Book: The Story of a Piece of Coal

E >> Edward A. Martin >> The Story of a Piece of Coal




[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.

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