THE STORY OF
A PIECE OF COAL
WHAT IT IS, WHENCE IT COMES,
AND WHITHER IT GOES
BY
EDWARD A. MARTIN, F.G.S.
1896
PREFACE.
The knowledge of the marvels which a piece of coal possesses within
itself, and which in obedience to processes of man's invention it is
always willing to exhibit to an observant enquirer, is not so widespread,
perhaps, as it should be, and the aim of this little book, this record of
one page of geological history, has been to bring together the principal
facts and wonders connected with it into the focus of a few pages, where,
side by side, would be found the record of its vegetable and mineral
history, its discovery and early use, its bearings on the great
fog-problem, its useful illuminating gas and oils, the question of the
possible exhaustion of British supplies, and other important and
interesting bearings of coal or its products.
In the whole realm of natural history, in the widest sense of the term,
there is nothing which could be cited which has so benefited, so
interested, I might almost say, so excited mankind, as have the wonderful
discoveries of the various products distilled from gas-tar, itself a
distillate of coal.
Coal touches the interests of the botanist, the geologist, and the
physicist; the chemist, the sanitarian, and the merchant.
In the little work now before the reader I have endeavoured to recount,
without going into unnecessary detail, the wonderful story of a piece of
coal.
E.A.M.
THORNTON HEATH,
_February_, 1896.
CONTENTS.
I. THE ORIGIN OF COAL AND THE PLANTS OF WHICH IT IS COMPOSED
II. A GENERAL VIEW OF THE COAL-BEARING STRATA
III. VARIOUS FORMS OF COAL AND CARBON
IV. THE COAL-MINE AND ITS DANGERS
V. EARLY HISTORY--ITS USE AND ITS ABUSE
VI. HOW GAS IS MADE--ILLUMINATING OILS AND BYE-PRODUCTS
VII. THE COAL SUPPLIES OF THE WORLD
VIII. THE COAL-TAR COLOURS
CHART SHEWING THE PRODUCTS OF COAL
GENERAL INDEX
LIST OF ILLUSTRATIONS.
FIG. 1. _Stigmaria_
" 2. _Annularia radiata_
" 3. _Rhacopteris inaequilatera_
" 4. Frond of _Pecopteris_
" 5. _Pecopteris Serlii_
" 6. _Sphenopteris affinis_
" 7. _Catamites Suckowii_
" 8. _Calamocladus grandis_
" 9. _Asterophyllites foliosa_
" 10. _Spenophyllum cuneifolium_
" 11. Cast of _Lepidodendron_
" 12. _Lepidodendron longifolium_
" 13. _Lepidodendron aculeatum_
" 14. _Lepidostrobus_
" 15. _Lycopodites_
" 16. _Stigmaria ficoides_
" 17. Section of _Stigmaria_
" 18. Sigillarian trunks in sandstone
" 19. _Productus_
" 20. _Encrinite_
" 21. Encrinital limestone
" 22. Various _encrinites_
" 23. _Cyathophyllum_
" 24. _Archegosaurus minor_
" 25. _Psammodus porosus_
" 26. _Orthoceras_
" 27. _Fenestella retepora_
" 28. _Goniatites_
" 29. _Aviculopecten papyraceus_
" 30. Fragment of _Lepidodendron_
" 31. Engine-house at head of a Coal-Pit
" 32. Gas Jet and Davy Lamp
" 33. Part of a Sigillarian trunk
" 34. Inside a Gas-holder
" 35. Filling Retorts by Machinery
" 36. "Condensers"
" 37. "Washers"
" 38. "Purifiers"
CHAPTER I.
THE ORIGIN OF COAL AND THE PLANTS OF
WHICH IT IS COMPOSED.
From the homely scuttle of coal at the side of the hearth to the
gorgeously verdant vegetation of a forest of mammoth trees, might have
appeared a somewhat far cry in the eyes of those who lived some fifty
years ago. But there are few now who do not know what was the origin of
the coal which they use so freely, and which in obedience to their demand
has been brought up more than a thousand feet from the bowels of the
earth; and, although familiarity has in a sense bred contempt for that
which a few shillings will always purchase, in all probability a stray
thought does occasionally cross one's mind, giving birth to feelings of a
more or less thankful nature that such a store of heat and light was long
ago laid up in this earth of ours for our use, when as yet man was not
destined to put in an appearance for many, many ages to come. We can
scarcely imagine the industrial condition of our country in the absence
of so fortunate a supply of coal; and the many good things which are
obtained from it, and the uses to which, as we shall see, it can be put,
do indeed demand recognition.
