BIGSBY — PALAEOZOIC ROCKS OF NEW YORK. 377 



no hornstone. This rock seems to Hall to be composed of finely- 

 levigated calcareous mud, with much silex introduced. 



It is distinguished from Onondaga Limestone by its greater com- 

 pactness, by its possessing more hornstone, and having no Crinoidea 

 nor Favosites (Hall) ; but as this distinction is not always easily 

 made, the two ought to be thrown into one group, which would then 

 be as useful and extensive an horizon as either the Niagara or the 

 Trenton (De Verneuil). The so-called "Seneca Limestone" is the 

 top or terminal portion of the Corniferous (Vanux. Rep. 144). 



This is a most persistent stratum, and is more uniform in mineral 

 character and extent than any other. It therefore becomes one of 

 the best planes or levels of reference in the whole New York system 

 (Hall). With this rock terminate all the important limestones of this 

 basin, the calcareous deposits of higher position being thin and local. 

 The subsequent sediments are quite different from that of the Corni- 

 ferous limestone ; and so, for the most part, are the organic remains. 



R. C. Taylor (Statistics of Coal) notices the existence of petroleum- 

 ponds in this rock near London, Canada West. 



TransUion. — It is intimately connected with Onondaga Limestone ; 

 they graduate into each other. 



Place. — It extends across New York from the Helderberg Moun- 

 tains to the River Niagara and westwards. 



Resting upon Onondaga Limestone, and supporting Marcellus Shale, 

 the Corniferous limestone is co-extensive with them in New York. 



It passes into the west from the mouth of the River Niagara, along 

 the north side of Lake Erie, in a broad belt which is projected north- 

 westwardly into Lake Huron and the countries W.S.W. of that body 

 of water, and, bifurcating, is again continued from the S.W. side of 

 Lake Erie southwards into Ohio and Illinois, in both which great 

 States this rock takes its proper place as a Lower Devonian lime- 

 stone of vast extent. 



Like Onondaga Limestone, this rock has been greatly denuded, as 

 about the Rivers Genesee and Niagara. The Lakes Seneca and Cayuga 

 also have been hollowed out of it, the hollow extending dry for many 

 miles north of the first-named lake. Three miles south of the village 

 of Seneca Falls, this limestone is broken by faults, partly from 

 denuding violence, and partly by the removal of subjacent gypseous 

 beds (?) (Hall, Rep. p. 161). 



Position. — Corniferous limestone has a general dip to the south- 

 west, and at one place at the rate of 27 feet to the mile (Vanuxem, 

 Rep. p. 141). 



Thickness. — The maximum is 80 feet in New York according to 

 one authority, but 71 feet according to Hall ; and usually it is much 

 less. 



Fossils. — Fossils are few here, and are mostly molluscs (Hall, 

 Rep. p. 163). 



The deposition of this limestone was the commencement of a 

 change by which the sea became too deep for corals. Those coral- 

 banks which had been formed in the Onondaga limestone were then 

 buried by calcareous mud derived from other quarters. 



