The Story of a Piece of Coal
by
Edward A. Martin
Part 1 out of 3
Produced by Miranda van de Heijning, Luiz Antonio de Souza and PG
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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.
CHAPTER II.
A GENERAL VIEW OF THE COAL-BEARING STRATA.
In considering the source whence coal is derived, we must be careful to
remember that coal itself is but a minor portion of the whole formation
in which it occurs. The presence of coal has indeed given the name to the
formation, the word "carboniferous" meaning "coal-bearing," but in taking
a comprehensive view of the position which it occupies in the bowels of
the earth, it will be necessary to take into consideration the strata in
which it is found, and the conditions, so far as are known, under which
these were deposited.
Geologically speaking, the Carboniferous formation occurs near the close
of that group of systems which have been classed as "palaeozoic," younger
in point of age than the well known Devonian and Old Red Sandstone
strata, but older by far than the Oolites, the Wealden, or the Cretaceous
strata.
In South Wales the coal-bearing strata have been estimated at between
11,000 and 12,000 feet, yet amongst this enormous thickness of strata,
the whole of the various coal-seams, if taken together, probably does not
amount to more than 120 feet. This great disproportion between the total
thickness and the thickness of coal itself shows itself in every
coal-field that has been worked, and when a single seam of coal is
discovered attaining a thickness of 9 or 10 feet, it is so unusual a
thing in Great Britain as to cause it to be known as the "nine" or
"ten-foot seam," as the case may be. Although abroad many seams are found
which are of greater thicknesses, yet similarly the other portions of the
formation are proportionately greater.
It is not possible therefore to realise completely the significance of
the coal-beds themselves unless there is also a knowledge of the
remaining constituents of the whole formation. The strata found in the
various coal-fields differ considerably amongst themselves in character.
There are, however, certain well-defined characteristics which find
representation in most of the principal coal-fields, whether British or
European. Professor Hull classifies these carboniferous beds as
follows:--
UPPER CARBONIFEROUS.
_Upper coal-measures._
Reddish and purple sandstones, red and grey clays and shales,
thin bands of coal, ironstone and limestone, with _spirorbis_
and fish.
_Middle coal-measures._
Yellow and gray sandstones, blue and black clays and shales,
bands of coal and ironstone, fossil plants, bivalves
and fish, occasional marine bands.
MIDDLE CARBONIFEROUS.
_Gannister beds_ or _Lower coal-measures._
_Millstone grit._ Flagstone series in Ireland.
_Yoredale beds._ Upper shale series of Ireland.
LOWER CARBONIFEROUS.
_Mountain limestone_.
_Limestone shale_.
Each of the three principal divisions has its representative in Scotland,
Belgium, and Ireland, but, unfortunately for the last-named country, the
whole of the upper coal-measures are there absent. It is from these
measures that almost all our commercial coals are obtained.
This list of beds might be further curtailed for all practical purposes
of the geologist, and the three great divisions of the system would thus
stand:--
Upper Carboniferous, or Coal-measures proper.
Millstone grit.
Lower Carboniferous, or Mountain limestone.
In short, the formation consists of masses of sandstone, shale, limestone
and coal, these also enclosing clays and ironstones, and, in the
limestone, marbles and veins of the ores of lead, zinc, and antimony, and
occasionally silver.
[Illustration: FIG. 18.--Sigillarian trunks in current-bedded sandstone.
St Etienne.]
As the most apparent of the rocks of the system are sandstone, shale,
limestone, and coal, it will be necessary to consider how these were
deposited in the waters of the carboniferous ages, and this we can best
do by considering the laws under which strata of a similar nature are now
being deposited as sedimentary beds.
A great proportion consists of sandstone. Now sandstone is the result of
sand which has been deposited in large quantities, having become
indurated or hardened by various processes brought to bear upon it. It is
necessary, therefore, first to ascertain whence came the sand, and
whether there are any peculiarities in its method of deposition which
will explain its stratification. It will be noticed at once that it bears
a considerable amount of evidence of what is called "current-bedding,"
that is to say, that the strata, instead of being regularly deposited,
exhibit series of wedge-shaped masses, which are constantly thinning out.
Sand and quartz are of the same chemical composition, and in all
probability the sand of which every sandstone in existence is composed,
appeared on this earth in its first solid form in the shape of quartz.
Now quartz is a comparatively heavy mineral, so also, therefore, will
sand be. It is also very hard, and in these two respects it differs
entirely from another product of sedimentary deposition, namely, mud or
clay, with which we shall have presently to deal when coming to the
shales. Since quartz is a hard mineral it necessarily follows that it
will suffer, without being greatly affected, a far greater amount of
wearing and knocking about when being transported by the agency of
currents and rivers, than will a softer substance, such as clay. An equal
amount of this wearing action upon clay will reduce it to a fine
impalpable silt. The grains of sand, however, will still remain of an
appreciable average size, and where both sand and clay are being
transported to the sea in one and the same stream, the clay will be
transported to long distances, whilst the sand, being heavier, bulk for
bulk, and also consisting of grains larger in size than grains of clay,
will be rapidly deposited, and form beds of sand. Of course, if the
current be a violent one, the sand is transported, not by being held in
suspension, but rather by being pushed along the bed of the river; such
an action will then tend to cause the sand to become powdered into still
finer sand.
When a river enters the sea it soon loses its individuality; it becomes
merged in the body of the ocean, where it loses its current, and where
therefore it has no power to keep in suspension the sediment which it had
brought down from the higher lands. When this is the case, the sand borne
in suspension is the first to be deposited, and this accumulates in banks
near the entrance of the river into the sea. We will suppose, for
illustration, that a small river has become charged with a supply of
sand. As it gradually approaches the sea, and the current loses its
force, the sand is the more sluggishly carried along, until finally it
falls to the bottom, and forms a layer of sand there. This layer
increases in thickness until it causes the depth of water above it to
become comparatively shallow. On the shallowing process taking place, the
current will still have a certain, though slighter, hold on the sand in
suspension, and will transport it yet a little further seaward, when it
will be thrown down, at the edge of the bank or layer already formed,
thus tending to extend the bank, and to shallow a wider space of
river-bed.
As a result of this action, strata would be formed, shewing
stratification diagonally as well as horizontally, represented in section
as a number of banks which had seemingly been thrown down one above the
other, ending in thin wedge-shaped terminations where the particular
supply of sediment to which each owed its formation had failed.
The masses of sandstone which are found in the carboniferous formation,
exhibit in a large degree these wedge-shaped strata, and we have
therefore a clue at once, both as to their propinquity to sea and land,
and also as to the manner in which they were formed.
[Illustration: FIG. 19.--_Productus_. Coal-measures.]
There is one thing more, too, about them. Just as, in the case we were
considering, we could observe that the wedge-shaped strata always pointed
away from the source of the material which formed them, so we can
similarly judge that in the carboniferous strata the same deduction holds
good, that the diagonally-pointing strata were formed in the same way,
and that their thinning out was simply owing to temporary failure of
sediment, made good, however, by a further deposition of strata when the
next supply was borne down.
It is scarcely likely, however, that sand in a pure state was always
carried down by the currents to the sea. Sometimes there would be some
silt mixed with it. Just as in many parts large masses of almost pure
sandstone have been formed, so in other places shales, or, as they are
popularly known by miners, "bind," have been formed. Shales are formed
from the clays which have been carried down by the rivers in the shape of
silt, but which have since become hardened, and now split up easily into
thin parallel layers. The reader has no doubt often handled a piece of
hard clay when fresh from the quarry, and has remembered how that, when
he has been breaking it up, in order, perhaps, to excavate a
partially-hidden fossil, it has readily split up in thin flakes or layers
of shaly substance. This exhibits, on a small scale, the chief
peculiarity of the coal shales.