Were our present forests uprooted and overthrown, to be covered by
sedimentary deposits such as those which cover our coal-seams, the amount
of coal which would be thereby formed for use in some future age, would
amount to a thickness of perhaps two or three inches at most, and yet, in
one coal-field alone, that of Westphalia, the 117 most important seams,
if placed one above the other in immediate succession, would amount to no
less than 294 feet of coal. From this it is possible to form a faint idea
of the enormous growths of vegetation required to form some of our
representative coal beds. But the coal is not found in one continuous
bed. These numerous seams of coal are interspersed between many thousands
of feet of sedimentary deposits, the whole of which form the
"coal-measures." Now, each of these seams represents the growth of a
forest, and to explain the whole series it is necessary to suppose that
between each deposit the land became overwhelmed by the waters of the sea
or lake, and after a long subaqueous period, was again raised into dry
land, ready to become the birth-place of another forest, which would
again beget, under similarly repeated conditions, another seam of coal.
Of the conditions necessary to bring these changes about we will speak
later on, but this instance is sufficient to show how inadequate the
quantity of fuel would be, were we dependent entirely on our own existing
forest growths.
However, we will leave for the present the fascinating pursuit of
theorising as to the how and wherefore of these vast beds of coal,
relegating the geological part of the study of the carboniferous system
to a future chapter, where will be found some more detailed account of
the position of the coal-seams in the strata which contain them. At
present the actual details of the coal itself will demand our attention.
Coal is the mineral which has resulted, after the lapse of thousands of
thousands of years, from the accumulations of vegetable material, caused
by the steady yearly shedding of leaves, fronds and spores, from forests
which existed in an early age; these accumulated where the trees grew
that bore them, and formed in the first place, perhaps, beds of peat; the
beds have since been subjected to an ever-increasing pressure of
accumulating strata above them, compressing the sheddings of a whole
forest into a thickness in some cases of a few inches of coal, and have
been acted upon by the internal heat of the earth, which has caused them
to part, to a varying degree, with some of their component gases. If we
reason from analogy, we are compelled to admit that the origin of coal is
due to the accumulation of vegetation, of which more scattered, but more
distinct, representative specimens occur in the shales and clays above
and below the coal-seams. But we are also able to examine the texture
itself of the various coals by submitting extremely thin slices to a
strong light under the microscope, and are thus enabled to decide whether
the particular coal we are examining is formed of conifers, horse-tails,
club-mosses, or ferns, or whether it consists simply of the accumulated
sheddings of all, or perhaps, as in some instances, of innumerable
spores.
In this way the structure of coal can be accurately determined. Were we
artificially to prepare a mass of vegetable substance, and covering it up
entirely, subject it to great pressure, so that but little of the
volatile gases which would be formed could escape, we might in the course
of time produce something approaching coal, but whether we obtained
lignite, jet, common bituminous coal, or anthracite, would depend upon
the possibilities of escape for the gases contained in the mass.
Everybody has doubtless noticed that, when a stagnant pool which contains
a good deal of decaying vegetation is stirred, bubbles of gas rise to the
surface from the mud below. This gas is known as marsh-gas, or light
carburetted hydrogen, and gives rise to the _ignis fatuus_ which hovers
about marshy land, and which is said to lure the weary traveller to his
doom. The vegetable mud is here undergoing rapid decomposition, as there
is nothing to stay its progress, and no superposed load of strata
confining its resulting products within itself. The gases therefore
escape, and the breaking-up of the tissues of the vegetation goes on
rapidly.
