334 WATSON AND CLINE IGNEOUS DIKES IX VIRGINIA 



coarse to fine and dense aphanitic. They are characterized by a variety 

 of texture — porphyritic, diabasic, and even granular — the latter ranging 

 from medium-grained in some of the nephelite syenite to dense aphanitic 

 in the basalts. TTith the exception of the diabase, each rock type may 

 show porphyritic texture. In the granite-felsophyre and nephelite syenite 

 the phenocrysts are composed chiefly of the felsic constituents, orthoclase 

 and albite in granite-felsophyre, and orthoclase in nepheline syenite. 

 Scant development of mafic minerals is also observed, a little biotite and 

 augite in granite-felsophyre, and a few plates of biotite with an occasional 

 crystal of hornblende in nepheline syenite. In the basic types (tesch- 

 enite, camptonite, and ba-salt) the phenocrysts are dominantly mafic min- 

 erals, including hornblende, augite, olivine, and less biotite. In addition 

 to the mafic minerals, analcite is frequently developed as phenocrysts in 

 the teschenite and camptonite. 



Age Relations of the igneous Rocks 



Because of the relationship of the dikes of the middle western district 

 of Virginia, they are all regarded to be of the same age. Since they are 

 found cutting the latest sedimentary rocks of the region, Mississippian. 

 the dikes must be post-Mississippian in age. Furthermore, since the 

 dikes show no evidence of having been subjected to the folding and fault- 

 ing of the Appalachian revolution, they are post-Paleozoic. On the basis 

 of distribution and structural relations, they appear, in light of our pres- 

 ent knowledge, to be Triassic, as has been pointed out in another section 

 of this paper. On the basis of lithologic character the diabases alone 

 afford any evidence. In this respect they are identical with the diabase 

 found in the numerous Triassic areas of the eastern United States. In 

 view of these facts, the writers regard the dikes of central western Vir- 

 ginia to be of Triassic age. 



