THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 12:
THE CARBONIFEROUS PERIOD.
Overlying the Devonian formation is the great and important series
of the Carboniferous Rocks, so called because workable
beds of coal are more commonly and more largely developed in this
formation than in any other. Workable coal-seams, however, occur
in various other formations (Jurassic, Cretaceous, Tertiary), so
that coal is not an exclusively Carboniferous product; whilst
even in the Coal-measures themselves the coal bears but a very
small proportion to the total thickness of strata, occurring
only in comparatively thin beds intercalated in a great series
of sandstones, shales, and other genuine aqueous sediments.
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Stratigraphically, the Carboniferous rocks usually repose
conformably upon the highest Devonian beds, so that the line of
demarcation between the Carboniferous and Devonian formations is
principally a palæontological one, founded on the observed
differences in the fossils of the two groups. On the other hand,
the close of the Carboniferous period seems to have been generally,
though not universally, signalised by movements of the crust of
the earth, so that the succeeding Permian beds often lie
unconformably upon the Carboniferous sediments.
Strata of Carboniferous age have been discovered in almost every
large land-area which has been sufficiently investigated; but
they are especially largely developed in Britain, in various
parts of the continent of Europe, and in North America. Their
general composition, however, is, comparatively speaking, so
uniform, that it will suffice to take a comprehensive view of
the formation without considering any one area in detail, though
in each region the subdivisions of the formation are known by
distinctive local names. Taking such a comprehensive view, it
is found that the Carboniferous series is generally divisible
into a Lower and essentially calcareous group (the
"Sub-Carboniferous" or "Carboniferous Limestone"); a Middle
and principally arenaceous group (the "Millstone Grit"); and an
Upper group, of alternating shales and sandstones, with workable
seams of coal (the "Coal-measures").
I. The Carboniferous, Sub-Carboniferous, or Mountain
Limestone Series constitutes the general base of the
Carboniferous system. As typically developed in Britain, the
Carboniferous Limestone is essentially a calcareous formation,
sometimes consisting of a mass of nearly pure limestone from
1000 to 2000 feet in thickness, or at other times of successive
great beds of limestone with subordinate sandstones and shales.
In the north of England the base of the series consists of pebbly
conglomerates and coarse sandstones; and in Scotland generally,
the group is composed of massive sandstones with a comparatively
feeble development of the calcareous element. In Ireland, again,
the base of the Carboniferous Limestone is usually considered
to be formed by a locally-developed group of grits and shales
(the "Coomhola Grits" and "Carboniferous Slate"), which attain
the thickness of about 5000 feet, and contain an intermixture
of Devonian with Carboniferous types of fossils. Seeing that the
Devonian formation is generally conformable to the Carboniferous,
we need feel no surprise at this intermixture of forms; nor does it
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appear to be of great moment whether these
strata be referred to the former or to the latter series. Perhaps
the most satisfactory course is to regard the Coomhola Grits and
Carboniferous Slates as "passage-beds" between the Devonian and
Carboniferous; but any view that may be taken as to the position
of these beds, really leaves unaffected the integrity of the
Devonian series as a distinct life-system, which, on the whole,
is more closely allied to the Silurian than to the Carboniferous.
In North America, lastly, the Sub-Carboniferous series is never
purely calcareous, though in the interior of the continent it
becomes mainly so. In other regions, however, it consists
principally of shales and sandstones, with subordinate beds of
limestone, and sometimes with this beds of coal or deposits of
clay-ironstone.
II. The Millstone Grit.—The highest beds of the
Carboniferous Limestone series are succeeded, generally with
perfect conformity, by a series of arenaceous beds, usually
known as the Millstone Grit. As typically developed in
Britain, this group consists of hard quartzose sandstones,
often so large-grained and coarse in texture as to properly
constitute fine conglomerates. In other cases there are regular
conglomerates, sometimes with shales, limestones, and thin beds
of coal—the thickness of the whole series, when well
developed, varying from 1000 to 5000 feet. In North America,
the Millstone Grit rarely reaches 1000 feet in thickness; and,
like its British equivalent, consists of coarse sandstones
and grits, sometimes with regular conglomerates. Whilst the
Carboniferous Limestone was undoubtedly deposited in a tranquil
ocean of considerable depth, the coarse mechanical sediments
of the Millstone Grit indicate the progressive shallowing of
the Carboniferous seas, and the consequent supervention of
shore-conditions.
