442 J. L. Campbell— Geology of Virginia. 
been found in them. In the portion near the river their dip 
varies from 25° to 50°. Lestimate their thickness at 180 feet. 
umber 6 is, in some respects, the most interesting of all 
the subdivisions of this Primal group. Itis the sandstone that 
“constitutes the type of this formation.” It differs from the 
beds already described in both its lithological and fossil pecu- 
liarities, (see July No., p. 22). It may well be called the 
“ Scolithus sandstone,” if we call the primal worms (?) that had 
their millions of habitations in this rock the “ Scohithus linearis.” 
Its entire thickness (including some quite brittle beds that 
underlie and overlie the more massive portion), is about 34 
feet. The dip at the base of the ridge, where the two rivers 
meet at the entrance of the gorge, is fully 65°, while it falls 
gradually to 40° before it reaches the summit—looking as if it 
mizht once have been one leg of a grand natural arch, which 
still stands up with one exposed face forming an almost perpen- 
dicular cliff nearly 800 feet in height. There is, however, 00 
point in this portion of the range where I have found it reach- 
ing beyond the northwestern line of ridges, of which it gener- 
ally forms the crest and the greater part of the western slope, 
as represented on the accompanying section. A part of this 
sandstone, with the next beds of slate and sandstone below 1t, 
has broken loose from the upper outcrop of the ledges on the 
S.W. side of the river, and slipped down the eastern face of the 
ridge without any great change of dip. This displaced mass 
may een as a ver conspicuous object ate opposite, 
though a little below the Cement Mills. It is apparently one 
; he next is the Canadian Period (3)—sometimes called, 
_ Middle Cambrian”—and, like the Primordial, belongs to the 
Lower Silurian Age. It has three epochs, Calciferous ioe 
be | : y charac 
ized by the b BroPence of one or more beds of hydraulic pet 
sechh 
stone. Where our section crosses, this limestone 1s quart 
