THE GEOLOGY OF SOUTH MOUNTAIN 343 
dikes in the eastern part of the mountain possibly extend farther 
south and may be the northern portions of dikes depicted on the 
Lancaster county maps. 
There is an oval mass of diabase northeast of Cushion Peak and 
north of Fritztown covering an area one and one-quarter miles east 
and west by one-half mile north and south. D’Invilliers' mentions 
a trap dike cutting the sandstone and limestone at this point, and 
infers a long northern extension of the same. Several of the above- 
mentioned maps likewise graphically express this view. This 
inference would seem to be erroneous, careful search failing to 
reveal the northeastern continuation of the igneous rock. This 
diabase mass Is probably a boss of the same age as the dikes of the 
mountain. 
SUMMARY 
This study has aimed to emphasize the following points: 
South Mountain structurally and topographically is a unit, its 
central core being an inlier of slightly metamorphosed acid and 
basic pre-Cambrian eruptives, flanked on the north by Cambrian 
sediments, and its southern base the local northern limit of the 
Triassic transgressional sea. During the Appalachian revolution 
the Cambrian quartzite, which may once have covered the summit, 
was up-arched and probably slightly overturned in the northwestern 
part of the mountain, while in the northeastern part it was over- 
turned upon the limestone of the Great Valley. The Triassic 
sediments along the southern base of the mountain show a slight 
tilting to the south. The diabase intrusions cut the Trias, and 
presumably belong to the same period of eruptive activity whose 
evidences are so common elsewhere throughout the East. The 
physiographic prominence of the mountain is due to the superior 
hardness of its rocks. 
The writer’s sincere thanks are due to Mr. Eby for the contri- 
bution of valuable data. Dr. M. E. Wadsworth, of the School of 
Mines, University of Pittsburgh, and Dr. C. R. Eastman, of the 
University of Pittsburgh and Carnegie Institute, have aided in 
suggestions; Mr. Jandorf has ably assisted in the field; and Dr. 
Wherry and Mr. Estabrook, of Lehigh University, have helped in 
correlation. _ 
1 OD. cit., Vil, 204. 