The formation of shales will now demand our attention. When a river is
carrying down with it a quantity of mud or clay, it is transported as a
fine, dusty silt, and when present in quantities, gives the muddy tint to
the water which is so noticeable. We can very well see how that silt will
be carried down in greater quantities than sand, since nearly all rivers
in some part of their course will travel through a clayey district, and
finely-divided clay, being of a very light nature, will be carried
forward whenever a river passes over such a district. And a very slight
current being sufficient to carry it in a state of suspension, it follows
that it will have little opportunity of falling to the bottom, until, by
some means or other, the current, which is the means of its conveyance,
becomes stopped or hindered considerably in its flow.
When the river enters a large body of water, such as the ocean or a lake,
in losing its individuality, it loses also the velocity of its current,
and the silt tends to sink down to the bottom. But being less heavy than
the sand, about which we have previously spoken, it does not sink all at
once, but partly with the impetus it has gained, and partly on account of
the very slight velocity which the current still retains, even after
having entered the sea, it will be carried out some distance, and will
the more gradually sink to the bottom. The deeper the water in which it
falls the greater the possibility of its drifting farther still, since in
sinking, it would fall, not vertically, but rather as the drops of rain
in a shower when being driven before a gale of wind. Thus we should
notice that clays and shales would exhibit a regularity and uniformity of
deposition over a wide area. Currents and tides in the sea or lake would
tend still further to retard deposition, whilst any stoppages in the
supply of silt which took place would give the former layer time to
consolidate and harden, and this would assist in giving it that bedded
structure which is so noticeable in the shales, and which causes it to
split up into fine laminae. This uniformity of structure in the shales
over wide areas is a well ascertained characteristic of the coal-shales,
and we may therefore regard the method of their deposition as given here
with a degree of certainty.
There is a class of deposit found among the coal-beds, which is known as
the "underclay," and this is the most regular of all as to the position
in which it is found. The underclays are found beneath every bed of coal.
"Warrant," "spavin," and "gannister" are local names which are sometimes
applied to it, the last being a term used when the clay contains such a
large proportion of silicious matter as to become almost like a hard
flinty rock. Sometimes, however, it is a soft clay, at others it is mixed
with sand, but whatever the composition of the underclays may be, they
always agree in being unstratified. They also agree in this respect that
the peculiar fossils known as _stigmariae_ abound in them, and in some
cases to such an extent that the clay is one thickly-matted mass of the
filamentous rootlets of these fossils. We have seen how these gradually
came to be recognised as the roots of trees which grew in this age, and
whose remains have subsequently become metamorphosed into coal, and it is
but one step farther to come to the conclusion that these underclays are
the ancient soils in which the plants grew.
No sketch of the various beds which go to form the coal-measures would be
complete which did not take into account the enormous beds of mountain
limestone which form the basis of the whole system, and which in thinner
bands are intercalated amongst the upper portion of the system, or the
true coal-measures.
Now, limestones are not formed in the same way in which we have seen that
sandstones and shales are formed. The last two mentioned owe their origin
to their deposition as sediment in seas, estuaries or lakes, but the
masses of limestone which are found in the various geological formations
owe their origin to causes other than that of sedimentary deposition.
In carboniferous times there lived numberless creatures which we know
nowadays as _encrinites_. These, when growing, were fixed to the bed of
the ocean, and extended upward in the shape of pliant stems composed of
limestone joints or plates; the stem of each encrinite then expanded at
the top in the shape of a gorgeous and graceful starfish, possessed of
numberless and lengthy arms. These encrinites grew in such profusion that
after death, when the plates of which their stems consisted, became
loosened and scattered over the bed of the sea, they accumulated and
formed solid beds of limestone. Besides the encrinites, there were of
course other creatures which were able to create the hard parts of their
structures by withdrawing lime from the sea, such as _foraminifera_,
shell-fish, and especially corals, so that all these assisted after death
in the accumulation of beds of limestone where they had grown and lived.
[Illustration: FIG. 20.--Encrinite.]
[Illustration: FIG. 21.--Encrinital limestone.]
There is one peculiarity in connection with the habitats of the
encrinites and corals which goes some distance in supplying us with a
useful clue as to the conditions under which this portion of the
carboniferous formation was formed. These creatures find it a difficult
matter, as a rule, to live and secrete their calcareous skeleton in any
water but that which is clear, and free from muddy or sandy sediment.
They are therefore not found, generally speaking, where the other
deposits which we have considered, are forming, and, as these are always
found near the coasts, it follows that the habitats of the creatures
referred to must be far out at sea where no muddy sediments, borne by
rivers, can reach them. We can therefore safely come to the conclusion
that the large masses of encrinital limestone, which attain such an
enormous thickness in some places, especially in Ireland, have been
formed far away from the land of the period; we can at the same time draw
the conclusion that if we find the encrinites broken and snapped asunder,
and the limestone deposits becoming impure through being mingled with a
proportion of clayey or sandy deposits, that we are approaching a
coast-line where perhaps a river opened out, and where it destroyed the
growth of encrinites, mixing with their dead remains the sedimentary
debris of the land.
[Illustration: FIG. 22.--Encrinites: various. Mountain limestone.]
We have lightly glanced at the circumstances attending the deposition of
each of the principal rocks which form the beds amongst which coal is
found, and have now to deal with the formation of the coal itself. We
have already considered the various kinds of plants and trees which have
been discovered as contributing their remains to the formation of coal,
and have now to attempt an explanation of how it came to be formed in so
regular a manner over so wide an area.
Each of the British coal-fields is fairly extensive. The Yorkshire and
Derbyshire coal-fields, together with the Lancashire coal-field, with
which they were at one time in geological connection, give us an area of
nearly 1000 square miles, and other British coal-fields show at least
some hundreds of square miles. And yet, spread over them, we find a
series of beds of coal which in many cases extend throughout the whole
area with apparent regularity. If we take it, as there seems every reason
to believe was the case, that almost all these coal-fields were not only
being formed at the same time, but were in most instances in continuation
with one another, this regularity of deposition of comparatively narrow
beds of coal, appears all the more remarkable.
The question at once suggests itself, Which of two things is probable?
Are we to believe that all this vegetable matter was brought down by some
mighty river and deposited in its delta, or that the coal-plants grew
just where we now find the coal?
Formerly it was supposed that coal was formed out of dead leaves and
trees, the refuse of the vegetation of the land, which had been carried
down by rivers into the sea and deposited at their mouths, in the same
way that sand and mud, as we have seen, are swept down and deposited. If
this were so, the extent of the deposits would require a river with an
enormous embouchure, and we should be scarcely warranted in believing
that such peaceful conditions would there prevail as to allow of the
layers of coal to be laid down with so little disturbance and with such
regularity over these wide areas. But the great objection to this theory
is, that not only do the remains still retain their perfection of
structure, but they are comparatively _pure,--i.e.,_ unmixed with
sedimentary depositions of clay or sand. Now, rivers would not bring down
the dead vegetation alone; their usual burden of sediment would also be
deposited at their mouths, and thus dead plants, sand, and clay would be
mixed up together in one black shaly or sandy mass, a mixture which would
be useless for purposes of combustion. The only theory which explained
all the recognised phenomena of the coal-measures was that the plants
forming the coal actually grew where the coal was formed, and where,
indeed, we now find it. When the plants and trees died, their remains
fell to the ground of the forest, and these soon turned to a black,
pasty, vegetable mass, the layer thus formed being regularly increased
year by year by the continual accumulation of fresh carbonaceous matter.
