158 SILICEOUS PETRIFACTIONS AND CRYSTALS.
is reason to be convinced that it has been introduced in a very fluid
state, or that it has been produced in thefe strata by the action of
some very fluid matter which has penetrated them. The proof of
this is, our finding ſhells, lithophyta, and other marine produc
tions, originally calcareous, but which are now completely pene
trated by this fort of matter, and seemingly converted into flint or
agate, without losing their external form. The fame fact is further
establifhed by the quantities of foffil wood penetrated and petrified
with this matter, found in many parts of the world. It is found in
all ſtates, from that of stony wood, still combustible, to that of pure
flint.
It also deserves notice, that the nodules of flint found in chalk
and lime, often in very strange forms, are not fimply lying there,
but seem forming, or else decaying, in that fituation, being always
furrounded by a crust, which changes gradually from a calcareous
to a filiceous nature.
Another curious fact relating to this stony fubstance is, that masses
of it are sometimes found which include water perfectly inclosed in
the hard ſtone. This has been observed in crystal, in common flint,
and in agate.
In whichsoever of thefe ftates the hard stones are found, when
masses of them occur that are not folid, but have vacuities within
them, in these we find their matter crystallized, and very often into
remarkably regular and transparent crystals. Such are often found
in hollows of veins, and of pebbles. They are generally columns
of six sides, terminated by a six-sided pyramid : Or sometimes they
are pyramids alone. All thefe crystals are chiefly found in cavities
of veins, or of hard stones.
I have yet to mention one other state or condition in which the
filica has been found, and that is, diffolved in water. There is one
very remarkable example of this in Iceland, particularly at the cele¬