10 Hall on the Strata and Organic Remains 
of the same species of Trilobites, Strophomena, Orthis, Del- 
thyris and Atrypa, while the nature of the deposit is unchang- 
ed, proves a condition in nowise essentially differing from the 
same period in New York. 
With the limestones, just described, terminate all the impor- 
tant calcareous formations of New York. 
The Marcellus Shales and Hamilton Group, consisting of . 
shales and shaly sandstones, succeed the limestone, forming an. 
important part of the series in New York. The thickness of 
the whole is scarcely less than one thousand feet, and it pro- 
bably contains more individual fossils than all the rocks and. 
groups below this point. 
In the eastern part of the State these formations consist of 
dark or black slaty shales, and dark or olive-greenish sandy 
shales, and impure sandstones ; towards the west the are- 
naceous matter diminishes, and there is a great increase of 
mud; and finally, in western New York, the whole con- 
sists of an immense development of grayish blue shales still 
. abounding in fossils. Farther west, as in Ohio, Indiana and 
Illinois, the lower member of this great group, consisting of 
black shale alone, is visible, having diminished from one 
thousand feet in thickness to one hundred or even fifty feet, in 
some places. The same group, from being highly fossiliferous, 
asin New York, has become apparently non-fossiliferous in 
its western extension, though in Obio a few species do occur." 
This group also furnishes one of the most interesting and 
instructive examples of the gradual change in lithological cha- 
racters and final disappearance of fossils, of any in the system. - 
As just remarked, the prevailing character of the mass in 
eastern New York is that of a sandy shale. The most nume- 
rous fossils in this part of the Stateare of the Cypricardia and 
. Modiola-like forms, with Nucula, and large numbers of Avicula ; 
the pe ores, as Delthyris, Orthis, &c., are in the minii 
ü $ De Clapp ai Dr Owen, Ga 
Die H E Nea tye he h di d fossils in this shale near th 