The chemical changes which have taken place in the beds of vegetation of
the carboniferous epoch, and which have transformed it into coal, are
even now but imperfectly understood. All we know is that, under certain
circumstances, one kind of coal is formed, whilst under other conditions,
other kinds have resulted; whilst in some cases the processes have
resulted in the preparation of large quantities of mineral oils, such as
naphtha and petroleum. Oils are also artificially produced from the
so-called waste-products of the gas-works, but in some parts of the world
the process of their manufacture has gone on naturally, and a yearly
increasing quantity is being utilised. In England oil has been pumped up
from the carboniferous strata of Coalbrook Dale, whilst in Sussex it has
been found in smaller quantities, where, in all probability, it has had
its origin in the lignitic beds of the Wealden strata. Immense quantities
are used for fuel by the Russian steamers on the Caspian Sea, the Baku
petroleum wells being a most valuable possession. In Sicily, Persia, and,
far more important, in the United States, mineral oils are found in great
quantity.
In all probability coniferous trees, similar to the living firs, pines,
larches, &c., gave rise for the most part to the mineral oils. The class
of living _coniferae_ is well known for the various oils which it
furnishes naturally, and for others which its representatives yield on
being subjected to distillation. The gradually increasing amount of heat
which we meet the deeper we go beneath the surface, has been the cause of
a slow and continuous distillation, whilst the oil so distilled has found
its way to the surface in the shape of mineral-oil springs, or has
accumulated in troughs in the strata, ready for use, to be drawn up when
a well has been sunk into it.
The plants which have gone to make up the coal are not at once apparent
to the naked eye. We have to search among the shales and clays and
sandstones which enclose the coal-seams, and in these we find petrified
specimens which enable us to build up in our mind pictures of the
vegetable creation which formed the jungles and forests of these
immensely remote ages, and which, densely packed together on the old
forest floor of those days, is now apparent to us as coal.
[Illustration: Fig. 2.--_Annularia radiata._ Carboniferous sandstone.]
A very large proportion of the plants which have been found in the
coal-bearing strata consists of numerous species of ferns, the number of
actual species which have been preserved for us in our English coal,
being double the number now existing in Europe. The greater part of these
do not seem to have been very much larger than our own living ferns, and,
indeed, many of them bear a close resemblance to some of our own living
species. The impressions they have left on the shales of the
coal-measures are most striking, and point to a time when the sandy clay
which imbedded them was borne by water in a very tranquil manner, to be
deposited where the ferns had grown, enveloping them gradually, and
consolidating them into their mass of future shale. In one species known
as the _neuropteris_, the nerves of the leaves are as clear and as
apparent as in a newly-grown fern, the name being derived from two Greek
words meaning "nerve-fern." It is interesting to consider the history of
such a leaf, throughout the ages that have elapsed since it was part of a
living fern. First it grew up as a new frond, then gradually unfolded
itself, and developed into the perfect fern. Then it became cut off by
the rising waters, and buried beneath an accumulation of sediment, and
while momentous changes have gone on in connection with the surface of
the earth, it has lain dormant in its hiding-place exactly as we see it,
until now excavated, with its contemporaneous vegetation, to form fuel
for our winter fires.
[Illustration: FIG. 3.--_Rhacopteris inaequilatera._ Carboniferous
limestone.]
Although many of the ferns greatly resembled existing species, yet there
were others in these ancient days utterly unlike anything indigenous to
England now. There were undoubted tree-ferns, similar to those which
thrive now so luxuriously in the tropics, and which throw out their
graceful crowns of ferns at the head of a naked stem, whilst on the bark
are the marks at different levels of the points of attachment of former
leaves. These have left in their places cicatrices or scars, showing the
places from which they formerly grew. Amongst the tree-ferns found are
_megaphyton_, _paloeopteris_, and _caulopteris_, all of which have these
marks upon them, thus proving that at one time even tree-ferns had a
habitat in England.
[Illustration: Fig. 4.--Frond of _Pecopteris._ Coal-shale.]
One form of tree-fern is known by the name of _Psaronius_, and this was
peculiar in the possession of masses of aerial roots grouped round the
stem. Some of the smaller species exhibit forms of leaves which are
utterly unknown in the nomenclature of living ferns. Most have had names
assigned to them in accordance with certain characteristics which they
possess. This was the more possible since the fossilised impressions had
been retained in so distinct a manner. Here before us is a specimen in a
shale of _pecopteris_, as it is called, (_pekos_, a comb). The leaf in
some species is not altogether unlike the well-known living fern
_osmunda_. The position of the pinnules on both sides of the central
stalk are seen in the fossil to be shaped something like a comb, or a
saw, whilst up the centre of each pinnule the vein is as prominent and
noticeable as if the fern were but yesterday waving gracefully in the
air, and but to-day imbedded in its shaly bed.
