exhibit a thin layer of dull, charred-looking substance, which is known
as "mineral charcoal."
Occasionally one of the faces of a lump of coal will present impressions,
which are obviously those of the stem, or leaves, of a plant; but though
hard mineral masses of pyrites, and even fine mud, may occur here and
there, neither sand nor pebbles are met with.
When the coal burns, the chief ultimate products of its combustion are
carbonic acid, water, and ammoniacal products, which escape up the
chimney; and a greater or less amount of residual earthy salts, which
take the form of ash. These products are, to a great extent, such as
would result from the burning of so much wood.
These properties of coal may be made out without any very refined
appliances, but the microscope reveals something more. Black and opaque
as ordinary coal is, slices of it become transparent if they are cemented
in Canada balsam, and rubbed down very thin, in the ordinary way of
making thin sections of non-transparent bodies. But as the thin slices,
made in this way, are very apt to crack and break into fragments, it is
better to employ marine glue as the cementing material. By the use of
this substance, slices of considerable size and of extreme thinness and
transparency may be obtained.[1]
[Footnote 1: My assistant in the Museum of Practical Geology, Mr. Newton,
invented this excellent method of obtaining thin slices of coal.]
Now let us suppose two such slices to be prepared from our lump of coal--
one parallel with the bedding, the other perpendicular to it; and let us
call the one the horizontal, and the other the vertical, section. The
horizontal section will present more or less rounded yellow patches and
streaks, scattered irregularly through the dark brown, or blackish,
ground substance; while the vertical section will exhibit mere elongated
bars and granules of the same yellow materials, disposed in lines which
correspond, roughly, with the general direction of the bedding of the
coal.
This is the microscopic structure of an ordinary piece of coal. But if a
great series of coals, from different localities and seams, or even from
different parts of the same seam, be examined, this structure will be
found to vary in two directions. In the anthracitic, or stone-coals,
which burn like coke, the yellow matter diminishes, and the ground
substance becomes more predominant, blacker, and more opaque, until it
becomes impossible to grind a section thin enough to be translucent;
while, on the other hand, in such as the "Better-Bed" coal of the
neighbourhood of Bradford, which burns with much flame, the coal is of a
far lighter, colour and transparent sections are very easily obtained. In
the browner parts of this coal, sharp eyes will readily detect multitudes
of curious little coin-shaped bodies, of a yellowish brown colour,
embedded in the dark brown ground substance. On the average, these little
brown bodies may have a diameter of about one-twentieth of an inch. They
lie with their flat surfaces nearly parallel with the two smooth faces of
the block in which they are contained; and, on one side of each, there
may be discerned a figure, consisting of three straight linear marks,
which radiate from the centre of the disk, but do not quite reach its
circumference. In the horizontal section these disks are often converted
into more or less complete rings; while in the vertical sections they
appear like thick hoops, the sides of which have been pressed together.
The disks are, therefore, flattened bags; and favourable sections show
that the three-rayed marking is the expression of three clefts, which
penetrate one wall of the bag.
The sides of the bags are sometimes closely approximated; but, when the
bags are less flattened, their cavities are, usually, filled with
numerous, irregularly rounded, hollow bodies, having the same kind of
wall as the large ones, but not more than one seven-hundredth of an inch
in diameter.
In favourable specimens, again, almost the whole ground substance appears
to be made up of similar bodies--more or less carbonized or blackened--
and, in these, there can be no doubt that, with the exception of patches
of mineral charcoal, here and there, the whole mass of the coal is made
up of an accumulation of the larger and of the smaller sacs.
But, in one and the same slice, every transition can be observed from
this structure to that which has been described as characteristic of
ordinary coal. The latter appears to rise out of the former, by the
breaking-up and increasing carbonization of the larger and the smaller
sacs. And, in the anthracitic coals, this process appears to have gone to
such a length, as to destroy the original structure altogether, and to
replace it by a completely carbonized substance.
