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UNIVEBSITY OF VIRGINIA PUBLICATIONS 



excess of CaO, the average ratio of total alkalies to lime being -4 to 1. In 

 the relation of K^O to ISTajO, the two are approximately equal. KjO 

 ranges from 1.48 per cent, to 5.63 per cent, with an average of 3.87 per 

 cent., and NajO from 1.95 per cent, to 5.09 per cent, with an average of 

 3.82 per cent. With only one exception (Maryland II), Na^O molecularly 

 either exceeds or is equal to K2O. 



The feldspar content is a mixture of potassic and sodic-lime varieties, 

 the latter being either equal to or greater than the former. Increase in total 

 feldspar is shown in the gTanites from Maryland southward to Georgia, ex- 

 cept Virginia, in which total feldspar is greater than for North or South 

 Carolina but less than for Georgia. Maryland shows the minimum and 

 Georgia the maximum of this constituent. Orthoclase including microcline is 

 least for North Carolina and greatest for Georgia. Plagioclase is least for 

 Maryland and greatest for Georgia. The ratio between orthoclase, includ- 

 ing microcline, and plagioclase (sodic-lime varieties), is greatest for North 

 Carolina and is approximately the same in each of the other four states. 

 Figure 2 illustrates diagrammatically the feldspar composition of the south- 

 ern granites by states. 



Mdryiand 



VirginiB 



t^orth CaroZ/na 



South Cero///73 



Georgia 



Figure 2. — Diagram illustrating feldspar composition of the southern granites by states. 

 The molecular ratio of potassic to sodic-lime feldspars is from 1 :1 to 

 1 :4, with a general average of 1 to 1.88. In a total of 39 analyses, 28 show 

 an orthoclase-plagioclase ratio of 1<2, and 11 a ratio of 1>2. The plagio- 

 clase is usually an acid oligoclase with some - albite, rarely a calcareous 

 oligoclase, and as shown above is usually less than double the potassic 

 feldspar. 



