and South Central Pennsylvania. 425 



) 



pared with those of the Chambersburg at Marion, Penn- 

 sylvania and at Strasburg, Virginia. 



It will be noticed that except for the absence of Ampyx 

 in the Rodman, this formation is more readily correlated 

 with the lower than the npper Chambersbnrg. As 

 Ampyx is found in both the lower and upper Chambers- 

 burg and is entirely absent in the Rodman, it is really 

 of no correlation value. On the other hand, since Chris- 

 tiania and Crypt olithus occur only in the upper Cham- 

 bersburg, and the majority of the Rodman types are 

 also found only in the lower Chambersburg, I believe 

 that these two zones are of the same age and that beds 

 representing the Nidulites zone and the upper Echinos- 

 phasrites zone are absent in central Pennsylvania. 



Ulrich has placed the strata at Chambersburg, which 

 contain the two Echinosphaerites zones and the interven- 

 ing Nidulites zone, in the Black River (Stones River), 

 while Raymond (op. cit., p. 252) assigns the same zones 

 to the Trenton. My own study of the problem shows 

 that of certain species in the Rodman, previously listed, 

 five are Trenton, eight are Stones River and Trenton 

 and four are confined to the Stones River alone. The 

 large number of Illsenidae present give the Rodman a 

 somewhat " Black River" appearance. The absence of 

 Cryptolithus and the fact that this trilobite does occur 

 at the base of the Salona would seem to preclude this 

 horizon from the Trenton. It is true that there are al- 

 ways from 40 to 100 feet of Trenton below the lowest 

 Cryptolithus beds in New York State and elsewhere, but 

 Raymond himself places the Athens (Normanskill) be- 

 low the Trenton in spite of the fact that this formation 

 contains Cryptolithus. Whether or not the whole of the 

 Chambersburg belongs in the Stones River group may 

 be open to question. It is fairly obvious, however, that 

 the lower Chambersburg and the Rodman are of the same 

 age. When the Valentine is absent, as at Roaring 

 Spring, the difference in lithology between the Rodman 

 and the Salona is certainly much greater than that 

 between the Rodman and the Carlim, and it is for this 

 reason, as well as on account of the fact that the fauna 

 of the Rodman is neither more Stones River than Tren- 

 ton in general appearance, that I prefer to place the 

 Rodman at the top of the Stones River group. 



Trenton Group. — Collie has placed all of the limestone 

 above his " Black River" in the Trenton, stating that 

 the fauna is decidedly Trenton in aspect. The Salona 