[Illustration: FIG. 5.--_Pecopteris Serlii_. Coal-shale.]
_Sphenopteris_, or "wedge-fern," is the name applied to another
coal-fern; _glossopteris_, or "tongue-leaf"; _cyclopteris_, or
"round-leaf"; _odonlopteris,_ or "tooth-leaf," and many others, show
their chief characteristics in the names which they individually bear.
_Alethopteris_ appears to have been the common brake of the coal-period,
and in some respects resembles _pecopteris_.
[Illustration: Fig. 6.--_Sphenopteris Affinis._ Coal-shale.]
In some species of ferns so exact are the representations which they have
impressed on the shale which contains them, that not only are the veins
and nerves distinctly visible, but even the fructification still remains
in the shape of the marks left by the so-called seeds on the backs of the
leaves. Something more than a passing look at the coal specimens in a
good museum will well repay the time so spent.
What are known as septarian nodules, or snake-stones, are, at certain
places, common in the carboniferous strata. They are composed of layers
of ironstone and sandstone which have segregated around some central
object, such as a fern-leaf or a shell. When the leaf of a fern has been
found to be the central object, it has been noticed that the leaf can
sometimes be separated from the stone in the form of a carbonaceous film.
Experiments were made many years ago by M. Goppert to illustrate the
process of fossilisation of ferns. Having placed some living ferns in a
mass of clay and dried them, he exposed them to a red heat, and obtained
thereby striking resemblances to fossil plants. According to the degree
of heat to which they were subjected, the plants were found to be either
brown, a shining black, or entirely lost. In the last mentioned case,
only the impression remained, but the carbonaceous matter had gone to
stain the surrounding clay black, thus indicating that the dark colour of
the coal-shales is due to the carbon derived from the plants which they
included.
Another very prominent member of the vegetation of the coal period, was
that order of plants known as the _Calamites_. The generic distinctions
between fossil and living ferns were so slight in many cases as to be
almost indistinguishable. This resemblance between the ancient and the
modern is not found so apparent in other plants. The Calamites of the
coal-measures bore indeed a very striking resemblance, and were closely
related, to our modern horse-tails, as the _equiseta_ are popularly
called; but in some respects they differed considerably.
Most people are acquainted with the horse-tail (_equisetum fluviatile)_
of our marshes and ditches. It is a somewhat graceful plant, and stands
erect with a jointed stem. The foliage is arranged in whorls around the
joints, and, unlike its fossil representatives, its joints are protected
by striated sheaths. The stem of the largest living species rarely
exceeds half-an-inch in diameter, whilst that of the calamite attained a
thickness of five inches. But the great point which is noticeable in the
fossil calamites and _equisetites_ is that they grew to a far greater
height than any similar plant now living, sometimes being as much as
eight feet high. In the nature of their stems, too, they exhibited a more
highly organised arrangement than their living representatives, having,
according to Dr Williamson, a "fistular pith, an exogenous woody stem,
and a thick smooth bark." The bark having almost al ways disappeared has
left the fluted stem known to us as the calamite. The foliage consisted
of whorls of long narrow leaves, which differed only from the fern
_asterophyllites_ in the fact that they were single-nerved. Sir William
Dawson assigns the calamites to four sub-types: _calamite_ proper,
_calamopitus, calamodendron_, and _eucalamodendron_.
[Image: FIG. 7.--Root of _Catamites Suckowii_. Coal-shale.]
[Image: FIG 8.--_Calamocladus grandis_. Carboniferous sandstone.]
Having used the word "exogenous," it might be as well to pay a little
attention, in passing, to the nomenclature and broad classification of
the various kinds of plants. We shall then doubtless find it far easier
thoroughly to understand the position in the scale of organisation to
which the coal plants are referable.
[Illustration: FIG. 9.--_Asterophyllites foliosa_. Coal-measures.]
The plants which are lowest in organisation are known as _Cellular_. They
are almost entirely composed of numerous cells built up one above the
other, and possess none of the higher forms of tissue and organisation
which are met with elsewhere. This division includes the lichens,
sea-weeds, confervae (green aquatic scum), fungi (mushrooms, dry-rot),
&c.
