INTRODUCTION. XV 



the New York Corniferous species, Odontocephalus selenurus of 

 Conrad. 



From this group of fossils, geologists will readily understand 

 that we have here a representation of the Upper Jlelderberg 

 series, probably consisting of a blending of the Corniferous and 

 Onondaga beds of the New York geologists. Consequently we 

 have added this formation, which had not been previously iden- 

 tified in our State, to the Illinois section. 



Above the Upper Helderberg rocks, just mentioned, we have 

 the Hamilton beds proper, well marked by characteristic fossils; 

 and over these, the Genesee division of the Hamilton group 

 (=" Black slate" of western geologists), which latter is suc- 

 ceeded by the Kinderhook and other Subcarboniferous rocks. 



In selecting names for the New York rocks, the surveyors of 

 that State very properly named each formation after the locality 

 where it is best developed ; the expressed object being, accord- 

 ing to Prof. Hall, "to give the name from the locality which 

 afforded the most complete and extensive exhibition of the 

 strata composing the group." Hence the great series of shales 

 and argillaceous sandstones, so extensively developed along the 

 Hudson river, above the Highlands, was called the "Hudson 

 river group." With the exception of Graptolites, few fossils 

 had been found in these rocks, though in a few little isolated 

 outliers, occupying synclinal axes, somewhere in the region of 

 the Hudson river,, supposed to belong to this group, and now 

 said by Prof. Hall to be "of insignificant extent," specimens 

 of Leptcena sericea, Orthis testudinaria, Asaphus and Trinucleus 

 had been found. From this latter fact, beds then known by 

 the local names Pulaski shales and sandstone, Lorraine shales, 

 etc., containing these and other fossils (as well as the associated 

 Utica slate), and holding a position above the Trenton group, 

 at localities farther west in New York, were by the New 

 York geologists referred to the Hudson river group, which they 

 consequently placed above the Trenton in their columnar series. 



