C. D. Walcott — Cambrian Rooks of Pennsylvania. 477 



the great quartzite masses, the roofing slates, etc., of Walcott's 

 upper, middle and lower Cambrian system."* 



When I began the investigation to ascertain, by stratigraphic 

 and paleontologic evidence, the geologic age of the South 

 Mountain quartzite and the associated schists and slates, I soon 

 discovered that there was very little prospect of finding the 

 true geologic succession in the northern portion of the moun- 

 tain, in Cumberland and York counties, owing to the folding 

 of the strata and also to the fact that there were a number of 

 westward thrusts of lower on higher beds, and that as a result 

 of this the central core of the Blue Ridge had been broken and 

 thrust over on the Lower Cambrian beds and, also, in places, 

 resting apparently conformably upon the latter, — all having an 

 eastward dip. The discovery of Olenellus with Hyoliihes com- 

 munis in the massive quartzite series in the Mt. Holly ridge, 

 just above Mt. Holly Springs in Cumberland county, proved 

 that the great western mass of quartzites of South Mountain, 

 with the interbedded shales, slates and conglomerates, were of 

 Lower Cambrian age ; but it did not throw light upon the geo- 

 logic age of the orthofelsite series of Frazer and the epidotic 

 rocks of Rogers. In company with Mr. Arthur Keith, of the 

 U. S. Geological Survey, who had mapped the Harper's Ferry 

 sheet, an examination was made across the ridges, from Mechan- 

 icstown, Md., to Monterey and westward to Pikesville, in 

 Franklin county, Pennsylvania. 



• On entering the gorge, a little west of Mechanicstown, on 

 the line of the Western Maryland R. R., an extended series of 

 shales and slates was passed, — all having a very high dip to 

 the southeast. About two miles from Mechanicstown, massive 

 quartzites were observed with a high dip to the east, and, 

 higher up in the gorge, there was a repetition of the slates 

 found east of the quartzites. This section, from the dip of 

 the quartzite, indicated a synclinal resting on a considerable 

 thickness of slates and shales. A series of sections, by Mr. 

 Keith, of the western, or Blue Ridge ridge extending from a 

 point eleven miles south of Mechanicstown to Harper's Ferry, 

 shows that this same synclinal structure prevails all along the 

 ridge, and that a synclinal fold of massive sandstone forms the 

 summit of the ridge, below which a series of shales rests 

 unconformably upon the subjacent crystalline rocks. j- The 

 synclinal structure is also shown for the quartzites of the 

 eastern or Cotoctin ridge. 



From a point two and one-half miles west of Mechanics- 

 town to Monterey, the road led across the epidotic schists of 



*Loc. cit, pp. 147, 148. 



f The structure of the Blue Ridge near Harper's Ferry. Bull. Geol. Soc. 

 America, vol. ii, 1891, pis. 4 and 5. 



r 