The division of _Vascular_ plants includes the far larger proportion of
vegetation, both living and fossil, and these plants are built up of
vessels and tissues of various shapes and character.
All plants are divided into (1) Cryptogams, or Flowerless, such as
mosses, ferns, equisetums, and (2) Phanerogams, or Flowering. Flowering
plants are again divided into those with naked seeds, as the conifers and
cycads (gymnosperms), and those whose seeds are enclosed in vessels, or
ovaries (angiosperms).
Angiosperms are again divided into the monocotyledons, as the palms, and
dicotyledons, which include most European trees.
Thus:--
-------------------------------------------------------------------
| (M.A. Brongniart). | |(Lindley). |
|CELLULAR | | |
| _Cryptogams_ (Flowerless) |Fungi, seaweeds, |Thallogens |
| | lichens | |
| | | |
|VASCULAR | | |
| _Cryptogams_ (Flowerless) |Ferns, equisetums, |Acrogens |
| | mosses, lycopodiums| |
| _Phanerogams_ (Flowering) | | |
| Gymnosperms (having |Conifers and |Gymnogens |
| naked seeds) | cycads | |
| Two or more Cotyledons | | |
| Angiosperms (having | | |
| enclosed seeds) | | |
| Monocotyledons |Palms, lilies, |Endogens |
| | grasses | |
| Dicotyledons |Most European |Exogens |
| | trees and shrubs | |
-------------------------------------------------------------------
Adolphe Brongniart termed the coal era the "Age of Acrogens," because, as
we shall see, of the great predominance in those times of vascular
cryptogamic plants, known in Dr Lindley's nomenclature as "Acrogens."
[Illustration: FIG. 10.--_Spenophyllum cuneifolium._ Coal-shale.]
Two of these families have already been dealt with, viz., the ferns
(_felices_), and the equisetums, (_calamites_ and _equisetites_), and we
now have to pass on to another family. This is that which includes the
fossil representatives of the Lycopodiums, or Club-mosses, and which goes
to make up in some coals as much as two-thirds of the whole mass.
Everyone is more or less familiar with some of the living Lycopodiums,
those delicate little fern-like mosses which are to be found in many a
home. They are but lowly members of our British flora, and it may seem
somewhat astounding at first sight that their remote ancestors occupied
so important a position in the forests of the ancient period of which we
are speaking. Some two hundred living species are known, most of them
being confined to tropical climates. They are as a rule, low creeping
plants, although some few stand erect. There is room for astonishment
when we consider the fact that the fossil representatives of the family,
known as _Lepidodendra_, attained a height of no less than fifty feet,
and, there is good ground for believing, in many cases, a far greater
magnitude. They consist of long straight stems, or trunks which branch
considerably near the top. These stems are covered with scars or scales,
which have been caused by the separation of the petioles or leaf-stalks,
and this gives rise to the name which the genus bears. The scars are
arranged in a spiral manner the whole of the way up the stem, and the
stems often remain perfectly upright in the coal-mines, and reach into
the strata which have accumulated above the coal-seam.
[Illustration: FIG. 11.--Cast of _lepidodendron_ in sandstone.]
Count Sternberg remarked that we are unacquainted with any existing
species of plant, which like the _Lepidodendron_, preserves at all ages,
and throughout the whole extent of the trunk, the scars formed by the
attachment of the petioles, or leaf-stalks, or the markings of the leaves
themselves. The yucca, dracaena, and palm, entirely shed their scales
when they are dried up, and there only remain circles, or rings, arranged
round the trunk in different directions. The flabelliform palms preserve
their scales at the inferior extremity of the trunk only, but lose them
as they increase in age; and the stem is entirely bare, from the middle
to the superior extremity. In the ancient _Lepidodendron_, on the other
hand, the more ancient the scale of the leaf-stalk, the more apparent it
still remains. Portions of stems have been discovered which contain
leaf-scars far larger than those referred to above, and we deduce from
these fragments the fact that those individuals which have been found
whole, are not by any means the largest of those which went to form so
large a proportion of the ancient coal-forests. The _lepidodendra_ bore
linear one-nerved leaves, and the stems always branched dichotomously and
possessed a central pith. Specimens variously named _knorria,
lepidophloios, halonia_, and _ulodendron_ are all referable to this
family.
