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 passage of
nothing but light and heat.
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