226 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



IV, and V) and of norites from Maryland (VI) and the Adirondacks 

 (VII and VIII). 



The area that has been studied in greatest detail in the Blue Ridge 

 proper lies north of James Elver Gap, on the two slopes of the Blue Ridge 

 in Amherst and Bockbridge counties. Other areas that have been studied 

 and published on are the Amherst-Nelson counties comagmatic area 40 

 near the southeast foot of the Blue Bidge and the Hemlock area 41 in 

 Floyd County, southwest Virginia. The gabbro of the Floyd County 

 area is a pyrrhotite-rich hornblende norite containing biotite and a little 

 olivine. 



Several areas of ga.bbroic rocks of the hornblendic variety have been 

 studied in the Bobinson Gap section, about 8 miles northeast of Balcony 

 Falls. One of the areas is located in Bockbridge County, on the north- 

 west slope of the Blue Bidge ; the others occur in Amherst County, on the 

 southeast slope of the ridge. The principal body of gabbro is found in 

 the Beverly Settlement, in Amherst County. It is essentially a horn- 

 blende norite, although a biotite facies occurs, but is not abundant. The 

 smaller areas of gabbroic rocks are described below, under pyroxenite. 

 The gabbroic rocks, including pyroxenite, and the syenite are intimately 

 associated, but the evidence is not yet conclusive as to what are their 

 exact field relations. Small bodies of the syenite are found in the larger 

 gabbro mass, while similar bodies of pyroxenite are associated with the 

 gabbro, from which they differ chiefly in the presence of scant feldspar 

 and the substitution of biotite for hornblende. Smaller areas of pyrox- 

 enite are distributed as dikelike bodies in the syenite and may represent 

 either independent intrusions as dikes or segregations as schlieren in the 

 syenite body. That the rocks are differentiates from a common magma 

 is entirely evident on mineralogical and chemical relationships, but 

 whether they represent separate intrusions or variants from differentiation 

 in place can not be definitely answered at this time. Further detailed 

 investigations of this igneous complex in the region immediately to the 

 north will be necessary before this question can be settled. In general 

 the chief difference noted in the gabbros and the more abundantly asso- 

 ciated type syenite is in the proportion and not in the kind of minerals, 

 for the important constituents in one type are usually present in greater 

 or less quantity in the other. 



MEGASCOPIC CHARACTER 



For the areas studied the gabbroic rocks are of dark gray to nearly 

 black color and of medium, even-granular texture. They show distinct 



*° Watson and Taber : Bull. III-A, Virginia Geol. Survey, 1913, pp. 42-234. 



tt T. h, Watson : Trans, Amer. Inst. Mng. Engrs., vol. xxxviii, 1907, pp. 683-697. 



