174 NEW YORK STATE MUSEUM 



f 



limestone, which, in bed III is fine grained, hard, and light 

 colored. Bed D marks, by its darker color, a tendency to return 

 to former conditions, and V and VI are impure and somewhat 

 shaly limestones. The changes in physical conditions thus indi- 

 cated are reflected, as one might expect, in the character and 

 condition of the organic remains. Thus, there is a gradual 

 increase in size as well as in number of species from bed I to 

 bed III, while the reverse is true from bed IV to VI, in which a 

 small lamellibranch becomes the mosit abundant fossil. These 

 are succeeded by a return to clear water conditions, for VII and 

 VIII are to an even greater extent than bed III pure hard lime- 

 stones containing a great abundance and variety of fossil 

 remains. As already noted, they represent the time of maxi- 

 mum development of the fauna. The change to muddier waters 

 and a Marcellus fauna is an abrupt one, as indicated by the thin 

 bedded dark shales and Liorhynchus fauna of bed F. 



CONCLUSION 



The base of the Marcellus is reported by Prof. I. P. Bishop 1 to be 

 20 feet below the limestone layers. The thickness of the lime- 

 stone is 8 feet, 4 inches, and the estimated thickness of the Mar- 

 cellus in western New York 140 feet. The top of the lime- 

 stone must therefore be not less than 111 feet below the base of 

 the Hamilton. Though occurring so far below the latter forma- 

 tion, the fossils of the Plumbottom creek limestones are, as 

 indicated by the foregoing lists, largely Hamilton species with 

 a few typical Marcellus forms and several persistent species 

 from the Onondaga limestone. 



Of the 72 species recognized from the limestone at Lancaster 

 54 occur in the Hamilton, 15 species are common to both Hamil- 

 ton and the Marcellus shales, eight are exclusively Marcellus, 11 

 have been reported from the Onondaga limestone, and two are 

 restricted to the Stafford limestone. The shales below and 

 above the limestone contain typical Marcellus fossils. It will 

 thus be seen that the conditions favorable to the formation of 

 the thin bedded dark shale, with its characteristically Marcellus 



1 Structural and economic geology of Erie county, p. 314. 



