C. D. Walcott — Cambrian Rocks of Pennsylvania. 475 



A glance at Dr. Frazer's map of York county shows that it 

 is probable that all of the limestones, quartzites and schists of 

 the central portion of the country are of Lower Cambrian age. 

 The Hellam quartzite ridge is, as stated by Dr. Frazer, evidently 

 an anticlinal ridge broken on the northwest side by a fault that 

 has brought the quartzite up against the higher horizons of the 

 shales and limestones The anticlinal structure apparently 

 extends to the southwest past York and towards Hanover.* 



The discovery of Lower Cambrian fossils in the compressed 

 synclinal of limestone in Lancaster county, south of Columbia, 

 indicates that the limestone on the west side of the river is of 

 the same geological age ; and that the shales and schists beneath 

 it (called chlorite schists, etc., by Frazer) are of Lower Cambrian 

 age ; and I doubt if there is a sedimentary rock, — other than 

 the Mesozoic New Red sandstone — of later age than the Cam- 

 brian in York county, unless it may possibly be the Peach 

 Bottom slate and chlorite schists of the southeastern corner of 

 the county ; and from the closely related structure of Lancaster 

 county it is probable that all of the Lancaster limestones will 

 fall within the Cambrian unless it be that some portions of 

 the upper series of limestone may pass into the Ordovician. 

 This generalization will also apply to the limestones of the 

 adjoining counties of Berks and Chester and, in fact, to the 

 entire extension of this series northeastward, to the Delaware. 

 All of the quartzites, that have been referred to the Potsdam, 

 will necessarily fall into the Lower Cambrian, as they are 

 beneath the limestones. 



When it is once considered that the quartzites, called the 

 Potsdam by the Pennsylvania Survey, are of Lower Cambrian 

 age; that a series of shales and limestones, superjacent to these, 

 are of Lower Cambrian age : that the Potsdam horizon of the 

 New York series is represented by limestones in the Auroral 

 series of Rogers ; and that the Calcif erous-Chazy terrane of the 

 New York section is represented only by the upper portions of 

 the Auroral limestones, geologists will have little difficulty in 

 determining the geologic horizons of the various outcrops of 

 quartzites, schists, shales and limestones. — provided careful 

 attention is paid to their sedimentary character and to the dis- 

 covery of occasional localities of fossils. 



South Mountain. 



Prof. Lesley states that " the South Mountains," separating 

 the Cumberland valley from the lower country of York and 



* Southeast of the Hellam hills the limestones appear to form a compressed 

 synclinal and this structure may extend to Hanover and beyond to the S.W. 

 My time was too limited to study the details of structure off of the line of the 

 sections mentioned in these notes. 



