666 Watson and Cline — Extrusive Basalt. 



noted which, from their relations to the associated sediments, 

 must be assigned to Lower Cambrian age. In this part of the 

 Blue Ridge proper the well-known pre-Cambrian basaltic rock 

 (Catoctin schist) which makes up a considerable part of the 

 mountain farther north does not occur, but instead the central 

 core of the main ridge is formed entirely of a quartz-bearing 

 hypersthene-syenite and related granitoid rocks not later than 

 Algonkian in age. 



The earliest sediments deposited on the eroded surface of 

 the pre-Cambrian igneous rocks, which form the central core 

 of the Blue Ridge in the James River section, are of Lower 

 Cambrian (Loudoun) age, and are made up of conglomerate 

 and arkose and liner material such as slate and shaly sandstone. 

 The Loudoun is succeeded conformably by a series of siliceous 

 and argillaceous sediments, the age of which has been deter- 

 mined on fossil grounds to be Lower Cambrian. They include 

 the Weverton sandstone, Harper's shale, and Antietam sand- 

 stone, which extend more or less continuously along the north- 

 west slope of the Blue Ridge in northern Virginia. 



At several points in the vicinity of James River Gap Cam- 

 brian sediments are arched in anticlinal fashion entirely over 

 the Blue Ridge and for short distances completely conceal the 

 pre-Cambrian igneous rocks. Remnants of this Cambrian 

 series of siliceous sediments are preserved in relatively small 

 synclinal basins in places on the southeastern slope of the Blue 

 Ridge. As a rule only the Loudoun member is found in these 

 positions, since in most places the later beds have been removed 

 by erosion. 



In the vicinity of James River Gap, however, the conditions 

 are somewhat unusual. Here we find in a complexly folded 

 synclinal basin near the southeastern base of the main ridge 

 the three lowest members of the series, including the Loudoun, 

 Weverton, and Harpers, with the Weverton abounding in bor- 

 ings of scolithus linearis. 



Well-defined occurrences of extrusive basalts are found within 

 the Loudoun near its contact with the granitoid rocks in the 

 James River Gap and vicinity. The one first observed by the 

 writers, and which shows most clearly the relations of the lava 

 both to the overlying and underlying sediments, is located 

 along the road slightly less than half a mile northwest of 

 Snowden Station. The geologic relations at this point are 

 shown in the accompanying section (fig. 1), which shows the 

 entire Loudoun conformably overlain by two massive beds of 

 Weverton quartzite, one of which contains numerous borings 

 of scolithus. The basement on which the Loudoun rests is the 

 usual granitoid rock forming the central core of the Blue Ridge 

 in this section, which at this point is pink in color after the 

 predominant feldspar. 



