C. D. Walcott — Cambrian Rocks of Pennsylvania. 469 



Art. LXIY. — Notes on the Cambrian Rocks of Pennsylvania 

 and Maryland, from the Susquehanna to the Potomac / by 

 Charles D. Walcott. 



[Read before the Philosophical Society of Washington, Oct. 29, 1892.] 



The special study of the Cambrian rocks of the Southern 

 Appalachians* was limited during the past field season to 

 an examination of some of the more important exposures of 

 the group in central Pennsylvania and Maryland. Within 

 this area between the Susquehanna and the Potomac rivers 

 two points have been the subject of investigation by geolo- 

 gists, and decided differences of opinion exist in regard to 

 the stratigraphy and the geological age of the rocks embraced 

 within them. One is that of South Mountain as it occurs in 

 Franklin, Cumberland, York and Adams counties, in Penn- 

 sylvania. The second is the area about Harper's Ferry, 

 .Virginia. After an examination of the published literature, 

 including geological sections and maps, it was decided to begin 

 work in York county on the Susquehanna and to extend it 

 southwest to the Potomac. Prior to this three days were 

 spent at Mount Holly Springs, in the northwestern part of 

 South Mountain, in a preliminary examination of the quartz- 

 ites exposed at that point. The discovery of the lower 

 Cambrian or Olenellus fauna in a synclinal trough near the 

 western foot of the mountain, gave an important datum point 

 which was afterwards of great service in work to the south in 

 Franklin county. 



York County. 

 Dr. Persifor Frazer considers the Lancaster limestones as 

 probably the equivalent of the Calciferous and Trenton lime- 

 stones of the New York series, and mentions that the York 

 limestone is a slender offshoot.f The Hellam quartzite of 

 York is called the " Chikis " quartzite, and is described as a 

 basal formation upon which a series of schists occur that to 

 the south " are, at all events, those slates in which the iron 

 ores of Lancaster and York are invariably found, the transition 

 series between the Primal and Auroral."^: Professor Lesley 

 describes in his final report§ the "Chiques" sandstone as the 

 same formation as the Hellam quartzite of York county (as 



* Notes on the Cambrian rocks of Virginia and the Southern Appalachians, 

 this Journal, vol. xliv, 1892, pp. 52-57. 



f 2d Geol. Survey of Penn. Report of Progress in 1877. Geology of Lan- 

 caster County, 1880, p. 4. % Loc. cit., p. 7. 



§ Geol. Survey of Penn. Summary Description of the Geology of Penn , vol. i, 

 1892, p. 165. 



