Watson and Cline — Extrusive Basalt. 669 



They possess, however, a distinct thin -bedded structure (lamina- 

 tion) and a perfect cleavage parallel to the bedding planes, giv- 

 ing very smooth surfaces. The rock in thin section is com- 

 posed of considerable terrigenous material, chiefly quartz grains 

 along with some feldspar fragments. The quartz grains are in 

 part rounded and in part angular, with the former in greatest 

 abundance. Feldspar fragments are altered largely to an 

 aggregate of secondary minerals, chiefly sericite, and in some 

 cases epidote. Magnetite mostly in small octahedra is very 

 abundant. Most of the thin section is composed of very fine- 

 grained material, much of which is sericite and quartz and 

 some of which appears to be microscopic needles of feldspar. 

 Some chlorite is also present. 



Specimens of the dark slates found on the northwest slope 

 of the Blue Ridge near the Rockbridge-Amherst counties line 

 and a short distance east of Balcony Rock are fine-grained 

 rocks, showing distinct bedding and a well-developed secondary 

 cleavage which cuts the bedding planes at an angle of about 45 

 degrees. The only mineral recognizable in the hand specimen 

 is pyrite which occurs as disseminated glistening grains along 

 the bedding planes. 



Microscopic examination shows that the rock contains a 

 large amount of feldspar in rounded and angular grains almost 

 entirely altered to an aggregate of secondary minerals, chiefly 

 sericite. Quartz occurs abundantly, but less so than feldspar, 

 usually in well-rounded grains. Zircon is fairly common. In 

 addition to these the coarser grained constituents of the rock 

 include magnetite much altered to leucoxene and fairly 

 abundant crystals of pyrite. This material, most of which is 

 certainly of terrigenous origin, is imbedded in a fine-grained 

 matrix very similar in character to the principal material com- 

 posing the slates associated with the amygclaloidal basalt one 

 and a half miles northwest of -Snowden. Although micro- 

 scopic examination does not establish a tuffaceous origin for this 

 rock, its stratigraphic position, which is identical with that of 

 the basalts along the southeastern slope of the Blue Ridge and 

 its resemblance otherwise to the basalts, suggests a probable 

 pyroclastic origin. 



The strong resemblance of some of the Cambrian basalt to 

 the Catoctin schist renders the distinction between the two 

 difficult on lithologic grounds alone. It is only where the 

 exact relations of the basalt to the Cambrian sediments can be 

 determined that the two basalts may be distinguished. They are 

 of similar origin and it is quite probable that the later basalt 

 represents the product of waning volcanic activity extending 

 upward into the Lower Cambrian from the Algonkian, which 

 was noted for widespread igneous activity in the Blue Ridge 

 region. 



University of Virginia, March 22, 1915. 



