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While the vegetable origin of coal is beyond question, two rival views
are current among geologists to account for the deposit of ancient
plant material in the form of coal-beds, such as we now find in the
earth. One school of geologists holds that the coal plants grew in
great lagoons and swamps, like the mangrove swamps of today, and that
the modern coal-beds mark the locations of these swamps. From time to
time these areas subsided and were flooded with water to such a depth
that the plants were killed. Eventually the decayed vegetation of the
former swamps was covered with a layer of mud or sand. Later a slow
upheaval of the ground brought these regions again to the surface; a
new swamp formed, only to be submerged again at a later period; and the
same process was repeated several times in the course of hundreds of
thousands of years.

The bulk of evidence seems to favor this view, but there is another.
Perhaps the coal-beds are not the sites of former swamps, but of
estuaries and ocean shores where the plant material settled down,
in still water, after a long drift down the ancient rivers from its
place of origin. It is not impossible that both explanations are
correct; some coal-beds having been formed in one way, and some in the
other. With the progress of time the deposits of sand were compacted
into sandstone, and the mud and clay into shale; while the layers of
vegetation were solidified by pressure, some of their constituents were
vaporized and expelled by heat, and the final product was coal.

The coal-measures abound in fossil plants of species long ago extinct,
and we also find the molds or casts of plants that have themselves
disappeared, leaving only their impressions in the mud by which they
were once enveloped. These records of ancient vegetation are mostly
found in the rocks just above and below the coal-beds, and not in the
coal itself.

The plants of the Carboniferous Period, during which most but not all
of the coal-beds were formed, bore a family likeness to certain kinds
of plants that flourish today. Many of them were ferns, ranging in size
from the smallest species up to great tree-ferns. Others resembled our
modern horsetails or scouring-rushes, with their fluted and jointed
stems, but these _calamites_, as the geologists call them, grew to
the size of trees, sometimes eighty or ninety feet in height. Some
plants of the coal age were like the modern cycads (intermediate in
appearance between tree-ferns and palms); some were like the ginkgo, a
tree with leaves like those of maidenhair fern, widely introduced into
this country from China and Japan. One of the commonest and largest
trees was the _lepidodendron_, closely resembling, except in its vastly
greater size, the club-moss or ground pine which we know so well as a
Christmas decoration.

The animal life of the period, of which, also, abundant fossil remains
are found, included mollusks, fishes, crustaceans, insects, spiders,
thousand-legs, snails, reptiles and lizards. Some of the insects were a
foot or more in length. Of cockroaches, alone, more than five hundred
species have been found in the coal-measures.

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