THE GEOLOGY OF SOUTH MOUNTAIN 339 
one doubtful exception discussed below. As the limestone enters 
only indirectly into the present problem no further description cf 
it will be given. Its Cambrian age has been shown by Walcott* 
and others.” 
Ordovician System; Hudson River Formation 
Norre.—This formation in southern Pennsylvania is correlated with the 
Martinsburg shale of West Virginia by the U.S. Geological Survey.3 
On the extreme western edge of the map at the village of Klein- 
feltersville and near the Triassic border appears a small area of 
slates, being probably the eastern end of an inlier of the Hudson 
River formation mentioned by Lesley? and shown on the maps of 
the Second Geological Survey. 
One-half mile west of Robesonia near the railroad a bluish slate 
lies either directly upon the Potsdam quartzite or in close juxta- 
position. This is the southern limit of a belt of Hudson River 
slates according to the maps of the Second Geological Survey. Dr. 
Wherry® has frequently found beds of slate intercalated in the 
Cambrian limestones and thinks that the Second Geological Survey 
was in error in mapping as Hudson River certain slate areas in the 
South Mountain region. The question as to the age of the slate at 
this locality may be regarded as not yet definitely settled. The 
possible existence here of a strike fault of some magnitude, cutting 
out the limestones and bringing Hudson River and Potsdam in 
contact, is admitted, but data relative to any such dislocation are 
t Op. cit., 29-33. 
2F. B. Peck, ‘‘The Cement Belt in Lehigh and Northampton Counties of Penn- 
sylvania. A Description of the Geological Formations,” Mines and Minerals, XXV, 
No. 2, 1904, 54; idem, Econ. Geol., III, No. 1, 1908, 41. 
G. W. Stose, ‘‘The Cambro-Ordovician Limestones of the Appalachian Valley in 
Southern Pennsylvania,” Jour. Geol., XVI, No. 8, 1908, 698-703. 
H. Justin Roddy, ‘‘The Lower Cambrian of Lancaster County, Pennsylvania”’ 
(abstract), Science, N.S., XXX, No. 769, 1909, 415. 
E. T. Wherry, “‘The Early Paleozoic of the Lehigh Valley District, Pennsylvania” 
(abstract), Science, N.S., XXX, No. 769, 1909, 416. 
3G. W. Stose, ‘‘Mercersburg-Chambersburg (Pa.) Folio,” U.S. Geol. Surv., No. 
170, 1909, Io. 
4 Op. cit., \xviii. 
5 Personal communication. 
