4« PALAEONTOLOGY OF NEW-YORK. 



But, to consider for the present the Hamilton group alone, we find it 

 in Eastern New-York to consist, at base, of the black Marcellus shale 

 already mentioned, including some bands of Goniatite limestone. Next 

 succeeds a hard, compact, calcareo-arenaceous shale, which, under 

 atmospheric influences, crumbles into angular fragments. This is fol- 

 lowed by more arenaceous bands, and by bands of soft slaty shale, 

 with arenaceous shale or argillaceous sandstone, and with some thin 

 bands of limestone, which are almost entirely composed of organic 

 remains. Towards the western part of New-York the coarser materials 

 gradually diminish, and we find an increasing proportion of soft shales, 

 with a more general diffusion of the calcareous matter, and the mass is 

 terminated by a limestone. Finally, from the Genesee river to the 

 western limits of the State, the entire group, above the Marcellus shale, 

 which is persistent, consists of dark soft shales and bands of limestones. 

 Thus the lithological characters are, at the east, an olive shale and 

 sandstone ; at the west, a grayish blue calcareous shale, with bands of 

 limestone. 



The contrast in fossil characters is equally strong. The great abun- 

 dance of the lamellibranchiate fossils, so characteristic of the group 

 in the eastern part of the State, gradually give place to a greater pro- 

 portion of Brachiopoda as we progress westward. The prevailing forms 

 of the east are aviculoid shells, with Modiolopsis, Nucula, etc., while 

 brachiopods are few ; while in the west the Brachiopoda predominate 

 over every thing else. 



L So different are the characters of this group in the eastern and the 

 western parts of the State, that were not the connection or absolute 

 sequence of the formation traceable throughout, doubts might arise in 

 regard to the identity of geological age. 



When we follow the Hamilton group in a westerly direction from 

 New-York, we find it in Canada West consisting of materials rather 

 more calcareous in character, and with thickness much diminished. 

 The same condition is true of the group in Ohio, where, to the east and 

 south of Sandusky, and elsewhere, it is seen with the underlying black 



