WITH THOSE OF NEW YORK. 289 



century to come, and I doubt not, from the numerous and efficient 

 observers now at work in this region, upon their native or adopted 

 soil, that all the most important details will soon be wrought out. 



From the want of a well-defined and acknowledged basis in 

 the West, it would always have been difficult, if not impossible, 

 to establish the identity from that direction eastward, and it re- 

 quired a knowledge of the New York rocks, in their wide geo- 

 graphical range and undisturbed position, to settle satisfactorily 

 the place of the western rocks. 



From the facts here stated the conclusion seems unavoidable, 

 that the character of fossils is, or may be, as variable as lithological 

 characters ; in fact, that the species depend in some degree upon 

 the nature of the material among which they lived. Fossil char- 

 acters, therefore, become of parallel importance to the lithological ; 

 and, in order to arrive at just conclusions, both must be studied in 

 connection, and localities of proximity examined. In the case of 

 the Hudson river gi'oup of shales and sandstones, in passing from 

 New York to Ohio, the lithological character is almost entirely 

 changed ; and at the same time also the most prominent and 

 abundant fossils are unlike those of the group in New York. 

 More careful examination, however, reveals the fossils which 

 characterize this group at the East, and also at the same time some 

 obscurely similar lithological characters. Similar lithological 

 changes, accompanied by lilce changes in fossils, occur in more 

 limited districts within the State of New York. 



The most marked and important changes, however, appear to 

 be in the higher rocks of the New York system. The Hamilton 

 group and Marcellus shale, which in New York have a thickness 

 of one thousand feet, have diminished to one hundred where last 

 examined ; and from being the group most prolific in fossils, as 

 it is in New York, it has become entirely barren of them. The 

 rocks forming the Portage and Chemung groups, which in their 

 greatest developement in New York are scarcely less than three 

 thousand feet in thickness, and in Pennsylvania much more, 

 have, in Indiana, diminished to as many hundred. The upper 

 of these groups, from being extremely fossiliferous, has become 

 almost destitute of these characters, so that, at the furthest extreme 