III. The Coal-measures.—The Coal-measures properly
so called rest conformably upon the Millstone Grit, and usually
consist of a vast series of sandstones, shales, grits, and coals,
sometimes with beds of limestone, attaining in some regions a total
thickness of from 7000 to nearly 14,000 feet. Beds of workable
coal are by no means unknown in some areas in the inferior group
of the Sub-Carboniferous; but the general statement is true,
that coal is mostly obtained from the true Coal-measures—the
largest known, and at present most productive coal-fields of the
world being in Great Britain, North America, and Belgium. Wherever
they are found, with limited exceptions, the Coal-measures present
a singular general uniformity of mineral composition. They
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consist, namely, of an indefinite alternation
of beds of sandstone, shale, and coal, sometimes with bands of
clay-ironstone or beds of limestone, repeated in no constant
order, but sometimes attaining the enormous aggregate thickness
of 14,000 feet, or little short of 3 miles. The beds of coal
differ in number and thickness in different areas, but they
seldom or never exceed one-fiftieth part of the total bulk of
the formation in thickness. The characters of the coal itself,
and the way in which the coal-beds were deposited, will be
briefly alluded to in speaking of the vegetable life of the
period. In Britain, and in the Old World generally, the
Coal-measures are composed partly of genuine terrestrial
deposits—such as the coal—and partly of sediments
accumulated in the fresh or brackish waters of vast lagoons,
estuaries, and marshes. The fossils of the Coal-measures in these
regions are therefore necessarily the remains either of terrestrial
plants and animals, or of such forms of life as inhabit fresh or
brackish waters, the occurrence of strata with marine fossils being
quite a local and occasional phenomenon. In various parts of North
America, on the other hand, the Coal-measures, in addition to
sandstones, shales, coal-seams, and bands of clay-ironstone,
commonly include beds of limestone, charged with marine remains,
and indicating marine conditions. The subjoined section (fig. 107)
gives, in a generalised form, the succession of the Carboniferous
strata in such a British area as the north of England, where
the series is developed in a typical form.
As regards the life of the Carboniferous period, we naturally
find, as has been previously noticed, great differences in different
parts of the entire series, corresponding to the different mode of
origin of the beds. Speaking generally, the Lower Carboniferous
(or the Sub-Carboniferous) is characterised by the remains of
marine animals; whilst the Upper Carboniferous (or Coal-measures)
is characterised by the remains of plants and terrestrial animals.
In all those cases, however, in which marine beds are found in
the series of the Coal-measures, as is common in America, then
we find that the fossils agree in their general characters with
those of the older marine deposits of the period.
Owing to the fact that coal is simply compressed and otherwise
altered vegetable matter, and that it is of the highest economic
value to man, the Coal-measures have been more thoroughly explored
than any other group of strata of equivalent thickness in the
entire geological series. Hence we have already a very extensive
acquaintance with the plants of the Carboniferous period;
and our knowledge on this subject is
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daily undergoing increase. It is not to be supposed, however,
that the remains of plants are found solely in Coal-measures;
GENERALIZED SECTION OF THE CARBONIFEROUS STRATA OF THE NORTH OF
ENGLAND.
Fig. 107.
for though most abundant towards the summit, they are found in
less numbers in all parts of the series. Wherever found, they
belong to the same great types of
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vegetation; but, before reviewing these, a few words must be
said as to the origin and mode of formation of coal.
The coal-beds, as before mentioned, occur interstratified with
shales, sandstones, and sometimes limestones; and there may,
within the limits of a single coal-field, be as many as 80 or
100 of such beds, placed one above the other at different levels,
and varying in thickness from a few inches up to 20 or 30 feet.
