668 Watson und Cline — Extrusive Basalt. 



vesicular appearance. Intimately associated with the basalt is 

 a dark-colored fine-grained slate showing distinct evidence of 

 bedding which is considered by the writers and described 

 below as a probable tuff partly composed of terrigenous 

 material. On the northwest slope of the Blue Ridge dark- 

 colored slates occur in James River Gap near the Rockbridge- 

 Amherst counties line a short distance east of Balcony Rock. 

 The slates show the same color and occupy the same strati- 

 graphic position as the basalts along the southeastern foot of 

 the mountain and although they contain much clastic material 

 which is certainly terrigenous in origin, the writers con- 

 sider that probably they are in part igneous and are to be 

 correlated, on this account, with the basalt on the southeast 

 slope of the mountain. Basaltic rocks also occur in the James 

 River Gap section at points east and south of Snowden. About 

 two miles south of east from Snowden and a short distance 

 northwest of the Jordan furnace, basaltic material occurs near 

 the contact of the granitoid rocks and Cambrian sediments ; 

 and about 2 miles south of James River near Peters Creek, 

 there is an abundant occurrence of basalt in a spur of Piney 

 Mountain, with outcrops of both Cambrian sediments and 

 syenite nearby. These two latter occurrences of basalt are 

 probably to be correlated with those of unmistakable Cambrian 

 age in the vicinity of Snowden, although their exact age rela- 

 tions could not be determined in the field. 



These basalts are dark-colored aphanitic rocks, usually show- 

 ing pronounced slaty cleavage. In part they are amygdaloidal 

 with the cavities either partially or completely filled with 

 epidote and quartz. In some of the more highly weathered 

 portions of the rock the amygdules have been largely removed, 

 leaving the walls of the cavities stained with limonite. In 

 some specimens the amygdules are abundant, but they are 

 usually small, only a few attaining a size as great as a quarter 

 of an inch in diameter. 



Microscopic study of thin sections of the basalt shows that 

 the rock has pronounced ophitic texture. While the lath- 

 shaped feldspars have largely altered to an aggregate of 

 secondary minerals, including chiefly quartz, sericite, kaolin, 

 and finely divided calcite, their original outlines are perfectly 

 preserved. With the exception of magnetite which is abundant 

 in small octahedra and shapeless grains, whatever dark con- 

 stituents the rock originally may have contained have entirely 

 altered. The partial alteration of the magnetite has resulted 

 in much leucoxene, which is an abundant constituent in all sec- 

 tions studied. The rock also contains considerable chlorite, some 

 epidote, and a small amount of secondary rutile. 



The dark-colored slates associated with amygdaloidal basalt 

 at the foot of Rocky Row Mountain about one -and a half miles 

 northwest of Snowden strongly resemble the basalts in color. 