Thus coal may be said, speaking broadly, to be composed of two
constituents: firstly, mineral charcoal; and, secondly, coal proper. The
nature of the mineral charcoal has long since been determined. Its
structure shows it to consist of the remains of the stems and leaves of
plants, reduced a little more than their carbon. Again, some of the coal
is made up of the crushed and flattened bark, or outer coat, of the stems
of plants, the inner wood of which has completely decayed away. But what
I may term the "saccular matter" of the coal, which, either in its
primary or in its degraded form constitutes by far the greater part of
all the bituminous coals I have examined, is certainly not mineral
charcoal; nor is its structure that of any stem or leaf. Hence its real
nature is at first by no means apparent, and has been the subject of much
discussion.
The first person who threw any light upon the problem, as far as I have
been able to discover, was the well-known geologist, Professor Morris. It
is now thirty-four years since he carefully described and figured the
coin-shaped bodies, or larger sacs, as I have called them, in a note
appended to the famous paper "On the Coalbrookdale Coal-Field," published
at that time, by the present President of the Geological Society, Mr.
Prestwich. With much sagacity, Professor Morris divined the real nature
of these bodies, and boldly affirmed them to be the spore-cases of a
plant allied to the living club-mosses.
But discovery sometimes makes a long halt; and it is only a few years
since Mr. Carruthers determined the plant (or rather one of the plants)
which produces these spore-cases, by finding the discoidal sacs still
adherent to the leaves of the fossilized cone which produced them. He
gave the name of _Flemingites gracilis_ to the plant of which the cones
form a part. The branches and stem of this plant are not yet certainly
known, but there is no sort of doubt that it was closely allied to the
_Lepidodendron_, the remains of which abound in the coal formation. The
_Lepidodendra_ were shrubs and trees which put one more in mind of an
_Araucaria_ than of any other familiar plant; and the ends of the
fruiting branches were terminated by cones, or catkins, somewhat like the
bodies so named in a fir, or a willow. These conical fruits, however, did
not produce seeds; but the leaves of which they were composed bore upon
their surfaces sacs full of spores or sporangia, such as those one sees
on the under surface of a bracken leaf. Now, it is these sporangia of the
Lepidodendroid plant _Flemingites_ which were identified by Mr.
Carruthers with the free sporangia described by Professor Morris, which
are the same as the large sacs of which I have spoken. And, more than
this, there is no doubt that the small sacs are the spores, which were
originally contained in the sporangia.
The living club-mosses are, for the most part, insignificant and creeping
herbs, which, superficially, very closely resemble true mosses, and none
of them reach more than two or three feet in height. But, in their
essential structure, they very closely resemble the earliest
Lepidodendroid trees of the coal: their stems and leaves are similar; so
are their cones; and no less like are the sporangia and spores; while
even in their size, the spores of the _Lepidodendron_ and those of the
existing _Lycopodium_, or club-moss, very closely approach one another.
Thus, the singular conclusion is forced upon us, that the greater and the
smaller sacs of the "Better-Bed" and other coals, in which the primitive
structure is well preserved, are simply the sporangia and spores of
certain plants, many of which were closely allied to the existing club-
mosses. And if, as I believe, it can be demonstrated that ordinary coal
is nothing but "saccular" coal which has undergone a certain amount of
that alteration which, if continued, would convert it into anthracite;
then, the conclusion is obvious, that the great mass of the coal we burn
is the result of the accumulation of the spores and spore-cases of
plants, other parts of which have furnished the carbonized stems and the
mineral charcoal, or have left their impressions on the surfaces of the
layer.
Of the multitudinous speculations which, at various times, have been
entertained respecting the origin and mode of formation of coal, several
appear to be negatived, and put out of court, by the structural facts the
significance of which I have endeavoured to explain. These facts, for
example, do not permit us to suppose that coal is an accumulation of
peaty matter, as some have held.
Again, the late Professor Quekett was one of the first observers who gave
a correct description of what I have termed the "saccular" structure of
coal; and, rightly perceiving that this structure was something quite
different from that of any known plant, he imagined that it proceeded
from some extinct vegetable organism which was peculiarly abundant
amongst the coal-forming plants. But this explanation is at once shown to
be untenable when the smaller and the larger sacs are proved to be spores
or sporangia.
