46 
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. 
^ 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 
