342 H. N. EATON 
Peck? found the quartzite overturned at one place, its beds dipping 
south toward gneiss. 
He? has shown that the limestones in the Cement Belt have been 
overturned to the north and northwest in several localities to such 
a degree as to make probable a crustal shortening of 6 to 1. 
The complicated structure of the South Mountain in York, 
Cumberland, Adams, and Franklin counties is generally conceded+ 
to be due to overthrust faulting acting from east to west, bringing 
pre-Cambrian volcanics in contact with lower Cambrian quartzite. 
Within recent years, however, one observer* has described it as 
an anticlinorium with Lower Cambrian strata “in unbroken 
sequence”? showing vertical or overturned dips on the northwest 
side. If the latter interpretation be correct, this mountain would 
seem to be similar, in major structural lineaments, to its smaller 
neighbor to the northeast described in the preceding pages. 
DIKES 
Three diabase dikes were traced for average distances of two 
miles, and others doubtless escaped observation. The general 
trend is northeast by southwest. The width of each can be 
inferred only approximately from surface float, being presumably 
but a few feet in the case of both the easternmost and westernmost 
dikes. The dike north of Laurel Ridge is the largest with a width 
of at least 200 feet. This dike cuts the Triassic conglomerate at 
Laurel Ridge, and thus its period of intrusion may easily be inferred. 
The dike rock is an olivine free diabase of fine texture. These 
tF, B. Peck, ‘‘Basal Conglomerate in Lehigh and Northampton Counties, 
Pennsylvania,” Geol. Soc. Amer., Bull., XIV (1903), 520. 
2F. B. Peck, ‘‘ Geology of the Cement Belt in Lehigh and Northampton Counties, 
Pennsylvania,” etc., Econ. Geol., III, No. 1, 1908, 52-55; idem, op. cit., Mines and 
Minerals, XXV, No. 2, 1904, 56. : : 
3C. D. Walcott, ‘Notes on the Cambrian Rocks of Pennsylvania and Maryland 
from the Susquehanna to the Potomac,” Am. Jour. Sci., 3d Ser., XLIV (1892), 477, 
479-80; F. Bascom, “‘ Volcanic Rocks of South Mountain, Pennsylvania,” U.S. Geol. 
Surv. Bull. No. 136, 1896, 30; E. T. Wherry, ‘‘The Copper Deposits of Franklin- 
Adams Counties, Pennsylvania,” Franklin Inst. Jour., February, 1911, 153 (hypo- 
thetical structure sections). 
4G. W. Stose, ‘‘The Sedimentary Rocks of South Mountain, Pennsylvania,” 
Jour. Geol., XIV, No. 3, 1906, 212, 216, 219-20; zdem, “‘ White Clays of South Moun- 
tain, Pennsylvania,” U.S. Geol. Surv. Bull., No. 315, 1907, 322-23. 
