426 R. M. Field — Middle Ordovician of Central 



or basal Trenton has been shown to be distinguishable, 

 both lithologically and f aunally, from the upper Carlim 

 and Coburn. The fact that the first Cryptolithus zone 

 occurs at the base of the Salona and not in the Rodman 

 probably indicates the base of the Trenton group. 

 Raymond (op. cit., p. 295) has correlated these beds with 

 the 20 feet of Hermitage in Kentucky, principally be- 

 cause of the occurrence of Cryptolithus at the base and 

 of Orthis tricenaria and Dinorthis pectinella throughout. 

 The succeeding formations of the Trenton group in Ken- 

 tucky, i. e., Bigsby, Catheys, etc., are not comparable 

 either lithologically or faunally to the Coburn. 



The recent discovery of the Parastrophia hemiplicata 

 zone just above the cross-bedded and ripple-marked 

 Salona is a fair indication of the beginning of the Middle 

 Trenton of New York State. 



There are three Parastrophia zones in the New York 

 Trenton, at the base, middle, and near the top. The 

 oldest or lowest species of the genus is somewhat similar 

 to the primitive type, P. pristina, found below the Rod- 

 man. The two species which are found in the middle 

 and upper zones are not distinguishable but both are 

 similar to the species found at the base of the Coburn. 

 Platystrophia, common in the middle and upper, and 

 Rafinesquina deltoidea in the upper Trenton of New 

 York, are absent in the Coburn. Collie reports a Platy- 

 strophia from the section at Bellefonte but I have been 

 unable to find it in any of the sections. 



The following composite section of the New York 

 Trenton is copied from Raymond. (Op. cit., p. 253.) 



Utica shale with Cryptolithus at top. 



g. Trenton. — Light-grey, coarse-grained coquina in 

 thick beds. Rafinesquina deltoidea, Hormotoma 

 trentonensis and other fossils 20 feet 



/. Thin-bedded, blue limestone with shaly partings. 

 Rafinesquina deltoidea the common characteristic 

 fossil . 92 " 



e. Thin-bedded, blue limestone with shaly partings. 

 Prasopora simidatrix and other common Trenton 

 fossils abundant 100 l ' 



d. Thin and thick-bedded limestone, dark in color and 

 fine grained. Diplograptus amplexicaulis a com- 

 mon fossil 35 



c. Thin-bedded, dark limestone with Triplecia extans 



