I have here a piece of coal (Fig. 45), which, though it has been
cut with some care so as to have a smooth face, is really in no
other way different from any ordinary lump which you can pick for
yourself out of the coal-scuttle. Our work to-day is to relate
the history of this black lump; to learn what it is, what it has
been, and what it will be.
It looks uninteresting enough at first sight, and yet if we
examine it closely we shall find some questions to ask even about
its appearance. Look at the smooth face of this specimen and see
if you can explain those fine lines which run across so close
together as to look like the edges of the leaves of a book. Try
to break a piece of coal, and you will find that it will split
much more easily along those lines than across the other way of
the lump; and if you wish to light a fire quickly you should
always put this lined face downwards so that the heat can force
its way up through these cracks and gradually split up the block.
Then again if you break the coal carefully along one of these
lines you will find a fine film of charcoal lying in the crack,
and you will begin to suspect that this black coal must have been
built up in very thin layers, with a kind of black dust between
them.
The next thing you will call to mind is that this coal burns and
gives flame and heat, and that this means that in some way
sunbeams are imprisoned in it; lastly, this will lead you to
think of plants, and how they work up the strength of the
sunbeams into their leaves, and hide black carbon in even the
purest and whitest substance they contain.
Is coal made of burnt plants, then? Not burnt ones, for if so it
would not burn again; but you may have read how the makers of
charcoal take wood and bake it without letting it burn, and then
it turns black and will afterwards make a very good fire; and so
you will see that it is probable that our piece of coal is made
of plants which have been baked and altered, but which have still
much sunbeam strength bottled up in them, which can be set free
as they burn.
If you will take an imaginary journey with me to a coal-pit near
Newcastle, which I visited many years ago, you will see that we
have very good evidence that coal is made of plants, for in all
coal-mines we find remains of them at every step we take.
Let us imagine that we have put on old clothes which will not
spoil, and have stepped into the iron basket (see Fig. 46) called
by the miners a cage, and are being let down the shaft to the
gallery where the miners are at work. Most of them will probably
be in the gallery b, because a great deal of the coal in a has
been already taken out. But we will stop in a because there we
can see a great deal of the roof and the floor. When we land on
the floor of the gallery we shall find ourselves in a kind of
tunnel with railway lines laid along it and trucks laden with
coal coming towards the cage to be drawn up, while empty ones are
running back to be loaded where the miners are at work. Taking
lamps in our hands and keeping out of the way of the trucks, we
will first throw the light on the roof, which is made of shale or
hardened clay. We shall not have gone many yards before we see
impressions of plants in the shale, like those in this specimen
(Fig. 47), which was taken out of a coal-mine at Neath in
Glamorganshire, a few days ago, and sent up for this lecture.
You will recognize at once the marks of ferns (a), for they look
like those you gather in the hedges of an ordinary country lane,
and that long striped branch (b) does not look unlike a reed, and
indeed it is something of this kind, as we shall see by-and-by.
You will find plenty of these impressions of plants as you go
along the gallery and look up at the roof, and with them there
will be others with spotted stems, or with stems having a curious
diamond pattern upon them, and many ferns of various kinds.
Next look down at your feet and examine the floor. You will not
have to search long before you will almost certainly find a piece
of stone like that represented in Fig. 48, which has also come
from Neath Colliery. This fossil, which is the cast of a piece
of a plant, puzzled those who found it for a very long time. At
last, however, Mr. Binney found the specimen growing to the
bottom of the trunk of one of the fossil trees with spotted
stems, called Sigillaria; and so proved that this curious pitted
stone is a piece of fossil root, or rather underground stem, like
that which we found in the primrose, and that the little pits or
dents in it are scars where the rootlets once were given off.
