IKTERJIEDIATE CHARACTER OF APPALACHIAX GRANITES 



17 



II. Medium-textured and medium gra}' biotite granite, Lassiter, and Petersburg 

 Granite Cos. quarries, Petersburg, Dinwiddie county. 



III. Fine-grained, darlc blue-gray biotite granite. McGowan, Netherwood, and 



Donald quarries, Chesterfield county, and Mitchell and Copeland quarry, 

 Henrico county, near Richmond. 



IV. Jledium coarse-textured and medium gray biotite granite. Netherwood, State 



( Old Dominion ) Granite Development Co., Krimm, and Middendorf quar- 

 ries, Chesterfield county, near Richmond. 

 V. Fine-grained, dark blue-gray biotite granite. Cartwright and Davis quarries, 



near Fredericksburg, Spottsylvania county. 

 VI. Mediiun-textured and medium gray, biotite granite. ilclntosh quarry. Ches- 

 terfield county, 5 miles west of Richmond. 

 VII. Medium coarse-textured, gray, biotite granite-gneiss. Middendorf (Belt Line 

 Railway) quarry, near Manchester, Chesterfield county. 



The average percentage of silica in the Virginia rocks, 70.57 per cent., 

 is essentially the same as that in the Maryland rocks, 70.89 per cent. The 

 range in this constitiient in the Virginia rocks is from 69.39 to 72.27 per 

 cent. Calculated from the complete analyses the percentages of quartz are as 

 follows: I, 28.44; II, 27.48; III, 28.56; IV, 27.60; V, 27.30; VI, 23.76; 

 and VII, 32.94; average, 28.01. 



Of the feldspars present in the Virginia rocks, microcline, which is gen- 

 erally shown in thin sections, is subject to greatest variation, and, as a rule, 

 is much less in amount than either orthoclase or plagioclase. An important 

 feature in the mineral composition of these rocks is the nearly constant 

 large amount of plagioclase present, as would be naturally inferred from the 

 percentages of NaoO and CaO in the table of partial analyses above. Extinc- 

 tion angles measured against the twinning planes of individual laths of 

 soda-lime feldspar usually indicate a plagioclase near oligoclase. Albite 

 is usuallj' present in subordinate amount, not as separate anhedra but inter- 

 grown with the orthoclase as microperthite. Anorthoclase, if present, has 

 not been identified. 



The mineral composition of the feldspars, calculated from the per- 

 centages of K2O, ]Sra,0, and CaO in table D, by allotting potash to ortho- 

 clase, and soda and lime to albite and anorthite. respectively, is given in 

 table E. 



E. Table of feldspar composition of Virginia granites. 

 (Numbers correspond to table D.) 



I. 



II. 



III. I IV. 



V. ! VI. VII. i Average. 



Orthoclase 



Albite 



Anorthite 



Total plagioclase 

 Total feldspar -__ 



AbnAnm ratio 



Or — plag. ratio— - 



29.46 

 29.34 

 7.78 

 37.12 I 

 66.58 

 4:1 

 1:1.26 



27.24 1 

 30.92 

 8.90 

 39.82 i 

 67.06 I 

 4:1 ; 

 1:1.46 



26.13 



29.85 



10.01 



39.86 



65.99 1 



3:1 



1:1.52 



28.36 

 29.34 

 9.17 

 38.51 

 66.87 

 3:1 

 1:1.35 



20.57 

 30.92 

 13.90 



44.82 

 65.39 

 2+:l 



1:2.1+11:1.7611:2.2 



25.02 

 33.54 

 10.56 

 44.10 

 69.12 

 3:1 



17.24 

 24.63 

 13.62 

 38.25 

 55.49 

 2:1 



24.86 



29.79 



10.56 



40.35 



65.21 



3:1 



1:1.86 



