Watson and Chine — Extrusive Basalt. 



667 



The basal bed of the Loudoun is an arkosic conglomerate 

 containing numerous well-rounded fragments of the under- 

 lying pink granitoid rock up to 2 inches or more in diameter, 

 imbedded in a finer-grained matrix of highly feldspathic sandy 

 material. The lower bed is 80± feet in thickness, and is suc- 

 ceeded by a sheet of fine-grained basaltic lava of 75± feet in 

 thickness which is well exposed along the road and in a small 

 ravine near by. The bottom portion of the lava sheet contains 

 numerous fragments of the underlying arkosic conglomerate, 

 while the upper surface is amygdaloidal in texture, the amyg- 

 dules being composed of epidote and quartz, chiefly the latter. 

 The lava sheet is succeeded by a second bed of arkosic con- 



FlG. 1. 



Weve rton 



Sandstone 



glomerate 65± feet thick, similar to the one on which it rests. 

 The basal part of the upper conglomerate contains numerous 

 fragments of the basalt, which passes upward into finer-grained 

 arkose and sandy shales that are ultimately succeeded conform- 

 ably by the fossiliferous Weverton quartzite. 



In addition to the occurrence of Cambrian basalt shown in 

 accompanying section (fig. 1), two others were noted near the 

 southeastern part of the Blue Ridge in the same vicinity and in 

 the same stratigraphic position. On the Grant tract about two 

 miles north of Snowden, slaty basaltic material was observed 

 associated with the Lower Cambrian sediments near their con- 

 tact with the pre-Cambrian granitoid rocks. About one-and-a- 

 half miles northwest of Snowden in a small syncline at the 

 southeastern foot of Rocky Row Mountain a small area of 

 Cambrian sediments occurs, a very good cross section of which 

 is exposed along the wagon road leading from Snowden to 

 Balcony Falls. In the southeastern limb of this syncline extru- 

 sive basalt, similar to that near Snowden, is interbedded with 

 the Loudoun sediments and separated from the pre-Cambrian 

 rocks by a slight thickness of arkosic conglomerate. The lava 

 at this point is in part highly amygdaloidal, but in the outcrop 

 the amygdules have been largely removed, giving the rock a 



