THE COAL, OR CARBONIFEROUS GROUP.
Carboniferous strata in the south-west of England--Superposition of
Coal-measures to Mountain limestone--Departure from this type in North
of England and Scotland--Section in South Wales--Underclays with
Stigmaria--Carboniferous Flora--Ferns, Lepidodendra, Calamites,
Asterophyllites, Sigillariae, Stigmariae--Coniferae--Endogens--Absence of
Exogens--Coal, how formed--Erect fossil trees--Parkfield Colliery--St.
Etienne, Coal-field--Oblique trees or snags--Fossil forests in Nova
Scotia--Brackish water and marine strata--Origin of Clay-iron-stone.
The next group which we meet with in the descending order is the
Carboniferous, commonly called "The Coal;" because it contains many beds
of that mineral, in a more or less pure state, interstratified with
sandstones, shales, and limestones. The coal itself, even in Great
Britain and Belgium, where it is most abundant, constitutes but an
insignificant portion of the whole mass. In the north of England, for
example, the thickness of the coal-bearing strata has been estimated at
3000 feet, while the various coal-seams, 20 or 30 in number, do not in
the aggregate exceed 60 feet.[308-A]
The carboniferous formation comprises two very distinct members: 1st, that
usually called the Coal-measures, of mixed freshwater, terrestrial, and
marine origin, often including seams of coal; 2dly, that named in England
the Mountain or Carboniferous limestone, of purely marine origin, and
containing corals, shells, and encrinites.
In the south-western part of our island, in Somersetshire and South Wales,
the three divisions usually spoken of by English geologists are:
1. Coal-measures { Strata of shale, sandstone, and grit, with
{ occasional seams of coal, from 600 to 12,000
{ feet thick.
2. Millstone grit { A coarse quartzose sandstone passing into a
{ conglomerate, sometimes used for millstones, with
{ beds of shale; usually devoid of coal;
{ occasionally above 600 feet thick.
3. Mountain or } A calcareous rock containing marine shells and
Carboniferous } corals; devoid of coal; thickness variable,
limestone } sometimes 900 feet.
The millstone grit may be considered as one of the coal sandstones of
coarser texture than usual, with some accompanying shales, in which coal
plants are occasionally found. In the north of England some bands of
limestone, with pectens, oysters, and other marine shells, occur in this
grit, just as in the regular coal-measures, and even a few seams of coal. I
shall treat, therefore, of the whole group, as consisting of two divisions
only, the Coal-measures and Mountain Limestone. The latter is found in the
southern British coal-fields, at the base of the system, or immediately in
contact with the subjacent Old Red Sandstone; but as we proceed northwards
to Yorkshire and Northumberland it begins to alternate with true
coal-measures, the two deposits forming together a series of strata about
1000 feet in thickness. To this mixed formation succeeds the great mass of
genuine mountain limestone.[309-A] Farther north, in the Fifeshire
coal-field in Scotland, we observe a still wider departure from the type of
the south of England, or a more complete intercalation of dense masses of
marine limestones with sandstones, and shales containing coal.
COAL-MEASURES.
In South Wales the coal-measures have been ascertained by actual
measurement to attain the extraordinary thickness of 12,000 feet, the beds
throughout, with the exception of the coal itself, appearing to have been
formed in water of moderate depth, during a slow but perhaps intermittent
depression of the ground, in a region to which rivers were bringing a
never-failing supply of muddy sediment and sand. The same area was
sometimes covered with vast forests, such as we see in the deltas of great
rivers in warm climates, which are liable to be submerged beneath fresh or
salt water should the ground sink vertically a few feet.
In one section near Swansea, in South Wales, where the total thickness of
strata is 3246 feet, we learn from Sir H. De la Beche that there are ten
principal masses of sandstone. One of these is 500 feet thick, and the
whole of them make together a thickness of 2125 feet. They are separated by
masses of shale, varying in thickness from 10 to 50 feet. The intercalated
coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of
them, which has two or three layers of clay interposed, attaining 9
feet.[309-B] At other points in the same coal-field the shales predominate
over the sandstones. The horizontal extent of some seams of coal is much
greater than that of others, but they all present one characteristic
feature, in having, each of them, what is called its _underclay_. These
underclays, co-extensive with every layer of coal, consist of arenaceous
shale, sometimes called firestone, because it can be made into bricks which
stand the fire of a furnace. They vary in thickness from 6 inches to more
than 10 feet; and Mr. Logan first announced to the scientific world in 1841
that they were regarded by the colliers in South Wales as an essential
accompaniment of each of the one hundred seams of coal met with in their
coal-field. They are said to form the _floor_ on which the coal rests; and
some of them have a slight admixture of carbonaceous matter, while others
are quite blackened by it.
