BIGSBY — PALEOZOIC BASIN OF NEW YORK. 433 



domain of the Trenton section. Their delicacy of organization has 

 rendered them intolerant of mineral change, and therefore typical. 



The Niagara fossils exhibit much greater variety in form, appendage, 

 and ornament than those of the lower stage. The Brachiopoda of 

 this section are less in number by one-third than those of Trenton ; 

 40 of them are new, out of 48 ; and they are very prolific of individuals. 



The genus Lingula has but one representative here. Or this and 

 Leptcena are each one-third of the Trenton number, while Atrypa 

 remains numerous, and Spirifer has multiplied fivefold. 9 out of 13 

 Trilobites, and 7 out of 10 Cephalopoda are typical. These are 

 striking facts. 



The Coralline Limestone of Schoharie is admitted by all to be a 

 part of the Niagara section. Although it has 30 fossils peculiar to 

 itself, principally Brachiopoda, Monomyaria, and Gasteropoda, it 

 has 7 important molluscs in common with the Niagara section ; viz. 

 Catenipora escharoides, Chcetetes Niagarensis, Stromatopora concen- 

 trica, Spirifer crispus, Atrypa reticularis, Comulites serpularius, and 

 Asaphus Blumenbachii. 



We must speak of the four Lower Helderberg formations in con- 

 nexion with the Group F, on account of their intimate relations with 

 it, although separated by 1000 feet of argillo-arenaceous strata (Onon- 

 daga-Salt group). 



Group H has been divided into Waterlime Rock (lowest), Lower 

 Pentamerus Limestone, Delthyris-Shaly Limestone, and Upper Pen- 

 tamerus Limestone. In the Synoptical View, these beds have been 

 shown to be of one epoch by their mineral and fossil characters, and 

 by conformableness. 



Though a local, and in some sense a supplementary group, con- 

 fined to the State of New York, these strata are distinct as a series, 

 overspreading considerable space, and nearly allied to the English 

 Wenlock, but possessing typical fossils, of which Delthyris-Shaly 

 Limestone alone has 30 ; the others show fewer, owing probably to 

 imperfect examination. 



The Onondaga-Salt Group, G. — The strongly- marked peculiarities 

 of this group, with its deposits of gypsum and its brine-springs, &c, 

 with its barrenness of fossils except about its calcareous capping, are 

 so clearly explained in the Synoptical View, and so conclusive of its 

 being a separate and unique series of sediments, that no further 

 remark upon them is required. 



The Silurian Stages of the New York Basin. 



These are three — the Lower, the Middle, and the Upper. We 

 shall speak of them in order. 



The Lower Stage. — In this part of the Central Palaeozoic Basin, 

 the lower stage does not rest upon beds which can in any sense be 

 considered as the North American representatives of the Longmynd 

 rocks of Wales. In the east of New York its beds issue, and proceed 

 westwards, from the metamorphic sheets of mineral matter which 

 floor most of New England, and which are in truth contemporaneous 