By this means a bed would be formed with regularity over a wide area; the
coal would be almost free from an admixture of sandy or clayey sediment,
and probably the rate of formation would be no more rapid in one part of
the forest than another. Thus there would be everywhere uniformity of
thickness. The warm and humid atmosphere, which it is probable then
existed, would not only have tended towards the production of an abnormal
vegetation, but would have assisted in the decaying and disintegrating
processes which went on amongst the shed leaves and trees.
When at last it was announced as a patent fact that every bed of coal
possessed its underclay, and that trees had been discovered actually
standing upon their own roots in the clay, there was no room at all for
doubt that the correct theory had been hit upon--viz., that coal is now
found just where the trees composing it had grown in the past.
But we have more than one coal-seam to account for. We have to explain
the existence of several layers of coal which have been formed over one
another on the same spot at successive periods, divided by other periods
when shale and sandstones only have been formed.
A careful estimate of the Lancashire coal-field has been made by
Professor Hull for the Geological Survey. Of the 7000 feet of
carboniferous strata here found, spread out over an area of 217 square
miles, there are on the average eighteen seams of coal.
This is only an instance of what is to be found elsewhere. Eighteen
coal-seams! what does this mean? It means that, during carboniferous
times, on no less than eighteen occasions, separate and distinct forests
have grown on this self-same spot, and that between each of these
occasions changes have taken place which have brought it beneath the
waters of the ocean, where the sandstones and shales have been formed
which divide the coal-seams from each other. We are met here by a
wonderful demonstration of the instability of the surface of the earth,
and we have to do our best to show how the changes of level have been
brought about, which have allowed of this game of geological see-saw to
take place between sea and land. Changes of level! Many a hard geological
nut has only been overcome by the application of the principle of changes
of level in the surface of the earth, and in this we shall find a sure
explanation of the phenomena of the coal-measures.
Great changes of the level of the land are undoubtedly taking place even
now on the earth's surface, and in assuming that similar changes took
place in carboniferous times, we shall not be assuming the former
existence of an agent with which we are now unfamiliar. And when we
consider the thicknesses of sandstone and shale which intervene beneath
the coal-seams, we can realise to a certain extent the vast lapses of
years which must have taken place between the existence of each forest;
so that although now an individual passing up a coal-mine shaft may
rapidly pass through the remains of one forest after another, the rise of
the strata above each forest-bed then was tremendously slow, and the
period between the growth of each forest must represent the passing away
of countless ages. Perhaps it would not be too much to say that the
strata between some of the coal-seams would represent a period not less
than that between the formation of the few tertiary coals with which we
are acquainted, and a time which is still to us in the far-away future.
The actual seams of coal themselves will not yield much information, from
which it will be possible to judge of the contour of the landmasses at
this ancient period. Of one thing we are sure, namely, that at the time
each seam was formed, the spot where it accumulated was dry land. If,
therefore, the seams which appear one above the other coincide fairly
well as to their superficial extent, we can conclude that each time the
land was raised above the sea and the forest again grew, the contour of
the land was very similar. This conclusion will be very useful to go
upon, since whatever decision may be come to as an explanation of one
successive land-period and sea-period on the same spot, will be
applicable to the eighteen or more periods necessary for the completion
of some of the coal-fields.
We will therefore look at one of the sandstone masses which occur between
the coal-seams, and learn what lessons these have to teach us. In
considering the formation of strata of sand in the seas around our
river-mouths, it was seen that, owing to the greater weight of the
particles of the sand over those of clay, the former the more readily
sank to the bottom, and formed banks not very far away from the land. It
was seen, too, that each successive deposition of sand formed a
wedge-shaped layer, with the point of the wedge pointing away from the
source of origin of the sediment, and therefore of the current which
conveyed the sediment. Therefore, if in the coal-measure sandstones the
layers were found with their wedges all pointing in one direction, we
should be able to judge that the currents were all from one direction,
and that, therefore, they were formed by a single river. But this is just
what we do not find, for instead of it the direction of the wedge-shaped
strata varies in almost every layer, and the current-bedding has been
brought about by currents travelling in every direction. Such diverse
current-bedding could only result from the fact that the spot where the
sand was laid down was subject to currents from every direction, and the
inference is that it was well within the sphere of influence of numerous
streams and rivers, which flowed from every direction. The only condition
of things which would explain this is that the sandstone was originally
formed in a closed sea or large lake, into which numerous rivers flowing
from every direction poured their contents.
Now, in the sandstones, the remains of numerous plants have been found,
but they do not present the perfect appearance that they do when found in
the shales; in fact they appear to have suffered a certain amount of
damage through having drifted some distance. This, together with the fact
that sandstones are not formed far out at sea, justify the safe
conclusion that the land could not have been far off. Wherever the
current-bedding shows itself in this manner we may be sure we are
examining a spot from which the land in every direction could not have
been at a very great distance, and also that, since the heavy materials
of which sandstone is composed could only be transported by being
impelled along by currents at the bed of the sea, and that in deep water
such currents could not exist, therefore we may safely decide that the
sea into which the rivers fell was a comparatively shallow one.
Although the present coal-fields of England are divided from one another
by patches of other beds, it is probable that some of them were formerly
connected with others, and a very wide sheet of coal on each occasion was
laid down. The question arises as to what was the extent of the inland
sea or lake, and did it include the area covered by the coal basins of
Scotland and Ireland, of France and Belgium? And if these, why not those
of America and other parts? The deposition of the coal, according to the
theory here advanced, may as well have been brought about in a series of
large inland seas and lakes, as by one large comprehensive sea, and
probably the former is the more satisfactory explanation of the two. But
the astonishing part of it is that the changes in the level of the land
must have been taking place simultaneously over these large areas,
although, of course, while one quarter may have been depressed beneath
the sea, another may have been raised above it.
In connection with the question of the contour of the land during the
existence of the large lakes or inland seas, Professor Hull has prepared,
in his series of maps illustrative of the Palaeo-Geography of the British
Islands, a map showing on incontestible grounds the existence during the
coal-ages of a great central barrier or ridge of high land stretching
across from Anglesea, south of Flint, Staffordshire, and Shropshire
coal-fields, to the eastern coast of Norfolk. He regards the British
coal-measures as having been laid down in two, or at most three, areas of
deposition--one south of this ridge, the remainder to the north of it. In
regard to the extent of the former deposits of coal in Ireland, there is
every probability that the sister island was just as favourably treated
in this respect as Great Britain. Most unfortunately, Ireland has since
suffered extreme denudation, notably from the great convulsions of nature
at the close of the very period of their deposition, as well as in more
recent times, resulting in the removal of nearly all the valuable upper
carboniferous beds, and leaving only the few unimportant
coal-beds to which reference has been made.
[Illustration: FIG. 23.--_Cyathophyllum_. Coral in encrinital limestone.]
We are unable to believe in the continuity of our coal-beds with those of
America, for the great source of sediment in those times was a continent
situated on the site of the Atlantic Ocean, and it is owing to this
extensive continent that the forms of _flora_ found in the coal-beds in
each country bear so close a resemblance to one another, and also that
the encrinital limestone which was formed in the purer depths of the
ocean on the east, became mixed with silt, and formed masses of shaly
impure limestone in the south-western parts of Ireland.
It must be noted that, although we may attribute to upheaval from beneath
the fact that the bed of the sea became temporarily raised at each period
into dry land, the deposits of sand or shale would at the same time be
tending to shallow the bed, and this alone would assist the process of
upheaval by bringing the land at least very near to the surface of the
water.
Each upheaval, however, could have been but a temporary arrest of the
great movement of crust subsidence which was going on throughout the coal
period, so that, at its close, when the last coal forest grew upon the
surface of the land, there had disappeared, in the case of South Wales, a
thickness of 11,000 feet of material.
