Sa Pee te ee Ome Er ee ye Renee eee ee ee 
J. L. Campbell—Silurian Formation in Virginia. 23 
debris of sandstone from the adjacent ridges just described. 
This is (g) on the section. It sometimes rises to a considerable 
height on the slope of the “scolithus bed,” especially where 
the dip is low; and in a few cases, as at Trish Creek, I have 
found it reaching the crest of the ridge. It is one of the rich- 
est repositories of iron ore in Virginia—especially brown hema- 
tite—and has valuable beds of manganese, one of which, near 
Waynesboro’, in Augusta county, is at present extensively 
worked. The ores ofthe Shenandoah Iron Works of Page 
county are obtained from this bed of shale. Although it 
abounds in iron ores, yet it has the peculiar feature of contain- 
ing a layer of ig so white as to be called “chalk” by the peo- 
ple of the region 
This brings us to the border of the limestones of the valley, 
and the plane of division between No. I and No. Thus we 
have passed over the Primordial Postal If it has here repre- 
~~ of both the Acadian and Potsdam epochs (which I 
doubt) the lowest shales and sandstones must represent the 
former, and the upper — and sandstones the latter. For 
the present, at least, [ shall regard the whole as belonging to 
the Po tsdam. The total gir varies considerably as we 
ascend the ridges. This is especially conspicuous in the beds 
of shale, and causes such a decided variation in the dip of the 
sandstones as to make them present the appearance in many 
laces of segments of broken arches; the dip varying as it does 
ae re and at ‘Balcony Falls from 65° at the base to 30° near the 
upper margin, or outcrop of the beds. This peculiarity has 
been caused either by an original thinning out of the beds 
towards their margin before they were upheaved, or by a 
: yi nt out of a portion of their material by the resistance 
ressure of the more unyielding beds of sandstone above 
ai d Bato at the period of upheaval. 
thrust, which was doubtless from the Blue Ridge 
pene the valley, seems to have been more powerful near the 
base than it was near the summit. Hence the steeper dip. 
below, which has become reversed in the limestones for seve- 
ral miles from the foot of the mountain. 
No. IL—The first natural subdivision (a) of the valley lime- 
stones may with propriety be called the “Hydraulic Forma- 
tion,” inasmuch as it abounds in hydraulic limestones through- 
out its whole length. It Seema noe several layers of 
very siliceous and argillaceous limes mcg from one 
another by beds of ene = moe a hales, and some 
soft sandstones. The of hy Sate ain is near the 
=r we = — ogee a hele it has been quarried for 
is only about twelve to fifteen 
fost. thick, ee aii sips seni to the N.W. Whe ere our section 
