J. L. Campbell—Silurian Formation in Virginia. 17 
southeast rising to heights ranging generally between 2000 and 
4000 feet above tide-level; and the North Mountain range on 
the northwest, almost equally high at many points. (2.) The 
axial line of the Blue Ridge (which consists chiefly of Archeean 
rocks) has but few gaps through which streams of water can 
ass. Not a single outlet of any considerable size is found for 
the waters of the valley through this ridge anywhere between 
Harper’s Ferry on the Potomac and Balcony Falls on the 
James—a distance of one hundred and fifty miles. The only 
other water-gaps are the one through which the Roanoke 
(afterwards the Staunton) River — towards the southeast, — 
1 
y which the waters of New 
and cherty ridges are ries erage of such dimensions that in 
t 
ry. 
(4) Any good map of Virginia will show that this valley is 
Am. Jour. ceetaeeee SERrEs, VoL. XVIII, No. 103,.—J ULY, 1879, 
