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.
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