I I 2 THOMAS L. WA TSON 



and the Coastal Plain sediments — and a short distance west of 

 the Carolina line. 



One and three quarter miles east of Appling, the county 

 seat of Columbia, is an outcrop of a coarse-grained porphyritic 

 granite. The feldspars are slightly pink in color with a some- 

 what greenish cast in places. The phenocrysts measure 20— 35 mm 

 in length and 5 — 1 5 mm broad; and commonly show the contact 

 type of Carlsbad twins. 



Microscopically, the rock is composed of a coarse-grained 

 groundmass of potash and plagioclase feldspars, orthoclase pre- 

 dominating, and quartz with biotite and occasional large plates 

 of muscovite. The phenocrysts are large tabular microperthitic 

 orthoclases. The anhedra of quartz vary in size and are badly 

 fractured. Laths of polysynthetically twinned plagioclase are 

 more abundant in this than in many of the other areas. Peg- 

 matitic intergrowths of quartz and feldspar are sparingly present. 

 The large feldspar phenocrysts contain abundant inclusions of 

 the groundmass minerals, especially biotite and plagioclase. 



The main porphyritic granite mass is one and one quarter 

 miles further east. The rock outcrops as a large doming-mass. 

 The porphyritic facies of the granite-mass is readily traceable 

 peripherally into an even granular medium coarse-textured 

 granite. The even granular facies of the rock mass is best 

 exposed along the public highway three miles slightly east of 

 south from Appling. Hand specimens of the rock from the 

 two exposures cannot be distinguished from each other. The 

 porphyritic feldspars in the principal exposure are larger but 

 show the same idiomorphic and other microscopic tendencies, 

 developed in the smaller one. 



A thin section of one of the phenocrysts from the main 

 outcrop showed the characteristic microcline structure, with 

 numerous inclusions of irregularly bounded crystals of all the 

 groundmass minerals. A chemical analysis of carefully selected 

 fragments of phenocrysts from this rock, yielded the writer the 

 results given in IVa, page 119. 



The phenocrysts are embedded in a close and firm, but 



