ed siliceous
crystals; a case which is not so common. I have them also attended with
circumstances of concretion and crystallization, which, besides being
extremely rare, are equally curious and interesting.

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. This fact, however,
is not still that of which we are now particularly to inquire; for this,
of which we are to treat, concerns only a cavity within this agate; now,
whatever may have been the origin of the agate itself, we are to show,
from what appears within its cavity, that the crystallizations which are
found in this place had arisen from a simply fluid state, and not from
that of any manner of solution.

The agates now in question are those of the coated kind, so frequent in
this country, called pebbles. Many of these are filled with a siliceous
crystallization, which evidently proceeds from the circumference
towards the centre. Many of them, again, are hollow. Those cavities are
variously lined with crystallized substances; and these are the object
of the present examination.

But before describing what is found within, it is necessary to attend to
this particular circumstance, that the cavity is perfectly inclosed with
many solid coats, impervious to air or water, but particularly with
the external cortical part, which is extremely hard, takes the highest
polish, and is of the most perfect solidity, admitting the passa