Edward A. Martin.

The Story of a Piece of Coal What It Is, Whence It Comes, and Whither It Goes online

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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, Saärbruck, 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°
to 42°), 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°) 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


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Online LibraryEdward A. MartinThe Story of a Piece of Coal What It Is, Whence It Comes, and Whither It Goes → online text (page 4 of 10)