8 



bulletin: museum of comparative zoology. 



CORRELATION 



Obviously this section should first be compared with that at Trenton 

 Falls, which is about sixty miles south of Martinsburg. Compar- 

 ing it with the composite Trenton Falls-Rathbone Brook section, 

 pubHshed by Ra;^Tnond, (Bull. M. C. Z., 1916, 56, p. 253), it appears 

 that the Trenton at Martinsburg is 129 feet thicker than at Trenton 

 Falls. The discovery of Cryptolithus at Martinsburg gives a much 

 better basis for comparison of the two sections than was previously 

 to be had. In the typical section, the highest layer with Cryptolithus 

 is seventy-three feet above the base, while at Martinsburg it is 100 

 feet above. At Trenton Falls, Rafinesquina deltoidea appears 228 

 feet above the base of the Trenton; at Martinsburg the lowest layer 

 containing this species is 300 feet above the base. In both sections 

 the strata between those characterized by these "guide fossils" are 

 layers of thinly bedded dark limestone alternating with beds of shale 

 one or two inches in thickness, the fauna in both cases being composed 

 of large numbers of the more common Trenton fossils, particularly 

 Prasopora simulatrix, Dalmanella, Plectambonites, Platystrophia, 

 Calymene, Ceraurus, and Isotelus. The similarity of the strata and 



