In this connection it is desirable to call attention to some interesting speculation of Mr. R. A. Daly, of the Canadian Survey (Amer. Journal Set., 4th
Series, Vol. 15, p. 269; Vol. 16, p. 107, and Vol. 20, p. 185), who has
defended the famous " assimilation theory" as to the origin of
granites. The theory does not stand well among lithologists, but it has
not been abandoned, either, by many distinguished students. It involves
some fundamental assumptions if extended to the possible limits of its
application, generally to acid intrusives. (An acid rock is a rock rich
in silica, a basic, one not so siliceous, and carrying high
percentages of lime, iron and magnesia.) The theory means the welling
out and intrusion of a basic magma into acid rocks which may be
crystalline schists and metamorphosed or simply consolidated sediments;
the basic rock " assimilates " or digests by fusion and absorption the
acid compound it invades, and there results an intermediate rock more
basic than the invaded strata, more acid than the invading eruptive,
and among such resultant rock sequela granites form a
conspicuous class. If then it can be assumed that the underlying fluid
rock masses of the earth are basic (ferro-magnesian, gabbroitic), that
these invade or are mingled with acid sediments overlying them, the
subsequently modified mixture, by separation, would yield "a thoroughly
granular acid rock," and this in turn, as a granitic effusion, tinder
the stimulus of pressure, would penetrate overlying fissures, faults,
and cavities, or actually " stope " its way upward, melting and
reinvolving superincumbent strata, and solidifying among these latter
as granite dikes, buttes, and Iaccolites.
Acid
or granitic effusions have been presented to us as vein masses in the
rocks of New York. They have almost certainly at times been poured out
in beds, which have again been lifted and folded with the inclosing
metamorphic schists, and they, too, seem to have irregularly penetrated
the schists, cutting them at various angles. It is permissible to
introduce here this assimilation theory with reference to their origin,
though