All of them, as Mr. Logan pointed out, are characterized by inclosing a
peculiar species of fossil vegetable called _Stigmaria_, to the exclusion
of other plants. It was also observed that, while in the overlying shales
or "roof" of the coal, ferns and trunks of trees abound without any
_Stigmariae_, and are flattened and compressed, those singular plants in the
underclays always retain their natural forms, branching freely, and sending
out their slender leaves, as they were formerly styled, through the mud in
all directions. Several species of _Stigmaria_ had long been known to
botanists, and described by them, before their position under each seam of
coal was pointed out. It was conjectured that they might be aquatic,
perhaps floating plants, which sometimes extended their branches and leaves
freely in fluid mud, and which were finally enveloped in the same mud.
CARBONIFEROUS FLORA.
These statements will suffice to convince the reader that we cannot arrive
at a satisfactory theory of the origin of coal till we understand the true
nature of _Stigmaria_; and in order to explain what is now known of this
plant, and of others which have contributed by their decay to produce coal,
it will be necessary to offer a brief preliminary sketch of the whole
carboniferous flora, an assemblage of fossil plants, with which we are
better acquainted than with any other which flourished antecedently to the
tertiary epoch. It should also be remarked that Goeppert has ascertained
that the remains of every family of plants scattered through the
coal-measures are sometimes met with in the pure coal itself, a fact which
adds greatly to the geological interest attached to this flora.
_Ferns._--The number of species of carboniferous plants hitherto
described amounts, according to M. Ad. Brongniart, to about 500. These
may perhaps be a fragment only of the entire flora, but they are enough
to show that the state of the vegetable world was then extremely
different from that now established. We are struck at the first glance
with the similarity of many of the ferns to those now living, and the
dissimilarity of almost all the other fossils except the coniferae. Among
the ferns, as in the case of _Pecopteris_ for example (fig. 351.), it is
not always easy to decide whether they should be referred to different
genera from those established for the classification of living species;
whereas, in regard to most of the other contemporary tribes, with the
exception of the coniferae, it is often difficult to guess the family,
or even the class, to which they belong. The ferns of the carboniferous
period are generally without organs of fructification, but in some
specimens these are well preserved. In the general absence of such
characters, they have been divided into genera, distinguished chiefly
by the branching of the fronds, and the way in which the veins of
the leaves are disposed. The larger portion are supposed to have been
of the size of ordinary European ferns, but some were decidedly
arborescent, especially the group called _Caulopteris_, by Lindley,
and the _Psaronius_ of the upper or newest coal-measures, before
alluded to (p. 307.).
[Illustration: Fig. 351. _Pecopteris lonchitica._ (Foss. Flo. 153.)]
[Illustration: Fig. 352. Sphenopteris. (Foss. Flo. 101.)
_a._ _Sphenopteris crenata._
_b._ The same, magnified.]
[Illustration: Fig. 353. _Caulopteris primaeva_, Lindley.]
All the recent tree-ferns belong to one tribe (_Polypodiaceae_), and to a
small number only of genera in that tribe, in which the surface of the
trunk is marked with scars, or cicatrices, left after the fall of the
fronds. These scars resemble those of _Caulopteris_ (see fig. 353.). No
less than 250 ferns have already been obtained from the coal strata; and
even if we make some reduction on the ground of varieties which have been
mistaken, in the absence of their fructification, for species, still the
result is singular, because the whole of Europe affords at present no more
than 50 indigenous species.
[3 Illustrations: Living tree-ferns of different genera. (Ad. Brong.)
Fig. 354. Tree-fern from Isle of Bourbon.
Fig. 355. _Cyathea glauca_, Mauritius.
Fig. 356. Tree fern from Brazil.]
[3 Illustrations: _Lepidodendron Sternbergii._ Coal-measures,
near Newcastle.
Fig. 357. Branching trunk, 49 feet long, supposed to have belonged to _L.
Sternbergii_. (Foss. Flo. 203.)
Fig. 358. Branching stem with bark and leaves of _L. Sternbergii_.
(Foss. Flo. 4.)
Fig. 359. Portion of same nearer the root; natural size. (Ibid.)]
_Lepidodendra._--These fossils belong to the family of _Lycopodiums_, yet
most of them grew to the size of large trees. The annexed figures represent
a large fossil _Lepidodendron_, 49 feet long, found in Jarrow Colliery,
near Newcastle, lying in shale parallel to the planes of stratification.
Fragments of others, found in the same shale, indicate, by the size of the
rhomboidal scars which cover them, a still greater magnitude. The living
club-mosses, of which there are about 200 species, are abundant in tropical
climates, where one species is sometimes met with attaining a height of 3
feet. They usually creep on the ground, but some stand erect, as the _L.
densum_, from New Zealand (fig. 360.).
[Illustration: Fig. 360. Lycopodium.
_a._ _Lycopodium densum_; banks of R. Thames, New Zealand.
_b._ branch, natural size.
_c._ part of same, magnified.]
In the carboniferous strata of Coalbrook Dale, and in many other
coal-fields, elongated cylindrical bodies, called fossil cones, named by
M. Adolphe Brongniart _Lepidostrobus_, are met with. (See fig. 361.)
They often form the nucleus of concretionary balls of clay-iron-stone,
and are well preserved, exhibiting a conical axis, around which a great
quantity of scales were compactly imbricated. The opinion of M.
