There is one fact more which is well worth our attention, being one
of those which are so general in the mineral regions. It is the
crystallizations which are found in close cavities of the most solid
bodies.
Nothing is more common than this appearance. Cavities are every where
found closely lined with crystallizations, of every different substance
which may be supposed in those places. These concretions are well known
to naturalists, and form part of the beautiful specimens which
are preserved in the cabinets of collectors, and which the German
mineralists have termed _Drusen_. I shall only particularise one
species, which may be described upon principle, and therefore may be
a proper subject on which to reason, for ascertaining the order of
production in certain bodies. This body, which we are now to examine, is
of the agate species.
We have now been considering the means employed by nature in
consolidating strata which were originally of an open structure; but in
perfectly solid strata we find bodies of agate, which have evidently
been formed in that place where they now are found.
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