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. But these natural
appearances would not even be explained by this dissolution and supposed
filtration of those substances. There is also required, _first_, A
cause for the separation of those different substances from the aqueous
menstruum in which they had been dissolved; _2dly_, An explanation of
the way in which a dissolved bitumen should be formed into round hard
bodies of the most solid structure; and, _lastly_, Some probable means
for this complicated operation being performed, below the bottom of the
ocean, in the close cavity of a marble stratum.

Thus, the additional proof, from the facts relating to the bituminous
substances, conspiring with that from the phenomena of other bodies,
affords the strongest corroboration of this opinion, that the various
concretions found in the internal parts of strata have not been
occasioned by means of aqueous solution, but by the power of heat and
operation of simple fusion, preparing those different substances to
concrete and crystallise in cooling.

The arguments which have been now employed for proving that strata have
been consolidated by the power of heat, or by the means of fusion, have
been drawn chiefly from the insoluble nature of those consolidating
substances in relation to water, which is the only general menstruum
that can be allowed for the mineral regions. But there are found, in
the mineral kingdom, many solid masses of saltgem, which is a soluble
substance. It may be now inquired, How far these masses, which are not
infrequent in the earth, tend either to confirm the present theory, or,
on the contrary, to give