198 WATSON AND CLINE ROCKS OP THE BLUE RIDGE REGION 



miles. The most southwesterly known exposure of the syenite is in 

 Floyd County and the most northeasterly one is in Warren County, a 

 short distance south of Front Royal. Exposures of the rock have been 

 observed in every county of the Blue Ridge region between Floyd and 

 Warren counties. 



The syenite frequently occurs in the northwest slope of the Blue Ridge, 

 where it may form the basement on which the Lower Cambrian sediments 

 rest. Usually it is more abundantly exposed on the southeast slope of 

 the ridge, since in this position the Cambrian sediments have been re- 

 moved to a large extent by erosion. It is not restricted, however, in dis- 

 tribution to the slopes of the Blue Ridge, but is noted in many localities 

 in the most elevated portions of the ridge. Along the western margin of 

 the Piedmont Plateau hypersthene syenite is the chief rock, forming 

 Tobacco Row Mountain in Amherst County, and is also noted in similar 

 positions in Madison and Greene counties. Hypersthene syenite forms 

 the Peaks of Otter, and it is very abundant in James River Gap and 

 vicinity, where it makes up large portions of the central core of the Blue 

 Ridge. 



MEGASCOPIC CHARACTER 



Normally the hypersthene syenite is of greenish color, which varies in 

 shade from a moderately light greenish to a dark greenish gray rock. It 

 shows granitic texture, which varies from medium fine to coarse granular, 

 and having usually a pronounced greasy or waxy appearance. In places 

 the rock is porphyritic in texture, with phenocrysts of orthoclase, rarely 

 of pyroxene. Variation in color of the fresh rock is noted, dependent on 

 the relative proportions of the felsic and mafic minerals, the former vary- 

 ing in amount from 75 per cent to 92 per cent. In structure the rock 

 is usually massive, but in places indistinct to pronounced foliation is 

 shown, due to the effects of pressure metamorphism. Weathered surfaces 

 are usually deeply pitted from inequality of resistance of the component 

 minerals, and the rock yields on complete decay a reddish brown soil 

 which is quite fertile. 



The syenite is composed essentially of orthoclase (microcline), plagio- 

 clase, and pyroxene, each of which may be recognized in hand specimens. 

 Feldspar is the most abundant constituent of the rock and is usually of 

 dark greenish gray color, with glistening cleavage surfaces, but decidedly 

 waxy on fracture surfaces. Quartz is nearly always present, but in vary- 

 ing amount, some ■ specimens showing very little, while others contain 

 considerable. Biotite occurs in places and in several localities it is pres- 

 ent in sufficient amount to be designated a characterizing accessory. 

 There are fades of the rock in which hornblende is an important con- 



