Carbon
derived from the decomposition of animal bodies is not uncommon;
though it never occurs in such quantity from this source as it
may do when it is derived from plants. Thus, many limestones are
more or less highly bituminous; the celebrated siliceous flags
or so-called "bituminous schists" of Caithness are impregnated
with oily matter apparently derived from the decomposition of the
numerous fishes embedded in them; Silurian shales containing
Graptolites, but destitute of plants, are not uncommonly
"anthracitic," and contain a small percentage of carbon derived
from the decay of these zoophytes; whilst the petroleum so largely
worked in North America has not improbably an animal origin.
That the fatty compounds present in animal bodies should more or
less extensively impregnate fossiliferous rock-masses, is only
what might be expected; but the great bulk of the carbon which
exists stored up in the earth's crust is derived from plants;
and the form in which it principally presents itself is that of
coal. We shall have to speak again, and at greater length, of
coal, and it is sufficient to say here that all the true coals,
anthracites, and lignites, are of organic origin, and consist
principally of the remains of plants in a more or less altered
condition. The bituminous shales which are found so commonly
associated with beds of coal also derive their carbon primarily
from plants; and the same is certainly, or probably, the case
with similar shales which are known to occur in formations younger
than the Carboniferous.
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