We have been hitherto considering fossil coal as formed of the
impalpable parts of inflammable bodies, united together by pressure, and
made to approach in various degrees to the nature of a chemical coal,
by means of subterranean heat; because, from the examination of those
strata, many of them have evidently been formed in this manner.
But vegetable bodies macerated in water, and then consolidated by
compression, form a substance of the same kind, almost undistinguishable
from some species of fossil coal. We have an example of this in our turf
pits or peat mosses; when this vegetable substance has been compressed
under a great load of earth, which sometimes happens, it is much
consolidated, and hardens, by drying, into a black body, not afterwards
dilutable or penetrated by water, and almost undistinguishable in
burning from mineralised bodies of the same kind.
Also, when fossil wood has been condensed by compression and changed by
the operation of heat, as it is frequently found in argillaceous strata,
particularly in the aluminous rock upon the coast of Yorkshire, it
becomes a jet almost undistinguishable from some species of fossil coal.
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