The Story of a Piece of Coal | Page 2

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