18 Ji. L. Campbell—Silurian Formation in Virginia. 
not single, whether viewed lengthwise or crosswise. From a 
few miles southwest of Winchester to a point nearly opposite 
Harrisonburg, it is divided into two subordinate valleys, by the 
Massanutton Mountains—a long belt of ridges of Upper Silu- 
rian and Devonian rocks that withstood the denuding agencies 
that uncovered so many hundreds of square miles of the 
wer Silurian limestones. Less extensive ridges also inter- 
(5.) Elevations.—At Harper's Ferry, where the Potomac 
leaves the Great Valley, the height above tide-level is only. 
about two hundred and forty feet; but when we reach the head 
waters of the Shenandoah, we have arrived at a water-shed 
having an average height of nearly 1800 feet. Then, in pass- 
ing on to the south corner of Rockbridge, we come to the 
“pass” of the James, at Balcony Falls, having an elevation of 
about 700 feet. The Roanoke Valley has about the same — 
: “f 
average elevation as that of the James Valley, 1200 feet; but 
on fising to the margin of New River Valley, near Christians- 
urg, in Montgomery County, we are about 2000 feet high; 
and on the southwest margin, at Mount Airy—the summit of _ 
the A. M. & O. Railroad—2600 feet. Many points on the Blue 
Ridge are not higher than this highest po of the great lime- 
or — At the Tennessee line the height is less than 
Bs ee) ‘ ; 
* This is often spoken of as if it were a single mountain—azd so it appears to 
reality, there are two short parallel ridges _ ; 
be as Ww in 
nearly a mile apart, cut off abruptly at both ends. 
