NORITE 231 



Summary of Classification 



Number. 





Symbol. 





Name. ■ 



I 



III. 



4'. (3)4. 



4. 



no name 



II 



I V. t ,3. 1. 2. 



3. 



roselandose 



III 



III. 



5. 4. 4. 





auvergnose 



IV 



'III. 



4'. 3. '4. 





vaalose 



V 



'III. 



5. 3'. 4. 





andose-ca mptonose 



VI 



III. 



5. 4. 3. 





auvergnose 



VII 



III. 



5. 3. 4. 





eamptonose 



VIII 



III. 



5. 3. 4. 





camptonose 







Pyroxentte 









DISTRIBUTION 





The several small areas of gabbroic rocks referred to under gabbro are 

 here classed as pyroxenite. The areas are in the same general region, but 

 are rather widely separated. One occurs in Eockbridge County near the 

 northwest slope of the Blue Eidge, about 3 miles southeast of Buena 

 Vista ; the other occurs in Amherst County, on the southeast slope of the 

 Blue Eidge, on the Eobinson Gap Eoad, three-quarters of a mile southeast 

 of the church in the Beverly Settlement. 



MICROSCOPIC CHARACTER 



In mineral composition the rock can not be considered a typical repre- 

 sentative of either the gabbro or the pyroxenite group, but must be re- 

 garded as a transitional or intermediate type, as confirmed by the analyses 

 on page 232. In some respects it is more closely allied with the gabbros 

 than with the pyroxenites, but because of its being essentially a pyroxene 

 aggregate with considerable biotite and subordinate feldspar it has 

 seemed best to group it with the pyroxenites. The rock is the same in 

 the several areas studied and differs essentially from the gabbro of the 

 same district in containing less feldspar and hornblende, but increased 

 amounts of augite (diallage), hypersthene, and biotite. Because of the 

 great preponderance of the dark-colored mafic minerals, the pyroxenite 

 is much darker in color (nearly black) than the normal gabbro. 



Microscopic study of thin sections indicated the same optical properties 

 of the minerals as for those in the gabbro described above. Augite 

 (diallage) is in excess of hypersthene and some original quartz and ortho- 

 clase occur. Biotite is more abundant than hornblende, the latter failing 

 entirely in several thin sections. Because of the presence of some feldspar 

 and much biotite, the rock is designated a biotite-feldspar-bearing py- 

 roxenite. 



