ch is termed the "under-clay," and in which are found
numerous roots of plants; whilst the strata immediately on the
top of the coal may be shaly or sandy, but in either case are
generally charged with the leaves and stems of plants, and often
have upright trunks passing vertically through them. When we
add to this that the coal itself is, chemically, nearly wholly
composed of carbon, and that its microscopic structure shows it
to be composed almost entirely of fragments of stems, leaves,
bark, seeds, and vegetable _debris_ derived from _land-plants_,
we are readily enabled to understand how the coal was formed.
The "_under-clay_" immediately beneath the coal-bed represents
an old land-surface--sometimes, perhaps, the bottom of a swamp
or marsh, covered with a luxuriant vegetation; the _coal bed_
itself represents the slow accumulation, through long periods,
of the leaves, seeds, fruits, stems, and fallen trunks of this
vegetation, now hardened and compressed into a fraction of its
original bulk by the pressure of the superincumbent rocks; and
the strata of sand or shale above the coal-bed--the so-called
"roof" of the coal--represent sediments quietly deposited as the
land, after a long period of repose, commenced to sink beneath
the sea. On this view, the rank and long-continued vegetation
which gave rise to each coal-bed was ultimately terminated by
a slow depression of the surface on which the plants grew. The
land-surface then became covered by the water, and aqueous sediments
were accumulated to a greater or less thickness upon the dense
mass of decaying vegetation below, enveloping any trunks of trees
which might still be in an erect position, and preserving between
their layers the leaves and branches of plants brought down from
the neighbouring land by streams, or blown into the wafer by the
wind. Finally, there set in a slow movement of elevation,--the
old land again reappeared above the water; a new and equally
luxuriant vegetation flourished upon the new land-surface; and
another coal-bed was accumulated, to be preserved ultimately in
a similar fashion. Some few beds of coal may have been formed by
drifted vegetable matter brought down into the ocean by rivers, and
deposited directly on the bottom of the sea; but in the majority
of cases the coal is undeniably the result of the slow growth and
decay of plants _in situ_: and as the plants of the coal are
not _marine_ plants, it is necessary to adopt some such