THE GEOLOGY OF SOUTH MOUNTAIN 341 
a few hundred feet of the gneiss. Wherever limestone was observed 
near the quartzite from Robesonia to Wernersville and farther 
eastward, the dips were universally south toward the mountain. 
On the Insane Asylum grounds a large outcrop of limestone occurs 
near the quartzite where the latter is narrowest. The beds here 
are vertical. : 
One and one-quarter miles south of Wernersville quartzite 
overlies limestone in a quarry, both dipping 60° S. The patch of 
limestone at this point probably is an outlier on the quartzite of a 
few acres’ extent along a synclinal axis, although the exact position 
of the quartzite-limestone boundary in this vicinity is in doubt, 
and the outcrop may possibly be on the boundary. 
From the above facts it would seem probable that along the 
northern side.of the mountain, particularly east of Robesonia, the 
Paleozoic sediments of the valley are overturned toward the north 
near their contact with the crystallines. This does not imply a 
great overlapping of the gneiss on the Paleozoics, and no fair 
interpreter could so construe the facts; but that, during the period 
of Appalachian folding when all the rocks were subjected to tan- 
gential pressure, the force acting from the south found expression 
in the northern part of South Mountain by an upward arching and 
slight overturn of the quartzite and limestone. 
The possible existence of strike faults along the northern slope 
has been carefully considered but no evidence on this point was 
found. The writer is aware that certain geologists have demon- 
strated the efficacy of faults in bringing portions of the pre- 
Cambrian range above the Paleozoics of the Great Valley, but at 
the locality in question it would seem safer to give preference to 
the folding hypothesis until the region has been mapped in greater 
detail. 
On the crest of the hill east of Fritztown the evidence in favor 
of overturning seems conclusive. The quartzite on both sides of 
the small pre-Cambrian inlier dips to the south. The structure of 
this hill thus epitomizes the structure of South Mountain. 
In connection with the above interpretation it is instructive to 
note the observations in counties to the east of one long familiar 
with the same formations and general structural conditions. Dr. 
