As a general rule, each bed of coal rests upon a bed of shale or
clay, which 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.
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