INTERMEDIATE CHARACTER OF APPALACHIAN GRANITES 5 



excess of the former) and sodic-lime (usually oligoclase, oligoclase-albite, and 

 some albite) varieties, the sodic-lime varieties Ijeing, with few exceptions, 

 either equal to or greater in amount than the potassic feldspar. .The molec- 

 ular ratio of potassic to sodic-lime feldspars ranges from 1 :1 to 1 :i, with an 

 average for the region as a whole, of 1 to 1.88. 



Computing in the usual way the feldspar composition from chemical 

 analyses, the limits of variation in the potassic and sodic-lime feldspars 

 ratio, and the average for each state, may be tabulated as follows : 



Limits of variation in ratio of potassic and sodic-lime feldspars in the south- 

 eastern Atlantic states granites. 



State. jLimits of Variation. 



Average. 



Maryland 



Virginia 



North Carolina 

 South Carolina 

 Georgia 



The extremes in the feldsf)ar ratio for the entire region are shown in the 

 rocks of Maryland. These are : Port Deposit granite-g-neiss (riesenose, 

 quartz-mica diorite), 1 of orthoelase to 3.9 of plagioclase; and Brookville 

 granite (liparose, quartz monzonite), 1 of orthoelase to 0.9 of plagioclase. 



Calculating all of the Na^O in the analyses as albite and the CaO as 

 anorthite, and adding the albite to the anorthite, a sodic-lime feldspar is 

 obtained which ranges from Ab,Anj (basic, or andesine oligoclase) to 

 AbijAui (oligoclase-albite), with a probable average of Ab^Anj, acid oligo- 

 clase. In some cases the potassic feldspar contains sufficient soda (approxi- 

 mately one- third the total jDotash) to be designated soda-orthoclase, as shown 

 in the analyses below of carefully selected potassic feldspar phenocrysts 

 from two of the Georgia porphyritic granites of widely-separated localities.'- 



Feldspar phenocrysts from porphyritic granite, McCollum Place, Coweta county, 

 Georgia. Bull. No. 9-A, Georgia Geol. Survey, 1902, pp. 92, 322. 



Feldspar plienocrvsts from porphyritic granite, Heggie Rock, Columbia county, 

 Georgia. Bull.'Xo. 9-A, Georgia Geol. Survey, 1902, pp. 235, 318. 



'Georgia Geological Survey, Bull. 9-A, 1902, p. 239. 