Brongniart is now generally adopted, that the _Lepidostrobus_ is
the fruit of _Lepidodendron_.
[Illustration: Fig. 361. _Lepidostrobus ornatus_, Brong.; half
nat. size. Shropshire.]
[Illustration: Fig. 362. _Calamites cannaeformis_, Schlot. (Foss. Flo. 79.)
Lower end with rootlets.]
[Illustration: Fig. 363. _Calamites Suckowii_, Brong.; natural size. Common
in coal throughout Europe.]
_Equisetaceae._--To this family belong two species of the genus
_Equisetites_, allied to the living "horse-tail" which now grows in marshy
grounds. Other species, which have jointed stems, depart more widely from
_Equisetum_, but are yet of analogous organization. They differed from it
principally in being furnished with a thin bark, which is represented in
the stem of _C. Suckowii_ (fig. 363.), in which it will be seen that the
striped external pattern does not agree with that left on the stone where
the bark is stripped off; so that if the two impressions were seen
separately, they might be mistaken for two distinct species.
The tallest living "horse-tails" are only 2 or 3 feet high in Europe, and
even in tropical climates only attain, as in the case of _Equisetum
giganteum_, discovered by Humboldt and Bonpland, in South America, a height
of about 5 feet, the stem being an inch in diameter. Several of the
Calamites of the coal acquired the height and dimensions of small trees.
[Illustration: Fig. 364. _Asterophyllites foliosa._ (Foss. Flo. 25.)
Coal-measures, Newcastle.]
_Asterophyllites._--In this family, M. Brongniart includes several genera,
and among them _Calamodendron_, _Asterophyllites_, and _Annularia_. The
graceful plant, represented in the annexed figure, is supposed to be the
branch of a shrub called _Calamodendron_, a new genus, divided off by
Brongniart from the _Calamites_ of former authors. Its pith and medullary
rays seem to show that it was dicotyledonous, and it appears to have been
allied, by the nature of its tissue, to the gymnogens, or, still more, to
the _Sigillaria_, which will next be mentioned.
_Sigillaria._--A large portion of the trees of the carboniferous period
belonged to this genus, of which about thirty-five species are known. The
structure, both internal and external, was very peculiar, and, with
reference to existing types, very anomalous. They were formerly referred,
by M. Ad. Brongniart, to ferns, which they resemble in the scalariform
texture of their vessels, and, in some degree, in the form of the
cicatrices left by the base of the leafstalks which have fallen off (see
fig. 365.). But with these points of analogy to cryptogamia, they combine
an internal organization much resembling that of cycads, and some of them
are ascertained to have had long linear leaves, quite unlike those of
ferns. They grew to a great height, from 30 to 60, or even 70 feet, with
regular cylindrical stems, and without branches, although some species were
dichotomous towards the top. Their fluted trunks, from 1 to 5 feet in
diameter, appear to have decayed rapidly in the interior, so as to become
hollow, when standing; when, therefore, they were thrown prostrate on the
mud, they were squeezed down and flattened. Hence, we find the bark of the
two opposite sides (now converted into bright shining coal) to constitute
two horizontal layers, one upon the other, half an inch, or an inch, in
thickness. These same trunks, when they are placed obliquely or vertically
to the planes of stratification, retain their original rounded form, and
are uncompressed, the cylinder of bark having been filled with sand, which
now affords a cast of the interior.
[Illustration: Fig. 365. _Sigillaria laevigata_, Brong.]
_Stigmaria._--This fossil, the importance of which has already been pointed
out, was formerly conjectured to be an aquatic plant. It is now ascertained
to be the root of _Sigillaria_. The connection of the roots with the stem,
previously suspected, on botanical grounds, by Brongniart, was first
proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The
fact has lately been shown, even more distinctly, by Mr. Richard Brown, in
his description of the _Stigmariae_ occurring in the underclays of the
coal-seams of the Island of Cape Breton, in Nova Scotia.
[Illustration: Fig. 366. Stigmaria attached to a trunk of
_Sigillaria_.[315-A]]
In a specimen of one of these, represented in the annexed figure (fig.
366.), the spread of the roots was 16 feet, and some of them sent out
rootlets, in all directions, into the surrounding clay.
The manner of attachment of the fibres to the stem resembles that of a ball
and socket joint, the base of each rootlet being concave, and fitting on to
a tubercle (see figs. 367 and 368.). Rows of these tubercles are arranged
spirally round each root, which have always a medullary cavity and woody
texture, much resembling that of _Sigillaria_, the structure of the
vessels being, like it, scalariform.
[Illustration: Fig. 367. Surface of another individual of same species,
showing form of tubercles. (Foss. Flo. 34.)]
[Illustration: Fig. 368. _Stigmaria ficoides_, Brong. One fourth of
nat. size. (Foss. Flo. 32.)]
