ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 825 



of the process is in no way different from the process of crys- 

 tallization 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 meta- 

 morphism 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 recog- 

 nized by the writer, and to the acid rocks possessing such a 

 groundmass the name quartz-porphryry is given. It is by no 

 means always possible to distinguish between a primary and sec- 

 ondary crystalline groundmass, hence no attempt is made to 

 draw a sharp line between the quartz-porphyries and the devitri- 

 fied 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, grano- 

 phyres with a micropegmatic groundmass, felsophyres, with a micro- 

 felsitic base, and vitrophyres (including pitchstones and pitchstone 

 porphyries), with a vitreous base. Foque and Levy use micro- 

 granitite, micropegmatite andporphyr petrosiliceuxas correspond- 

 ing 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 



