ONONDAGA-SALT GROUP. 



ONONDAGA-SALT GROUP. 



339 



Since the publication of my report on the Fourth Geological District of New- York, in 1843, 

 I have had but small opportunity of making farther investigations for fossils in this group. In 

 all the localities, within the State, which have been examined, however, no new species have 

 been discovered. Even where they do occur, the surface markings are so much obliterated that 

 it is nearly impossible to characterize the species in a satisfactory manner. Several of those 

 figured in the report cited above are too obscure to be regarded as characteristic and important 

 species, until other specimens shall be found. There are moreover other considerations, which 

 induce me to hesitate as to the propriety of attempting to characterize a formation like that of 

 the Onondaga-salt group, by fossils, until a much larger number shall have been found. The 

 rarity of specimens, the few species yet recognized, and the imperfect condition of these, a 

 condition induced by the nature of the formation, all show that the period was one unfa- 

 vorable to the existence of organized beings ; and the few which occur near the base of the 

 formation might perhaps be regarded as a continuation of species from the upper part of th« 

 Niagara group, and those near the termination of the group may belong to those forms called 

 into existence after the cessation of the causes giving origin to the greater part of this series, 

 and during the period of gradual change which ushered in the formations of the Tentaculite lime- 

 stone and the succeeding rocks. In support of this view in reference to the upper strata we have 

 the fact that the higher drab-colored layers of the Onondaga-salt group graduate into the Tenta- 

 culite limestone in some places ; while in the western part of the State, where the latter rock is 

 not recognizable, the line of demarkation between the drab layers and the succeeding limestone 

 is very conspicuous. In the western part of New- York, the only well marked fossil of these 

 higher beds of the Onondaga-salt group is the Eurypterus, several specimens of which have 

 been found near Williamsville in Erie county. Within a few months past, my friend Ledyard 

 LiNCKLAEN, esquire, of Cazenovia, has sent me a specimen containing a part of the head of an 

 Eurypterus, and several impressions of the Spirifer peculiar to the Tentaculite limestone. 

 The character of the rock is intermediate in color, texture and composition, between' the 

 ordinary drab-colored layers of the Onondaga-salt group, and those of the Tentaculite lime- 

 stone. The conditions most favorable to the development of this peculiar fossil, the Eurypterus, 

 evidently existed just at the close of a period marked by extensive deposits of mud and saline 

 matter, the greater portion of which had been deposited, and an increased proportion of calca- 

 reous matter taking place in the sediment when the organism in question came into existence. 

 This example may not be an unusual one ; and indeed we well know that species often pass 

 the appaj-ent or physical limits of formations, being little affected by the incipient changes 

 which have finally produced a distinct formation characterized by numerous organisms. In the 

 present instance we have but a single species, so far as we know, and it may perhaps be equally 

 claimed for both groups. 



[Paljeontoloot — Vol. II.] 43 