_Conifers._--The coniferous trees of this period are referred to five
genera; the woody structure of some of them showing that they were allied
to the Araucarian division of pines, more than to any of our common
European firs. Some of their trunks exceeded 44 feet in height.
_Endogens._--Hitherto but few monocotyledonous plants have been
discovered in the coal-strata. Most of these consist of fruits referred
by some botanists to palms. The three-sided nuts, called
_Trigonocarpum_, seven species of which are known, appear to have the
best claim to rank as palms, although M. Ad. Brongniart entertains some
doubt even as to their being monocotyledons.
_Exogens._
The entire absence, so far as our paleontological investigations have
hitherto gone, of ordinary dicotyledons or exogens in the coal measures, is
most remarkable. Hence, M. Adolphe Brongniart has called this period the
age of acrogens, in consequence of the vast preponderance of ferns and
_Lepidodendra_.[316-A] Nevertheless, a forest of the period, now under
consideration, may have borne a considerable resemblance to those woody
regions of New Zealand, in which ferns, arborescent and herbaceous, and
lycopodiums, with many coniferae, abound.
The comparative proportion of living ferns and _Araucariae_, in Norfolk
Island, to all the other plants, appears to be very similar to that
formerly borne by these tribes respectively in a forest of the coal-period.
I have already stated that Professor Goeppert, after examining the fossil
vegetables of the coal-fields of Germany, has detected, in beds of pure
coal, remains of plants of every family hitherto known to occur fossil in
the coal. Many seams, he remarks, are rich in _Sigillaria_,
_Lepidodendron_, and _Stigmaria_, the latter in such abundance, as to
appear to form the bulk of the coal. In some places, almost all the plants
are calamites, in others ferns.[316-B]
_Coal, how formed--Erect trees._--I shall now consider the manner in
which the above-mentioned plants are imbedded in the strata, and how
they may have contributed to produce coal. "Some of the plants of our
coal," says Dr. Buckland, "grew on the identical banks of sand, silt,
and mud, which, being now indurated to stone and shale, form the strata
that accompany the coal; whilst other portions of these plants have been
drifted to various distances from the swamps, savannahs, and forests
that gave them birth, particularly those that are dispersed through the
sandstones, or mixed with fishes in the shale beds." "At Balgray, three
miles north of Glasgow," says the same author, "I saw in the year 1824,
as there still may be seen, an unequivocal example of the stumps of
several stems of large trees, standing close together in their native
place, in a quarry of sandstone of the coal formation."[317-A]
Between the years 1837 and 1840, six fossil trees were discovered in the
coal-field of Lancashire, where it is intersected by the Bolton railway.
They were all in a vertical position, with respect to the plane of the
bed, which dips about 15 deg. to the south. The distance between the first
and the last was more than 100 feet, and the roots of all were imbedded
in a soft argillaceous shale. In the same plane with the roots is a bed
of coal, eight or ten inches thick, which has been ascertained to extend
across the railway, or to the distance of at least ten yards. Just above
the covering of the roots, yet beneath the coal seam, so large a
quantity of the _Lepidostrobus variabilis_ was discovered inclosed in
nodules of hard clay, that more than a bushel was collected from the
small openings around the base of the trees (see figure of this genus,
p. 313.). The exterior trunk of each was marked by a coating of friable
coal, varying from one quarter to three quarters of an inch in
thickness; but it crumbled away on removing the matrix. The dimensions
of one of the trees is 15-1/2 feet in circumference at the base, 7-1/2
feet at the top, its height being 11 feet. All the trees have large
spreading roots, solid and strong, sometimes branching, and traced to a
distance of several feet, and presumed to extend much farther. Mr.
Hawkshaw, who has described these fossils, thinks that, although they
were hollow when submerged, they may have consisted originally of hard
wood throughout; for solid dicotyledonous trees, when prostrated in
tropical forests, as in Venezuela, on the shore of the Caribbean Sea,
were observed by him to be destroyed in the interior, so that little
more is left than an outer shell, consisting chiefly of the bark. This
decay, he says, goes on most rapidly in low and flat tracks, in which
there is a deep rich soil and excessive moisture, supporting tall
forest-trees and large palms, below which bamboos, canes, and minor
palms flourish luxuriantly. Such tracts, from their lowness, would be
most easily submerged, and their dense vegetation might then give rise
to a seam of coal.[317-B]
In a deep valley near Capel-Coelbren, branching from the higher part of the
Swansea valley, four stems of upright _Sigillariae_ were seen, in 1838,
piercing through the coal-measures of S. Wales; one of them was 2 feet in
diameter, and one 13 feet and a half high, and they were all found to
terminate downwards in a bed of coal. "They appear," says Sir H. De la
Beche, "to have constituted a portion of a subterranean forest at the epoch
when the lower carboniferous strata were formed.[318-A]
In a colliery near Newcastle, say the authors of the Fossil Flora, a great
number of _Sigillariae_ were placed in the rock as if they had retained the
position in which they grew. Not less than thirty, some of them 4 or 5 feet
in diameter, were visible within an area of 50 yards square, the interior
being sandstone, and the bark having been converted into coal. The roots of
one individual were found imbedded in shale; and the trunk, after
maintaining a perpendicular course and circular form for the height of
about 10 feet, was then bent over so as to become horizontal. Here it was
distended laterally, and flattened so as to be only one inch thick, the
flutings being comparatively distinct.[318-B] Such vertical stems are
familiar to our miners, under the name of coal-pipes. One of them, 72 feet
in length, was discovered, in 1829, near Gosforth, about five miles from
Newcastle, in coal-grit, the strata of which it penetrated. The exterior of
the trunk was marked at intervals with knots, indicating the points at
which branches had shot off. The wood of the interior had been converted
into carbonate of lime; and its structure was beautifully shown by cutting
transverse slices, so thin as to be transparent. (See p. 40.)
