106 THOMAS. L. WATSON 



and epidote derived from the alteration of the feldspars and 

 biotite. Bent and curved filaments of rutile are quite abundant 

 as inclusions in the larger quartz crystals. The effects of slight 

 pressure metamorphism are evident in the lines of fracture and 

 undulous extinction common to the larger quartz and feldspar 

 individuals. 



The Warren county porphyritic granite area. — Two somewhat 

 extensive outcrops of foliated porphyritic granite occur in the 

 middle eastern portion of Warren county, approximately ten 

 miles from each other, in an almost east and west direction. 

 These are known as the Holder's-Mill and Brinkley-Place granite 

 masses, respectively, and marked W-H on the map. 



The rock has a pronounced secondary foliated structure. 

 The quartz and feldspar crystals are drawn out and inclosed 

 between the biotite layers, forming at times distinct " augen " of 

 the two liarht-colored minerals. The rock contains abundant black 

 biotite plates arranged along somewhat parallel lines. The quartz 

 and feldspar grains are greatly squeezed and mashed, and are 

 more or less drawn out in directions parallel with the biotite lay- 

 ers, as a result of metamorphic action. The porphyritic granite 

 of this area owes its foliated structure, therefore, to pressure 

 metamorphism, 1 so common to many igneous rock masses in 

 those regions subjected to mountain-building forces. Hence it 

 is derived or secondary and not primary or fluidal. 



The feldspar phenocrysts are composed principally of micro- 

 perthitic orthoclase with some microcline I 5-20 mm long ; are white 

 opaque to pink in color ; contain numerous inclusions of biotite 

 plates, and exhibit the usual habit of Carlsbad twins. They are 

 prevailingly irregular in crystal outline and badly fractured from 

 subsequent intense metamorphism. The porphyritic feldspars 

 are embedded in a coarse-grained groundmass of quartz, feld- 

 spar and biotite. The groundmass feldspathic constituent con- 

 sists of the potash feldspars with microperthitic structures, and 



1 Gregory has established, according to origin, three classes of gneisses, namely, 

 metapyrigen-gneisses, clastic-gneisses, and fluxion-gneisses. See Quart. Jour. Geol. 

 Soc. (London), 1894, P. 2 66 ; Daly, R. A. Jour. Geol. (Chicago), Vol. V, p. 780. 