Some, once more, have imagined that coal was of submarine origin; and
though the notion is amply and easily refuted by other considerations, it
may be worth while to remark, that it is impossible to comprehend how a
mass of light and resinous spores should have reached the bottom of the
sea, or should have stopped in that position if they had got there.
At the same time, it is proper to remark that I do not presume to suggest
that all coal must needs have the same structure; or that there may not
be coals in which the proportions of wood and spores, or spore-cases, are
very different from those which I have examined. All I repeat is, that
none of the coals which have come under my notice have enabled me to
observe such a difference. But, according to Principal Dawson, who has so
sedulously examined the fossil remains of plants in North America, it is
otherwise with the vast accumulations of coal in that country.
"The true coal," says Dr. Dawson, "consists principally of the flattened
bark of Sigillarioid and other trees, intermixed with leaves of Ferns and
_Cordaites_, and other herbaceous _débris_, and with fragments of decayed
wood, constituting 'mineral charcoal,' all these materials having
manifestly alike grown and accumulated where we find them."[2]
[Footnote 2: _Acadian Geology_, 2nd edition, p. 135.]
When I had the pleasure of seeing Principal Dawson in London last summer,
I showed him my sections of coal, and begged him to re-examine some of
the American coals on his return to Canada, with an eye to the presence
of spores and sporangia, such as I was able to show him in our English
and Scotch coals. He has been good enough to do so; and in a letter dated
September 26th, 1870, he informs me that--
"Indications of spore-cases are rare, except in certain coarse shaly
coals and portions of coals, and in the roofs of the seams. The most
marked case I have yet met with is the shaly coal referred to as
containing _Sporangites_ in my paper on the conditions of accumulation of
coal ("Journal of the Geological Society," vol. xxii. pp. 115, 139, and
165). The purer coals certainly consist principally of cubical tissues
with some true woody matter, and the spore-cases, &c., are chiefly in the
coarse and shaly layers. This is my old doctrine in my two papers in the
"Journal of the Geological Society," and I see nothing to modify it. Your
observations, however, make it probable that the frequent _clear spots_
in the cannels are spore-cases."
Dr. Dawson's results are the more remarkable, as the numerous specimens
of British coal, from various localities, which I have examined, tell one
tale as to the predominance of the spore and sporangium element in their
composition; and as it is exactly in the finest and purest coals, such as
the "Better-Bed" coal of Lowmoor, that the spores and sporangia obviously
constitute almost the entire mass of the deposit.
Coal, such as that which has been described, is always found in sheets,
or "seams," varying from a fraction of an inch to many feet in thickness,
enclosed in the substance of the earth at very various depths, between
beds of rock of different kinds. As a rule, every seam of coal rests upon
a thicker, or thinner, bed of clay, which is known as "under-clay." These
alternations of beds of coal, clay, and rock may be repeated many times,
and are known as the "coal-measures"; and in some regions, as in South
Wales and in Nova Scotia, the coal-measures attain a thickness of twelve
or fourteen thousand feet, and enclose eighty or a hundred seams of coal,
each with its under-clay, and separated from those above and below by
beds of sandstone and shale.
The position of the beds which constitute the coal-measures is infinitely
diverse. Sometimes they are tilted up vertically, sometimes they are
horizontal, sometimes curved into great basins; sometimes they come to
the surface, sometimes they are covered up by thousands of feet of rock.
But, whatever their present position, there is abundant and conclusive
evidence that every under-clay was once a surface soil. Not only do
carbonized root-fibres frequently abound in these under-clays; but the
stools of trees, the trunks of which are broken off and confounded with
the bed of coal, have been repeatedly found passing into radiating roots,
still embedded in the under-clay. On many parts of the coast of England,
what are commonly known as "submarine forests" are to be seen at low
water. They consist, for the most part, of short stools of oak, beech,
and fir-trees, still fixed by their long roots in the bed of blue clay in
which they originally grew. If one of these submarine forest beds should
be gradually depressed and covered up by new deposits, it would present
just the same characters as an under-clay of the coal, if the
_Sigillaria_ and _Lepidodendron_ of the ancient world were substituted
for the oak, or the beech, of our own times.
