46 GUIDE TO LOCALITIES. 



These species (with, the exception of the first) and a number 

 of others from Gay Head are found in the middle Cretacean of 

 other districts ; to which, and more particularly to the horizon of 

 the Amboy clays, these beds are referred. Professor Marsh, how- 

 ever, considers these beds to be the equivalent of the Potomac 

 formation of Maryland; which he regards, from its vertebrate re- 

 mains, as of Jurassic age. Total thickness probably less than one 

 hundred and fifty feet (Woodworth). 



Marine upper Cretacean. 



These beds, exposed in the Indian hill district (described be- 

 low), are probably represented at Gay Head, appearing strati- 

 graphically beneath the Miocene beds (Woodworth). No fossils 

 have been reported from the Gay Head section. Shaler consid- 

 ered the Indian hill beds as of middle Cretacean age. 



Eocene (absent). 

 Neocene. 



Miocene. From collectionsmadeby Woodworth and others, Dall 

 identified the Greensand beds and underlying Osseous conglomer- 

 ate as of Miocene age. In his paper will be found a list of spe- 

 cies, which include eight vertebrates in addition to numerous 

 unidentified remains of osseous fishes, and three crustaceans, 

 Archwoplax signifera, A. ? sp. aud Balanus (? 2^'>'oteus). Fourteen 

 species of mollusca are reported from Gay Head, and three addi- 

 tional ones from Chilmark. The genera include Pecten, Yoldia, 

 Nucula, Astarte, Crassatella, Cardium, Venus, Cytherea, Tellina, 

 Macoma, Mya and others. The shells are represented mainly by 

 internal, or in a few cases by external, casts. 



The Osseous conglomerate, twelve to eighteen inches thick, is 

 seen just north of the Devil's Den. It contains black chert peb- 

 bles bearing corals, crinoid stems, graptolites and shells ; which 

 indicate, according to Walcott,that they are derived from Silurian 

 (Ordovician ?) strata. The principal fossils of the conglomerate 

 are the vertebrse, jaw -fragments, ribs, paddles, and head bones of 

 cetaceans, and masses of lignite derived from the erosion of the 

 Potomac series (Woodworth). The overlying Greensand varies 

 up to ten feet in thickness. The lower beds are green, the upper 

 rusty brown. The cast of Macoma lyellii are in the attitude of 



