wiftly 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
res