As a general rule, each bed of coal rests upon a bed of shale or
clay, which is termed the "under-clay," and in which are found
numerous roots of plants; whilst the strata immediately on the
top of the coal may be shaly or sandy, but in either case are
generally charged with the leaves and stems of plants, and often
have upright trunks passing vertically through them. When we
add to this that the coal itself is, chemically, nearly wholly
composed of carbon, and that its microscopic structure shows it
to be composed almost entirely of fragments of stems, leaves,
bark, seeds, and vegetable débris derived from
land-plants, we are readily enabled to understand how the
coal was formed. The "under-clay" immediately beneath the
coal-bed represents an old land-surface—sometimes, perhaps,
the bottom of a swamp or marsh, covered with a luxuriant vegetation;
the coal bed itself represents the slow accumulation,
through long periods, of the leaves, seeds, fruits, stems, and
fallen trunks of this vegetation, now hardened and compressed
into a fraction of its original bulk by the pressure of the
superincumbent rocks; and the strata of sand or shale above the
coal-bed—the so-called "roof" of the coal—represent
sediments quietly deposited as the land, after a long period of
repose, commenced to sink beneath the sea. On this view, the rank
and long-continued vegetation which gave rise to each coal-bed
was ultimately terminated by a slow depression of the surface on
which the plants grew. The land-surface then became covered by
the water, and aqueous sediments were accumulated to a greater
or less thickness upon the dense mass of decaying vegetation
below, enveloping any trunks of trees which might still be in an
erect position, and preserving between their layers the leaves
and branches of plants brought down from the neighbouring land by
streams, or blown into the wafer by the wind. Finally, there set in
a slow movement of elevation,—the old land again reappeared
above the water; a new and equally luxuriant vegetation flourished
upon the new land-surface; and another coal-bed was accumulated,
to be preserved ultimately in a similar fashion. Some few beds
of coal may have been formed by drifted vegetable matter brought
down into the ocean by rivers,
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and deposited
directly on the bottom of the sea; but in the majority of cases
the coal is undeniably the result of the slow growth and decay of
plants in situ: and as the plants of the coal are not
marine plants, it is necessary to adopt some such theory
as the above to account for the formation of coal-seams. By this
theory, as is obvious, we are compelled to suppose that the vast
alluvial and marshy flats upon which the coal-plants grew were
liable to constantly-recurring oscillations of level, the successive
land-surfaces represented by the successive coal-beds of any
coal-field being thus successively buried beneath accumulations
of mud or sand. We have no need, however, to suppose that these
oscillations affected large areas at the same time; and geology
teaches us that local elevations and depressions of the land
have been matters of constant occurrence throughout the whole
of past time.
All the varieties of coal (bituminous coal, anthracite; cannel-coal,
&c.) show a more or less distinct "lamination"—that is to
say, they are more or less obviously composed of successive thin layers,
differing slightly in colour and texture. All the varieties of coal,
also, consist chemically of carbon, with varying proportions
of certain gaseous constituents and a small amount of incombustible
mineral or "ash." By cutting thin and transparent slices of coal,
we are further enabled, by means of the microscope, to ascertain
precisely not only that the carbon of the coal is derived from
vegetables, but also, in many cases, what kinds of plants, and what
parts of these, enter into the formation of coal. When examined
in this way, all coals are found to consist more or less entirely
of vegetable matter; but there is considerable difference in
different coals as to the exact nature of this. By Professor
Huxley it has been shown that many of the English coals consist
largely of accumulations of rounded discoidal sacs or bags, which
are unquestionably the seed-vessels or "spore-cases" of certain
of the commoner coal-plants (such as the Lepidodendra).
The best bituminous coals seem to be most largely composed of
these spore-cases; whilst inferior kinds possess a progressively
increasing amount of the dull carbonaceous substance which is
known as "mineral charcoal," and which is undoubtedly composed
of "the stems and leaves of plants reduced to little more than
their carbon." On the other hand, Principal Dawson finds that
the American coals only occasionally exhibit spore-cases to any
extent, but consist principally of the cells, vessels, and fibres
of the bark, integumentary coverings, and woody portions of the
Carboniferous plants.
The number of plants already known to have existed
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during the Carboniferous period is so great, that
nothing more can be done here than to notice briefly the typical
and characteristic groups of these—such as the
Ferns, the Calamites, the Lepidodendroids, the Sigillarioids,
and the Conifers.
In accordance with M. Brongniart's generalisation, that the
Palæozoic period is, botanically speaking, the "Age of
Acrogens," we find the Carboniferous plants to be still mainly
referable to the Flowerless or "Cryptogamous" division of the
vegetable kingdom. The flowering or "Phanerogamous" plants,
which form the bulk of our existing vegetation, are hardly known,
with certainty, to have existed at all in the Carboniferous era,
except as represented by trees related to the existing Pines and
Fig. 108.—Odontopteris Schlotheimii. Carboniferous,
Europe and North America.
