaria. One-tenth natural size.]
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.
[Illustration: Fig. 43.--Lepidodendron Sternbergii.]
[Illustration: 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 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.
[Illustration: Fig. 45.--Lepidostrobus variabilis.]
[Illustration: Fig. 46.--Lepidodendron elegans.]