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ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.
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of the process is in no way different from the process of crystallization in a fluid magma, save in the rapidity of the action, and is of both a physical and chemical character. It is not the purpose of this paper to discuss the other evidences of metamorphism in the South Mountain rocks. There is ample proof that both dynamic and statical metamorphism were wide spread. While the former would, by shearing, obliterate the original structures of a glassy rock and produce a slate, the latter might be an important initiatory and accelerating factor in the process of devitrification of the glassy rocks.

Nomenclature.—The character of the acid rocks has been briefly presented, and there remains to be considered a name or names which shall be descriptive of them. While the possibility of devitrification can hardly be doubted, the fact that a finely crystalline aggregate of quartz and feldspar may also be the direct product of consolidation from a molten magma is equally recognized by the writer, and to the acid rocks possessing such a groundmass the name quartz-porphyry is given. It is by no means always possible to distinguish between a primary and secondary crystalline groundmass, hence no attempt is made to draw a sharp line between the quartz-porphyries and the devitrified rhyolites.

The typical ancient originally glassy acid volcanic should be distinguished in some way by the name from the typical ancient originally holocrystalline acid volcanic. Is there any name now in use which does this? A great variety of terms has been applied to the acid type of the older volcanic rocks. Under the general group of quartz-porphyries, Rosenbusch classifies them as microgranites, with a microgranitic groundmass, granophyres with a micropegmatic groundmass, felsophyres, with a microfelsitic base, and vitrophyres (including pitchstones and pitchstone porphyries), with a vitreous base. Foqué and Lévy use microgranitite, micropegmatite and porphyr petrosiliceux as corresponding terms. By British petrographers these acid rocks have been termed hornstones, claystones, and claystone porphyries, felsites, quartz-felsites, and felsites porphyries, agreeing in this respect

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