232 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



CHEMICAL COMPOSITION AND CLASSIFICATION 



A chemical analysis of the biotite-feldspar-bearing pyroxenite from the 

 northwest slope of the Blue Eidge in Eockbri'dge County is given in 

 column I below. Analyses of two other well known pyroxenites — one 

 from Baltimore County, Maryland (II), and one from Webster, North 

 Carolina (III) — are tabulated for comparison. I is a feldspar-biotite- 

 bearing hypersthene-cliallage mixture, II a bronzite-diopside aggregate, 

 and III is a bronzite-diallage mixture. Thus on the basis of mineral 

 composition each of the rocks represents a different type of pyroxenite, 

 a difference clearly expressed chemically in the analyses below. In the 

 Virginia rock (analysis I) the presence of feldspar manifests itself in 

 the increased percentages of alumina, lime, and alkalies. The greater 

 richness of the Maryland (II) and the North Carolina (III) rocks in the 

 enstatite molecule (bronzite, which is hypersthene in the Virginia rock) 

 accounts for the greatly increased percentage of magnesia, which is about 

 double that of the Virginia rock. 



The difference in composition of the three rocks is further shown in 

 the table of norms below. As indicated by their positions in the quanti- 

 tative system, the Virginia rock is a dofemane, while the Maryland and 

 North Carolina rocks are perfemanes. 



Analyses of Pyroxenite 



I II III 



Si0 2 50.08 50. SO 55.14 



A1 2 3 6.56 3.40 .66 



Fe 2 3 1.56 1.39 3.48 



FeO 8.94 8.11 4.73 



• MgO 12.95 22.77 26.66 



OaO 16.14 12.31 8.39 



Na 2 .89 trace .30 



K 2 .46 trace none 



h 2 o+ ".'!!.'!!.'.'!!!!"!""!'!"!•.'!"] '.32 [ - 52 - 38 



Ti0 2 1.90 none trace 



P 2 5 19 trace .23 



MnO 40 .17 .03 



Cr 2 3 .32 .25 



CI .24 



S 37 trace 



100.95 100.03 100.25 



I. Biotite-feldspar-bearing pyroxenite from northwest slope of the Blue 

 Ridge in Robinsons Gap, Rockbridge County, Virginia. J. G. Dinwid- 

 die, analyst. 



