bstances. Hence such violent ebullition in volcanos, and hence the
emission of so much pumice-stone and ashes, which are of the same
nature.

In the body of our whin-stone, on the contrary, there is no mark of
calcination or vitrification. We frequently find in it much calcareous
spar, or the _terra calcarea aerata_, which had been in a melted state
by heat, and had been crystallized by congelation into a sparry form.
Such is the _lapis amygdaloides_, and many of our whin-stone rocks,
which contain pebbles crystallized and variously figured, both
calcareous, siliceous, and of a mixture in which both these substances
form distinct parts. The specimens of this kind, which I have from the
whin-stone or porphyry rock of the Calton-hill, exhibit every species
of mineral operation, in forming jasper, figured agate, and marble; and
they demonstrate, that this had been performed by heat or fusion.

I do not mean to say, that this demonstration is direct; it is
conditional, and proceeds upon the supposition, that the basaltic or
porphyry rock, in which those specimens are found, is a body which had
been in a melted state. Now, this is a supposition for which I have
abundance of evidence, were it required; but naturalists are now
sufficiently disposed to admit that proposition; they even draw
conclusions from this fact, which, I think, they are not sufficiently
warranted in doing; that is, from this appearance, they infer the former
existence of volcanos in those places. For my part, though I have made
the most strict examination, I never saw any vestige of such an event.
That there are, in other countries, evident marks of volcanos which
have been long extinguished, is unquestionably true; but naturalists,
imagining that there are no other marks of subterraneous fire and
fusion, except in the production of a lava, attribute to a volcano, as
a cause, these effects, which only indicate the exertion of that power
which might have been the cause of a volcano.

If the theory now given be just, a rock of marble is no less a mark
of subterraneous fire and fusion, than that of the basaltes; and the
flowing of basaltic streams among strata broken and displaced, affords
the most satisfactory evidence of those operations by which the body of
our land had been elevated above the surface of the sea; but it gives no
proof that the eruptive force of mineral vapours had been discharged in
a burning mountain. Now, this discharge is essential i