However, we will leave for the present the fascinating pursuit of
theorising as to the how and wherefore of these vast beds of coal,
relegating the geological part of the study of the carboniferous system
to a future chapter, where will be found some more detailed account of
the position of the coal-seams in the strata which contain them. At
present the actual details of the coal itself will demand our attention.
Coal is the mineral which has resulted, after the lapse of thousands of
thousands of years, from the accumulations of vegetable material, caused
by the steady yearly shedding of leaves, fronds and spores, from forests
which existed in an early age; these accumulated where the trees grew
that bore them, and formed in the first place, perhaps, beds of peat; the
beds have since been subjected to an ever-increasing pressure of
accumulating strata above them, compressing the sheddings of a whole
forest into a thickness in some cases of a few inches of coal, and have
been acted upon by the internal heat of the earth, which has caused them
to part, to a varying degree, with some of their component gases. If we
reason from analogy, we are compelled to admit that the origin of coal is
due to the accumulation of vegetation, of which more scattered, but more
distinct, representative specimens occur in the shales and clays above
and below the coal-seams. But we are also able to examine the texture
itself of the various coals by submitting extremely thin slices to a
strong light under the microscope, and are thus enabled to decide whether
the particular coal we are examining is formed of conifers, horse-tails,
club-mosses, or ferns, or whether it consists simply of the accumulated
sheddings of all, or perhaps, as in some instances, of innumerable
spores.
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