20 J. L. Campbell—Silurian Formation in Virginia. 
Here, then, we have a plateau, rather than a valley, with an 
average elevation above the sea of about 1200 or 1300 feet. 
This is much above the average elevation of the Mississippi 
Valley. It is in reality a of the great belt of uplift that 
constitutes the Appalachian Range, but erosive agencies have ~ 
stripped it of the greater part of its mountain-making masses. 
The Blue Ridge, which now forms its southeast border, was 
once the shore-line of the great primal ocean that covered the 
Mississippi Valley (including “ Appalachia”) during the re- 
mote ages of geological histo 
At present the streams of water in the valley tend towards — 
the southeast margin all the way from the Potomac to Salem, © 
in Roanoke County. This is most strikingly the case in the — 
basins drained by the Roanoke and the James Rivers, thus indi- 
cating less elevation on that side than on the other. I think we 
shall learn hereafter that this is most probably the result of — 
same rocks forming subordinate limestone valleys, but they 
must be left out of our present discussion. 
—My purpose is to give in the first place a section — 
divisions and sub-divisions, and some leading peculiarities of 
each, will, I think, illustrate the geology of this middle part of — 
the State in a manner, and to an extent, not hitherto attempted — 
by any one. 
I am indebted to the partial survey of Virginia, made under — 
the direction of the venerable and distinguished geologist, Pro- 
fessor W. B. Rogers, for guidance and aid in my own investi-— : 
gations and for many of the facts contained in this communica- 
tion. The line of section here given has been carefully explored 
and re-explored throughout its whole extent, several times. 
crosses a portion of the valley not heretofore represented in 
section, so far as I know; and while it may be regarded, to a 
certain extent, as typical of this region of the State for some 
miles on each side of its’ line, it presents some peculiarities 
worthy of special notice. These will be discu in future. 
For the present a general description must suffice. 
ffi 
The southeast extremity is on the slope of the Blue Ridge _ 
beyond Robinson’s Gap, and extends one mile past the line _ 
~onsoemommmm 
