QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 191) 



stituent. Garnet of red color is developed in grains and crystals of the 

 syenite of certain localities in Eoanoke County and of those in Greene 

 and adjoining counties in the northern Blue Eidge. 



MICROSCOPIC CHARACTER 



Normative feldspar ranges in amount from 56 per cent to 66 per cent. 

 The varieties of orthoelase, microcline, albite, and calcic andesine make 

 up the feldspar content of the rock. Of these andesine is the chief feld- 

 spar, although orthoelase or microcline may equal or exceed it in amount 

 in some thin sections. Like the rocks of other areas which it most 

 closely resembles and with which it is compared in subsequent pages of 

 this paper, a noteworthy feature of the soda-lime feldspars in the rock is 

 the frequent absence of twinning lamella?, which might readily be mis- 

 taken for orthoelase. Microcline is the dominant variety of feldspar in 

 some thin sections. Albite occurs in most thin-sections studied, not as 

 separate individuals, but intergrown with the potash varieties as mocro- 

 perthite. Andesine is developed in anhedral forms and is frequently 

 twinned, both on the albite and pericline laws. Its composition, deter- 

 mined by Larsen on four specimens of the rock collected from different 

 localities, by measurements on the rhombic section and of the index of 

 refraction, is as follows : 



I AbiAii! 



11 Ab 63 An 37 



HI Ab 62 An 38 



IV Ab 70 An 30 



V Ab 07 An 33 



I. Specimen of syenite collected near the southeast foot of the Blue Ridge, 



in Browns Gap, Albemarle County, Virginia. 

 II. Specimen of syenite collected near the southeast foot of Tobacco Row 

 Mountain, in the vicinity of Elon, Amherst County, Virginia. 



III. Specimen of syenite collected on northwest slope of the northern Blue 



Ridge, near the boundary between Madison and Greene counties, Vir- 

 ginia. 



IV. Specimen of syenite collected from the base of northeast slope of the 



Peaks of Otter, Bedford County, Virginia. 

 V. Specimen of syenite collected iy 2 miles east of Vinton, Roanoke County, 

 Virginia. 



A comparison of these results clearly shows thai the dominant variety 

 of soda-lime feldspar (plagioclase) present in the rock of widely separated 

 occurrences is very uniform in composition, corresponding in each case 

 to a calcic andesine. Tt is important in this connection to note that the 