Of the many remarkable things in connection with coal-beds, not the least
is the state of purity in which coal is found. On the floor of each
forest there would be many a streamlet or even small river which would
wend its way to meet the not very distant sea, and it is surprising at
first that so little sediment found its way into the coal itself. But
this was cleverly explained by Sir Charles Lyell, who noticed, on one of
his visits to America, that the water of the Mississippi, around the rank
growths of cypress which form the "cypress swamps" at the mouths of that
river, was highly charged with sediment, but that, having passed through
the close undergrowth of the swamps, it issued in almost a pure state,
the sediment which it bore having been filtered out of it and
precipitated. This very satisfactorily explained how in some places
carbonaceous matter might be deposited in a perfectly pure state, whilst
in others, where sandstone or shale was actually forming, it might be
impregnated by coaly matter in such a way as to cause it to be stained
black. In times of flood sediment would be brought in, even where pure
coal had been forming, and then we should have a thin "parting" of
sandstone or shale, which was formed when the flood was at its height. Or
a slight sinking of the land might occur, in which case also the
formation of coal would temporarily cease, and a parting of foreign
matter would be formed, which, on further upheaval taking place, would
again give way to another forest growth. Some of the thicker beds have
been found presenting this aspect, such as the South Staffordshire
ten-yard coal, which in some parts splits up into a dozen or so smaller
beds, with partings of sediment between them.
In the face of the stupendous movements which must have happened in order
to bring about the successive growth of forests one above another on the
same spot, the question at once arises as to how these movements of the
solid earth came about, and what was the cause which operated in such a
manner. We can only judge that, in some way or other, heat, or the
withdrawal of heat, has been the prime motive power. We can perceive,
from what is now going on in some parts of the earth, how great an
influence it has had in shaping the land, for volcanoes owe their
activity to the hidden heat in the earth's interior, and afford us an
idea of the power of which heat is capable in the matter of building up
and destroying continents. No less certain is it that heat is the prime
factor in those more gradual vertical movements of the land to which we
have referred elsewhere, but in regard to the exact manner in which it
acts we are very much in the dark. Everybody knows that, in the majority
of instances, material substances of all kinds expand under the influence
of heat, and contract when the source of heat is withdrawn. If we can
imagine movements in the quantity of heat contained in the solid crust,
the explanation is easy, for if a certain tract of land receive an
accession of heat beneath it, it is certain that the principal effect
will be an elevation of the land, consequent on the expansion of its
materials, with a subsequent depression when the heat beneath the tract
in question becomes gradually lessened. Should the heat be retained for a
long period, the strata would be so uplifted as to form an anticlinal, or
saddle-back, and then, should subsequent denudation take place, more
ancient strata would be brought to view. It was thus in the instance of
the tract bounded by the North and South Downs, which were formerly
entirely covered by chalk, and in the instance of the uprising of the
carboniferous limestone between the coal-fields of Lancashire,
Staffordshire, and Derbyshire.
How the heat-waves act, and the laws, if any, which they obey in their
subterranean movements, we are unable to judge. From the properties which
heat possesses we know that its presence or absence produces marked
differences in the positions of the strata of the earth, and from
observations made in connection with the closing of some volcanoes, and
the opening up of fresh earth-vents, we have gone a long way towards
establishing the probability that there are even now slow and ponderous
movements taking place in the heat stored in the earth's crust, whose
effects are appreciably communicated to the outside of the thin rind of
solid earth upon which we live.
Owing to the great igneous and volcanic activity at the close of the
deposition of the carboniferous system of strata, the coal-measures
exhibit what are known as _faults_ in abundance. The mountain limestone,
where it outcrops at the surface, is observed to be much jointed, so much
so that the work of quarrying the limestone is greatly assisted by the
jointed structure of the rock. Faults differ from joints in that, whilst
the strata in the latter are still in relative position on each side of
the joint, they have in the former slipped out of place. In such a case
the continuation of a stratum on the opposite side of a fault will be
found to be depressed, perhaps a thousand feet or more. It will be seen
at once how that, in sinking a new shaft into a coal-seam, the
possibility of an unknown fault has to be brought into consideration,
since the position of the seam may prove to have been depressed to such
an extent as to cause it to be beyond workable depth. Many seams, on the
other hand, which would have remained altogether out of reach of mining
operations, have been brought within workable depth by a series of
_step-faults_, this being a term applied to a series of parallel faults,
in none of which the amount of down-throw is great.
The amount of the down-throw, or the slipping-down of the beds, is
measured, vertically, from the point of disappearance of a layer to an
imaginary continuation of the same layer from where it again appears
beyond the fault. The plane of a fault is usually more or less inclined,
the amount of the inclination being known as the _hade_ of the fault, and
it is a remarkable characteristic of faults that, as a general rule, they
hade to the down-throw. This will be more clearly understood when it is
explained that, by its action, a seam of coal, which is subject to
numerous faults, can never be pierced more than once by one and the same
boring. In mountainous districts, however, there are occasions when the
hade is to the up-throw, and this kind of fault is known as an _inverted
fault_.
Lines of faults extend sometimes for hundreds of miles. The great Pennine
Fault of England is 130 miles long, and others extend for much greater
distances. The surfaces on both sides of a fault are often smooth and
highly polished by the movement which has taken place in the strata. They
then show the phenomenon known as _slicken-sides_. Many faults have
become filled with crystalline minerals in the form of veins of ore,
deposited by infiltrating waters percolating through the natural
fissures.
In considering the formation and structure of the better-known
coal-bearing beds of the carboniferous age, we must not lose sight of the
fact that important beds of coal also occur in strata of much more recent
date. There are important coal-beds in India of Permian age. There are
coal-beds of Liassic age in South Hungary and in Texas, and of Jurassic
age in Virginia, as well as at Brora in Sutherlandshire; there are coals
of Cretaceous age in Moravia, and valuable Miocene Tertiary coals in
Hungary and the Austrian Alps.
Again, older than the true carboniferous age, are the Silurian
anthracites of Co. Cavan, and certain Norwegian coals, whilst in New
South Wales we are confronted with an assemblage of coal-bearing strata
which extend apparently from the Devonian into Mesozoic times.
Still, the age we have considered more closely has an unrivalled right to
the title, coal appearing there not merely as an occasional bed, but as a
marked characteristic of the formation.
The types of animal life which are found in this formation are varied,
and although naturally enough they do not excel in number, there are yet
sufficient varieties to show probabilities of the existence of many with
which we are unfamiliar. The highest forms yet found, show an advance as
compared with those from earlier formations, and exhibit amphibian
characteristics intermediate between the two great classes of fishes and
reptiles. Numerous specimens proper to the extinct order of
_labyrinthodontia_ have been arranged into at least a score of genera,
these having been drawn from the coal-measures of Newcastle, Edinburgh,
Kilkenny, Saaerbruck, Bavaria, Pennsylvania, and elsewhere. The
_Archegosaurus,_ which we have figured, and the _Anthracosaurus,_ are
forms which appear to have existed in great numbers in the swamps and
lakes of the age. The fish of the period belong almost entirely to the
ancient orders of the ganoids and placoids. Of the ganoids, the great
_megalichthys Hibberti_ ranges throughout the whole of the system.
Wonderful accumulations of fish remains are found at the base of the
system, in the bone-bed of the Bristol coal-field, as well as in a
similar bed at Armagh. Many fishes were armed with powerful conical
teeth, but the majority, like the existing Port Jackson shark, were
possessed of massive palates, suited in some cases for crushing, and in
others for cutting.
