200 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



feldspar-bearing members of the titanium-bearing series of rocks 8 in 

 Amherst and Nelson counties, Virginia, which forms a part of the larger 

 Blue Eidge petrographic province, contain as the dominant feldspar a 

 calcic andesine of similar composition to that of the rock here described. 

 The relationship of these rocks is discussed later. 



Computing the feldspar composition in the usual way from analyses of 

 the syenite given on page 202, the results may be tabulated as follows : 



Normative Feldspar Composition of the Blue Ridge, Virginia, Hypersthene 



Syenite 



I II III IV V 



Orthoclase 21.68 25.02 22.24 7.78 22.80 



Albite 28.82 20.44 20.44 37.20 23.58 



Anorthite 15.57 15.29 13.34 10.29 18.35 



Total plagioclase 44.39 35.73 33.78 4T.49 41.93 



Total feldspar 66.07 60.75 56.02 55.27 64.73 



Orthoclase-plagioclase ratio ... lto2 1 to 1.4 1 to 1.5 lto6 1 to 1.8 



An examination of this table shows that the ratio of normative ortho- 

 clase (microcline) to normative plagioclase ranges from 1 : 1.4 to 1 : 6, a 

 variation which is in agreement with measurements made on thin sections 

 under the microscope. The feldspar frequently exhibits distinct evidence 

 of pressure metamorphism in bent and broken lamellae of plagioclase, 

 optical disturbance, and in some cases granulation. 



Pyroxene is the chief mafic constituent of the syenite. Both ortho- 

 rhombic and monoclinic varieties occur, the former being dominant in 

 most cases. Augite may fail in a few thin sections of the rock, but 

 hypersthene very rarely. The two pyroxenes are frequently intergrown 

 with each other. They are very similar in color and can only be distin- 

 guished in nrany cases by their optical properties. 



The optical properties prove hypersthene to be the orthorhombic 

 pyroxene present. It varies from colorless to reddish, exhibiting fairly 

 strong absorption in the more deeply colored forms of the mineral. Tt 

 is developed in stout, irregular-shaped forms, usually elongated in the 

 direction of the c axis and frequently containing rounded inclusions of 

 apatite and ilmenite. Platelike inclusions are abundantly developed in 

 both the monoclinic and orthorhombic pyroxenes of some thin sections. 



The hypersthene is pleochroic in shades, varying from reddish brown 

 to pale yellowish brown. It alters into a fibrous pleochroic mineral, with 

 the long axis of the fibers oriented parallel with the c axis of the hypers- 

 thene. Measured on the long axis of the fibers, the extinction is about 



8 T. L. Watson and S. Taber : Geology of the titanium and apatite deposits of Vir- 

 ginia. Bull. III-A, Virginia Geol. Survey, 1913. 



