£56 
PALAEONTOLOGY OP NEW-YORK. 
Bracliiopoda and Lamellibranchiata, though there are usually few of any other 
class. But these are not equally distributed, either vertically or horizontally. 
There is one fact, however, which can scarcely fail to impress the collector of 
fossils in this group of strata; which is, that in going westward, certain forms 
which are abundant in some localities become rare or disappear altogether, so that 
sometimes localities not very far removed from each other give almost entirely a 
different set of species. Certain species which are common in Schoharie, Broome, 
Tioga and Chemung counties, I have not seen in Cattaraugus and Chautauque 
counties; while many species which are common in the western counties are quite 
rare' or unknown to me in Tioga, Tompkins, and the counties east of these. Al¬ 
though we may attribute this view in some part to our imperfect collections, it is 
nevertheless in a great degree true. 
Reasoning upon the nature and origin of the sediments, as well as upon these 
observed conditions, we might expect to find a changing fauna as we recede from 
the ancient coast line furnishing these materials, and which were then swept 
into the wide ocean to the westward. While in some of the more eastern localities 
we find species of the Hamilton group apparently mingling with those of the 
Chemung group, the higher beds of Cattaraugus and Chautauque counties give 
us an association of fossils having a more carboniferous aspect than those of the 
higher beds in the eastern counties of the State. 
As we proceed farther to the westward, these differences become more and more 
marked; and in connexion with the contemporaneity of the sedimentary forma¬ 
tions in distant localities holding dissimilar species, we must consider the gradual 
lithological changes which have affected the character of the fauna. There can be 
no longer any question that the higher arenaceous and argillaceous formations of 
New-York and the adjacent portions of Pennsylvania and Ohio, when traced in a 
southwesterly direction, become intercalated with calcareous bands; while the 
coarse sediments give out, or are replaced in a great degree by calcareous or 
argillo-calcareous deposits, containing some of the same species of fossils, with 
an accession of other forms adapted to the changed conditions of life.* 
In the extreme southwestern extension of the Pakeozoic series, the interval 
between the Upper Helderberg group and the Coal Measures, which in the North 
is occupied by the Hamilton, Portage, Chemung and Catskill formations con¬ 
stituting so marked a feature in New-York, Pennsylvania and Ohio, is there filled 
by calcareous accumulations that have been considered as belonging exclusively to 
* I have already shown a similar condition existing at the period of the Coal Measures; where some 
calcareous bands of a few feet thickness in Ohio, Pennsylvania and Virginia, become expanded so that, 
together with the associated calcareous shales, they embrace almost the entire formation towards the 
Rocky Mountains and in the far west and southwestern regions of the United States and in Mexico. 
