LOWER SILURIAN. 209 



region outside of the present Atlantic border of the continent, were 

 above tide-level at the time (see p. 418), it would have been a conti- 

 nental barrier against both waves and currents. 



With the opening of the Cincinnati era, sediments again were de- 

 posited over New York and the Appalachians, and some change of 

 level had, therefore, taken place. But, as the formation of lime- 

 stones was continued in the Mississippi basin, and also in the St. 

 Lawrence bay (at Anticosti), the change did not affect essentially 

 these regions. If the Atlantic barrier, above alluded to, were a fact 

 in the Trenton era, an oscillation of level submerging it, and raising 

 toward the surface another parallel region more to the west, where 

 the Appalachians now stand, would have opened again the New 

 York and Appalachian area to the ocean, and so might have occasioned 

 the transition to sedimentary accumulations. 



Climate. — No proof that a diversity of zones of climate prevailed 

 over the globe is observable in the fossils of the Trenton period, or 

 of any part of the Lower Silurian era, so far as yet studied. The fol- 

 lowing species, common in the United States, and occurring at least as 

 far south as Tennessee and Alabama, have been found in the strata of 

 northern North America, near Lake Winnipeg : Strophomena alter- 

 nata,- Lepteena sericea ?, Maclurea magna, Pleurotomaria lenticularis ?, 

 Calymene senaria, Chcetetes Lycoperdon, Receptaculites Neptuni. 



The mild temperature of the Arctic regions is further evident from 

 the occurrence of the following United States and European species 

 on King William's Island, North Devon, and at Depot Bay, in Bel- 

 lot's Strait (lat. 72°, long. 94°), — Ohcetetes lycoperdon, Orthoceras 

 moniliforme H., Receptaculites Neptuni De France, Ormoceras crebri- 

 septum II., Huronia vertebralis Stokes ; besides Maclurea Arctica 

 Haughton, near the Chazy species M. magna. Moreover, the forma- 

 tion of thick strata of limestone shows that life like that of lower 

 latitudes not only existed there, but nourished in profusion. 



Life. — Exterminations. — At the close of the Chazy epoch, its 

 species, with few exceptions, disappeared, for the rocks of the Trenton 

 epoch contain a different range of species. No facts have been ob- 

 served to explain the nature of the catastrophe that intervened 

 between the two epochs. Such a fact as this — that sinking the 

 coral islands of the Pacific three hundred feet would destroy the reef- 

 forming Corals of those islands — may have some bearing on the 

 subject. The geographical changes introducing the Cincinnati epoch 

 appear to have had some connection with the partial destruction of 

 the Trenton species that then occurred. A large number of species 

 are continued on from the Trenton into the Cincinnati group, wherever 

 the rocks of the latter, like those of the former, are limestones. But, 

 14 



