theory_;[256] but I may state that it is now demonstrable in some
countries that fossiliferous formations, some of them of the age of the
Silurian strata, as near Christiana in Norway, others belonging to the
Oolitic period, as around Carrara in Italy, have been converted
partially into gneiss, mica-schist, and statuary marble. The
transmutation has been effected apparently by the influence of
subterranean heat, acting under great pressure, or by chemical and
electrical causes operating in a manner not yet understood, and which
have been termed _Plutonic_ action, as expressing, in one word, all the
modifying causes which may be brought into play at great depths, and
under conditions never exemplified at the surface. To this Plutonic
action the fusion of granite itself in the bowels of the earth, as well
as the superinducement of the metamorphic texture into sedimentary
strata, must be attributed; and in accordance with these views the age
of each metamorphic formation may be said to be twofold, for we have
first to consider the period when it originated, as an aqueous deposit,
in the form of mud, sand, marl, or limestone; secondly, the date at
which it acquired a crystalline texture. The same strata, therefore,
may, according to this view, be very ancient in reference to the time of
their deposition, and very modern in regard to the period of their
assuming the metamorphic character.

_No proofs that these crystalline rocks were produced more abundantly at
remote periods_.--Several modern writers, without denying the truth of
the Plutonic or metamorphic theory, still contend that the crystalline
and non-fossiliferous formations, whether stratified or unstratified,
such as gneiss and granite, are essentially ancient as a class of rocks.
They were generated, say they, most abundantly in the primeval state of
the globe, since which time the quantity produced has been always on the
decrease, until it became very inconsiderable in the Oolitic and
Cretaceous periods, and quite evanescent before the commencement of the
tertiary epoch.

Now the justness of these views depends almost entirely on the question
whether granite, gneiss, and other rocks of the same order ever
originated at the surface, or whether, according to the opinions above
adopted, they are essentially subterranean in their origin, and
therefore entitled to the appellation of _hypogene_. If they were formed
superficially in their present state, and as copiousl