BIGSBY PALAEOZOIC BASIN OF NEW YORK. 



431 



by far the most fossiliferous. But these last (Chazy and Birdseye 

 Limestones) retain their relative positions over considerable spaces, 

 and are the products of the same natural causes ; they are also bound 

 together by a common organic facies (Sharpe), and by sixteen import- 

 ant species (in nine genera) of Silurian life, common to some two, 

 at least, of the members of this group. 



Fossils common to the Three Members of Group B. 



Fossils. 



Stromatocerium rugosum 

 Chaetetes Lycoperdon ... 



Lingula obtusa 



(Crania) quadrata... 



ovata 



Leptaena alternata 



Murchisonia angustata ... 

 perangulata 



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Pleurotomaria umbilicata * 



Orthoceras duplex * ... 



vertebrate '* ... 



Ormoceras gracile 

 Illaeniis Arcturus . 



crassicauda . 



Trentonensis. 



Bellerophon bilobatus * 



The peculiar Cephalopod, Ormoceras gracile, is one of these. The 

 Chazy Limestone is remarkable for containing another species, to- 

 gether with a variety ; they are both typical. Its Zoophytes and 

 Bryozoa have only five species each ; but individuals so abound that 

 the rock is often a mere coral-reef. 



On the rocky floor of some shallows near the mouth of the 

 Madawaska, a tributary of the River Ottawa in Canada, Sir "W. 

 Logan met with a profusion of finely-developed fossils of these three 

 limestones — existing therefore in common. Thus it is in Silurian 

 rocks as in the secondary and tertiary. The Lias, for instance, of 

 Yorkshire is divided into zones, each distinguished by its own fossils ; 

 but in following the same strata to considerable distances (as in the 

 Silurian case), the organisms typical of one member are seen to be- 

 come common to several formations, and thus to combine all into one 

 group, the Liassic (Murchison). 



Group C consists of the two very wide-spread sections, Utica Slate 

 and Hudson-River group. It is the uppermost aggregate of the 

 lower stage. Utica Slate has been deposited in quiet waters, as we 

 learn from its homogeneous character and the smoothness of its 

 grain ; while the Hudson- River strata were laid down in varying 

 depths, and often among conflicting currents ; for we find in them 

 numerous beds of conglomerates and breccias, both quartzose and 

 calcareous, the latter laden with Trenton fossils. 



In fact, a definition of the Hudson-River group in some localities 

 would describe the Oneida Conglomerate, the stratum next succeeding. 

 This vast deposit prevails over such wide spaces, and resembles, as 

 just mentioned, the last section so closely, that Professor Rogers and 