[Illustration: FIG. 24.--_Archegosaurus minor_. Coal-measures.]
[Illustration: FIG. 25.--_Psammodus porosus_. Crushing palate of a fish.]
[Illustration: FIG. 26.--_Orthoceras_. Mountain limestone.]
In the mountain limestone we see, of course, the predominance of marine
types, encrinital remains forming the greater proportion of the mass.
There are occasional plant remains which bear evidence of having drifted
for some distance from the shore. But next to the _encrinites_, the
corals are the most important and persistent. Corals of most beautiful
forms and capable of giving polished marble-like sections, are in
abundance. _Polyzoa_ are well represented, of which the lace-coral
(_fenestella_) and screw-coral (_archimedopora_) are instances.
_Cephalopoda_ are represented by the _orthoceras_, sometimes five or six
feet long, and _goniatites_, the forerunner of the familiar _ammonite_.
Many species of brachiopods and lammellibranchs are met with. _Lingula_,
most persistent throughout all geological time, is abundant in the
coal-shales, but not in the limestones. _Aviculopecten_ is there abundant
also. In the mountain limestone the last of the trilobites (_Phillipsia_)
is found.
[Illustration: FIG. 27.--_Fenestella retipora_. Mountain limestone.]
[Illustration: FIG. 28.--_Goniatites_. Mountain limestone.]
We have evidence of the existence in the forests of a variety of
_centipede_, specimens having been found in the erect stump of a hollow
tree, although the fossil is an extremely rare one. The same may be said
of the only two species of land-snail which have been found connected
with the coal forests, viz., _pupa vetusta_ and _zonites priscus_, both
discovered in the cliffs of Nova Scotia. These are sufficient to
demonstrate that the fauna of the period had already reached a high stage
of development. In the estuaries of the day, masses of a species of
freshwater mussel (_anthracosia_) were in existence, and these have left
their remains in the shape of extensive beds of shells. They are familiar
to the miner as _mussel-binds_, and are as noticeable a feature of this
long ago period, as are the aggregations of mussels on every coast at
the present day.
[Illustration: FIG. 29.--_Aviculopecten papyraceus_. Coal-shale.]
CHAPTER III.
VARIOUS FORMS OF COAL AND CARBON.
In considering the various forms and combinations into which coal enters,
it is necessary that we should obtain a clear conception of what the
substance called "carbon" is, and its nature and properties generally,
since this it is which forms such a large percentage of all kinds of
coal, and which indeed forms the actual basis of it. In the shape of
coke, of course, we have a fairly pure form of carbon, and this being
produced, as we shall see presently, by the driving off of the volatile
or vaporous constituents of coal, we are able to perceive by the residue
how great a proportion of coal consists of carbon. In fact, the two have
almost an identical meaning in the popular mind, and the fact that the
great masses of strata, in which are contained our principal and most
valuable seams of coal, are termed "carboniferous," from the Latin
_carbo_, coal, and _fero_, I bear, tends to perpetuate the existence of
the idea.
There is always a certain, though slight, quantity of carbon in the air,
and this remains fairly constant in the open country. Small though it may
be in proportion to the quantity of pure air in which it is found, it is
yet sufficient to provide the carbon which is necessary to the growth of
vegetable life. Just as some of the animals known popularly as the
_zoophytes_, which are attached during life to rocks beneath the sea, are
fed by means of currents of water which bring their food to them, so the
leaves, which inhale carbon-food during the day through their
under-surfaces, are provided with it by means of the currents of air
which are always circulating around them; and while the fuel is being
taken in beneath, the heat and light are being received from above, and
the sun supplies the motive power to digestion.
It is assumed that it is, within the knowledge of all that, for the
origin of the various seams and beds of coaly combinations which exist in
the earth's crust, we must look to the vegetable world. If, however, we
could go so far back in the world's history as the period when our
incandescent orb had only just severed connection with a
gradually-diminishing sun, we should probably find the carbon there, but
locked up in the bonds of chemical affinities with other elements, and
existing therewith in a gaseous condition. But, as the solidifying
process went on, and as the vegetable world afterwards made its
appearance, the carbon became, so to speak, wrenched from its
combinations, and being absorbed by trees and plants, finally became
deposited amongst the ruins of a former vegetable world, and is now
presented to us in the form of coal.
We are able to trace the gradual changes through which the pasty mass of
decaying vegetation passed, in consequence of the fact that we have this
material locked up in various stages of carbonisation, in the strata
beneath our feet. These we propose to deal with individually, in as
unscientific and untechnical a manner as possible.
First of all, when a mass of vegetable matter commences to decay, it soon
loses its colour. There is no more noticeable proof of this, than that
when vitality is withdrawn from the leaves of autumn, they at once
commence to assume a rusty or an ashen colour. Let the leaves but fall to
the ground, and be exposed to the early frosts of October, the damp mists
and rains of November, and the rapid change of colour is at once
apparent. Trodden under foot, they soon assume a dirty blackish hue, and
even when removed they leave a carbonaceous trace of themselves behind
them, where they had rested. Another proof of the rapid acquisition of
their coaly hue is noticeable in the spring of the year. When the trees
have burst forth and the buds are rapidly opening, the cases in which the
buds of such trees as the horse-chestnut have been enclosed will be found
cast off, and strewing the path beneath. Moistened by the rains and the
damp night-mists, and trodden under foot, these cases assume a jet black
hue, and are to all appearance like coal in the very first stages of
formation.
But of course coal is not made up wholly and only of leaves. The branches
of trees, twigs of all sizes, and sometimes whole trunks of trees are
found, the last often remaining in their upright position, and piercing
the strata which have been formed above them. At other times they lie
horizontally on the bed of coal, having been thrown down previously to
the formation of the shale or sandstone, which now rests upon them. They
are often petrified into solid sandstone themselves, whilst leaving a
rind of coal where formerly was the bark. Although the trunk of a tree
looks so very different to the leaves which it bears upon its branches,
it is only naturally to be supposed that, as they are both built up after
the same manner from the juices of the earth and the nourishment in the
atmosphere, they would have a similar chemical composition. One very
palpable proof of the carbonaceous character of tree-trunks suggests
itself. Take in your hand a few dead twigs or sticks from which the
leaves have long since dropped; pull away the dead parts of the ivy which
has been creeping over the summer-house; or clasp a gnarled old monster
of the forest in your arms, and you will quickly find your hand covered
with a black smut, which is nothing but the result of the first stage
which the living plant has made, in its progress towards its condition as
dead coal. But an easy, though rough, chemical proof of the constituents
of wood, can be made by placing a few pieces of wood in a medium-sized
test-tube, and holding it over a flame. In a short time a certain
quantity of steam will be driven off, next the gaseous constituents of
wood, and finally nothing will be left but a few pieces of black brittle
charcoal. The process is of course the same in a fire-grate, only that
here more complete combustion of the wood takes place, owing to its being
intimately exposed to the action of the flames. If we adopt the same
experiment with some pieces of coal, the action is similar, only that in
this case the quantity of gases given off is not so great, coal
containing a greater proportion of carbon than wood, owing to the fact
that, during its long burial in the bowels of the earth, it has been
acted upon in such a way as to lose a great part of its volatile
constituents.
From processes, therefore, which are to be seen going on around us, it is
easily possible to satisfy ourselves that vegetation will in the long run
undergo such changes as will result in the formation of coal.
