100 THOMAS L. WATSON 



The size, development and character of the phenocrysts in 

 the porphyritic portions of the areas; and the gradation from 

 one rock facies into another of the same composition shown in 

 the same rock-mass, unquestionably point to contemporaneous 

 growth with the groundmass constituents, and have, therefore, 

 been formed in place and not at greater depths — intratelluric. In 

 the present paper, it is proposed, therefore, to give a brief 

 description and summary of the individual porphyritic granite 

 areas ; and to state the reasons for the belief in the contemporane- 

 ous origin of the phenocrysts with the groundmass constituents, 

 based on a careful field and laboratory study of these rocks. 



DESCRIPTION OF INDIVIDUAL AREAS 



The Fayette-Campbell- Coweta counties porphyritic granite area.' 1 — 

 This area, marked C-C on the accompanying map, is thirty 

 miles southwest from Atlanta and occupies contiguous portions 

 of Campbell, Coweta and Fayette counties. The rock-outcrops 

 are usually small and in the nature of bowlder and flat-surface 

 masses. The exposures are most numerous in the vicinity of 

 Palmetto and Coweta Stations on the Atlanta and West Point 

 railroad, and near Line Creek in Fayette county. Specimens of 

 the porphyritic rock collected from the outcrop near Line Creek 

 in Fayette county are somewhat lighter in color than similar 

 ones from the Palmetto-Coweta portions of the massif. The 

 ratio of quartz to biotite in the Line Creek outcrop is visibly 

 greater than in the Campbell-Coweta exposures, as indicated in 

 analyses II and XI, p. 119. The rock, however, is generally 

 quite uniform over the entire area. Several good contacts of 

 the partially decayed porphyritic granite and mica-schist are 

 exposed along the wagon roads and in the cuts of the railroad, 

 traversing the area. The field relationships of the two rocks 

 indicate, that the porphyritic granite is the younger rock ; 

 intruded into the overlying schist, and exposed subsequently by 

 erosion. 



The rock is a very coarse-grained porphyritic-biotite granite 

 'Eighteenth Annual Report, U. S. Geol. Surv., 1898, pp. 551-572. 



