194 watson and cline rocks of the blue ridge region 



Introduction 



The Blue Ridge, which forms the extreme eastern member of the Appa- 

 lachian Mountains, constitutes one of the principal topographic divisions 

 of the Appalachian ranges. In Virginia the Blue Eidge Mountains form 

 a fairly continuous and well defined ridge extending from Harpers Ferry 

 southwestward entirely across the State. At Harpers Perry the Blue 

 Eidge Mountains are narrow, and in elevation are less than 1,000 feet 

 above sealevel; but southwestward through Virginia the ridge becomes 

 broader and higher, and attains its greatest width in North Carolina. 

 Heights of more than 4,000 feet above the sea are reached at several 

 points in Virginia. 



The Blue Eidge is composed of a central core of igneous rocks, flanked 

 on the northwest side by the folded sedimentary series of Cambro-Ordo- 

 vician rocks of the Great Valley province. The basal member of this 

 series is a quartzite (Weverton), which extends for much of the distance 

 as a range of hills along the Avest flank of the main ridge at an altitude 

 equal in some cases to that of the Blue Eiclge. Eemnants of the Cam- 

 brian series of sediments are also preserved in places along the southeast 

 slope of the Blue Eidge and at several points in the vicinity of James 

 Eiver Gap. The sediments are arched in anticlinal fashion entirely over 

 the ridge, completely concealing for short distances the central core of 

 igneous rocks. The southeast slope merges into the Piedmont Plateau, 

 along which, in places, are groups of outlying Ioav ridges that have been 

 isolated by erosion from the main ridge, but exhibiting as a role similar 

 rock types. 



In the middle and northern parts of the Blue Eidge and the adjacent 

 portions of the Piedmont Plateau in Virginia one of the dominant igneous 

 rocks of granitoid type is a quartz-bearing pyroxene syenite. The igneous 

 complex, of which pyroxene syenite is the chief type, may represent a 

 Precambrian batholithic intrusion, exposed at intervals for a distance of 

 150 miles in a belt up to 30 miles or more in width.' Differentiation of 

 the syenite magma has given rise to a variety of related rocks, some of 

 which are of particular interest. 



Studies of the igneous complex forming the central core of the Blue 

 Eidge in middle and northern Virginia are sufficiently advanced to indi- 

 cate that the rock types exhibit certain kinships which mark them as 

 differentiates from a common magma, and that this igneous complex, 

 designated by the writers as the Blue Eidge petrographic province, shows 

 certain important differences in mineralogy and chemistry from the 

 igneous rocks which enter into the composition of the Piedmont Plateau 

 to the east. 



