MIDDLE AND UPPER OEDQVICIAN. 177 



magna and thin, beds of black chert that weather into small rectangular blocks; 

 an upper series of thin-bedded pure limestone; and a lower series of interbedded 

 massive pure beds and magnesian layers. From the middle division 22 species 

 of fossils have been obtained, among which there are eight which occur in the 

 middle Chazy of the Champlain Valley. As a whole, the fossils as well as the 

 lithologic character of the formation are practically the same as those of the Stones 

 River group of Tennessee. 



The Chambersburg limestone is the uppermost division of the Shenandoah limestone. 

 It is characterized throughout the area by fossiliferous thin-bedded limestones with argillaceous 

 partings. It varies in thickness across the' strike from a maximum of 600 feet in the Chambers- 

 burg belt to about 100 feet in the McConnellsburg Cove. 



Its most typical development is in the Chambersburg belt, tliroughout which fossils are 

 abundant. The following section in the railroad cut IJ miles west of Kauffman is the most 

 complete continuous section in this belt: 



Black ahale (Martinsburg). 



Largely concealed, but probably chiefly shale (near the top are black carbonaceous limestone with 

 concholdal fractirre, shaly dark crystalline limestone, thin sandstone, and .10 feet of coarse crys- 

 talline limestone containing Lingulas) 150 



Calcareous shale and limestone 100 



Nodular clayey limestone 50 



Dark platy limestone '. 94 



Compact dark limestone, very fossiliferous i 108 



Cobbly limestone containing numerous nidulites, Bryozoa, and a layer of cystid heads 105 



607 



The "cobbly" .character of the weathered outcrop of certain of the beds, due to a wavy 

 lamination, or clay parting that crosses the bedding at a high angle and, on weathering, gives 

 rise to rounded lenticular masses resembling rough cobbles, is one of the noticeable features of 

 , this formation. The upper 200 feet of the formation is composed largely of shale with inter- 

 bedded thin fossiliferous limestones. 



********* 



Nearly everywhere this formation yields on careful search an abundance and great variety 

 of fossils, and those from the Chambersburg quadrangle differ from those obtained in the Mercers- 

 burg quadrangle. In the Chambersburg belt the formation may be divided into four faunal 

 zones. 



********* 



In the Mercersburg quadrangle four faunal zones are also distinguished, but they do not 

 correspond with those of the Chambersburg belt. 



In southeastern Pennsylvania and Maryland the Ordovician strata are over- 

 thrust by pre-Cambrian schist and metamorphosed to phyllite and schist. The 

 complex relations have. been worked out by Bascom"-®^ and Mathews,"® who 

 distinguish the established divisions of the section in adjacent fossiliferous districts. 



The Cambrian and Lower Ordovician formations of the Philadelphia district 

 (Chickies quartzite and Shenandoah limestone) are described in Chapter III (pp. 

 111-113). Bascom ^**' continues the description of the Ordovician formations as 

 follows : 



Further sedimentation in Ordovician time is represented by the Octoraro. This is the last 

 known Paleozoic deposit in this region and, like the preceding Paleozoic sediments, is dynami- 

 cally metamorphosed and free from igneous intrusions. 



The Octoraro schist is largely confined to the South Valley Hills, pincbing out to the north- 

 east and expanding to the southwest. Outliers of the mica schist occur north of Berwyn and of 

 48011°— 12— 12 