[Illustration: FIG. 12.--_Lepidodendron longifolium._ Coal-shale.]
[Illustration: FIG. 13.--_Lepidodendron aculeatum_ in sandstone.]
In some strata, as for instance that of the Shropshire coalfield,
quantities of elongated cylindrical bodies known as _lepidostrobi_ have
been found, which, it was early conjectured, were the fruit of the giant
club-mosses about which we have just been speaking. Their appearance can
be called to mind by imagining the cylindrical fruit of the maize or
Indian corn to be reduced to some three or four inches in length. The
sporangia or cases which contained the microscopic spores or seeds were
arranged around a central axis in a somewhat similar manner to that in
which maize is found. These bodies have since been found actually
situated at the end of branches of _lepidodendron_, thus placing their
true nature beyond a doubt. The fossil seeds (spores) do not appear to
have exceeded in volume those of recent club-mosses, and this although
the actual trees themselves grew to a size very many times greater than
the living species. This minuteness of the seed-germs goes to explain the
reason why, as Sir Charles Lyell remarked, the same species of
_lepidodendra_ are so widely distributed in the coal measures of Europe
and America, their spores being capable of an easy transportation by the
wind.
[Illustration: FIG. 14.--_Lepidostrobus._ Coal-shale.]
One striking feature in connection with the fruit of the _lepidodendron_
and other ancient representatives of the club-moss tribe, is that the
bituminous coals in many, if not in most, instances, are made up almost
entirely of their spores and spore-cases. Under a microscope, a piece of
such coal is seen to be thronged with the minute rounded bodies of the
spores interlacing one another and forming almost the whole mass, whilst
larger than these, and often indeed enclosing them, are flattened
bag-like bodies which are none other than the compressed sporangia which
contained the former.
[Illustration: FIG. 15.--_Lycopodites_. Coal sandstone.]
Now, the little Scottish or Alpine club-moss which is so familiar,
produces its own little cones, each with its series of outside scales or
leaves; these are attached to the bags or spore-cases, which are crowded
with spores. Although in miniature, yet it produces its fruit in just the
same way, at the terminations of its little branches, and the spores, the
actual germs of life, when examined microscopically, are scarcely
distinguishable from those which are contained in certain bituminous
coals. And, although ancient club-mosses have been found in a fossilised
condition at least forty-nine feet high, the spores are no larger than
those of our miniature club-mosses of the present day.
The spores are more or less composed of pure bitumen, and the bituminous
nature of the coal depends largely on the presence or absence of these
microscopic bodies in it. The spores of the living club-mosses contain so
much resinous matter that they are now largely used in the making of
fireworks, and upon the presence of this altered resinous matter in coal
depends its capability of providing a good blazing coal.
At first sight it seems almost impossible that such a minute cause should
result in the formation of huge masses of coal, such an inconceivable
number of spores being necessary to make even the smallest fragment of
coal. But if we look at the cloud of spores that can be shaken from a
single spike of a club-moss, then imagine this to be repeated a thousand
times from each branch of a fairly tall tree, and then finally picture a
whole forest of such trees shedding in due season their copious showers
of spores to earth, we shall perhaps be less amazed than we were at first
thought, at the stupendous result wrought out by so minute an object.
Another well-known form of carboniferous vegetation is that known as the
_Sigillaria_, and, connected with this form is one, which was long
familiar under the name of _Stigmaria_, but which has since been
satisfactorily proved to have formed the branching root of the
sigillaria. The older geologists were in the habit of placing these
plants among the tree-ferns, principally on account of the cicatrices
which were left at the junctions of the leaf-stalks with the stem, after
the former had fallen off. No foliage had, however, been met with which
was actually attached to the plants, and hence, when it was discovered
that some of them had long attenuated leaves not at all like those
possessed by ferns, geologists were compelled to abandon this
classification of them, and even now no satisfactory reference to
existing orders of them has been made, owing to their anomalous
structure. The stems are fluted from base to stem, although this is not
so apparent near the base, whilst the raised prominences which now form
the cicatrices, are arranged at regular distances within the vertical
grooves.
