438 J, L. Campbell— Geology of Virginia. 
Conceive a vertical plane with its edge resting on a line rep- 
resented by the broken line of the map, marked “S.E.,” and 
“N.W.,” and having a height of 1500 feet above the bed of 
the river. Then imagine all the outcropping faces and edges 
of all the eroded rocks of the gorge, and al] that the plane itself 
would cut (including those of Salling’s Mountain), to be pic- 
tured on the plane, and you will have a mental conception of 
what the section is designed to represent. 
The student of geology will find here a somewhat ag 
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but a very interesting problem for solution. By 
careful observations along the canal and bed of the river, and 
also by the turnpike that crosses the mountain near the canal, 
very satisfactory conclusions may be reached. In the gorge 
we have the rocks of two distinct eras so meeting as to enable 
us to study not only their composition and structure, but also 
their relative positions, and some of the metamorphic influences 
they have exerted upon one another. These two eras are, (1 
the Archean, represented on the accompanying section by the 
rocks on the right marked G, S, and 1 a, 6; (2) a portion of 
the Lower Silurian covering the remainder of the section. 
Let us begin at the base of the Archean. Here we find two 
