202 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



every thin section. Of these lesser minerals, apatite and magnetite are 

 the most important, because of their constant presence in greater or less 

 amount in all thin sections studied. Apatite occurs in anheclral and 

 euhedral forms as inclusions in other minerals, while magnetite is devel- 

 oped in grains of fairly large size and commonly shows alteration to 

 leucoxene, both along the border and along the fracture lines. Kutile is 

 rare except as needlelike inclusions in the felsic minerals. 



CHEMICAL COMPOSITION AND CLASSIFICATION 



The chemical composition of the Blue Eidge hypersthene syenite is 

 shown in the five analyses of the subjoined table made on specimens of 

 the rock collected from as many different localities. The variation in 

 silica of nearly 8 per cent is due chiefly to the variable amount of quartz 

 in the rock as confirmed by microscopic study. The alkalies, soda and 

 potash, show some variation; but in all cases, except IV, which is dosodic, 

 the rock is sodipotassic. Titania and phosphoric anhydride, especially 

 the former, are high for this type of rock, characteristic features, but more 

 emphasized in the composition of the Amherst-Nelson counties eomag- 

 matic area of the same general region. 



Analyses of Hypersthene Syenite from the Blue Ridge Mountains, Virginia 



Si0 2 68.21 



Al 2 O s 



Fe 2 3 



FeO 



MgO 



CaO 



Na 2 



K 2 



H 2 0— 



H 2 0+ 



Ti0 2 



P 2 5 



MnO 



BaO 



CO, • 



S trace 



I 



II 



III 



IV 



V 



68.21 



65.88 



63.76 



61.08 



60.52 



15.33 



14.15 



13.04 



12.43 



a 16.99 s 



.81 



.77 



1.36 



1.86 



.60 



3.62 



5.29 



5.64 



6.66 



6.53 



.68 



1.48 



1.37 



1.44 



1.59 



2.90 



3.42 



4.30 



5.32 



4.58 



3.38 



2.42 



2.36 



4.42 



2.83 



3.72 



4.19 



3.76 



1.34 



3.91 



.14 



.23 



.10 



.06 



.88 



.33 



.41 



.14 



.37 





1.01 



1.75 



2.85 



4.26 



n. d. 



.42 



.39 



.88 



1.01 



.74 



none 



.33 



.19 



.41 



.25 



none 



none 



none 



none 





none 



none 



none 



none 





trace 



none 



.06 



.09 







100.15 100.71 100.11 100.75 



99.42 



T. Hypersthene syenite from the southeast foot of the Blue Ridge, in Browns 

 Gap, Albemarle County, Virginia. J. G. Dinwiddie, analyst. 



a Includes Ti0 2 . 



9 Watson and Taber : Bull. III-A, Virginia Geological Survey, 1913. 



