130 THE CAMBRIAN. [nuu-ftl. 



Dr. Frazor divides the Potsdam sandstone formation into an upper and a lower. 

 * * * The upper, or Kennett rock division, he identifies with Mr. Hall's Edge Hill 

 rock. 



The lower is the Toughkenamon division. Dr. Frazer himself says 

 that the latter, or the lower division, " is composed of weathered particles 

 of a gneiss or syenite loosely compacted and laminated." The second or 

 Kennett rock is the upper division and "it is a thin-bedded rock, lying 

 in plates of fairly parallel sides, composed of fragments of white limpid 

 quartzite, and is generally large- grained. " l 



Prof. Lesley quotes Prof. Rogers's description of the Primal series as 

 it occurs at various localities in the county, together with many of 

 Dr Frazer's observations ; stating that he described three principal and 

 a few very subordinate and doubtful areas of the Primal rock north of 

 the Chester Valley. One important conclusion of Dr. Frazer's is here 

 quoted : 



In mentioning thus the " Primal," the quartzite and quartzose sandstone alone are 

 considered here. It will be seen further on that abundaat evidence sustains the view 

 that the greater part if not all of the weathered feldspar porphyries, conglomerates, 

 &c, in the townships both north and south of the Chester Valley are really Lower 

 Potsdam. The effect of this would be to add a border of these rocks to the area of 

 the (silicious) Potsdam as at present indicated on the map, and thus contract by this 

 much the remaining area of real Azoic or Hypozoic. 2 



The report of Mr. E. V. D'Invilliers upon the geology of the South 



Mountain belt of Berks County contains an extended description of the 



u Potsdam" sandstone, or No. 1 of Rogers's classification. The opening 



paragraph throws a doubt upon the correlation that has been made of 



this lower sandstone with the Potsdam sandstone of New York : 



It would be safer to name this formation the Beading Sandstone; but in the descrip- 

 tions of its outcrops along the Little Lehigh, the Lehigh, and the Delaware Rivers, 

 in vol. i of this report, it has been called Potsdam Sandstone, taking for granted that 

 any sand formation underneath the Magnesian limestones of the Great Valley must 

 be the same sand formation which in northern New York underlies the Corniferous 

 [Calciferous?], Chazy, and other limestones of the Mohawk Valley. 3 



The lower sandstone or conglomerate and the lower Primal slates of 



Rogers were not recognized in Berks County. 



The lowest bed of the sandstone, always seen resting on the gneiss, is a coarse con- 

 glomerate of angular quartz rock fragments of all sizes in a silicous paste. * * * 

 The term sub-Potsdam conglomerate would express its position underneath the Pots- 

 dam quartzite white sandstone proper, belonging to it by conformity, and separated 

 from the gneiss by nonconformity. 4 



A detailed description of the outcrop of the sandstones and their 

 general character follows. Certain areas of slate exist in places be- 

 tween the sandstone and the Magnesian limestone and are referred to 

 the Primal series. Where the slate is not present the limestone is iin- 



1 Geological notes in the several townships of Chester County. Second Geol. Surv. Penn. The ge- 

 ology of Chester County. C4, 1883, p. 307. 



2 The geology of Chester County, after the surveys of Eogers, Frazer, and Hall. Second Geol. Surv. 

 Penn.,CCCC; 1883, p. 159. 



3 The geology of the South Mountain Belt of Berks County. Second Geol. Surv. of Penn., D3, 1883, 

 vol. 2, p. 99. 



•Op-oit.,.p.lQ0. 



