THE ORIGIN OF THE PHENOCRYSTS 1 1 3 



coarse-grained groundmass of flesh-colored feldspars tinged a 

 slight greenish cast, somewhat dark smoky quartz, and biotite. 

 The porphyritic feldspar crystals make up nearly one half of the 

 total rock. The feldspars are white and opaque rather than 

 pink in color over the greater part of the exposure. This rock 

 very closely resembles that from Greene county in the hand 

 specimens. Here, as in the areas described above, the feld- 

 spathic constituent consists of the potash and soda-lime feld- 

 spars, with the potash varieties predominating. The porphyritic 

 feldspars are chiefly orthoclase with some microcline, carrying 

 inclusions of all the groundmass minerals. The included biotite 

 shreds are visible macroscopically. Some of the largest plagio- 

 clase inclusions in the orthoclase phenocrysts carry, in turn, 

 microscopic inclusions of quartz and other groundmass minerals. 

 Twinning according to the Carlsbad and albite laws among the 

 included feldspar species is commonly observed. 



Biotite shows its usual characteristic optical properties, and 

 is partially altered to chlorite and epidote. Muscovite is spar- 

 ingly present as foils intergrown with the biotite. Sporadic 

 accessory magnetite and apatite occur. 



The Pike county granite area. — The Pike county granite area, 

 marked P on the accompanying map, includes fifty or more acres 

 of exposed flat-surface rock in the northwest part of the county. 

 The porphyritic facies of the rock gradually passes into the even- 

 textured medium-grained granite. Only a small proportion of 

 this area, however, shows the porphyritic texture. 



The rock is a medium-grained biotite granite, varying from 

 even granular to porphyritic in texture, showing a partial gneis- 

 soid structure in places. 



Microscopically, the porphyritic portions of the rock are 

 composed of orthoclase phenocrysts in a coarse-grained ground- 

 mass of quartz, microperthitic orthoclase, microcline and some 

 soda-lime feldspar, biotite, and occasional intergrown shreds of 

 muscovite. Biotite is the chief accessory mineral. It is deep 

 brown to yellow in color, with strong absorption, and is inter- 

 grown with some muscovite. It is more or less altered to dark 