In a tropical forest, at the present day, the trunks of fallen trees, and
the stools of such trees as may have been broken by the violence of
storms, remain entire for but a short time. Contrary to what might be
expected, the dense wood of the tree decays, and suffers from the ravages
of insects, more swiftly than the bark. And the traveller, setting his
foot on a prostrate trunk, finds that it is a mere shell, which breaks
under his weight, and lands his foot amidst the insects, or the reptiles,
which have sought food or refuge within.
The trees of the coal forests present parallel conditions. When the
fallen trunks which have entered into the composition of the bed of coal
are identifiable, they are mere double shells of bark, flattened together
in consequence of the destruction of the woody core; and Sir Charles
Lyell and Principal Dawson discovered, in the hollow stools of coal trees
of Nova Scotia, the remains of snails, millipedes, and salamander-like
creatures, embedded in a deposit of a different character from that which
surrounded the exterior of the trees. Thus, in endeavouring to comprehend
the formation of a seam of coal, we must try to picture to ourselves a
thick forest, formed for the most part of trees like gigantic club-
mosses, mares'-tails, and tree-ferns, with here and there some that had
more resemblance to our existing yews and fir-trees. We must suppose
that, as the seasons rolled by, the plants grew and developed their
spores and seeds; that they shed these in enormous quantities, which
accumulated on the ground beneath; and that, every now and then, they
added a dead frond or leaf; or, at longer intervals, a rotten branch, or
a dead trunk, to the mass.
A certain proportion of the spores and seeds no doubt fulfilled their
obvious function, and, carried by the wind to unoccupied regions,
extended the limits of the forest; many might be washed away by rain into
streams, and be lost; but a large portion must have remained, to
accumulate like beech-mast, or acorns, beneath the trees of a modern
forest.
But, in this case it may be asked, why does not our English coal consist
of stems and leaves to a much greater extent than it does? What is the
reason of the predominance of the spores and spore-cases in it?
A ready answer to this question is afforded by the study of a living
full-grown club-moss. Shake it upon a piece of paper, and it emits a
cloud of fine dust, which falls over the paper, and is the well-known
Lycopodium powder. Now this powder used to be, and I believe still is,
employed for two objects which seem, at first sight, to have no
particular connection with one another. It is, or was, employed in making
lightning, and in making pills. The coats of the spores contain so much
resinous matter, that a pinch of Lycopodium powder, thrown through the
flame of a candle, burns with an instantaneous flash, which has long done
duty for lightning on the stage. And the same character makes it a
capital coating for pills; for the resinous powder prevents the drug from
being wetted by the saliva, and thus bars the nauseous flavour from the
sensitive papilla; of the tongue.
But this resinous matter, which lies in the walls of the spores and
sporangia, is a substance not easily altered by air and water, and hence
tends to preserve these bodies, just as the bituminized cerecloth
preserves an Egyptian mummy; while, on the other hand, the merely woody
stem and leaves tend to rot, as fast as the wood of the mummy's coffin
has rotted. Thus the mixed heap of spores, leaves, and stems in the coal-
forest would be persistently searched by the long-continued action of air
and rain; the leaves and stems would gradually be reduced to little but
their carbon, or, in other words, to the condition of mineral charcoal in
which we find them; while the spores and sporangia remained as a
comparatively unaltered and compact residuum.
There is, indeed, tolerably clear evidence that the coal must, under some
circumstances, have been converted into a substance hard enough to be
rolled into pebbles, while it yet lay at the surface of the earth; for in
some seams of coal, the courses of rivulets, which must have been living
water, while the stratum in which their remains are found was still at
the surface, have been observed to contain rolled pebbles of the very
coal through which the stream has cut its way.
