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.
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 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 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.
[Illustration: Fig. 107. GENERALIZED SECTION OF THE CARBONIFEROUS
STRATA OF THE NORTH OF ENGLAND.]
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 daily undergoing increase. It
is not to be supposed, however, that the remains of plants are
found solely in Coal-measures; 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
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, 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 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.
[Illustration: Fig. 108.--_Odontopteris Schlotheimii_. Carboniferous,
Europe and North America.]
[Illustration: Fig. 109.--_Calamites cannoeformis_. Carboniferous
Rocks, Europe and North America.]
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 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 which date from the Devonian. The simply
herbaceous ferns are extremely numerous, and belong to such
widely-distributed and largely-represented genera as _Neuropteris,
Odontopteris_ (fig. 108), _Alethopteris, Pecopteris, Sphenopteris,
Hymenophyllites_, &c.
[Footnote 18: Whilst the vegetation of the Coal-period was mainly
a terrestrial one, aquatic plants are not unknown. Sea-weeds
(such as the _Spirophyton cauda-Galli_) are common in some of
the marine strata; whilst coal, according to the researches of
the Abbé Castracane, is asserted commonly to contain the siliceous
envelopes of Diatoms.]
The fossils known as _Calamites_ (fig. 109) are very common 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 (_Equisetaceoe_) 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."
[Illustration: 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.]
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 modern
Club-mosses (_Lycopodiaceoe_). They are amongst the commonest
and most characteristic of the Carboniferous plants; and the
majority of the "spore-cases" so commonly found in the coal appear
to have been derived from the cones of Lepidodendroids.
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).
[Illustration: Fig. 111.--Fragment of the external surface of
_Sigillaria Groeseri_, showing the ribs and leaf-scars. The left-hand
figure represents a small portion enlarged. Carboniferous, Europe.]
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 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 _Sigillarioe_, standing 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.
[Illustration: Fig. 112.--_Stigmaria ficoides_. Quarter natural
size. Carboniferous.]
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 _Stigmarioe_ (fig. 112) ordinarily present themselves in
the form of long, compressed or rounded 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 _Stigmarioe_ are
generally found ramifying in the "under-clay," which forms the
floor of a bed of coal, and which represents the ancient soil
upon which the _Sigillarioe_ grew.
[Illustration: Fig. 113.--_Trigonocarpon ovatum_. Coal-measures,
Britain. (After Liudley and Hutton.)]
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, Paloeoxylon, 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 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.