These "coal-pipes" are much dreaded by our miners, for almost every year in
the Bristol, Newcastle, and other coal-fields, they are the cause of fatal
accidents. Each cylindrical cast of a tree, formed of solid sandstone, and
increasing gradually in size towards the base, and being without branches,
has its whole weight thrown downwards, and receives no support from the
coating of friable coal which has replaced the bark. As soon, therefore, as
the cohesion of this external layer is overcome, the heavy column falls
suddenly in a perpendicular or oblique direction from the roof of the
gallery whence coal has been extracted, wounding or killing the workman who
stands below. It is strange to reflect how many thousands of these trees
fell originally in their native forests in obedience to the law of gravity;
and how the few which continued to stand erect, obeying, after myriads of
ages, the same force, are cast down to immolate their human victims.
It has been remarked, that if, instead of working in the dark, the miner
was accustomed to remove the upper covering of rock from each seam of coal,
and to expose to the day the soils on which ancient forests grew, the
evidence of their former growth would be obvious. Thus in South
Staffordshire a seam of coal was laid bare in the year 1844, in what is
called an open work at Parkfield Colliery, near Wolverhampton. In the
space of about a quarter of an acre the stumps of no less than 73 trees
with their roots attached appeared, as shown in the annexed plan (fig.
369.), some of them more than 8 feet in circumference. The trunks, broken
off close to the root, were lying prostrate in every direction, often
crossing each other. One of them measured 15, another 30 feet in length,
and others less. They were invariably flattened to the thickness of one or
two inches, and converted into coal. Their roots formed part of a stratum
of coal 10 inches thick, which rested on a layer of clay 2 inches thick,
below which was a second forest, resting on a 2-foot seam of coal. Five
feet below this again was a third forest with large stumps of
_Lepidodendra_, _Calamites_, and other trees.
[Illustration: Fig. 369. Ground-plan of a fossil forest, Parkfield
Colliery, near Wolverhampton, showing the position of 73 trees in a
quarter of an acre.[319-A]]
In the account given, in 1821, by M. Alex. Brongniart of the coal-mine
of Treuil, at St. Etienne, near Lyons, he states, that distinct
horizontal strata of micaceous sandstone are traversed by vertical
trunks of monocotyledonous vegetables, resembling bamboos or large
_Equiseta_.[319-B] Since the consolidation of the stone, there has been
here and there a sliding movement, which has broken the continuity of
the stems, throwing the upper parts of them on one side, so that they
are often not continuous with the lower.
From these appearances it was inferred that we have here the monuments
of a submerged forest. I formerly objected to this conclusion,
suggesting that, in that case, all the roots ought to have been found at
one and the same level, and not scattered irregularly through the mass.
I also imagined that the soil to which the roots were attached should
have been different from the sandstone in which the trunks are enclosed.
Having, however, seen calamites near Pictou, in Nova Scotia, buried at
various heights in sandstone and in similar erect attitudes, I have now
little doubt that M. Brongniart's view was correct. These plants seem
to have grown on a sandy soil, liable to be flooded from time to time,
and raised by new accessions of sediment, as may happen in swamps near
the banks of a large river in its delta. Trees which delight in marshy
grounds are not injured by being buried several feet deep at their base;
and other trees are continually rising up from new soils, several feet
above the level of the original foundation of the morass. In the banks
of the Mississippi, when the water has fallen, I have seen sections of a
similar deposit in which portions of the stumps of trees with their
roots _in situ_ appeared at many different heights.[320-A]
[Illustration: Fig. 370. Section showing the erect position of fossil trees
in coal sandstone at St. Etienne. (Alex. Brongniart.)]
When I visited, in 1843, the quarries of Treuil above-mentioned, the fossil
trees seen in fig. 370. were removed, but I obtained proofs of other
forests of erect trees in the same coal-field.
[Illustration: Fig. 371. Inclined position of a fossil tree, cutting
through horizontal beds of sandstone, Craigleith quarry, Edinburgh. Angle
of inclination from _a_ to _b_ 27 deg..]
