WAM30TT.J PENNSYLVANIA. 131 



mediately above the Potsdam sandstone. "Its place in the series is 

 next above the Potsdam sandstone formation No. I." l 



In a lecture upon the geology of Philadelphia, Prof. H. Carvill Lewis 

 describes the Primal slates and Potsdam sandstone as follows: 



On top of the gneiss and at the base of that great group of fossil iferous rocks known 

 as the Paleozoic System, is a peculiar formation in the middle strata of which occur 

 the oldest fossils yet found in the vicinity of Philadelphia. The lower part of the 

 formation is the pale sandy slate which forms Edge Hill and Barren Hill, and the 

 northern base of Chestnut Hill. The slates are closely folded and stand often almost 

 perpendicular. # * * A sandstone, which from its great development at Potsdam, 

 N. Y., has been named from that place, overlies the slates and often contains long 

 tubular fossils, known as Scolithus linearis, which appear to be the casts of worm- 

 boles. * * * On top of this fossiliferous sandstone is a series of soft, iron-bearing 

 shales, often decomposed into variegated clays, carrying extensive beds of iron ore 

 (limonite). 2 



The "Potsdam" saudstone is described by Prof. Fred. Prime, jr., as 

 it occurs in Lehigh and Northampton Counties. It rarely exceeds 25 

 feet in thickness, and there are no underlying rocks of any thickness 

 between it and the unconformably subjacent Laurentian rocks. The 

 Primal upper slate of Rogers is classed with the Siluro-Cambrian lime- 

 stone. 3 



Prof. J. P. Lesley's hand atlas of the sixty-seven counties of Pennsyl- 

 vania contains a description of the rocks referred to the Potsdam 

 quartzite, No. I, of Rogers, in each of the counties in which they occur 5 

 and the geographic distribution is shown upon the maps of the counties. 

 The text presents the best general statement of the distribution of the 

 strata now referred to the Cambrian in Pennsylvania that has yet been 

 published. 4 



Prof. Lesley's resume" was followed in 1886 by an important contri- 

 bution by Dr. Persifor Frazer entitled u A sketch of the Geology of 

 York County, Pennsylvania." In adopting the classification of the 

 Geological Congress for the system he places under the heading "Cam- 

 bric" the Hellam quartzite or Potsdam sandstone. 5 The lower series 

 or talcose slates of Rogers are considered in all probability as identical 

 with the Azoic schists ; and it is stated that abundant instances occur 

 of the unconformable contact of the quartzite upon the supposed equiv- 

 alent schists in Chester County. On the geological map accompanying 

 this memoir the details and distribution of the Potsdam formation are 

 delineated. The talcose schist of Rogers, or the upper member of his 

 Primal series, is referred to the "Siluric" as "hydro-mica schists." 6 



Dr. T. Sterry Hunt objects to the interpretation of Prof. Rogers, that 



^p.cit.p. 137. 



2 The geology of Philadelphia. Franklin Inst. Jour., 3d sor., vol. 85, 1883, p. 425. 

 s Geology of Lehigh and Northampton Counties. Second Geol. Snrv. Penn., D3, vol. 1, 1883, pp. 210, 

 212. 



4 A geological hand atlas of the sixty-seven counties of Pennsylvania, embodying the results of the 

 Verity field work of the survey, from 1874 to 1881. Second Geol. Snrv. Penn., X, 1885, pp. cxii. 



5 General notes. Sketch on the geology of York County, Pennsylvania. Am. Phil. Soc, Proc, voL 

 23, 1886, p. 398. 



•Op.cit.,p.400. 



