re, and conform to those erroneous ideas which first
observations, or inaccurate knowledge of minerals, may have suggested to
former naturalists.

It is the dissolution and concretion of siliceous substance, no
doubt, that gives such difficulty to our naturalists in explaining
petrifaction: they have, however, something apparently in their favour,
which it may be proper now to mention.

In the _first_ place, although siliceous substance is not soluble, so
far as we know, by simple water, it is soluble by means of alkaline
substance; consequently, it is possible that it may be dissolved in the
earth.

_Secondly_, The water of Giezer in Iceland, actually petrifies bodies
which are alternately imbibed with that hot water and exposed to the
air. This water, therefore, not only contains siliceous substance in a
dissolved state, but deposits this again, either by means of cooling,
or being aerated, or of evaporating. Consequently, without knowing the
principle upon which it proceeds, we here perceive a natural operation
by which siliceous petrifaction may be performed.

_Lastly_, We have another principle for the dissolution of siliceous
substance. This is the fluor acid which volatilises the siliceous
substance. This, however, requires certain conditions, which cannot be
found as a general cause in the mineral regions.

Thus we would seem to have every thing necessary for explaining the
concretion and crystallization of siliceous bodies, provided we could
find the proper conditions requisite for that operation; for whether it
shall be by means of acid or alkaline substances that siliceous matter
is to be dissolved, volatilised, and transported from one place to
another, it is necessary that those dissolving substances should be
present upon those occasions. Nor is it sufficient only to dissolve the
siliceous substance which is to be transported; the necessary conditions
for the concretion again of the dissolved substances, whatever these may
be, are also absolutely required for this operation. Now, though those
requisite conditions may be, upon many occasions, allowed in the earth,
it is not according to the theory of our modern naturalists, who explain
petrifaction upon the principles of simple infiltration of water,
that any advantage can be taken of those conditions; nor are natural
appearances to be explained without employing more complicated chemical
agents in the mineral regions.

To this subject of the petrifa