rom a regular coal stratum; and what is more
remarkable, in this stratum is contained a true plumbago, Farther up the
country, the Earl of Dumfries has also a mine containing plumbago along
with other coal strata; and though the plumbago of these two mines have
not all the softness and beauty of the mineral of the same species from
Cumberland, they are nevertheless perfect plumbago.

I have a specimen of steatetical whinstone or basaltes from some part of
Cumberland, in which is contained many nodules of the most perfect and
beautiful plumbago. It is dispersed through this stone in rounded masses
of all sizes from a nut to a pin's head; and many of these are mixed
with pyrites. There is therefore reason to believe that this plumbago
had been in fusion.

Now, if we consider that every species of coal and every species of
plumbago are equally, that is, perfectly combustible, and yield, in
burning, the same volatile principles, differing only perhaps a little
in the small quantity of fixed matter which remains, we shall be
inclined to believe, that they have all the same origin in a vegetable
substance; and that they are diversified by some very small composition
of other matter. This being allowed, one thing is certain, that it is by
the operation of mineral fire or heat that those combustible substances,
however composed, have been brought to their present state of coal,
although we are ignorant of the circumstances by which their differences
and their peculiar chemical and mechanical qualities have been produced.

Let us resume in a few words. There is not perhaps one substance in the
mineral kingdom by which the operation of subterraneous heat is, to
common understanding, better exemplified than that of mineral coal.
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