Those strata are evidently a deposit of inflammable substances which all
come originally from vegetable bodies. In this state of their formation,
those coal strata must all be oleagenous or bituminous. In many of them,
however, these volatile parts are found wanting; and, the stratum is
found in the state of the most perfect coal or caput mortuum. There, is,
I presume, no other means to be found by which this eminent effect could
be produced, except by distillation; and, this distillation perhaps
proceeded under the restraining force of an immense compression.
To this theory it must not be objected, that all the strata of coal,
which are found in the same place or neighbourhood, are not reduced to
that caput mortuum or perfect coaly state. The change from a bituminous
to a coaly substance can only take place in proportion as the
distillation of the volatile parts is permitted. Now this distillation
must be permitted, if any passage can be procured from the inflammable
body submitted to the operation of subterraneous heat; and, one stratum
of coal may find vent for the passage of those vapours, through some
crevice which is not open to another.
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