BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 345 



have nearly disappeared, though in some portions of the succeeding 

 slate they are again abundant." (Hall, Pal. i. p. 191.) 



Six only of invertebrate animals have been transmitted from 

 earlier groups to the Trenton Limestone, which sends on only 45, 

 the great majority perishing. 



Fossils typical. — These are 211 in number, leaving thus about 

 one-sixth as either derived from, or transmitted to other groups. 

 All the Echinoderms are typical ; 55 Brachiopoda, 31 Gasteropoda, 

 and 42 Cephalopoda. For further detail consult Table II. 



Fossils occurrent in Europe. — (See Table of fossils common to both 

 hemispheres.) — Chsetetes petropolitanus ; Stromatopora concentrica; Pti- 

 lodictya acuta; Atrypa protea; Leptaena alternata; L. depressa; L. 

 Stroph.grandis; Lingula attenuata; L. obtusa; Orthis biforata; O. parva; 

 O. rugosa; O. testudinaria ; O. Verneuilli; Spirifer biforatus; S. lynx; 

 Terebratula bidentata; T. reticularis; Calymene Fischeri; C. punctata; 

 C. senaria; Ceraurus pleurexanthemus ; Illaenus crassicauda; Isotelus 

 gigas ; Lichas laciniata ; Phacops Dalmanni ; Trinucleus Caractaci ; Mo- 

 diolopsis modiolaris ; Nucula levata ; N. post striata ; Subulites elongata ; 

 Pleurotomaria lenticularis ; P. percarinata ; Porcellia ornata ; Cyrtoceras 

 multicameratum ; Orthoceras arcuohratum ; O. bilineatum ; O. commune ; 

 O. vertebrale ; Bellerophon bilobatus. 



The Cystidea occur in the Lower Silurian stage both in America 

 and Europe. 



Fossils recurrent in New York. — These are 38 in number, of 

 which 22 only recur once, and that almost always in the next group, 

 or next but one : 1 5 recur twice. The one remaining passes on 

 into the Devonian system, like Orbicula lamellosa. As might be 

 expected, we find among these recurrents four Graptolites, twelve 

 Brachiopoda, nine Trilobites, six Lamellibranchiata, and two Gastero- 

 poda. Trocholites is only found here and in the Pulaski Sandstones 

 of the Hudson- River group. 



Utica Slate. 



Mineral Character. — This subdivision is composed of the same 

 material that separates the dark limestone-layers of the Trenton 

 Limestone. It is a dark, bluish-black, or carbonaceous shale, green 

 in Ohio and the West, weathering brownish and chocolate-coloured. 

 It contains no fragments of other rocks ; but in many places there 

 are thin beds of impure limestone. It is metamorphosed into a 

 roofing-slate frequently in the first-named geological district (with 

 Graptolites). 



Transition. — This is very gradual, and, as a general rule, from 

 the Trenton Limestone ; but near Split Rock, Whallon's Bay, 

 Lake Champlain, as well as in some other places, it lies against and 

 upon Hypogene Rock, overlapping it in wrinkles and curvatures at 

 the line of junction, as we sometimes see Trenton Limestone do in 

 like circumstances. 



Place. — Utica Slate is one of the constants, and always interposed 

 between Trenton Limestone and the Hudson-River group through- 



