I o 4 THOMA S L. WA TSON 



depot, with the exposures more or less continuous from this 

 point, for eleven miles, in a northeastwardly direction, along the 

 Georgia railroad. The area lies near the merging of the crystal- 

 line rocks beneath the Coastal Plain sands and clays — the Fall- 

 line (area marked S on the map). It is elliptical in shape 

 with its longer diameter eleven miles in length, trending north- 

 east-southwest. The granitic rock outcrops as bowlder and flat- 

 surface masses, frequently containing four to five acres of 

 exposed rock in one body. 



The rock texture is prevailingly porphyritic, grading, in 

 many cases, into a non-porphyritic, even-grained granite of the 

 same mineral and chemical composition. In several quarries the 

 rock shows in places a somewhat pronounced gneissoid struct- 

 ure. Some half dozen quarries have been extensively worked 

 in various places over the area, within a few miles northeast of 

 Sparta. 



The rock is prevailingly a coarse-grained, medium gray, por- 

 phyritic biotite-granite. The phenocrysts are composed of the 

 potash feldspars having a pronounced pinkish cast which dis- 

 appears in thin section. They are flat tabular in crystal form, 

 averaging 20 mm in length parallel to the clinopinacoid (oio). 

 Carlsbad twins of the contact type are common. The pheno- 

 crysts are further characterized by numerous inclusions of all 

 the groundmass minerals. 



The porphyritic feldspars are embedded in a coarse-grained 

 groundmass of quartz, potash and soda-lime feldspars and biotite, 

 with some accessory muscovite, chlorite, apatite, zircon, epidote 

 and scattered grains of magnetite. Microperthitic structures are 

 common to the potash feldspars. Micropegmatitic structures 

 are less frequently observed, than in some of the other areas. 

 The rock from the Sparta area differs from that of the Campbell- 

 Coweta- Fayette counties area in containing less biotite, and hence, 

 lighter in color; in the phenocrysts being idiomorphic instead of 

 allotriomorphic in crystal outline, and usually of a pinkish cast 

 rather than white in color. Microcline is, alike, variable for the 

 two areas in thin sections of the rock examined from various 



