THE GEOLOGY OF SOUTH MOUNTAIN 333 
district, the last of the chain of the Highlands. Between the 
Schuylkill and Cumberland County, this is the only representative 
of the South Mountains of our state. The tract is about nine miles 
long and two wide, and extends from the Cacoosing into Millbaugh 
Hill.” The same writer’ mentions the distribution of the Primal 
strata along the northern slope. The mountain’s topographic and 
structural features were thus early recognized by this master 
observer. i 
D’Invilliers,? in 1883, mentions Millbaugh Hill in a number of 
places in his report upon Berks County, giving minor structural 
details but no full description of the mountain. 
The existence of gneiss and Potsdam on the mountain is briefly 
noted by Lesley? in 1885, in summarizing the geological structure 
of the state. 
If we turn to geological maps of the region a discordance of view 
is manifest. The writer has not seen the atlas accompanying the 
Rogers Survey reports. Our information is derived wholly from 
the maps of the Second Survey. 
A map of Lancaster County,’ published in 1878, shows the 
Triassic series covering the entire northern part of the county and 
contiguous parts of Lebanon and Berks. 
A map of the Reading region,’ published in 1883, includes only 
the eastern portion of the mountain at Cushion Peak, which is 
shown to consist of Potsdam sandstone cut by a large trap dike on 
the northern slope. Potsdam sandstone likewise covers the hill 
east of Fritztown and extends for some distance eastward. 
In 1884 appeared a map of Berks County® which depicted the 
areal geology of South Mountain in a fairly accurate manner. 
t [bid., 202. 
2 E. V. d’Invilliers, “‘The Geology of the South Mountain Belt of Berks County,” 
Second Geol. Survey Pennsylvania, D 3, Vol. II, Pt. 1, 1883, vii, 50, 135-36, 204, 347. 
3J. P. Lesley, ““A Geological Hand Atlas of the Sixty-seven Counties of 
Pennsylvania,” etc., zbid., 1885, xxv, Ixviii. 
4 Persifor Frazer, asst. geologist, ‘‘Grand Atlas, Geological Map of Lancaster 
County,” ibid., 1878; scale, 1 in.=2 mi. 
5 “Geological Index Map to the Topographical Map of the Durham and Reading 
Hills or South Mountains,” zbid., 1883; scale, 1 in.=2 mi. 
6 “Geological Map of Berks County, Compiled from the Surveys of F. Prime, E. 
V. d’Invilliers, R. H. Sanders, and Others,” zbid., 1884; scale, 1 in.=2 mi. 