Firs, and possibly by the Cycads or "false palms."[18] Amongst
the "Cryptogams," there is no more striking or beautiful group of
Carboniferous plants than the Ferns. Remains of these are
found all through the Carboniferous, but in exceptional numbers
in the Coal-measures, and include both herbaceous forms like the
majority of existing species, and arborescent forms resembling
the living Tree-ferns of New Zealand. Amongst the latter, together
with some new types, are examples of the genera Psaronius
and Caulopteris, both of
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which date from the Devonian. The simply herbaceous ferns are
extremely numerous, and belong to such widely-distributed and
Fig. 109.—Calamites cannœformis. Carboniferous
Rocks, Europe and North America.
largely-represented genera as Neuropteris, Odontopteris
(fig. 108), Alethopteris, Pecopteris, Sphenopteris,
Hymenophyllites, &c.
The fossils known as Calamites (fig. 109) are very common
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in the Carboniferous deposits, and have given
occasion to an abundance of research and speculation. They present
themselves as prostrate and flattened striated stems, or as similar
uncompressed stems growing in an erect position, and sometimes
attaining a length of twenty feet or more. Externally, the stems
are longitudinally ribbed, with transverse joints at regular
intervals, these joints giving origin to a whorl or branchlets,
which mayor may not give origin to similar whorls of smaller
branchlets still. The stems, further, were hollow, with transverse
partitions at the joints, and having neither true wood nor bark,
but only a thin external fibrous shell. There can be little doubt
but that the Calamites are properly regarded as colossal
representatives of the little Horse-tails (Equisetaceœ)
of the present day. They agree with these not only in the general
details of their organisation, but also in the fact that the fruit
was a species of cone, bearing "spore-cases" under scales.
According to Principal Dawson, the Calamites "grew in
dense brakes on the sandy and muddy flats, subject to inundation,
or perhaps even in water; and they had the power of budding out
from the base of the stem, so as to form clumps of plants, and
also of securing their foothold by numerous cord-like roots
proceeding from various heights on the lower part of the stem."
The Lepidodendroids, represented mainly by the genus
Lepidodendron itself (fig. 110), were large tree-like
plants, which attain their maximum in the Carboniferous period,
but which appear to commence in the Upper Silurian, are well
represented in the Devonian, and survive in a diminished form into
the Permian. The trunks of the larger species of Lepidodendron
at times reach a length of fifty feet and upwards, giving off
branches in a regular bifurcating manner. The bark is marked
with numerous rhombic or oval scars, arranged in quincunx order,
and indicating the points where the long, needle-shaped leaves
were formerly attached. The fruit consisted of cones or spikes,
carried at the ends of the branches, and consisting of a central
axis surrounded by overlapping scales, each of which supports
a "spore-case" or seed-vessel. These cones have commonly been
described under the name of Lepidostrobi. In the structure
of the trunk there is nothing comparable to what is found in
existing trees, there being a thick bark surrounding a zone
principally composed of "scalariform" vessels, this in turn
enclosing a large central pith. In their general appearance the
Lepidodendra bring to mind the existing Araucarian Pines;
but they are true "Cryptogams," and are to be regarded as a
gigantic extinct type of the
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modern Club-mosses (Lycopodiaceœ). They are amongst
the commonest and most characteristic of the Carboniferous
Fig. 110.—Lepidodendron Sternbergii, Carboniferous,
Europe. The central figure represents a portion of the trunk with
its branches, much reduced in size. The right-hand figure is a
portion of a branch with the leaves partially attached to it; and
the left-hand figure represents the end of a branch bearing a cone
of fructification.
plants; and the majority of the "spore-cases" so commonly found
in the coal appear to have been derived from the cones of
Lepidodendroids.
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The so-called Sigillanoids, represented
mainly by Sigillaria itself (fig. 111), were no less
abundant and characteristic of the Carboniferous forests than the
Lepidodendra. They commence their existence, so far as
known, in the Devonian period, but they attain their maximum in the
Carboniferous; and—unlike the Lepidodendroids—they are
not known to occur in the Permian period. They are comparatively
gigantic in size, often attaining a height of from thirty to
fifty feet or more; but though abundant and well preserved, great
divergence of opinion prevails as to their true affinities. The
name of Sigillarioids (Lat. sigilla, little seals
or images) is derived from the fact that the bark is marked with
seal-like impressions or leaf-scars (fig. 111).
