HUDSON RIVER BEOS NEAR ALBANY 561 



tion as a whole dwindled to an insignificant thickness, or 

 that only a short span of the Trenton period is represented in the 

 lower Mohawk valley by limestone, presumably the earliest por- 

 tion, as in the Hudson valley. The facts at hand do not permit 

 a choice between the two alternatives, but the thinning out of the 

 limestone, together with the entire absence of the Dicellograptus 

 fauna in the Mohawk valley, suggests the presence of a barrier,, 

 perhaps by a shallowing of the sea as indicated by the thin forma- 

 tion of limestone in the region of the lower Mohawk, in early 

 Trenton time. The very peculiar Trenton fauna of the limestone 

 conglomerate of K-ysedorph hill and Moordener kill, characterized 

 by the occurrence of P 1 e c t a m b o n i t e s s e r i c e a var. 

 aspera, Plectambonites aff . gibbosa, Chris- 

 tiania, Eccyliopterus, Ampyx and R e m o p 1 e u- 

 rides, indicates a great faunistic difference between the Hud- 

 son valley and Mohawk valley regions even before the deposition 

 of the Dicellograptus zone, and at the time of the deposition of 

 the basal Trenton limestone beds. 



DISCONTINUITY OF FAUNISTIC SUCCESSION IN TREN- 



TON AND UTICA BEDS 



A fact apparently incongruous with the separation of the Nor- 

 mans kill and the Utica shales by the middle and upper Trenton 

 beds is the discontinuity of the faunistic succession in these Tren- 

 ton beds; for, while the lower Dicellograptus fauna disappears 

 in the middle Trenton shales, a small part of the graptolite fauna 

 of the Dicellograptus zone reappears in the Utica shale. It is 

 this observation which induced Whitfield and Walcott to connect 

 the Normans kill with the Utica shale. On the other hand, the 

 graptolite, Diplograptus amplexicaulis, common 

 in the middle Trenton, disappears in the Utica shale and is said 

 to reappear in the Lorraine beds. The latter fact is in accordance 

 with the known return of other Trenton forms in Lorraine times. 

 The explanation of this alternating recession and return of grap- 

 tolite faunas in the Hudson valley region seems to lie in the dis- 

 tribution of the faunas and the character of the associate forms. 



