212 WATSON AND CLINE- — ROCKS OF THE BLUE RIDGE REGION 



Summary of Classification 



Number. Symbol. Name. 



I I. 5. 3. 4 piedmontose 



II II. 4. 3. 4 tonalose 



When the Blue Ridge type of rock here described is compared with the 

 rutile-bearing type of syenite (andesine anorthosite) of Amherst and 

 Nelson counties, Virginia, the dissimilarity is seen to he equally as great 

 as in the akerites, yet striking similarity in mineral composition in part 

 is indicated. Both types are characterized by andesine as the dominant 

 feldspar, but of a very unusual kind in the Nelson County rock, which is 

 composed of andesine antiperthite with inclosed orthoclase (microcline) 

 spindles. Feldspar intergrowths occur, but seem not to be characteristic 

 of the main Blue Eidge type. Hypersthene, although largely altered to 

 uralitic hornblende in the Nelson County rock, is a constituent of both 

 types. Neither monoclinic pyroxene (augite) nor primary hornblende 

 and biotite, mineral components of the Blue Eidge type, are known to 

 occur in the Nelson County rock. Both types are quartz-bearing, but the 

 quartz of the Blue Eidge rock is gray, while that of the Nelson County 

 rock is deep blue, due to a crowding of its substance with abundant very 

 minute needles of rutile, which are essentially absent from the quartz of 

 the former type. 



The dominant facies of the Nelson County rock is composed largely of 

 feldspar, averaging more than 85 per cent of this mineral, the principal 

 variety of which is andesine, with important amounts of orthoclase 

 (microcline), ranging up to 20 per cent; subordinate quartz, and negli- 

 gible amounts of mafic minerals. It is a rutile quartz-bearing anorthosite 

 composed of andesine antiperthite and is a dosodic piedmontose. The 

 border or hornblendic facies of the rock-mass contains less feldspar, the 

 dominant variety of which is andesine, but more quartz of deep blue color, 

 and considerable uralitic hornblende derived from hypersthene. This 

 facies of the rock corresponds to a dosodic tonalose. 



The rocks of the Amherst-Nelson counties comagmatic area, of which 

 syenite (andesine anorthosite) is a principal type, are so related, both as 

 to geographic position and broad geologic relations, as to leave no doubt 

 of their belonging to the larger Blue Eidge petrographic province. 



COMPARISON WITH PYROXENE SYENITE OF THE ADIRONDACKS 



The Adirondack highland consists entirely of a Precambrian rock com- 

 plex rimmed by early Paleozoics. The Grenville series, which is Pre- 

 cambrian, is the only sedimentary series yet recognized in the region, and, 

 so far as known, it is older than the associated igneous rocks, which consist 

 of anorthosites, syenites, granites, and gabbros — all more or less meta- 



