of the Older Formations of the sbinited States. 9 
with the exception of nodules and iei layers of chert, 
which increases in proportion as we advance westward. 
The Onondaga Salt Group, which in New York, at the point 
of its greatest development is about one thousand feet thick, has 
greatly diminished at the west, and in all situations where I have 
examined it, is but an insignificant mass. It retains, it is true, 
the essential characters of the same in New York, but from 
being destitute of fossils is of less interest than many of the 
other rocks, and of little consequence for illustrating the pre- 
sent subject. 
Helderberg Limestones. Succeeding to the Salt Group we 
have an extensive series of limestones which are well developed 
at the Helderberg and Schoharie. These are highly fossiliferous, 
abounding in the Brachiopoda, Cephalopoda, Trilobites, Crin- 
oidea, Corals, &c. We are greatly indebted to my friend 
John Gebhard, Jr. for the extensive knowledge we. possess of 
those fossils, and to him,more than to all others, is due the 
credit of bringing to light the numerous and beautiful collec- 
tions of the organic remains of that region. 
These limestones, except the two upper ones, all disappear 
before reaching the western limits of New York. The two 
higher divisions of the series are well developed in Ohio, 
Indiana, and Kentucky, and réappear on the Mississippi River. 
"The general character of the rock remains the same, and the - 
fossils are identical, proving that throughout this. wide range 
of country the condition of the ocean bed was uniform, and 
the character of its inhabitants the same. Specimens, collect- 
. ed in Ohio, in Indiana, or Kentucky, can scarcely be 
guished from those of the Helderberg ; the color of the mass, 
Lis true, is somewhat lighter, but otherwise thore i is no percep- 
difference 
: The E eohi selenurus and a species et Calymene 
(C. crassimarginata) seem to be quite as characteristic at the 
west and south-west as in New York. It is true that some of 
the New York fossils Sea els at the west, and it is also true 
A421 
2 thet other nak Still, the occurrence 