The structural facts are such as to leave no alternative but to adopt the
view of the origin of such coal as I have described, which has just been
stated; but, happily, the process is not without analogy at the present
day. I possess a specimen of what is called "white coal" from Australia.
It is an inflammable material, burning with a bright flame and having
much the consistence and appearance of oat-cake, which, I am informed
covers a considerable area. It consists, almost entirely, of a compacted
mass of spores and spore-cases. But the fine particles of blown sand
which are scattered through it, show that it must have accumulated,
subaërially, upon the surface of a soil covered by a forest of
cryptogamous plants, probably tree-ferns.
As regards this important point of the subaërial region of coal, I am
glad to find myself in entire accordance with Principal Dawson, who bases
his conclusions upon other, but no less forcible, considerations. In a
passage, which is the continuation of that already cited, he writes:--
"(3) The microscopical structure and chemical composition of the beds of
cannel coal and earthy bitumen, and of the more highly bituminous and
carbonaceous shale, show them to have been of the nature of the fine
vegetable mud which accumulates in the ponds and shallow lakes of modern
swamps. When such tine vegetable sediment is mixed, as is often the case,
with clay, it becomes similar to the bituminous limestone and calcareo-
bituminous shales of the coal-measures. (4) A few of the under-clays,
which support beds of coal, are of the nature of the vegetable mud above
referred to; but the greater part are argillo-arenaceous in composition,
with little vegetable matter, and bleached by the drainage from them of
water containing the products of vegetable decay. They are, in short,
loamy or clay soils, and must have been sufficiently above water to admit
of drainage. The absence of sulphurets, and the occurrence of carbonate
of iron in connection with them, prove that, when they existed as soils,
rain-water, and not sea-water, percolated them. (5) The coal and the
fossil forests present many evidences of subaërial conditions. Most of
the erect and prostrate trees had become hollow shells of bark before
they were finally embedded, and their wood had broken into cubical pieces
of mineral charcoal. Land-snails and galley-worms (_Xylobius_) crept into
them, and they became dens, or traps, for reptiles. Large quantities of
mineral charcoal occur on the surface of all the large beds of coal. None
of these appearances could have been produced by subaqueous action. (6)
Though the roots of the _Sigillaria_ bear more resemblance to the
rhizomes of certain aquatic plants; yet, structurally, they are
absolutely identical with the roots of Cycads, which the stems also
resemble. Further, the _Sigillarioe_ grew on the same soils which
supported Conifers, _Lepidodendra_, _Cordaites_, and Ferns-plants which
could not have grown in water. Again, with the exception perhaps of some
_Pinnularioe_, and _Asterophyllites_, there is a remarkable absence from
the coal measures of any form of properly aquatic vegetation. (7) The
occurrence of marine, or brackish-water animals, in the roofs of coal-
beds, or even in the coal itself, affords no evidence of subaqueous
accumulation, since the same thing occurs in the case of modern submarine
forests. For these and other reasons, some of which are more fully stated
in the papers already referred to, while I admit that the areas of coal
accumulation were frequently submerged, I must maintain that the true
coal is a subaërial accumulation by vegetable growth on soils, wet and
swampy it is true, but not submerged."
I am almost disposed to doubt whether it is necessary to make the
concession of "wet and swampy"; otherwise, there is nothing that I know
of to be said against this excellent conspectus of the reasons for
believing in the subaërial origin of coal.
But the coal accumulated upon the area covered by one of the great
forests of the carboniferous epoch would in course of time, have been
wasted away by the small, but constant, wear and tear of rain and streams
had the land which supported it remained at the same level, or been
gradually raised to a greater elevation. And, no doubt, as much coal as
now exists has been destroyed, after its formation, in this way. What are
now known as coal districts owe their importance to the fact that they
were areas of slow depression, during a greater or less portion of the
carboniferous epoch; and that, in virtue of this circumstance, Mother
Earth was enabled to cover up her vegetable treasures, and preserve them
from destruction.