Watson and Cline — Extrusive Basalt. 665 



Art. L. — Extrusive Basalt of Cambrian Age in the Blue 

 Ridge of Virginia ; by Thomas L. Watson and Justus H. 

 Cline, University of Virginia. 



The James River Gap section of the Blue Ridge of Vir- 

 ginia early attracted the attention of geologists, because of the 

 splendid exposures of the rocks and the excellent opportunities 

 offered for their study. As early as 1835 Professor William 

 Barton Rogers, State Geologist of Virginia, was attracted to 

 this section and frequent references are made to it in his 

 annual reports on the Geology of the Virginias. In later years 

 important contributions to our knowledge of the geology of 

 this section were published by Professors Fontaine, J. L. and 

 H. D. Campbell, and others, and our present general concep- 

 tions of the geology of the Blue Ridge in middle Virginia are 

 based largely on the results obtained by these investigators in 

 the James River Gap. 



These investigators have shown that the Blue Ridge in this 

 section is composed of a central core of pre-Cambrian granitoid 

 igneous rocks, chiefly syenitic in character, flanked on both the 

 southeast and northwest sides by siliceous sediments. On the 

 northwestern slope the sediments were designated by Rogers 

 as Formation No. 1 (Lower Cambrian), but the corresponding 

 sediments on the southeastern slope were considered as prob- 

 able pre-Cambrian (Huron ian) in age, because chiefly of their 

 more highly metamorphosed condition and the great difference 

 in thickness of the beds as compared with those on the north- 

 western slope. All doubt, however, as to the age of the sedi- 

 ments on the southeastern slope was dispelled in 1884: when 

 Professor J. L. Campbell reported that borings of scolithus 

 linearis had been discovered by H. D. Campbell in the 

 quartzites which underlie the slates of the Snowden slate belt.* 

 This discovery correlated the sediments on the two sides of the 

 Blue Ridge in this section and at the same time definitely estab- 

 lished its anticlinal structure. In 1892 Walcottf discovered, 

 on the south side of James River opposite Gilmore, the Ole- 

 nellus fauna in a calcareous sandstone above the Scolithus 

 quartzite of the Balcony Falls section, which established the 

 Lower Cambrian age of these sediments. 



During the progress of recent investigations by the Virginia 

 Geological Survey in the vicinity of James River Gap in the 

 Blue Ridge, well-defined occurrences of extrusive basalt were 



* Campbell, J. L. : Geology of the Blue Eidge near Balcony Falls, Virginia, 

 a Modified View, this Journal (3). vol. xxviii. pp. 221-223, 1884. 



f Walcott, C. T> : Notes on the Cambrian Rocks of Virginia and the South- 

 ern Appalachians, this Journal (3), vol. lxiv, pp. 52-57, 18S2. 