When they have remained standing for some length of time, and the strata
have been allowed quietly to accumulate around the trunks, they have
escaped compression. They were evidently, to a great extent, hollow like
a reed, so that in those trees which still remain vertical, the interior
has become filled up by a coat of sandstone, whilst the bark has become
transformed into an envelope of an inch, or half an inch of coal. But
many are found lying in the strata in a horizontal plane. These have been
cast down and covered up by an ever-increasing load of strata, so that
the weight has, in the course of time, compressed the tree into simply
the thickness of the double bark, that is, of the two opposite sides of
the envelope which covered it when living.
_Sigillarae_ grew to a very great height without branching, some
specimens having measured from 60 to 70 feet long. In accordance with
their outside markings, certain types are known as _syringodendron_,
_favularia_, and _clathraria_. _Diploxylon_ is a term applied to an
interior stem referable to this family.
[Illustration: FIG. 16.--_Stigmaria ficoides_. Coal-shale.]
But the most interesting point about the _sigillariae_ is the root. This
was for a long time regarded as an entirely distinct individual, and the
older geologists explained it in their writings as a species of succulent
aquatic plant, giving it the name of _stigmaria_. They realized the fact
that it was almost universally found in those beds which occur
immediately beneath the coal seams, but for a long time it did not strike
them that it might possibly be the root of a tree. In an old edition of
Lyell's "Elements of Geology," utterly unlike existing editions in
quality, quantity, or comprehensiveness, after describing it as an
extinct species of water-plant, the author hazarded the conjecture that
it might ultimately be found to have a connection with some other
well-known plant or tree. It was noticed that above the coal, in the
roof, stigmariae were absent, and that the stems of trees which occurred
there, had become flattened by the weight of the overlying strata. The
stigmariae on the other hand, abounded in the _underclay_, as it is
called, and were not in any way compressed but retained what appeared to
be their natural shape and position. Hence to explain their appearance,
it was thought that they were water-plants, ramifying the mud in every
direction, and finally becoming overwhelmed and covered by the mud
itself. On botanical grounds, Brongniart and Lyell conjectured that they
formed the roots of other trees, and this became the more apparent as it
came to be acknowledged that the underclays were really ancient soils.
All doubt was, however, finally dispelled by the discovery by Mr Binney,
of a sigillaria and a stigmaria in actual connection with each other, in
the Lancashire coal-field.
Stigmariae have since been found in the Cape Breton coal-field, attached
to Lepidodendra, about which we have already spoken, and a similar
discovery has since been made in the British coal-fields. This,
therefore, would seem to shew the affinity of the sigillaria to the
lepidodendron, and through it to the living lycopods, or
club-mosses.
Some few species of stigmarian roots had been discovered, and various
specific names had been given to them before their actual nature was made
out. What for some time were thought to be long cylindrical leaves, have
now been found to be simply rootlets, and in specimens where these have
been removed, the surface of the stigmaria has been noticed to be covered
with large numbers of protuberant tubercles, which have formed the bases
of the rootlets. There appears to have also been some special kind of
arrangement in their growth, since, unlike the roots of most living
plants, the tubercles to which these rootlets were attached, were
arranged spirally around the main root. Each of these tubercles was
pitted in the centre, and into these the almost pointed ends of the
rootlets fitted, as by a ball and socket joint.
[Illustration: FIG. 17--_Section of stigmaria_.]
"A single trunk of _sigillaria_ in an erect forest presents an epitome of
a coal-seam. Its roots represent the _stigmaria_ underclay; its bark the
compact coal; its woody axis, the mineral charcoal; its fallen leaves and
fruits, with remains of herbaceous plants growing in its shade, mixed
with a little earthy matter, the layers of coarse coal. The condition of
the durable outer bark of erect trees, concurs with the chemical theory
of coal, in showing the especial suitableness of this kind of tissue for
the production of the purer compact coals."--(Dawson, "Structures in
Coal.")
There is yet one other family of plants which must be mentioned, and
which forms a very important portion of the constituent _flora_ of the
coal period. This is the great family of the _coniferae_, which although
differing in many respects from the highly organised dicotyledons of the
present day, yet resembled them in some respects, especially in the
formation of an annual ring of woody growth.
