We see the very same thing happen under our eye, and precisely in the
same manner. When a fluid mass of any mineral or metallic substance is
made to congeal by sudden cooling on the outside, while the mass within
is fluid, a cavity is thus sometimes formed by the contraction of the
contained fluid; and in this cavity are found artificial _druzen_, as
they may be called, being crystallizations similar to those which the
mineral cavities exhibit in such beauty and perfection.
Petrification and consolidation, in some degree, may doubtless be
performed, in certain circumstances, by means of the solution of
calcareous earth; but the examples given by M. de Luc, of those bodies
of lime-stone and agate petrified in the middle of strata of loose or
sandy materials, are certainly inexplicable upon any other principle
except the fusion of those substances with which the bodies are
petrified[35].
[Note 35: Vid. Lettre 28 et Lettre 103. Lettres Physiques et Morales.]
This subject deserves the strictest attention; I propose it as a
touchstone for every theory of petrification or perfect consolidation.
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