Externally, the trunks of Sigillaria present strong
longitudinal ridges, with vertical alternating rows of oval
leaf-scars indicating the points where the leaves were originally
Fig. 111.—Fragment of the external surface of Sigillaria
Grœseri, showing the ribs and leaf-scars. The left-hand
figure represents a small portion enlarged. Carboniferous,
Europe.
attached. The trunk was furnished with a large central pith, a thick
outer bark, and an intermediate woody zone,—composed, according
to Dawson, partly of the disc-bearing fibres so characteristic of
Conifers; but, according to Carruthers, entirely made up of the
"scalariform" vessels characteristic of Cryptogams. The size of the
pith was very great, and the bark seems to have been the most
durable portion of the trunk. Thus we have evidence that in many
cases the stumps and "stools" of Sigillariœ, standing
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upright in the old Carboniferous swamps, were
completely hollowed out by internal decay, till nothing but an
exterior shell of bark was left. Often these hollow stumps became
ultimately filled up with sediment, sometimes enclosing the remains
of galley-worms, land-snails, or Amphibians, which formerly found in
the cavity of the trunk a congenial home; and from the sandstone or
shale now filling such trunks some of the most interesting fossils
of the Coal-period have been obtained. There is little certainty as
to either the leaves or fruits of Sigillaria, and there
is equally little certainty as to the true botanical position of
these plants. By Principal Dawson they are regarded as being
probably flowering plants allied to the existing "false palms"
or "Cycads," but the high authority of Mr Carruthers is to
be quoted in support of the belief that they are Cryptogamic,
and most nearly allied to the Club-mosses.
Leaving the botanical position of Sigillaria thus undecided,
we find that it is now almost universally conceded that the fossils
originally described under the name of Stigmaria are the
roots of Sigillaria, the actual connection between
the two having been in numerous instances demonstrated in an
unmistakable manner. The Stigmariœ (fig. 112) ordinarily
present themselves in the form of long, compressed or rounded
Fig. 112.—Stigmaria ficoides. Quarter natural size.
Carboniferous.
fragments, the external surface of which is covered with rounded
pits or shallow tubercles, each of which has a little pit or
depression in its centre. From each of these pits there proceeds,
in perfect examples, a long cylindrical rootlet; but in many cases
these have altogether disappeared. In their internal structure,
Stigmaria exhibits a central pith surrounded by a sheath of
scalariform vessels, the whole enclosed in a cellular envelope.
The Stigmariœ are generally found ramifying in
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the "under-clay," which forms the floor of a
bed of coal, and which represents the ancient soil upon which the
Sigillariœ
grew.
The Lepidodendroids and Sigillaroids, though the first
were certainly, and the second possibly, Cryptogamic or flowerless
plants, must have constituted the main mass of the forests of
the Coal period; but we are not without evidence of the existence
at the same time of genuine "trees," in the technical sense of
this term—namely, flowering plants with large woody stems. So
far as is certainly known, all the true trees of the Carboniferous
formation were Conifers, allied to the existing Pines and
Firs. They are recognised by the great size and concentric woody
rings of their prostrate, rarely erect trunks, and by the presence
of disc-bearing fibres in their wood, as demonstrated by the
microscope; and the principal genera which have been recognised are
Dadoxylon, Palœoxylon, Araucarioxylon, and Pinites.
Their fruit is not known with absolute certainty, unless it be
represented, as often conjectured, by Trigonocarpon (fig.
113). The fruits known under this name are nut-like, often of
Fig. 113.—Trigonocarpon ovatum. Coal-measures, Britain.
(After Liudley and Hutton.)
considerable size, and commonly three- or six-angled. They probably
originally possessed a fleshy envelope; and if truly referable
to the Conifers, they would indicate that these ancient
evergreens produced berries instead of cones, and thus resembled
the modern Yews rather than Pines. It seems, further, that the
great group of the Cycads, which are nearly allied to the
Conifers, and which attained such a striking prominence in
the Secondary period, probably commenced its existence during the
Coal period; but these anticipatory forms are comparatively few
in number, and for the most part of somewhat dubious affinities.
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