hether more
simple or more compound, directing itself by its internal principle of
attraction, and affecting mechanically those that are concreting around
it.

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.
First, There are found, among argillaceous strata, insulated bodies of
iron-stone, perfectly consolidated; secondly, There are found, in strata
of chalk and lime-stone, masses of insulated flints; thirdly, There
are found, in strata of sea sand, masses of that sand cemented by a
siliceous substance; fourthly, In the midst of blocks of sand-stone,
there are found masses of loose or pure sand inclosed in crystallised
cavities; and in this sand are found insulated masses of crystallised
spar, including within them the sand, but without having the sparry
or calcareous crystallization disturbed by it. There are also other
globular masses of the same kind, where the sparry crystallization is
either not to be observed, or appears only partially[36]: And now,
lastly, In strata of shell-sand, there are found masses of consolidated
lime-stone or marble. In all those cases, the consolidated bodies are
perfectly insulated in the middle of strata, in which they must of
necessity have been petrified or consolidated; the stratum around the
bodies has not been affected by the petrifying substance, as there
is not any vestige of it there; and here are examples of different
substances, al