16 J. L. Campbell—Silurian Formation in Virginia. 
while at the time of the observations in table III, there was an 
unusually strong current from the north or northwest, whic 
reached to a height of more than 10,000 feet, and swept 
over the United States from the Pacific Ocean to the Atlantic. 
The cases enumerated in table III are remarkable on account 
of the long continuance of the movement of storm centers from 
north to south, but the published volumes of the Signal Service 
observations show many other cases in which storms pursued a 
similar course for twenty-four hours or more. 
In preparing the materials for this article, I have been assisted 
by Mr. Henry A. Hazen, a graduate of Dartmouth College of 
the class of 1871. 
Art. Il.—Silurian Formation in Central Virginia ; by J. L. 
CAMPBELL, Washington and Lee University. 
Limits.—W hat is known as the “Great Valley of Virginia” 
occupies a belt of country extending entirely across the State 
from the Tennessee line on the southwest to the Potomac on 
the northeast—including Jefferson and part of Berkeley County, 
now a portion of West Virginia. It has mountain boundaries 
throughout its whole extent. On its southeastern margin it is 
separated from what is called “Piedmont Virginia,” by the : 
Blue Ridge and its southwest prolongations, Poplar-Camp and 
Iron Mountains. On the northwest side we find a somewhat 
irregular line of broken ridges bearing different names at differ- 
ent points. Through several of the southwest valley counties 
it is called “Walker's Mountain.” In Botetourt, Rockbridge © | 
and Augusta, it is called “North Mountain,” while through 
the remainder of the distance to the Potomac it is called 
“Little North Mountain.” The length of the Valley, from the 
Tennessee line to the Potomac, is about three hundred and thirty 
miles. Near its southwest extremity, in Washington County, it _ 
is about twelve or fifteen miles wide, and becoming gradually _ 
wider it extends towards the northeast. We find it in Rock- — 
bridge and Augusta varying in breadth from twenty to twenty- — 
five miles. Its total area, embracing the contiguous mountain 
2 ts on each side, is not much short of 6,000 square miles. 
ts 
Ls iphy.—With the exception of a limited belt 
occupied by the Massanutton range in its northeast parts, and — 
some strips covered by outliers of North and Walker’s Moun- 
tains, this extensive zone has for its surface one continuous _ 
ia a 
oF SE ee ae 
dge on the — 
