J. L. Campbell—Silurian Formation in Virginia. 27 
both ways from one line of section. The lower and older 
rocks of No. II are found = their own normal order) meee 
ing the newer of No. III }, which dip beneath them. At 
number of points between Kerr’s Creek — Collier’s Chalk 
two considerable streams that run out from the N. Mountain 
at the opposite extremities of the House Moosic ridges, this 
dipping of the newer under the older rocks may be seen along 
a line of very considerable regularity. 
Our general description has now extended to the horizon 
between the Lower and Upper Silurian. 
o. IV, the equivalent of the Medina group, is composed of 
very adnebe sandstones that are the chief mountain-making 
rocks along the northwest margin of the valley, and through- 
out a belt of twenty or twenty-five miles wide, parallel with it. 
It may be represented under three subdivisions. The lower 
one of which (a), is a very hard, light gray, sometimes white, , 
sandstone, distinctly conglomerate in many places, and so 
durable as to present long lines of precipices where the strata 
crop out on the faces of the mountains. The middle member 
(6).of this group is a dark brownish purple sandstone with beds 
of interstratified shales of the same color. Shells in the sand- 
stones, and fucoids in the shales, are conspicuous features of 
this division. A third member (c) is much lighter in color 
than 6, but darker than a. Some of the harder layers havea 
pinkish hue, while the softer and more brittle, especially near 
the top, where they border on No. V, are brown and yellowish 
brown in color. hile this group, as it appears on the two 
ridges of House Mountain, rests upon a nearly horizontal base, 
in the North Mountain its position is changed to that of a 
steep northwest dip. 
The general pressure that acted from the Blue Ridge side of 
the valley towards the northwest, seems to have lifted the 
House Mountain ridges somewhat above what was the original 
level of the surrounding region, and, at the same time, to have 
broken off and pushed back the edges that now form the crest _ 
of North Mountain. But while the section represents the gen- 
eral result, it will be found on examination, that there are a 
number of local and limited irregularities in the form of con- 
‘tortions and fractures that codid not be exhibited on a scale 
representing so much space within so short alimit. So, also, it 
has here, apparently, a greater degree of symmetry on the sur- 
face, than e denuding praict to which it has been sub jected, 
have given it. Butin this regard, also, the irregularities are 
too numerous and limited to find a place on the section. 
The strata of this group all thin off as they extend farther 
towards the interior basin of the coal regions. They also vary 
— in thickness where they crop out along the margin of the 
