564 NEW YORK STATE MUSEUM 



were yery unfavorable to the Trenton fauna, then flourishing in 

 the clear waters on the continental platform. During the Utica 

 epoch the mud-bearing currents encroached on the eastern part 

 of the continent and drove out a part of the Trenton fauna.^ 



DISCUSSION OF THE VALIDITY OF THE TERM, " HUD- 

 SON RIVER GROUP " 



The demonstration of the presence of Trenton, Utica and Lor- 

 raine faunas in the shales of the Hudson river valley, necessitates 

 a renewed investigation of the validity of the old term, " Hudson 

 river group", or its modification, " Hudson group", as proposed 

 by Walcott for all the beds between the Trenton limestone and 

 the Upper Siluric. As the term has been so ably and forcibly 

 defended by such authorities as Hall (17) and Walcott (36a), its 

 validity can be questioned only after obtaining new facts. These, 



^A parallel case to the continued appearance of genera in the Quebec 

 Dicellograptus shales and the conglomerate of Rysedorph hill, which have 

 disappeared in homotaxial beds of other regions of eastern North America, 

 is the continuation of Trenton forms which nowhere else have been ob- 

 served to go above the Trenton, asParastrophia, hemiplicata, 

 Gyclospira bisulcata, Clionychia undata Spyro- 

 ceras bilineatum, Cyrtoceras annul a turn, Crypto^ 

 graptus tricornis, and Climacograptus eaudatus, 

 into beds which by the general character of their fossil contents are char- 

 acterized as being of Utica age. This phenomenon has been observed in 

 several localities, at the Dudley observatory by Dr Beecher, in 

 two outcrops on Green Island, one on Van Schaick island, and one 

 south of Mechanicsville by the writer. The beds near Mechanicsville 

 may be here excluded as being evidently transitional between the Tren- 

 ton and Utica epochs, or of oldest Utica age, but the other beds lie at or 

 near the top of the Utica terrane and yet contain Trenton forms still. 

 This peculiarity in the composition of the Utica faunas is, to the writer's 

 knowledge, restricted to the Hudson valley region, and therefore, marks a 

 regional difference in the character of the Utica faunas. This conclusion 

 is supported by the distribution of Corynoides curtus mentioned 

 before. This graptolite disappears in the Utica beds of the middle 

 Mohawk valley, while it fills the beds of the lower Mohawk and the 

 Hudson valley, and is also common in the Utica shale just above the Tren- 

 ton at the Panton shore of Lake Champlain. Also the variety of P 1 e c- 

 tambonites sericea with wrinkled cardinal margin, mentioned 

 before^ is a fossil which extends in this region from the lowest Trenton 

 into tne upper Utica beds, and is rarely oDserved outside of it. 



