BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 383 



turn made up of Cypricardia, Turbo y Bellerophon, and Orthocerata, 

 without any other Mollusc. It is a continuation of a like stratum on 

 the east, but with other shells superadded. Apparently there were 

 distinct periods of formation (Hall). 



Fossils typical. — These are 131, and they are principally Brachio- 

 poda and Lamellibranchiata. 



Fossils occurrent in Europe. — They are 38 in number in this 

 single and imperfectly-examined group. They represent most of the 

 genera. 



Of these 38 species, 21 are members of the Devonian system in 

 their respective countries, 2 belong to the Wenlock Limestone, and 1 

 to the Upper Ludlow. The remaining 14 have not yet been pro- 

 perly traced. It is easy nevertheless to perceive that the Hamilton 

 group represents the Devonian system in Europe ; but the particular 

 stage to which it best refers has yet to be ascertained. 



Fossils recurrent in New York. — For particulars, see the Table of 

 Devonian Recur rency in New York. 



We observe here, that of the 29 recurrents of this group, none 

 come from its immediate predecessor. Eight are derived from the 

 Corniferous Limestone, although usually the mineral constitutions of 

 the rocks differ. 



Eighteen Hamilton fossils, out of 1 9 which pass upwards, are found 

 again in the Chemung group, mineralogically a very similar set of 

 strata. Only one species proceeds into the Tully Limestone. 



The Hamilton maintains more frequent relations with the neigh- 

 bouring groups than any other, both upwards and downwards ; but, 

 unlike the Onondaga and Tully Limestones, &c, none of its fossils 

 are recurrents from the Silurian system. 



Tully Limestone. 



Mineral Character. — This stratum marks prominently the end of 

 the fossiliferous shales of the Hamilton group ; but, being so slender, 

 small, and barren, it scarcely deserves a separate denomination. 



It is a thick-bedded deposit, close-grained and compact in parts. 

 It is often a mixture of shaly and calcareous matter, — the latter pre- 

 vailing, and often sole. The usual colour of this rock is blue or 

 nearly black. 



Tully Limestone is the last Devonian stratum in which lime forms 

 an essential part, — the lithological character of the products above 

 being similar throughout, and different from those below. 



This contrast of character is more marked toward the west end of 

 New York (Hall) than it is further east ; and finally on its eastern 

 extreme there is a greater mineral similarity. Here a few Lower De- 

 vonian fossils creep into the uppermost group of Middle Devonian 

 (Hall, p. 213). 



Place. — Tully Limestone only occupies a small part of the State ; 

 and is particularly well seen on the Cayuga and Seneca Lakes, undu- 

 lating along their shores in broad low arches. It commences in the 

 east, near Smyrna, Chenango County, and is seen no further west 



