Vestiges of the Natural History of Creation (Chapter 6)
by Robert Chambers
Secondary Rocks.
Era of the Carboniferous Formation.
Land Formed. Commencement of Land Plants.
e now enter upon a new great epoch in the history of our
globe. There was now dry land. As a consequence of this fact, there was
fresh water, for rain, instead of immediately returning to the sea, as
formerly, was now gathered in channels of the earth, and became springs,
rivers, and lakes. There was now a theatre for the existence of land plants
and animals, and it remains to be inquired if these accordingly were
produced.
The Secondary Rocks, in which our further researches are
to be prosecuted, consist of a great and varied series, resting, generally
unconformably, against flanks of the upturned primary rocks, sometimes
themselves considerably inclined, at others, forming extensive basin-like
beds, nearly horizontal; in many places, much broken up and shifted by
disturbances from below. They have all been formed out of the materials of
the older rocks, by virtue of the wearing power of air and water, which is
still every day carrying down vast quantities of the elevated matter of the
globe into the sea. But the separate strata are each much more distinct in
the matter of its composition than might be expected. Some are siliceous or
arenaceous (sandstones), composed mainly of fine grains from the quartz
rocks—the most abundant of the primary strata. Others are
argillaceous—clays, shales, &c., chiefly derived, probably, from the
slate beds of the primary series. Others are calcareous, derived from the
early limestone. As a general feature, they are softer and less crystalline
than the primary rocks, as if they had endured less of both heat and
pressure than the senior formation. There are beds (coal) formed
solely of vegetable matter, and some others in which the main ingredient
is particles of iron, (the iron black band.) The secondary rocks
are quite as communicative with regard to their portion of the earth's
history as the primitive were.
The first, or lowest, group of the secondary rocks is
called the Carboniferous Formation, from the remarkable feature of
its numerous interspersed beds of coal. It commences with the beds of the
mountain limestone, which, in some situations, as in Derbyshire and
Ireland, are of great thickness, being alternated with chert (a siliceous
sandstone), sandstones, shales, and beds of coal, generally of the harder
and less bituminous kind (anthracite), the whole being covered in
some places by the millstone grit, a siliceous conglomerate composed of the
detritus of the primary rocks. The mountain limestone, attaining in England
to a depth of eight hundred yards, greatly exceeds in volume any of the
primary limestone beds, and shews an enormous addition of power to the
causes formerly suggested as having produced this substance. In fact,
remains of corals, crinoidea, and shells, are so abundant in it, as to
compose three-fourths of the mass in some parts. Above the mountain
limestone commence the more conspicuous coal beds, alternating with
sandstones, shales, beds of limestone, and ironstone. Coal is altogether
composed of the matter of a terrestrial vegetation, transmuted by pressure.
Some fresh-water shells have been found in it, but few of marine origin,
and no remains of those zoophytes and crinoidea so abundant in the mountain
limestone and other rocks. Coal beds exist in Europe, Asia, and America,
and have hitherto been esteemed as the most valuable of mineral
productions, from the important services which the substance renders in
manufactures and in domestic economy. It is to be remarked, that there are
some local variations in the arrangement of coal beds. In France, they rest
immediately on the granite and other primary rocks, the intermediate strata
not having been found at those places. In America, the kind called
anthracite occurs among the slate beds, and this species also abounds more
in the mountain limestone than with us. These last circumstances only shew
that different parts of the earth's surface did not all witness the same
events of a certain fixed series exactly at the same time. There had been
an exhibition of dry land about the site of America, a little earlier than
in Europe.
Some features of the condition of the earth during the
deposition of the-carboniferous group, are made out with a clearness which
must satisfy most minds. First we are told of a time when carbonate of lime
was formed in vast abundance at the bottoms of profound seas, accompanied
by an unusually large population of corals and encrinites; while in some
parts of the earth there were patches of dry land, covered with a luxuriant
vegetation. Next we have a comparatively brief period of volcanic
disturbance, (when the conglomerate was formed.) Then the causes favourable
to the so abundant production of limestone, and the large population of
marine acrita, decline, and we find the masses of dry land increase in
number and extent, and begin to bear an amount of forest vegetation, far
exceeding that of the most sheltered tropical spots of the present surface.
The climate, even in the latitude of Baffin's Bay, was torrid, and perhaps
the atmosphere contained a larger charge of carbonic acid gas (the material
of vegetation) than it now does. The forests or thickets of the period,
included no species of plants now known upon earth. They mainly consisted
of gigantic shrubs, which are either not represented by any existing types,
or are akin to kinds which are now only found in small and lowly forms.
