334 H. N. EATON 
According to the authors the central core consists of pre-Cambrian 
rocks bounded on the north, east, and west by Potsdam sandstone 
and on the south by the Trias. The above-mentioned trap dike at 
Cushion Peak is also shown. 
The Hand Atlas," in 1885, shows the main discrepancies above 
noted between the Berks and Lancaster county maps. The 
Lebanon County map shows Potsdam sandstone in the eastern 
corner. 
The Lebanon County map,’ issued in 1892, follows the Hand 
Atlas in regard to the distribution of the Potsdam. 
According to the state map of 1893, Potsdam sandstone covers 
the entire mountain and the hill east of Fritztown, and is cut by 
the trap dike at Cushion Peak. 
GEOLOGY 
From the preceding references it is evident that there has long 
been recognized on South Mountain an inlier of pre-Cambrian 
rocks surrounded by Cambrian sediments on the north, east, and 
west, and by Triassic sediments on the south. 
The writer visited the region in the spring of 1911 accompanied 
by Mr. M. L. Jandorf of York, Pa. All available outcrops were 
studied. Exposures are of infrequent occurrence, however, owing 
to the covering of residual soil. 
The boundary between pre-Cambrian and Potsdam on the west 
and northwest coincides with the foot of the eastward-facing scarp 
previously mentioned and the valley of a northern branch of 
Cocalico Creek. East of Eagle Peak the boundary extends in a 
straight line a trifle south of east for a distance of two and one-half 
miles bringing the pre-Cambrian down nearly to the level of the 
valley; thence following the base of the mountain southeastward 
and rising near the saddle west of Cushion Peak. From here it 
trends southwest for one and one-quarter miles, meeting the Trias 
at Laurel Ridge. 
« “A Geological Hand Atlas of the Sixty-seven Counties of Pennsylvania,” Second 
Geol. Survey Pennsylvania, 1885, scale, 1 in. =6 mi. 
2,R. H. Sanders, ‘‘Geological Map of Lebanon County,” ibid., 1892; scale, 
Tin. 2) rm 
3A. D. W. Smith, ‘‘ Geological Map of Pennsylvania,” zbid., 1893; scale, 1 in. = 
6 mi. 