There are certain parts in most countries, and particularly in Ireland,
where masses of vegetation have undergone a still further stage in
metamorphism, namely, in the well-known and famous peat-bogs. Ireland is
_par excellence_ the land of bogs, some three millions of acres being
said to be covered by them, and they yield an almost inexhaustible supply
of peat. One of the peat-bogs near the Shannon is between two and three
miles in breadth and no less than fifty in length, whilst its depth
varies from 13 feet to as much as 47 feet. Peat-bogs have in no way
ceased to be formed, for at their surfaces the peat-moss grows afresh
every year; and rushes, horse-tails, and reeds of all descriptions grow
and thrive each year upon the ruins of their ancestors. The formation of
such accumulations of decaying vegetation would only be possible where
the physical conditions of the country allowed of an abundant rainfall,
and depressions in the surface of the land to retain the moisture. Where
extensive deforesting operations have taken place, peat-bogs have often
been formed, and many of those in existence in Europe undoubtedly owe
their formation to that destruction of forests which went on under the
sway of the Romans. Natural drainage would soon be obstructed by fallen
trees, and the formation of marsh-land would follow; then with the growth
of marsh-plants and their successive annual decay, a peaty mass would
collect, which would quickly grow in thickness without let or hindrance.
In considering the existence of inland peat-bogs, we must not lose sight
of the fact that there are subterranean forest-beds on various parts of
our coasts, which also rest upon their own beds of peaty matter, and very
possibly, when in the future they are covered up by marine deposits, they
will have fairly started on their way towards becoming coal.
Peat-bogs do not wholly consist of peat, and nothing else. The trunks of
such trees as the oak, yew, and fir, are often found mingled with the
remains of mosses and reeds, and these often assume a decidedly coaly
aspect. From the famous Bog of Allen in Ireland, pieces of oak, generally
known as "bog-oak," which have been buried for generations in peat, have
been excavated. These are as black as any coal can well be, and are
sufficiently hard to allow of their being used in the manufacture of
brooches and other ornamental objects. Another use to which peat of some
kinds has been put is in the manufacture of yarn, the result being a
material which is said to resemble brown worsted. On digging a ditch to
drain a part of a bog in Maine, U.S., in which peat to a depth of twenty
feet had accumulated, a substance similar to cannel coal itself was
found. As we shall see presently, cannel coal is one of the earliest
stages of true coal, and the discovery proved that under certain
conditions as to heat and pressure, which in this case happened to be
present, the materials which form peat may also be metamorphosed into
true coal.
Darwin, in his well-known "Voyage in the _Beagle_" gives a peculiarly
interesting description of the condition of the peat-beds in the Chonos
Archipelago, off the Chilian coast, and of their mode of formation. "In
these islands," he says, "cryptogamic plants find a most congenial
climate, and within the forest the number of species and great abundance
of mosses, lichens, and small ferns, is quite extraordinary. In Tierra
del Fuego every level piece of land is invariably covered by a thick bed
of peat. In the Chonos Archipelago where the nature of the climate more
closely approaches that of Tierra del Fuego, every patch of level ground
is covered by two species of plants (_Astelia pumila_ and _Donatia
megellanica_), which by their joint decay compose a thick bed of elastic
peat.
"In Tierra del Fuego, above the region of wood-land, the former of these
eminently sociable plants is the chief agent in the production of peat.
Fresh leaves are always succeeding one to the other round the central
tap-root; the lower ones soon decay, and in tracing a root downwards in
the peat, the leaves, yet holding their places, can be observed passing
through every stage of decomposition, till the whole becomes blended in
one confused mass. The Astelia is assisted by a few other plants,--here
and there a small creeping Myrtus (_M. nummularia_), with a woody stem
like our cranberry and with a sweet berry,--an Empetrum (_E. rubrum_),
like our heath,--a rush (_Juncus grandiflorus_), are nearly the only ones
that grow on the swampy surface. These plants, though possessing a very
close general resemblance to the English species of the same genera, are
different. In the more level parts of the country the surface of the peat
is broken up into little pools of water, which stand at different
heights, and appear as if artificially excavated. Small streams of water,
flowing underground, complete the disorganisation of the vegetable
matter, and consolidate the whole.
"The climate of the southern part of America appears particularly
favourable to the production of peat. In the Falkland Islands almost
every kind of plant, even the coarse grass which covers the whole surface
of the land, becomes converted into this substance: scarcely any
situation checks its growth; some of the beds are as much as twelve feet
thick, and the lower part becomes so solid when dry that it will hardly
burn. Although every plant lends its aid, yet in most parts the Astelia
is the most efficient.
"It is rather a singular circumstance, as being so very different from
what occurs in Europe, that I nowhere saw moss forming by its decay any
portion of the peat in South America. With respect to the northern limit
at which the climate allows of that peculiar kind of slow decomposition
which is necessary for its production, I believe that in Chiloe (lat. 41 deg.
to 42 deg.), although there is much swampy ground, no well characterised peat
occurs; but in the Chonos Islands, three degrees farther southward, we
have seen that it is abundant. On the eastern coast in La Plata (lat.
35 deg.) I was told by a Spanish resident, who had visited Ireland, that he
had often sought for this substance, but had never been able to find any.
He showed me, as the nearest approach to it which he had discovered, a
black peaty soil, so penetrated with roots as to allow of an extremely
slow and imperfect combustion."
The next stage in the making of coal is one in which the change has
proceeded a long way from the starting-point. _Lignite_ is the name which
has been applied to a form of impure coal, which sometimes goes under the
name of "brown coal." It is not a true coal, and is a very long way from
that final stage to which it must attain ere it takes rank with the most
valuable of earth's products. From the very commencement, an action has
being going on which has caused the amount of the gaseous constituents to
become less and less, and which has consequently caused the carbon
remaining behind to occupy an increasingly large proportion of the whole
mass. So, when we arrive at the lignite stage, we find that a
considerable quantity of volatile matter has already been parted with,
and that the carbon, which in ordinary living wood is about 50 per cent.
of the whole, has already increased to about 67 per cent. In most
lignites there is, as a rule, a comparatively large proportion of
sulphur, and in such cases it is rendered useless as a domestic fuel. It
has been used as a fuel in various processes of manufacture, and the
lignite of the well-known Bovey Tracey beds has been utilised in this way
at the neighbouring potteries. As compared with true coal, it is
distinguished by the abundance of smoke which it produces and the choking
sulphurous fumes which also accompany its combustion, but it is largely
used in Germany as a useful source of paraffin and illuminating oils. In
Silesia, Saxony, and in the district about Bonn, large quantities of
lignite are mined, and used as fuel. Large stores of lignite are known to
exist in the Weald of the south-east of England, and although the mining
operations which were carried on at one time at Heathfield, Bexhill, and
other places, were failures so far as the actual discovery of true coal
was concerned, yet there can be no doubt as to the future value of the
lignite in these parts, when England's supplies of coal approach
exhaustion, and attention is turned to other directions for the future
source of her gas and paraffin oils.
Beside the Bovey Tracey lignitic beds to which we have above referred,
other tertiary clays are found to contain this early promise of coal. The
_eocene_ beds of Brighton are an important instance of a tertiary
lignite, the seam of _surturbrand_, as it is locally called, being a
somewhat extensive deposit.
We have now closely approached to true coal, and the next step which we
shall take will be to consider the varieties in which the black mineral
itself is found. The principal of these varieties are as follows, against
each being placed the average proportion of pure carbon which it
contains:--
Splint or Hard Coal, 83 per cent.;
Cannel, Candle or Parrott Coal, 84 per cent.;
Cherry or Soft Coal, 85 per cent.;
Common Bituminous, or Caking Coal, 88 per cent.;
Anthracite, Blind Coal, Culm, Glance, or Stone Coal, from South
Wales, 93 per cent.
As far as the gas-making properties of the first three are concerned, the
relative proportions of carbon and volatile products are much the same.