That these forests grew upon a Polynesia, or multitude of small islands, is
considered probable, from similar vegetation being now found in such
situations within the tropics. With regard to the circumstances under which
the masses of vegetable matter were transformed into successive coal
strata, geologists are divided. From examples seen at the present day, at
the mouths of such rivers as the Mississippi, which traverse extensive
sylvan regions, and from other circumstances to be adverted to, it is held
likely by some that the vegetable matter, the rubbish of decayed forests,
was carried by rivers into estuaries, and there accumulated in vast natural
rafts, until it sunk to the bottom, where an overlayer of sand or mud would
prepare it for becoming a stratum of coal. Others conceive that the
vegetation first went into the condition of a peat moss, that a sink in the
level then exposed it to be overrun by the sea, and covered with a layer of
sand or mud; that a subsequent uprise made the mud dry land, and fitted it
to bear a new forest, which afterwards, like its predecessor, became a bed
of peat; that, in short, by repetitions of this process, the alternate
layers of coal, sandstone, and shale, constituting the carboniferous group,
were formed. It is favourable to this last view that marine fossils are
scarcely found in the body of the coal itself, though abundant in the shale
layers above and below it; also that in several places erect stems of trees
are found with their roots still fixed in the shale beds, and crossing the
sandstone beds at almost right angles, shewing that these, at least, had
not been drifted from their original situations. On the other hand, it is
not easy to admit such repeated risings and sinkings of surface as would be
required, on this hypothesis, to form a series of coal strata. Perhaps we
may most safely rest at present with the supposition that coal has been
formed under both classes of circumstances, though in the latter only as an
exception to the former.
Upwards of three hundred species of plants have been
ascertained to exist in the coal formation; but it is not necessary to
suppose that the whole contained in that system are now, or ever will be
distinguished. Experiments shew that some great classes of plants become
decomposed in water in a much less space of time than others, and it is
remarkable that those which decompose soonest, are of the classes found
most rare, or not at all, in the coal strata. It is consequently to be
inferred that there may have been grasses and mosses at this era, and many
species of trees, the remains of which had lost all trace of organic form
before their substance sunk into the mass of which coal was formed. In
speaking, therefore, of the vegetation of this period, we must bear in mind
that it may have comprehended forms of which we have no memorial.
Supposing, nevertheless, that, in the main, the
ascertained vegetation of the coal system is that which grew at the time of
its formation, it is interesting to find that the terrestrial botany of our
globe begins with classes of comparatively simple forms and structure. In
the ranks of the vegetable kingdom, the lowest place is taken by plants of
cellular tissue, and which have no flowers, (cryptogamia,) as
lichens, mosses, fungi, ferns, sea-weeds. Above these stand plants of
vascular tissue, and bearing flowers, in which again there are two great
subdivisions; first, plants having one seed-lobe, (monocotyledons,)
and in which the new matter is added within, (endogenous,) of which
the cane and palm are examples; second, plants having two seed-lobes,
(dicotyledons,) and in which the new matter is added on the outside
under the bark, (exoyenous,) of which the pine, elm, oak, and most
of the British forest-trees are examples; these subdivisions also ranking
in the order in which they are here stated. Now it is clear that a
predominance of these forms in succession marked the successive epochs
developed by fossil geology; the simple abounding first, and the complex
afterwards.
Two-thirds of the plants of the carboniferous era are of
the cellular or cryptogamic kind, a proportion which would probably be much
increased if we knew the whole Flora of that era The ascertained
dicotyledons, or higher-class plants, are comparatively few in this
formation; but it will be found that they constantly increased as the globe
grew older.
The master-form or type of the era was the fern,
or breckan, of which about one hundred and thirty species have already been
ascertained as entering into the composition of coal.[1] The fern is a plant which thrives best in warm,
shaded, and moist situations. In tropical countries, where these conditions
abound, there are many more species than in temperate climes, and some of
these are arborescent, or of a tree-like size and luxuriance.[2] The ferns of the coal strata have been of this
magnitude, and that without regard to the parts of the earth where they are
found. In the coal of Baffin's Bay, of Newcastle, and of the torrid zone
alike, are the fossil ferns arborescent, shewing clearly that, in that era,
the present tropical temperature, or one even higher, existed in very high
latitudes.