_Snags._--In 1830, a slanting trunk was exposed in Craigleith quarry,
near Edinburgh, the total length of which exceeded 60 feet. Its diameter
at the top was about 7 inches, and near the base it measured 5 feet in
its greater, and 2 feet in its lesser width. The bark was converted into
a thin coating of the purest and finest coal, forming a striking
contrast in colour with the white quartzose sandstone in which it lay.
The annexed figure represents a portion of this tree, about 15 feet
long, which I saw exposed in 1830, when all the strata had been removed
from one side. The beds which remained were so unaltered and undisturbed
at the point of junction, as clearly to show that they had been
tranquilly deposited round the tree, and that the tree had not
subsequently pierced through them, while they were yet in a soft state.
They were composed chiefly of siliceous sandstone, for the most part
white; and divided into laminae so thin, that from six to fourteen of
them might be reckoned in the thickness of an inch. Some of these thin
layers were dark, and contained coaly matter; but the lowest of the
intersected beds were calcareous. The tree could not have been hollow
when imbedded, for the interior still preserved the woody texture in a
perfect state, the petrifying matter being, for the most part,
calcareous.[321-A] It is also clear, that the lapidifying matter was not
introduced laterally from the strata through which the fossil passes, as
most of these were not calcareous. It is well known that, in the
Mississippi and other great American rivers, where thousands of trees
float annually down the stream, some sink with their roots downwards,
and become fixed in the mud. Thus placed, they have been compared to a
lance in rest; and so often do they pierce through the bows of vessels
which run against them, that they render the navigation extremely
dangerous. Mr. Hugh Miller mentions four other huge trunks exposed in
quarries near Edinburgh, which lay diagonally across the strata at an
angle of about 30 deg., with their lower or heavier portions downwards, the
roots of all, save one, rubbed off by attrition. One of these was 60 and
another 70 feet in length, and from 4 to 6 feet in diameter.
[Illustration: Fig. 372. Section of the cliffs of the South Joggins,
near Minudie, Nova Scotia.]
The number of years for which the trunks of trees, when constantly
submerged, can resist decomposition, is very great; as we might suppose
from the durability of wood, in artificial piles, permanently covered by
water. Hence these fossil snags may not imply a rapid accumulation of beds
of sand, although the channel of a river or part of a lagoon is often
filled up in a very few years.
_Nova Scotia._--One of the finest examples in the world of a succession of
fossil forests of the carboniferous period, laid open to view in a natural
section, is that seen in the lofty cliffs bordering the Chignecto Channel,
a branch of the Bay of Fundy, in Nova Scotia.[321-B]
In the annexed section (fig. 372.), which I examined in July, 1842, the
beds from _c_ to _i_ are seen all dipping the same way, their average
inclination being at an angle of 24 deg. S.S.W. The vertical height of the
cliffs is from 150 to 200 feet; and between _d_ and _g_, in which space I
observed seventeen trees in an upright position, or, to speak more
correctly, at right angles to the planes of stratification, I counted
nineteen seams of coal, varying in thickness from 2 inches to 4 feet. At
low tide a fine horizontal section of the same beds is exposed to view on
the beach. The thickness of the beds alluded to, between _d_ and _g_, is
about 2,500 feet, the erect trees consisting chiefly of large _Sigillariae_,
occurring at ten distinct levels, one above the other; but Mr. Logan, who
afterwards made a more detailed survey of the same line of cliffs, found
erect trees at seventeen levels, extending through a vertical thickness of
4,515 feet of strata; and he estimated the total thickness of the
carboniferous formation, with and without coal, at no less than 14,570
feet, every where devoid of marine organic remains.[322-A] The usual height
of the buried trees seen by me was from 6 to 8 feet; but one trunk was
about 25 feet high and 4 feet in diameter, with a considerable bulge at the
base. In no instance could I detect any trunk intersecting a layer of coal,
however thin; and most of the trees terminated downwards in seams of coal.
Some few only were based in clay and shale, none of them in sandstone. The
erect trees, therefore, appeared in general to have grown on beds of coal.
In some of the underclays I observed _Stigmaria_.
[Illustration: Fig. 373. Fossil tree at right angles to planes of
stratification. Coal measures, Nova Scotia.]
In regard to the plants, they belonged to the same genera, and most of them
to the same species, as those met with in the distant coal-fields of
Europe. In the sandstone, which filled their interiors, I frequently
observed fern leaves, and sometimes fragments of _Stigmaria_, which had
evidently entered together with sediment after the trunk had decayed and
become hollow, and while it was still standing under water. Thus the tree,
_a b_, fig. 373., the same which is represented at _a_, fig. 374., or in
the bed _e_ in the larger section, fig. 372., is a hollow trunk 5 feet 8
inches in length, traversing various strata, and cut off at the top by a
layer of clay 2 feet thick on which rests a seam of coal (_b_, fig. 374.)
