422 R. M. Field — Middle Ordovician of Central 



These two formations have, therefore, been placed nnder 

 the group name of Trenton. 



There is no donbt that the above rough correlation is 

 warranted, but the manner in which the separate forma- 

 tions of central Pennsylvania are to be correlated with 

 the neighboring ones is still open to question. The prin- 

 cipal argument concerns the location of the dividing line 

 between the Stones River and the Trenton groups : Shall 

 the Rodman be placed at the top of the Stones River or 

 at the base of the Trenton? 



Stones River. — The formations above the Loysburg 

 and below the Rodman, i. e., Carlim, Valentine and 

 Center Hall, have been shown to be related in age and 

 mode of origin. The Valentine thins to the east and its 

 beds are successively replaced by the upper beds of the 

 Carlim, the younger of which appears to be synchronous 

 with the Center Hall at Bellefonte. Collie has stated 

 that the Stones River group includes the fossiliferous, 

 fairly pure limestone which follows the Beekmantown, 

 and the superjacent quarry-rock which he likens to the 

 Birdseye zone of New York State. He makes no men- 

 tion of the Center Hall, however, and what is more 

 important, he also makes no mention of the thin but 

 highly characteristic faunal zone above the pure quarry- 

 rock. Recent and more intensive study of the Appala- 

 chian province shows that there is considerable variation 

 in the limestone of Stones River age. If the Carlim is 

 a bedded reef and the Valentine a lagoon or offshore 

 phase, we may account for the slight lithological and 

 faunal variations to the north, south and west as due to 

 such local conditions as distance from shore, presence 

 or absence of marine currents, character of the under- 

 lying formations, and possible disturbing influence of 

 large rivers bearing argillaceous or siliceous impurities. 

 To ascribe a different age to each phase is a doubtful 

 proceeding in view of even the little that we know regard- 

 ing the varied distribution of bottom types in the south 

 Atlantic coral latitudes. 



The characteristic fossils of the Carlim are shown in 

 the following table : 