In the swamps and ditches of England there grows a plant
called the horse-tait (equisetum), having a succulent, erect,
jointed stem, with slender leaves, and a scaly catkin at the top. A second
large section of the plants of the carboniferous era were of this kind
(equisetaceæ), but, like the fern, reaching the magnitudes of
trees. While existing equiseta rarely exceed three feet in height, and the
stems are generally under half an inch in diameter, their kindred, entombed
in the coal beds, seem to have been generally fourteen or fifteen feet
high, with stems from six inches to a foot in thickness. Arborescent plants
of this family, line the arborescent ferns, now grow only in tropical
countries, and their being found in the coal beds in all latitudes is
consequently held as an additional proof, that at this era a warm climate
was extended much farther to the north than at present. It is to be
remarked that plants of this kind (forming two genera, the most abundant of
which is the calamites) are only represented on the present surface
by plants of the same family: the species which flourished at
this era gradually lessen in number as we advance upwards in the series of
rocks, and disappear before we arrive at the tertiary formation.
The club-moss family (lycopodiaceæ) are
other plants of the present surface, usually seen in a lowly and creeping
form in temperate latitudes, but presenting species which rise to a greater
magnitude within the tropics. Many specimens of this family are found in
the coal beds; it is thought they have contributed more to the substance of
the coal than any other family. But, like the ferns and equisetaceae, they
rise to a prodigious magnitude. The lepidodendra (so the fossil genus is
called) have probably been from sixty-five to eighty feet in height, having
at their base a diameter of about three feet, while their leaves measured
twenty inches in length. In the forests of the coal era, the lepidodendra
would enjoy the rank of firs in our forests, affording shade to the only
less stately ferns and calamites. The internal structure of the stem, and
the character of the seed-vessels, shew them to have been a link between
single-lobed and double-lobed plants, a fact worthy of note, as it favours
the idea that, in vegetable, as well as animal creation, a progress has
been observed, in conformity with advancing conditions. It is also curious
to find a missing link of so much importance in a genus of plants which has
long ceased to have a living place upon earth.
The other leading plants of the coal era are without
representatives on the present surface, and their characters are in general
less clearly ascertained. Amongst the most remarkable are—the
sigillaria, of which large stems are very abundant, shewing that the
interior has been soft, and the exterior fluted with separate leaves
inserted in vertical rows along the flutings—and the stigmaria,
plants apparently calculated to flourish in marshes or pools, having a
short, thick, fleshy stem, with a dome-shaped top, from which sprung
branches of from twenty to thirty feet long. Amongst monocotyledons were
some palms, (flabellaria and nœggerathia,) besides a few
not distinctly assignable to any class.
The dicotyledons of the coal are comparatively few,
though on the present surface they are the most numerous sub-class. Besides
some of doubtful affinity, (annularia, asterophyllites, &c.,)
there were a few of the pine family, which seem to have been the highest
class of trees of this era, and are only as yet found in isolated cases,
and in sandstone beds. The first discovered lay in the Craigleith quarry,
near Edinburgh, and consisted of a stem about two feet thick, and
forty-seven feet in length. Others have since been found, both in the same
situation, and at Newcastle. Leaves and fruit being wanting, an ingenious
mode of detecting the nature of these trees was hit upon by Mr. Witham of
Lartington. Taking thin polished cross slices of the stem, and subjecting
them to the microscope, he detected the structure of the wood to be that
of a cone-bearing tree, by the presence of certain "reticulations" which
distinguish that family, in addition to the usual radiating and concentric
lines. That particular tree was concluded to be an araucaria, a species now
found in Norfolk Island, in the South Sea, and in a few other remote
situations. The coniferæ of this era form the dawn of dicotyledenous
trees, of which they may be said to be the simplest type, and to which, it
has already been noticed, the lepidodendra are a link from the
monocotyledons. The concentric rings of the Craigleith and other
coniferæ of this era have been mentioned. It is interesting to find in
these a record of the changing seasons of those early ages, when as yet
there were no human beings to observe time or tide. They are clearly
traced; but it is observed that they are more slightly marked than is the
case with their family at the present day, as if the changes of temperature
had been within a narrower range.
Such was the vegetation of the carbonigenous era,
composed of forms at the bottom of the botanical scale, flowerless,
fruitless, but luxuriant and abundant beyond what the most favoured spots
on earth can now shew. The rigidity of the leaves of its, plants, and the
absence of fleshy fruits and farinaceous seeds, unfitted it to afford
nutriment to animals; and, monotonous in its forms, and destitute of
brilliant colouring, its sward probably unenlivened by any of the smaller
flowering herbs, its shades uncheered by the hum of insects, or the music
of birds, it must have been but a sombre scene to a human visitant. But
neither man nor any other animals were then in existence to look for such
uses or such beauties in this vegetation. It was serving other and equally
important ends, clearing (probably) the atmosphere of matter noxious to
animal life, and storing up mineral masses which were in long subsequent
ages to prove of the greatest service to the human race, even to the extent
of favouring the progress of its civilization.