1 foot thick. On this coal again stood two large trees (_c_ and _d_), while
at a greater height the trees _f_ and _g_ rest upon a thin seam of coal
(_e_), and above them is an underclay, supporting the 4-foot coal.
[Illustration: Fig. 374. Erect fossil trees. Coal-measures, Nova Scotia.]
If we now return to the tree first mentioned (fig. 373.), we find the
diameter (_a b_) 14 inches at the top and 16 inches at the bottom, the
length of the trunk 5 feet 8 inches. The strata in the interior
consisted of a series entirely different from those on the outside. The
lowest of the three outer beds which it traversed consisted of purplish
and blue shale (_c_, fig. 373.), 2 feet thick, above which was sandstone
(_d_) 1 foot thick, and, above this, clay (_e_) 2 feet 8 inches. But, in
the interior, were nine distinct layers of different composition: at the
bottom, first, shale 4 inches, then sandstone 1 foot, then shale 4
inches, then sandstone 4 inches, then shale 11 inches, then clay (_f_)
with nodules of ironstone 2 inches, then pure clay 2 feet, then
sandstone 3 inches, and, lastly, clay 4 inches. Owing to the outward
slope of the face of the cliff, the section (fig. 373.) was not exactly
perpendicular to the axis of the tree; and hence, probably, the apparent
sudden termination at the base without a stump and roots.
In this example the layers of matter in the inside of the tree are more
numerous than those without; but it is more common in the coal-measures
of all countries to find a cylinder of pure sandstone,--the cast of the
interior of a tree, intersecting a great many alternating beds of shale
and sandstone, which originally enveloped the trunk as it stood erect in
the water. Such a want of correspondence in the materials outside and
inside, is just what we might expect if we reflect on the difference of
time at which the deposition of sediment will take place in the two
cases; the imbedding of the tree having gone on for many years before
its decay had made much progress.
The high tides of the Bay of Fundy, rising more than 60 feet, are so
destructive as to undermine and sweep away continually the whole face of
the cliffs, and thus a new crop of erect trees is brought into view
every three or four years. They are known to extend over a space between
two and three miles from north to south, and more than twice that
distance from east to west, being seen in the banks of streams
intersecting the coal-field.
In Cape Breton, Mr. Richard Brown has observed in the Sydney coal-field a
total thickness of coal-measures, without including the underlying
millstone grit, of 1843 feet, dipping at an angle of 8 deg.. He has published
minute details of the whole series, showing at how many different levels
erect trees occur, consisting of _Sigillaria_, _Lepidodendron_, _Calamite_,
and other genera. In one place eight erect trunks, with roots and rootlets
attached to them, were seen at the same level, within a horizontal space 80
feet in length. Beds of coal of various thickness are interstratified. Some
of the associated strata are ripple-marked, with impressions of rain-drops.
Taking into account forty-one clays filled with roots of _Stigmaria_ in
their natural position, and eighteen layers of upright trees at other
levels, there is, on the whole, clear evidence of at least fifty-nine
fossil forests, ranged one above the other, in this coal-field, in the
above-mentioned thickness of strata.[324-A]
The fossil shells in Cape Breton and in the Nova Scotia section (fig.
372.), consisting of _Cypris_, _Unio_ (?), _Modiola_, _Microconchus
carbonarius_ (see fig. 375.), and _Spirorbis_, seem to indicate brackish
water; but we ought never to be surprised if, in pursuing the same stratum,
we come to a fresh or purely marine deposit; for this will depend upon our
taking a direction higher up or lower down the ancient river or delta
deposit. When the Purbeck beds of the Wealden were described in Chap.
XVIII., I endeavoured to explain the intimate connection of strata formed
at a river's mouth, or in the tranquil lagoons of the delta, or in the sea,
after a slight submergence of the land, with its dirt-beds.
In the English coal-fields the same association of fresh, or rather
brackish water with marine strata, in close connection with beds of coal of
terrestrial origin, has been frequently recognized. Thus, for example, a
deposit near Shrewsbury, probably formed in brackish water, has been
described by Sir R. Murchison as the youngest member of the carboniferous
series of that district, at the point where the coal-measures are in
contact with the Permian or "Lower New Red." It consists of shales and
sandstones about 150 feet thick, with coal and traces of plants; including
a bed of limestone, varying from 2 to 9 feet in thickness, which is
cellular, and resembles some lacustrine limestones of France and Germany.
It has been traced for 30 miles in a straight line, and can be recognized
at still more distant points. The characteristic fossils are a small
bivalve, having the form of a _Cyclas_, a small _Cypris_ (fig. 376.), and
the microscopic shell of an annelid of an extinct genus called
_Microconchus_ (fig. 375.), allied to _Serpula_ or _Spirorbis_.
In the lower coal-measures of Coalbrook Dale, the strata, according to Mr.
