330 WATSON AND CL1NE IGNEOUS DIKES IN VIRGINIA 



general petrologic relations of the rock types to each other. These rela- 

 tions (regional characters) are briefly summarized under (a) chemical, 

 (b) mineralogical, and (c) textural characters. 



The rocks of the region are naturally grouped into two principal sub- 

 divisions : ( 1 ) Those in which the f elsic constituents are dominant, in- 

 cluding felsophyre, quartz gabbro (norite), and nepheline syenite; and 

 (2) those in which the mafic constituents are dominant, including dia- 

 base and basalt," teschenite, and camptonite. The rocks show certain evi- 

 dences of kinship in chemical and mineral ogical composition, and to a less 

 extent in texture, which mark them as differentiates from a common or 

 parent magma, and are interpreted broadly as representing a group or 



Mossy Creek 



— © 



Mr Solon 



^f 1 * * Mi/es ®Mosco» 



Figure 5. — Diagram illustrating kinship Relations of Dikes in northern Augusta County, 

 Virginia, on the Basis of relative Distribution 



(a') Nepheline syenite containing biotite, a little hornblende and augite as mane 

 minerals, intimately associated with (c7) teschenite containing the same mafic minerals; 

 (a) nepheline syenite containing hornblende as principal mafic mineral associated with 

 (c) camptonite in which hornblende is the similar dominant mineral; (b) quartz gabbro 

 (norite). The powder of each of these rocks, except (b) quartz gabbro, when boiled 

 with hydrochloric acid, filtered and evaporated, yields copious gelatinous silica. 



series of complementary dikes. This relation does not admit of reason- 

 able doubt for the closely parallel series of nepheline syenite, quartz 

 gabbro, teschenite, and camptonite dikes found in the northern part of 

 Augusta County, 4 miles southwest from Bridgewater (see map, figure 1, 

 and figure 5). Figure 5 illustrates most strikingly this relation. 



CHEMICAL CHARACTERS 



The chief chemical characters of the rocks of the region as developed 

 from the table of analyses below may be summarized as follows : Silica 

 with one exception (I) is generally low and deficient. Alumina is fairly 

 uniform except in II and III (nepheline syenites, miaskose), where it 



