436 J. L. Campbell— Geology of Virginia. 
line both ways, this ridge is flanked by Archzan rocks on the 
southeast and Primordial rocks on the northwest—the latter 
resting unconformably upon the former. (2) Skirting the north- 
west side of this leading ridge, and parallel with it, are two 
well defined lines of broken ridges that have evidently been 
once continuous, but now consist of short, abruptly terminating 
mountains, of rounded dome-like hills, and of rugged conical 
peaks. ese all have a frame-work of Primordial sandstones, 
with the less durable shales of the same period lying along 
their flanks or filling the depressions between them. Of these 
lines of ridges the one bordering on the great limestone valley, 
heretofore described, (see July No.), is by far the most con- 
spicuous, and the most uniform in its physical features. It 
consists essentially of the durable masses of the Upper Pots- 
dam sandstones, so durable that many parts of it have main- 
tained a height almost equal to that of the main ridge, the ave- 
rage height of which, in this region, somewhat exceeds 2500 
fect. The mean bearing of this portion of the range is about 
a ee os 
Salling’s Mountain, seen on the left of the map, is an out- 
lying ridge of Primordial sandstones and slates, cut off at its 
northeastern end by the North River, and at its southwestern 
end by James River. It is separated from the principal chain 
by a narrow synclinal valley of limestone (Lower Silurian), 
most of which is concealed from view by an extensive bed 0 
alluvium, accumulated by the two rivers that meet here; but 
accumulated originally in a Y-shaped lake, through which they 
seem to have flowed at some former period of their history. 
two rivers above mentioned, traverse the little valley 
obliquely, and meet ata very obtuse angle just where their 
waters, as one united stream, enter the deep gorge or cafion by 
which they pass through the mountain range. Just below 
their junction are mills for grinding hydraulic lime burnt from 
the ledges that crop out a little higher up the James River. 
“Balcony Falls” is the name given to a succession of “ rapids, 
beginning about half-a-mile below the Cement Mills, and con- 
tinuing to the southeast limit of the gorge. The river here 1s 
ve feet —? tide level. He 
Feology.— The foregoing outline of the topography of the 
.Tegion will enable A, mina to understand more clearly 1ts 
geological peculiarities, and to interpret more readily than he 
otherwise could, the ideal section accompanying the map. 