The conifers are those trees which, as the name would imply, bear their
fruit in the form of cones, such as the fir, larch, cedar, and others.
The order is one which is familiar to all, not only on account of the
cones they bear, and their sheddings, which in the autumn strew the
ground with a soft carpet of long needle-like leaves, but also because of
the gum-like secretion of resin which is contained in their tissues. Only
a few species have been found in the coal-beds, and these, on examination
under the microscope, have been discovered to be closely related to the
araucarian division of pines, rather than to any of our common firs. The
living species of this tree is a native of Norfolk Island, in the
Pacific, and here it attains a height of 200 feet, with a girth of 30
feet. From the peculiar arrangement of the ducts in the elongated
cellular tissue of the tree, as seen under the microscope, the fossil
conifers, which exhibit this structure, have been placed in the same
division.
The familiar fossil known to geologists as _Sternbergia_ has now been
shown to be the cast of the central pith of these conifers, amongst which
may be mentioned _cordaites, araucarites_, and _dadoxylon._. The central
cores had become replaced with inorganic matter after the pith had shrunk
and left the space empty. This shrinkage of the pith is a process which
takes place in many plants even when living, and instances will at once
occur, in which the stems of various species of shrubs when broken open
exhibit the remains of the shrunken pith, in the shape of thin discs
across the interval cavity.
We might reasonably expect that where we find the remains of fossil
coniferous trees, we should also meet with the cones or fruit which they
bear. And such is the case. In some coal-districts fossil fruits, named
_cardiocarpum_ and _trigonocarpum_, have been found in great quantities,
and these have now been decided by botanists to be the fruits of certain
conifers, allied, not to those which bear hard cones, but to those which
bear solitary fleshy fruits. Sir Charles Lyell referred them to a Chinese
genus of the yew tribe called _salisburia_. Dawson states that they are
very similar to both _taxus_ and _salisburia._. They are abundant in some
coal-measures, and are contained, not only in the coal itself, but also
in the sandstones and shales. The under-clays appear to be devoid of
them, and this is, of course, exactly what might have been expected,
since the seeds would remain upon the soil until covered up by vegetable
matter, but would never form part of the clay soil itself.
In connection with the varieties which have been distinguished in the
families of the conifers, calamites, and sigillariae, Sir William Dawson
makes the following observations: "I believe that there was a
considerably wide range of organisation in _cordaitinae_ as well as in
_calamites_ and _sigillariae_, and that it will eventually be found that
there were three lines of connection between the higher cryptogams
(flowerless) and the phaenogams (flowering), one leading from the
lycopodes by the _sigillariae_, another leading by the _cordaites_, and
the third leading from the _equisetums_ by the _calamites_. Still further
back the characters, afterwards separated in the club-mosses,
mare's-tails, and ferns, were united in the _rhizocarps_, or, as some
prefer to call them, the heterosporous _filicinae_."
In concluding this chapter dealing with the various kinds of plants which
have been discovered as contributing to the formation of
coal-measures, it would be as well to say a word or two concerning the
climate which must have been necessary to permit of the growth of such an
abundance of vegetation. It is at once admitted by all botanists that a
moist, humid, and warm atmosphere was necessary to account for the
existence of such an abundance of ferns. The gorgeous waving
tree-ferns which were doubtless an important feature of the landscape,
would have required a moist heat such as does not now exist in this
country, although not necessarily a tropical heat. The magnificent giant
lycopodiums cast into the shade all our living members of that class, the
largest of which perhaps are those that flourish in New Zealand. In New
Zealand, too, are found many species of ferns, both those which are
arborescent and those which are of more humble stature. Add to these the
numerous conifers which are there found, and we shall find that a forest
in that country may represent to a certain extent the appearance
presented by a forest of carboniferous vegetation. The ferns, lycopods,
and pines, however, which appear there, it is but fair to add, are mixed
with other types allied to more recent forms of vegetation.
There are many reasons for believing that the amount of carbonic acid gas
then existing in the atmosphere was larger than the quantity which we now
find, and Professor Tyndall has shown that the effect of this would be to
prevent radiation of heat from the earth. The resulting forms of
vegetation would be such as would be comparable with those which are now
reared in the green-house or conservatory in these latitudes. The gas
would, in fact, act as a glass roof, extending over the whole world.