The animal remains of this era are not numerous, in
comparison with those which go before, or those which come after. The
mountain limestone, indeed, deposited at the commencement of it, abounds
unusually in polypiaria and crinoiclea; but when we ascend to the coal-beds
themselves, the case is altered, and these marine remains altogether
disappear. We have then only a limited variety of conchifers and shell
mollusks, with fragments of a few species of fishes, and these are rarely
or never found in the coal seams, but in the shales alternating with them.
Some of the fishes are of a sauroid character, that is, partake of the
nature of the lizard, a genus of the reptilia, a land class of animals, so
that we may be said here to have the first approach to a kind of animals
calculated to breathe the atmosphere. Such is the Megalichthys Hibbertii,
found by Dr. Hibbert Ware, in a limestone bed of fresh-water origin,
underneath the coal at Burdiehouse, near Edinburgh. Others of the same kind
have been found in the coal measures in Yorkshire, and in the low coal
shales at Manchester. This is no more than might be expected, as
collections of fresh water now existed, and it is presumable that they
would be peopled. The chief other fishes of the coal era are named
palæothrissum, palæoniscus, diperdus.
Coal strata are nearly confined to the group termed the
carboniferous formation. Thin beds are not unknown afterwards, but they
occur only as a rare exception. It is therefore thought that the most
important of the conditions which allowed of so abundant a terrestrial
vegetation, had ceased about the time when this formation was closed. The
high temperature was not one of the conditions which terminated, for there
are evidences of it afterwards; but probably the superabundance of
carbonic acid gas supposed to have existed during this era was expended
before its close. There can be little doubt that the infusion of a large
dose of this gas into the atmosphere at the present day would be attended
by precisely the same circumstances as in the time of the carboniferous
formation. Land animal life would not have a place on earth; vegetation
would be enormous; and coal strata would be formed from the rest
accumulations of woody matter, which would gather in every sea, near the
mouths of great rivers On the exhaustion of the superabundance of carbonic
acid gas, the coal formation would cease, and the earth might again become
a suitable theatre of being for land animals.
The termination of the carboniferous formation is marked
by symptoms of volcanic violence, which some geologists have considered to
denote the close of one system of things and the beginning of another.
Coal beds generally lie in basins, as if following the curve of the bottom
of seas. But there is no such basin which is not broken up into pieces,
some of which have been tossed up on edge, others allowed to sink, causing
the ends of strata to be in some instances many yards, and in a few several
hundred feet, removed from the corresponding ends of neighbouring
fragments. These are held to be results of volcanic movements below, the
operation of which is further seen in numerous upbursts and intrusions of
volcanic rock (trap). That these disturbances took place about the close of
the formation, and not later, is shewn in the fact of the next higher group
of strata being comparatively undisturbed. Other symptoms of this time of
violence are seen in the beds of conglomerate which occur amongst the first
strata above the coal. These, as usual, consist of fragments of the elder
rocks, more or less worn from being tumbled about in agitated water, and
laid down in a mud paste, afterwards hardened. Volcanic disturbances break
up the rocks; the pieces are worn in seas; and a deposit of conglomerate is
the consequence. Of porphyry, there are some such pieces in the
conglomerate of Devonshire, three or four tons in weight. It is to be
admitted for strict truth that, in some parts of Europe, the carboniferous
formation is followed by superior deposits, without the appearance of such
disturbances between their respective periods; but apparently this case
belongs to the class of exceptions already noticed.[3]
That disturbance was general, is supported by the further and important
fact of the destruction of many forms of organic being previously
flourishing, particularly of the vegetable kingdom.
Notes
The principal families are named sphenopteris,
neuropteris, and pecopteris.
A specimen from Bengal, in the staircase of the
British Museum, is forty-five feet high.
"Some of the most considerable dislocations of
the border of the coal fields of Coalbrookdale and Dudley happened after
the deposition of a part of the new red sandstone; but it is certain that
those of Somersetshire and Gloucestershire were completed before the date
of that rock."—Philips.
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[ Robert Chambers,
Vestiges of the
Natural History of Creation, 1st edition, 1844; Reprinted in James Secord,
ed., Chicago: University of Chicago Press, 1994, pp. 76-93. ]
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