82               GEOLOGY OF NEW YORK CITY
	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