THE ORIGIN OF THE PHENOCRYSTS 105 



places. The relative proportion of the component minerals is, 

 including all species present, feldspar > quartz > biotite. 



Like the former area, the Sparta porphyritic granite is sur- 

 rounded on all sides by mica-schist ; but the great depth of 

 residual decay of the granite and schist, renders exposures of 

 the contact between the two impossible. The field relationships 

 however, indicate, as in the above area, that the granite is the 

 younger rock, intrusive into the then overlying schist. 



The Baldivin county porphyritic granite area. — Large bowlder 

 outcrops of a coarse-grained porphyritic granite occur three 

 miles southeast from Milledgeville, the county seat of Baldwin. 

 Like the Sparta area, the Baldwin county porphyritic granite mass 

 is located near the line of contact (Fall-line) between the Pied- 

 mont crystallines and the Coastal Plain sediments (area marked 

 B on the map). 



Hand specimens of the granite from Baldwin county are 

 indistinguishable from similar specimens of the Columbia county 

 porphyritic mass, described below. The rock is a very coarse- 

 grained porphyritic granite, composed of an aggregate of inter- 

 locking quartz and feldspars — orthoclase with microperthitic 

 structures, microcline and plagioclase- — with intergrown shreds 

 of biotite. The rock varies in color from medium to dark gray. 

 Both microcline and orthoclase occur in the groundmass and as 

 porphyritically developed minerals. The phenocrysts measure 

 in extreme cases 30-40 mm long and 5-1 o mm broad. They are 

 prevailingly idiomorphic in form; flat, tabular parallel to the 

 clinopinacoid (010), and commonly twinned according to the 

 Carlsbad law. Abundant inclusions of black biotite foliae are 

 plainly visible to the unaided eye in the feldspar phenocrysts ; 

 while the microscope shows additional numerous irregularly 

 bounded quartz and feldspar grains without definite orientation. 



The groundmass is composed of an abundance of white 

 opaque feldspars, slightly dark-colored smoky quartz, and biotite 

 plates measuring 2~5 mm in diameter. The microscopic accesso- 

 ries are primary inclusions of apatite and zircon and scattered 

 grains of magnetite, with some secondary muscovite, chlorite 