Everybody knows a piece of cannel coal when it is seen, how it appears
almost to have been once in a molten condition, and how it breaks with a
conchoidal fracture, as opposed to the cleavage of bituminous coal into
thin layers; and, most apparent and most noticeable of all, how it does
not soil the hands after the manner of ordinary coal. It is at times so
dense and compact that it has been fashioned into ornaments, and is
capable of receiving a polish like jet. From the large percentage of
volatile products which it contains, it is greatly used in gasworks.
Caking coal and the varieties of coal which exist between it and
anthracite, are familiar to every householder; the more it approaches the
composition of the latter the more difficult it is to get it to burn, but
when at last fairly alight it gives out great heat, and what is more
important, a less quantity of volatile constituents in the shape of gas,
smoke, ammonia, ash and sulphurous acid. For this reason it has been
proposed to compel consumers to adopt anthracite as _the_ domestic coal
by Act of Parliament. Certainly by this means the amount of impurities in
the air might be appreciably lessened, but as it would involve the
reconstruction of some millions of fire-places, and an increase in price
in consequence of the general demand for it, it is not likely that a
government would be so rash as to attempt to pass such a measure; even if
passed, it would probably soon become as dead and obsolete and impotent
as those many laws with which our ancestors attempted, first to arrest,
and then to curb the growth in the use of coal of any sort. Anthracite is
not a "homely" coal. If we use it alone it will not give us that bright
and cheerful blaze which English-speaking people like to obtain from
their fires.
It is a significant fact, and one which proves that the various kinds of
coal which are found are nothing but stages begotten by different degrees
of disentanglement of the contained gases, that where, as in some parts,
a mass of basalt has come into contact with ordinary bituminous coal, the
coal has assumed the character of anthracite, whilst the change has in
some instances gone so far as to convert the anthracite into graphite.
The basalt, which is one of the igneous rocks, has been erupted into the
coal-seam in a state of fusion, and the heat contained in it has been
sufficient to cause the disentanglement of the gases, the extraction of
which from the coal brings about the condition of anthracite and
graphite.
The mention of graphite brings us to the next stage. Graphite, plumbago,
or, as it is more commonly called, black-lead, which, we may say in
passing, has nothing of lead about it at all, is best known in the shape
of that very useful and cosmopolitan article, the black-lead pencil. This
is even purer carbon than anthracite, not more than 5 per cent. of ash
and other impurities being present. It is well-known by its grey metallic
lustre; the chemist uses it mixed with fire-clay to make his crucibles;
the engineer uses it, finely powdered, to lubricate his machinery; the
house-keeper uses it to "black-lead" her stoves to prevent them from
rusting. An imperfect graphite is found inside some of the hottest
retorts from which gas is distilled, and this is used as the negative
element in zinc and carbon electricity-making cells, whilst its use as
the electrodes or carbons of the arc-lamp is becoming more and more
widely adopted, as installations of electric light become more general.
One great source of true graphite for many years was the famous mine at
Borrowdale, in Cumberland, but this is now almost exhausted. The vein lay
between strata of slate, and was from eight to nine feet thick. As much
as L100,000 is said to have been realised from it in one year. Extensive
supplies of graphite are found in rocks of the Laurentian age in Canada.
In this formation nothing which can undoubtedly be classed as organic has
yet been discovered. Life at this early period must have found its home
in low and humble forms, and if the _eozooen_ of Dawson, which has been
thought to represent the earliest type of life, turns out after all not
to be organic, but only a deceptive appearance assumed by certain of the
strata, we at least know that it must have been in similarly humble forms
that life, if it existed at all, did then exist. We can scarcely,
therefore, expect that the vegetable world had made any great advance in
complexity of organism at this time, otherwise the supplies of graphite
or plumbago which are found in the formation, would be attributed to
dense forest growths, acted upon, after death, in a similar manner to
that which awaited the vegetation which, ages after, went to form beds of
coal. At present we know of no source of carbon except through the
intervention and the chemical action of plants. Like iron, carbon is
seldom found on the earth except in combination. If there were no growth
of vegetation at this far-away period to give rise to these deposits of
graphite, we are compelled to ask ourselves whether, perchance, there did
not then exist conditions of which we are not now cognisant on the earth,
and which allowed graphite to be formed without assistance from the
vegetable kingdom. At present, however, science is in the dark as to any
other process of its formation, and we are left to assume that the
vegetable growth of the time was enormous in quantity, although there is
nothing to show the kind of vegetation, whether humble mosses or tall
forest trees, which went to constitute the masses of graphite. Geologists
will agree that this is no small assumption to make, since, if true, it
may show that there was an abundance of vegetation at a time when animal
life was hidden in one or more very obscure forms, one only of which has
so far been detected, and whose very identity is strongly doubted by
nearly all competent judges. At the same time there _may_ have been an
abundance of both animal and vegetable life at the time. We must not
forget that it is a well-ascertained fact that in later ages, the minute
seed-spores of forest trees were in such abundance as to form important
seams of coal in the true carboniferous era, the trees which gave birth
to them being now classed amongst the humble _cryptogams_, the ferns, and
club-mosses, &c. The graphite of Laurentian age may not improbably have
been caused by deposits of minute portions of similar lowly specimens of
vegetable life, and if the _eozooen_ the "dawn-animalcule," does represent
the animal life of the time, life whose types were too minute to leave
undoubted traces of their existence, both animal life and vegetable life
may be looked upon as existing side by side in extremely humble forms,
neither as yet having taken an undoubted step forward in advance of the
other in respect to complexity of organism.
[Illustration: FIG 30.--_Lepidodendron_. Portion of Sandstone stem after
removal of bark of a giant club-moss]
There is but one more form of carbon with which we have to deal in
running through the series. We have seen that coal is not the _summum
bonum_ of the series. Other transformations take place after the stage of
coal is reached, which, by the continued disentanglement of gases,
finally bring about the plumbago stage.
What the action is which transforms plumbago or some other form of carbon
into the condition of a diamond cannot be stated. Diamond is the purest
form of carbon found in nature. It is a beautiful object, alike from the
results of its powers of refraction, as also from the form into which its
carbon has been crystallised. How Nature, in her wonderful laboratory,
has precipitated the diamond, with its wonderful powers of spectrum
analysis, we cannot say with certainty. Certain chemists have, at a great
expense, produced crystals which, in every respect, stand the tests of
true diamonds; but the process of their production at a great expense has
in no way diminished the value of the natural product.
The process by which artificial diamonds have been produced is so
interesting, and the subject may prove to be of so great importance, that
a few remarks upon the process may not be unacceptable.
The experiments of the great French chemist, Dumas, and others,
satisfactorily proved the fact, which has ever since been considered
thoroughly established, that the diamond is nothing but carbon
crystallised in nearly a pure state, and many chemists have since been
engaged in the hitherto futile endeavour to turn ordinary carbon into the
true diamond.
Despretz at one time considered that he had discovered the process, which
consisted in his case of submitting a piece of charcoal to the action of
an electric battery, having in his mind the similar process of
electrolysis, by which water is divided up into the two gases, hydrogen
and oxygen. He obtained a microscopic deposit on the poles of the
battery, which he pronounced to be diamond dust, but which, a long time
after, was proved to be nothing but graphite in a crystallised state.
This was, however, certainly a step in the right direction.
The honour of first accomplishing the task fell to Mr Hannay, of Glasgow,
who succeeded in producing very small but comparatively soft diamonds, by
heating lampblack under great pressure, in company with one or two other
ingredients. The process was a costly one, and beyond being a great
scientific feat, the discovery led to little result.
