IOS THOMAS L. WATSON 



tabular in outline and twinned according to the Carlsbad law, 

 and shows an abundance of large included biotite shreds (see 



Fig- 3)- 



Still a third exposure of granitoid rock showing a pronounced 

 porphyritic texture in places, occurs six to eight miles south of 

 the Holder's Mill-Brinkley Place area. It differs quite strongly 

 from the Holder's Mill-Brinkley Place granite in the porphyritic 

 texture being less strongly marked and finer grained ; in contain- 

 ing less biotite, and is massive instead of foliated in structure. 

 It further differs from the former area in containing a larger 

 proportion of plagioclase, and a smaller percentage of microcline 

 feldspars. Only one included grain of microcline was found in 

 a number of thin sections studied of this rock. The feldspar 

 phenocrysts are deep pink in color, 5-1 o mm long, displaying the 

 usual characteristic twinning and cleavage ; and carry numerous 

 inclusions of all the groundmass minerals. 



The Gree?ie county porphyritic granite area. — The Greene county 

 porphyritic granite area, marked G on the accompan3'ing map, 

 includes at least 100 acres in the main granite outcrop, located 

 ten miles south of Greensboro, in the southern part of the county. 

 The main central exposure is in the form of a low flat doming- 

 mass with a roughened and irregular surface, and partially cov- 

 ered, in places, with a thick growth of cedars (see Fig. 4) . 



An even granular medium coarse-grained granite outcrops in 

 bowlder form three miles south of Greensboro, and is continuous 

 along the public highway from this point to the main porphyritic 

 granite mass (see Fig. 5 ) . The true granitic facies of the rock mass 

 grades interiorly into the typical porphyritic granite facies. The 

 even-grained granitic portion of the mass representing the outer 

 or peripheral zone, indicates some variation in mineral composi- 

 tion, from place to place, along a north and south section. The 

 zone nearest the porphyritic area, two and one half miles north 

 therefrom, and showing absence of all trace of porphyritic tex- 

 ture, is a medium-grained biotite granite agreeing, microscop- 

 ically, in mineral composition, and in chemical composition as 

 well, with the porphyritic granite. 



