The strata of fossil
coal are found in almost every intermediate state, as well as in those
of bitumen and charcoal. Of the one kind is that fossil coal which melts
or becomes fluid upon receiving heat; of the other, is that species of
coal, found both in Wales and Scotland, which is perfectly infusible in
the fire, and burns like coals, without flame or smoke. The one species
abounds in oily matter, the other has been distilled by heat, until it
has become a _caput mortuum_, or perfect coal.
The more volatile parts of these bituminous bodies are found in their
separate state on some occasions. There is a stratum of limestone in
Fifeshire, near Raith, which, though but slightly tinged with a black
colour, contains bituminous matter, like pitch, in many cavities, which
are lined with calcareous spar crystallised. I have a specimen of such
a cavity, in which the bitumen is in sphericles, or rounded drops,
immersed in the calcareous spar.
Now, it is to be observed, that, if the cavity in the solid limestone or
marble, which is lined with calcareous crystals containing pyrites, had
been thus encrusted by means of the filtration of water, this water must
have dissolved calcareous spar, pyrites, and bitumen.
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