Prestwich, often change completely within very short distances, beds of
sandstone passing horizontally into clay, and clay into sandstone. The
coal-seams often wedge out or disappear; and sections, at places nearly
contiguous, present marked lithological distinctions. In this single field,
in which the strata are from 700 to 800 feet thick, between forty and
fifty species of terrestrial plants have been discovered, besides several
fishes and trilobites of forms distinct from those occurring in the
Silurian strata. Also upwards of forty species of mollusca, among which are
two or three referred to the freshwater genus _Unio_, and others of marine
forms, such as _Nautilus_, _Orthoceras_, _Spirifer_, and _Productus_. Mr.
Prestwich suggests that the intermixture of beds containing freshwater
shells with others full of marine remains, and the alternation of coarse
sandstone and conglomerate with beds of fine clay or shale containing the
remains of plants, may be explained by supposing the deposit of Coalbrook
Dale to have originated in a bay of the sea or estuary into which flowed a
considerable river subject to occasional freshes.[325-A]
[2 Illustrations: Freshwater Fossils--Coal.
Fig. 375.
_a._ _Microconchus carbonarius_.
_b._ var. of same; nat. size, and magnified.
Fig. 376. _Cypris inflata_, natural size, and magnified. Murchison.[325-B]]
In the Edinburgh coal-field, at Burdiehouse, fossil fishes, mollusca,
and cypris, very similar to those in Shropshire and Staffordshire, have
been found by Dr. Hibbert.[325-C] In the coal-field also of Yorkshire
there are freshwater strata, some of which contain shells referred to
the genus _Unio_; but in the midst of the series there is one thin but
very widely spread stratum, abounding in fishes and marine shells, such
as _Ammonites Listeri_ (fig. 377.), _Orthoceras_, and _Avicula
papyracea_, Goldf. (fig. 378.)[325-D]
[Illustration: Fig. 377. _Ammonites Listeri_, Sow.]
[Illustration: Fig. 378. _Avicula papyracea_, Goldf. (_Pecten
papyraceus_, Sow.)]
No similarly intercalated layer of marine shells has been noticed in the
neighbouring coal-field of Newcastle, where, as in South Wales and
Somersetshire, the marine deposits are entirely below those containing
terrestrial and freshwater remains.[326-A]
_Clay-iron-stone._--Bands and nodules of clay-iron-stone are common in
coal-measures, and are formed, says Sir H. De la Beche, of carbonate of
iron, mingled mechanically with earthy matter, like that constituting the
shales. Mr. Hunt, of the Museum of Practical Geology, instituted a series
of experiments to illustrate the production of this substance, and found
that decomposing vegetable matter, such as would be distributed through all
coal strata, prevented the farther oxidation of the proto-salts of iron,
and converted the peroxide into protoxide by taking a portion of its oxygen
to form carbonic acid. Such carbonic acid, meeting with the protoxide of
iron in solution, would unite with it and form a carbonate of iron; and
this mingling with fine mud, when the excess of carbonic acid was removed,
might form beds or nodules of argillaceous iron-stone.[326-B]
FOOTNOTES:
[308-A] Phillips; art. "Geology," Encyc. Britan.
[309-A] Sedgwick, Geol. Trans., Second Series, vol. iv.; and Phillips,
Geol. of Yorksh. part 2.
[309-B] Memoirs of Geol. Survey, vol. i. p. 195.
[315-A] The trunk in this case is referred by Mr. Brown to _Lepidodendron_,
but his illustrations seem to show the usual markings assumed by
_Sigillaria_ near its base.
[316-A] For terminology of classification of plants, see above,
note, p. 223.
[316-B] Quart. Geol. Journ., vol. v., Mem., p. 17.
[317-A] Anniv. Address to Geol. Soc., 1840.
[317-B] Hawkshaw, Geol. Soc. Proceedings, Nos. 64. and 69.
[318-A] Geol. Report on Cornwall, &c. p. 143.
[318-B] Lindley and Hutton, Foss. Flo. part 6. p. 150.
[319-A] See papers by Messrs. Beckett and Ick. Proceed. in Geol. Soc.,
vol. iv. p. 287.
[319-B] Annales des Mines, 1821.
[320-A] Principles of Geol., 8th ed., p. 215.
[321-A] See figures of texture, Witham, Foss. Veget., pl. 3.
[321-B] See Lyell's Travels in N. America, vol. ii. p. 179.
[322-A] Quart. Geol. Journ., vol. ii. p. 177.
[324-A] Geol. Quart. Journ., vol. ii. p. 393.; and vol. vi. p. 115.
[325-A] Prestwich, Geol. Trans., 2d Series, vol. v. p. 440. Murchison,
Silurian System, p. 105.
[325-B] Silurian System, p. 84.
[325-C] Trans. Roy. Soc. Edin. vol. xiii. Horner, Edin. New Phil.
Journ., April, 1836.
[325-D] Phillips; art. "Geology," Encyc. Metrop., p. 590.
[326-A] Phillips; art. "Geology," Encyc. Metrop., p. 592.
[326-B] Memoirs of Geol. Survey, pp. 51. 255, &c.