Let the leaves but fall to
the ground, and be exposed to the early frosts of October, the damp mists
and rains of November, and the rapid change of colour is at once
apparent. Trodden under foot, they soon assume a dirty blackish hue, and
even when removed they leave a carbonaceous trace of themselves behind
them, where they had rested. Another proof of the rapid acquisition of
their coaly hue is noticeable in the spring of the year. When the trees
have burst forth and the buds are rapidly opening, the cases in which the
buds of such trees as the horse-chestnut have been enclosed will be found
cast off, and strewing the path beneath. Moistened by the rains and the
damp night-mists, and trodden under foot, these cases assume a jet black
hue, and are to all appearance like coal in the very first stages of
formation.
But of course coal is not made up wholly and only of leaves. The branches
of trees, twigs of all sizes, and sometimes whole trunks of trees are
found, the last often remaining in their upright position, and piercing
the strata which have been formed above them. At other times they lie
horizontally on the bed of coal, having been thrown down previously to
the formation of the shale or sandstone, which now rests upon them. They
are often petrified into solid sandstone themselves, whilst leaving a
rind of coal where formerly was the bark. Although the trunk of a tree
looks so very different to the leaves which it bears upon its branches,
it is only naturally to be supposed that, as they are both built up after
the same manner from the juices of the earth and the nourishment in the
atmosphere, they would have a similar chemical composition.
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