The Story of a Piece of Coal | Page 3

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