A young French chemist, M. Henri Moissau, has since come to the front,
and the diamonds which he has produced have stood every test for the true
diamond to which they could be subjected; above all, the density of the
product is 3.5, _i.e._, that of the diamond, that of graphite reaching 2
only.
He recognised that in all diamonds which he had consumed--and he consumed
some L150 worth in order to assure himself of the fact--there were always
traces of iron in their composition. He saw that iron in fusion, like
other metals, always dissolves a certain quantity of carbon. Might it not
be that molten iron, cooling in the presence of carbon, deep in volcanic
depths where there was little scope for the iron to expand in assuming
the solid form, would exert such tremendous pressure upon the particles
of carbon which it absorbed, that these would assume the crystalline
state?
He packed a cylinder of soft iron with the carbon of sugar, and placed
the whole in a crucible filled with molten iron, which was raised to a
temperature of 3000 deg. by means of an electric furnace. The soft cylinder
melted, and dissolved a large portion of the carbon. The crucible was
thrown into water, and a mass of solid iron was formed. It was allowed
further to cool in the open air, but the expansion which the iron would
have undergone on cooling, was checked by the crucible which contained
it. The result was a tremendous pressure, during which the carbon, which
was still dissolved, was crystallised into minute diamonds.
These showed themselves as minute points which were easily separable from
the mass by the action of acids. Thus the wonderful transformation from
sugar to the diamond was accomplished.
It should be mentioned that iron, silver, and water, alone possess the
peculiar property of expanding when passing from the liquid to the solid
state.
The diamonds so obtained were of both kinds. The particles of white
diamond resembled in every respect the true brilliant. But there was also
an appreciable quantity of the variety known as the "black diamond."
These diamonds seem to approximate more closely to carbon as we are most
familiar with it. They are not considered as of such value as the
transparent form, but they are still of considerable commercial value.
The _carbonado_, as this kind is called, possesses so great a degree of
hardness that by means of it it is possible to bore through the hardest
rocks. The diamond drill, used for boring purposes, is furnished around
the outer edge of the cylinder of the "boring bit," as it is called, with
perhaps a dozen black diamonds, together with another row of Brazilian
diamonds on the inside. By the rotation of the boring tool the sharp
edges of the diamonds cut their way through rocks of all degrees of
hardness, leaving a core of the rock cut through, in the centre of the
cylindrical drill. It is found that the durability of the natural edge of
the diamond is far greater than that of the edge caused by _artificial_
cutting and trimming. The cutting of a pane of glass by means of a ring
set with an artificially-cut diamond, cannot therefore be done without
injuring to a slight extent the edge of the stone.
The diamond is the hardest of all known substances, leaving a scratch on
any substance across which it may be drawn. Yet it is one whose form can
be changed, and whose hardness can be completely destroyed, by the simple
process of combustion. It can be deprived of its high lustre, and of its
power of breaking up by refraction the light of the sun into the various
tints of the solar spectrum, simply by heating it to a red heat, and then
plunging it into a jar of oxygen gas. It immediately expands, changes
into a coky mass, and burns away. The product left behind is a mixture of
carbon and oxygen, in the proportions in which it is met with in
carbonic-anhydride, or, carbonic acid gas deprived of its water. This is
indeed a strange transformation, from the most valuable of all our
precious stones to a compound which is the same in chemical constituents
as the poisonous gas which we and all animals exhale. But there is this
to be said. Probably in the far-away days when the diamond began to be
formed, the tree or other vegetable product which was its far-removed
ancestor abstracted carbonic acid gas from the atmosphere, just as do our
plants in the present day. By this means it obtained the carbon wherewith
to build up its tissues. Thus the combustion of the diamond into
carbonic-anhydride now is, after all, only a return to the same compound
out of which it was originally formed. How it was formed is a secret:
probably the time occupied in the formation of the diamond may be counted
by centuries, but the time of its re-transformation into a mass of coky
matter is but the work of seconds!
There is another form of carbon which was formerly of much greater
importance than it is now, and which, although not a natural product, is
yet deserving of some notice here. Charcoal is the substance referred to.
In early days the word "coal," or, as it was also spelt, "cole," was
applied to any substance which was used as fuel; hence we have a
reference in the Bible to a "fire of coals," so translated when the
meaning to be conveyed was probably not coal as we know it. Wood was
formerly known as coal, whilst charred wood received the name of
charred-coal, which was soon corrupted into charcoal. The
charcoal-burners of years gone by were a far more flourishing community
than they are now. When the old baronial halls and country-seats depended
on them for the basis of their fuel, and the log was a more frequent
occupant of the fire-grate than now, these occupiers of midforest were a
people of some importance.
We must not overlook the fact that there is another form of charcoal,
namely, animal charcoal or bone-black. This can be obtained by heating
bones to redness in closed iron vessels. In the refining of raw sugar the
discoloration of the syrup is brought about by filtering it through
animal-charcoal; by this means the syrup is rendered colourless.
When properly prepared, charcoal exhibits very distinctly the rings of
annual growth which may have characterised the wood from which it was
formed. It is very light in consequence of its porous nature, and it is
wonderfully indestructible.
But its greatest, because it is its most useful property, is undoubtedly
the power which it has of absorbing great quantities of gas into itself.
It is in fact what may be termed an all-round purifier. It is a
deodoriser, a disinfectant, and a decoloriser. It is an absorbent of bad
odours, and partially removes the smell from tainted meat. It has been
used when offensive manures have been spread over soils, with the same
object in view, and its use for the purification of water is well known
to all users of filters. Some idea of its power as a disinfectant may be
gained by the fact that one volume of wood-charcoal will absorb no less
than 90 volumes of ammonia, 35 volumes of carbonic anhydride, and 65
volumes of sulphurous anhydride.
Other forms of carbon which are well-known are (1) coke, the residue left
when coal has been subjected to a great heat in a closed retort, but from
which all the bye-products of coal have been allowed to escape; (2) soot
and lamp-black, the former of which is useful as a manure in consequence
of ammonia being present in it, whilst the latter is a specially prepared
soot, and is used in the manufacture of Indian ink and printers' ink.
CHAPTER IV.
THE COAL-MINE AND ITS DANGERS.
It is somewhat strange to think that where once existed the solitudes of
an ancient carboniferous forest now is the site of a busy underground
town. For a town it really is. The various roads and passages which are
cut through the solid coal as excavation of a coal-mine proceeds,
represent to a stranger all the intricacies of a well-planned town. Nor
is the extent of these underground towns a thing to be despised. There is
an old pit near Newcastle which contains not less than fifty miles of
passages. Other pits there are whose main thoroughfares in a direct line
are not less than four or five miles in length, and this, it must be
borne in mind, is the result of excavation wrought by human hands and
human labour.
So great an extent of passages necessarily requires some special means of
keeping the air within it in a pure state, such as will render it fit for
the workers to breathe. The further one would go from the main
thoroughfare in such a mine, the less likely one would be to find air of
sufficient purity for the purpose. It is as a consequence necessary to
take some special steps to provide an efficient system of ventilation
throughout the mine. This is effectually done by two shafts, called
respectively the downcast and the upcast shaft. A shaft is in reality a
very deep well, and may be circular, rectangular or oval in form. In
order to keep out water which may be struck in passing through the
various strata, it is protected by plank or wood tubbing, or the shaft is
bricked over, or sometimes even cast-iron segments are sunk. In many
shafts which, owing to their great depth, pass through strata of every
degree of looseness or viscosity, all three methods are utilised in turn.
In Westphalia, where coal is worked beneath strata of more recent
geological age, narrow shafts have been, in many cases, sunk by means of
boring apparatus, in preference to the usual process of excavation, and
the practice has since been adopted in South Wales. In England the usual
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