6 UNIVEESITT OF VIRGINIA PUBLICATIONS 



Omitting non-feldspathic constiti-Teiits and allotting the potash to ortho- 

 clase, and the soda and lime to albite and anorthite, respectively, the analj'ses 

 show the following range and average in feldspar composition for the region 

 as a whole. 



Name. 



Orthoclase 

 Albite — 

 Anorthite 



Range. 



Average. 



8.90 to 33.36 



16.24 to 42.97 



1.67 to 13.90 



25.4 

 32.7 



8.7 



In the quantitative classification^ of igneous rocks, all analyses of the 

 rocks of the region given on subsequent pages are distributed between rangs 

 1, 2, and 3, and subrangs 3 and 4 of class I (2 in class II), as follows: 

 Peralkalic, 4 ; domalkalic, 29 ; alkalicalcic, 5 ; sodipotassic, 29 ; dosodic, 7 ; 

 between sodipotassic and dosodic, 2. 



Microperthite is generally present in the rocks, but anorthoclase, if 

 present, has not been identified. 



The free quartz in the rocks has been computed from the complete 

 analyses for each state except Georgia, and the results may be tabulated as 

 follows : 



Limits of variation and average percentage of quart:: in southeastern- 

 Atlantic states granites. 



State. 



Percentage of quartz. 



Average- 



Maryland 



Virginia 



North Carolina 

 South Carolina 



13.6 to 41.2 

 28.8 to 32.9 

 14.6 to 43.5 

 19.3 to 30.8 



30.4 

 28.0 

 32.8 

 24.9 



Total silica averages 71.18 per cent, for the entire region, with a varia- 

 tion between extremes of more than 11 per cent.; 66.01 per cent. (North 

 Carolina) and 77.19 per cent (North Carolina). 



Biotite, subject to variation in amount, is, with few exceptions, a con- 

 stituent in the rocks of every locality within the region. Muscovite is a 

 frequent associate of biotite in many localities, the two micas appearing 



^Quantitative Classification of Igneous Eooks. By Cross, Iddings, Pirsson, and 

 Washington. Cliieago, 1903. 