Whole masses of these root-stems, with ribbon-like roots lying
scattered near them, are found buried in the layer of clay called
the underclay which makes the floor of the coal, and they prove
to us that this underclay must have been once the ground in which
the roots of the coal-plants grew. You will feel still more sure
of this when you find that there is not only one straight gallery
of coal, but that galleries branch out right and left, and that
everywhere you find the coal lying like a sandwich between the
floor and the roof, showing that quite a large piece of country
must be covered by these remains of plants all rooted in the
underclay.
But how about the coal itself? It seems likely, when we find
roots below and leaves and stems above, that the middle is made
of plants, but can we prove it? We shall see presently that it
has been so crushed and altered by being buried deep in the
ground that the traces of leaves have almost been destroyed,
though people who are used to examining with the microscope, can
see the crushed remains of plants in thin slices of coal.
But fortunately for us, perfect pieces of plants have been
preserved even in the coal-bed itself. Do you remember our
learning in Lecture IV, that water with lime in it petrifies
things, that is, leaves carbonate of lime to fill up grain by
grain the fibres of an animal or plant as the living matter
decays, and so keeps an exact representation of the object?
Now, it so happens that in a coal-bed at South Ouram, near
Halifax, as well as in some other places, carbonate of lime
trickled in before the plants were turned into coal, and made
some round nodules in the plant-bed, which look like cannon-
balls. Afterwards, when all the rest of the bed was turned into
coal, these round balls remained crystallized, and by cutting
thin transparent slices across the nodule we can distinctly see
the leaves and stems and curious little round bodies which make
up the coal. Several such sections may be seen at the British
Museum, and when we compare these fragments of plants with those
which we find above and below the coal-bed, we find that they
agree, thus proving that coal is made of plants, and of those
plants whose roots grew in the clay floor, while their heads
reached up far above where the roof now is.
The next question is, what kind of plants were these? Have we
anything like them living in the world now? You might perhaps
think that it would be impossible to decide this question from
mere petrified pieces of plants. But many men have spent their
whole lives in deciphering all the fragments that could be found,
and though the section given in Fig. 49 may look to you quite
incomprehensible, yet a botanist can reed it as we read a book.
For example, at S and L, where stems are cut across, he can learn
exactly how they were build up inside, and compare them with the
stems of living plants, while the fruits cc and the little round
spores lying near them, tell him their history as well as if he
had gathered them from the tree. In this way we have learnt to
know very fairly what the plants of the coal were like, and you
will be surprised when I tell you that the huge trees of the
coal-forests, of which we sometimes find trunks in the coal-mines
from ten to fifty feet long, are only represented on the earth
now by small insignificant plants, scarcely ever more than two
feet, and often not many inches high.
Have you ever seen the little club moss or Lycopodium which grows
all over England, but chiefly in the north, on heaths and
mountains? At the end of each of its branches it bears a cone
made of scaly leaves; and fixed to the inside of each of these
leaves is a case called a sporangium, full of little spores or
moss-seeds, as we may call them, though they are not exactly like
true seeds. In one of these club-mosses called Selaginella, the
cases near the bottom of the cone contain large spores, while
those near the top contain a powdery dust. These spores are full
of resin, and they are collected on the Continent for making
artificial lightning in the theatres, because they flare when
lighted.
Now this little Selaginella is of all living plants the one most
like some of the gigantic trees of the coal-forests. If you look
at this picture of a coal-forest (Fig. 51), you will find it
difficult perhaps to believe that those great trees, with diamond
markings all up the trunk, hanging over from the right to the
left of the picture, and covering all the top with their boughs,
could be in any way relations of the little Selaginella; yet we
find branches of them in the beds above the coal, bearing cones
larger but just like Selaginella cones; and what is most curious,
the spores in these cones are of exactly the same kind and not
any larger than those of the club-mosses.
These trees are called by botanists Lepidodendrons, or scaly
trees; there are numbers of them in all coal-mines, and one trunk
has been found 49 feet long. Their branches were divided in a
curious forked manner and bore cones at the ends. The spores
which fell from these cones are found flattened in the coal, and
they may be seen scattered about in the coal-ball.