210 WATSON AND CLINE— ROCKS OF THE BLUE RIDGE REGION 



common magma. They are characterized by prominence of apatite and 

 the titanium minerals, ilmenite, rutile, and in a few places titaniferous 

 magnetite; opalescent blue quartz; pyroxene (hypersthene) or secondary 

 hornblende (uralite) derived from pyroxene as the dominant mafic min- 

 eral; and in most of the feldspar-bearing types by andesine antiperthite, 

 with inclosed orthoclase (microcline) spindles as the dominant feldspar. 

 They have been intensely, but unequally, metamorphosed, and in most of 

 them complete or partial foliation has been developed. 



The principal rock types that have been mapped by the Virginia 

 Geological Survey include (1) biotite-quartz monzonite gneiss with 

 variant schists, (2) syenite (andesine anorthosite), referred to as pegma- 

 tite in all publications previous to Bulletin III-A of the Virginia Geolog- 

 ical Survey; (3) gabbro, (4) nelsonite, and (5) diabase. Intermediate 

 gradations are observed between most of these types, and usually the 

 dominant minerals in one type form the. subordinate minerals in the 

 others. Of the five rock types enumerated syenite is of most importance 

 in this paper. 



The syenite covers an area of about 20 square miles. Tt is, as a rule, 

 closely crystalline and in places gneissoid or even schistose in structure. 

 In some places along its southeastern border the rock exhibits abnormally 

 coarse texture. It is composed chiefly of feldspar and blue quartz, but 

 contains also, in places near the margin, pyroxene (hypersthene usually 

 altered to hornblende), rutile, and lesser amounts of ilmenite and apatite. 

 The ratio of these minerals varies greatly in different parts of the rock- 

 mass, but feldspar is the dominant mineral, except in portions of the 

 border zone. Two facies of the rock which apparently grade into each 

 other are recognized — a feldspathic variety corresponding to dosodic 'pied- 

 montose and a hornblendic variety, which is developed chiefly as a border 

 zone and corresponds to dosodic tonalose. The central and larger portion 

 of the mass consists almost exclusively of feldspar, with only scattered 

 grains of blue quartz and scarcely any visible rutile or other minerals. 

 The principal feldspar is a calcic soda variety (andesine) corresponding 

 to about Ab 65 An 35 and containing intergrowths of microcline oriented 

 parallel to the twin lamellae. In the feldspathic facies of the rock norma- 

 tive orthoclase (microcline) ranges from 16.68 to 23.35 per cent, with an 

 average of 19.88 per cent. Total normative feldspar averages 88.63 per 

 cent. 



The composition of the syenite is shown in the chemical analyses given 

 below, which should be compared with those of the Blue Ridge syenite on 

 page 202. 



