TRENTON AND HUDSON PERIODS. 225 



strata of limestone shows that life like that of lower latitudes not 

 only existed there, but nourished in tropical profusion. 



Life. — Exterminations. — The Chazy epoch, the commencement 

 of the Trenton period, opened with a new fauna, wholly dis- 

 tinct in species from that which preceded it. At its close these 

 species, with few exceptions, disappear, and the Trenton epoch 

 begins with an independent life. No facts have been observed to 

 explain the nature of the catastrophe that intervenes 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 geo- 

 graphical changes introducing the Hudson period appear to have 

 had some connection with the partial destruction of the Trenton 

 species that then occurred. A large number of species are conti- 

 nued on from the Trenton into the Hudson period wherever the 

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

 where the latter are shales, — that is, wherever the great geogra- 

 phical change alluded to took place, which we have suggested may 

 have been a subsidence destructive to the life, — there the species 

 were almost wholly changed, and a new fauna appeared, and one 

 fitted for the muddy bottom, and, therefore, including many Con- 

 chifers with the Brachiopods, and but few Crinoids. 



With the close of the Trenton period there was nearly a total 

 extinction of all the existing species throughout the great interior 

 continental basin and the Appalachian region. Not over eight or 

 ten species are known to have survived through the changes that 

 followed introductory to the Upper Silurian. In the eastern 

 border basin, at Anticosti, a much larger number of species conti- 

 nued on, — at least thirty ; and there, as is explained beyond, lime- 

 stones still had uninterrupted progress, while fragmental rocks 

 spread largely over the rest of the continent. 



The number of Lower Silurian species that are known to have 

 become extinct in the American seas from the beginning of the 

 Potsdam to the end of the Hudson period is about 850 ; and, 

 adding 400 for undescribed species (which is not too large an addi- 

 tion), the whole number will be about 1250. (Billings.) 



Many genera also disappeared in this time ; and some of them 

 ore enumerated in connection with the observations on the Pots- 

 dam period. Among Mollusks, the genus Maclurea, containing 

 large species, is one of the most prominent. But, besides genera, 

 a whole family, in the case of the Graptolites, approaches its extinc- 

 tion. These species have their commencement and culmination in 

 the Lower Silurian. Nearly all the genera of Cystideans also become 



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