Another famous tree which grew in the coal-forests was the one
whose roots we found in the floor or underclay of the coal. It
has been called Sigillaria, because it has marks like seals
(sigillum, a seal) all up the trunk, due to the scars left by the
leaves when they fell from the tree. You will see the
Sigillarias on the left-hand side of the coal-forest picture,
having those curious tufts of leaves springing out of them at the
top. Their stems make up a great deal of the coal, and the bark
of their trunks is often found in the clays above, squeezed flat
in lengths of 30, 60, or 70 feet. Sometimes, instead of being
flat the bark is still in the shape of a trunk, and the interior
is filled with sane; and then the trunk is very heavy, and if the
miners do not prop the roof up well it falls down and kills those
beneath it. Stigmaria is the root of the Sigillaria, and is
found in the clays below the coal. Botanists are not yet quite
certain about the seed-cases of this tree, but Mr. Carruthers
believes that they grew inside the base of the leaves, as they do
in the quillwort, a small plant which grows at the bottom of our
mountain lakes.
But what is that curious reed-like stem we found in the piece of
shale (see Fig. 47)? That stem is very important, for it
belonged to a plant called a Calamite, which, as we shall see
presently, helped to sift the earth away from the coal and keep
it pure. This plant was a near relation of the "horsetail," or
Equisetum, which grows in our marshes; only, just as in the case
of the other trees, it was enormously larger, being often 20 feet
high, whereas the little Equisetum, Fig. 52, is seldom more than
a foot, and never more than 4 feet high in England, though in
tropical South America they are much higher. Still, if you have
ever gathered "horsetails," you will see at once that those trees
in the foreground of the picture (Fig. 51), with leaves arranged
in stars round the branches, are only larger copies of the little
marsh-plants; and the seed-vessels of the two plants are almost
exactly the same.
These great trees, the Lepidodendrons, the Sigillarias, and the
Calamites, together with large tree-ferns, are the chief plants
that we know of in the coal-forests. It seems very strange at
first that they should have been so large when their descendants
are now so small, but if you look at our chief plants and trees
now, you will find that nearly all of them bear flowers, and this
is a great advantage to them, because it tempts the insects to
bring them the pollen-dust, as we saw in the last lecture.
Now the Lipidodendrons and their companions had no true flowers,
but only these seed-cases which we have mentioned; but as there
were no flowering plants in their time, and they had the ground
all to themselves, they grew fine and large. By-and-by, however,
when the flowering plants came in, these began to crowd out the
old giants of the coal-forests, so that they dwindled and
dwindled from century to century till their great-great-
grandchildren, thousands of generations after, only lift up their
tiny heads in marshes and on heaths, and tell us that they were
big once upon a time.
And indeed they must have been magnificent in those olden days,
when they grew thick and tall in the lonely marshes where plants
and trees were the chief inhabitants. We find no traces in the
clay-beds of the coal to lead us to suppose that men lived in
those days, nor lions, nor tigers, nor even birds to fly among
the trees; but these grand forests were almost silent, except
when a huge animal something like a gigantic newt or frog went
croaking through the marsh, or a kind of grasshopper chirruped on
the land. But these forms of life were few and far between,
compared to the huge trees and tangled masses of ferns and reeds
which covered the whole ground, or were reflected in the bosom of
the large pools and lakes round about which they grew.
And now, if you have some idea of the plants and trees of the
coal, it is time to ask how these plants became buried in the
earth and made pure coal, instead of decaying away and leaving
behind only a mixture of earth and leaves?
To answer this question, I must ask you to take another journey
with me across the Atlantic to the shores of America, and to land
at Norfolk in Virginia, because there we can see a state of
things something like the marshes of the coal-forests. All round
about Norfolk the land is low, flat, and marshy, and to the south
of the town, stretching far away into North Carolina, is a large,
desolate swamp, no less than forty miles long and twenty-five
broad. The whole place is one enormous quagmire, overgrown with
water-plants and trees. The soil is as black as ink from the
old, dead leaves, grasses, roots, and stems which lie in it; and
so soft, that everything would sink into it, if it were not for
the matted roots of the mosses, ferns, and other plants which
bind it together. You may dig down for ten or fifteen feet, and
find nothing but peat made of the remains of plants which have
lived and died there in succession for ages and ages, while the
black trunks of the fallen trees lie here and there, gradually
being covered up by the dead plants.
The whole place is so still, gloomy, and desolate, that it goes
by the name of the "Great Dismal Swamp," and you see we have here
what might well be the beginning of a bed of coal; for we know
that peat when dried becomes firm and makes an excellent fire,
and that if it were pressed till it was hard and solid it would
not be unlike coal. If, then, we can explain how this peaty bed
has been kept pure from earth, we shall be able to understand how
a coal-bed may have been formed, even though the plants and trees
which grow in this swamp are different from those which grew in
the coal-forests.
The explanation is not difficult; streams flow constantly, or
rather ooze into the Great Dismal Swamp from the land that lies
to the west, but instead of bringing mud in with them as rivers
bring to the sea, they bring only clear, pure water, because, as
they filter for miles through the dense jungle of reeds, ferns,
and shrubs which grow round the marsh, all the earth is sifted
out and left behind. In this way the spongy mass of dead plants
remains free from earthy grains, while the water and the shade of
the thick forest of trees prevent the leaves, stems, etc., from
being decomposed by the air and sun. And so year after year as
the plants die they leave their remains for other plants to take
root in, and the peaty mass grows thicker and thicker, while tall
cedar trees and evergreens live and die in these vast, swampy
forests, and being in loose ground are easily blown down by the
wind, and leave their trunks to be covered up by the growing moss
and weeds.
Now we know that there were plenty of ferns and of large
Calamites growing thickly together in the coal-forests, for we
find their remains everywhere in the clay, so we can easily
picture to ourselves how the dense jungle formed by these plants
would fringe the coal-swamp, as the present plants do the Great
Dismal Swamp, and would keep out all earthy matter, so that year
after year the plants would die and form a thick bed of peat,
afterwards to become coal.