BIGSBY PALEOZOIC ROCKS OF NEW YORK. 287 



sively Eussian out of 392, or 48 per cent.* ; James HaU finds 369 

 new species out of a total of 900, or about 40 per cent. ; while, of 

 Sir C. Lyell's small collection of American fossils, five-sixths are new 

 to Europe, according to the late Daniel Sharpe. In the same way- 

 General Portlock, in his Summary of the Tyrone and Fermanagh 

 Fossils, finds 105 forms (principally Crustacea, Cephalopoda, and Bi- 

 myaria), out of 188, peculiar to Ireland, or 54 per cent.f. Each of 

 these countries therefore presents a geographic province. Professor 

 J. Phillips calculates that, out of 430 of the fossils of the adjacent 

 districts of Woolhope and Abherley, only 96 are common to them 

 both, giving the remarkably large per-centage of 76 to the re- 

 stricted fossilsj. Lockport, in the State of New York, and Drum- 

 mond Island in Lake Huron, both on the same geologic horizon, 

 differ widely in their extinct faunae ; and it is a most remarkable 

 fact, that the Scandinavian and Bohemian basins, about 800 miles 

 apart, have but 1 per cent, of their very numerous Trilobites in com- ■ 

 mon, and only 5 per cent, on their whole faunae. We must remem- 

 ber that there are a multitude of causes which make the species of 

 the same epoch vary in its different localities, — such as depth or 

 dilution of sea-water, the contents and rates of currents, and other 

 agencies already alluded to. In the Lake of Stennis in the Orkney 

 Islands, however, we have a beautiful instance of the facility with 

 which certain marine geneia. (Cardiacece and Mytili) have continued 

 to live on amid their new associates, the Limncece and Neritince of 

 fresh waters, while the others perished. This lake has been gra- 

 dually converted from a salt-water loch into a freshwater or marshy 

 tract §. 



The second kind of life-centre, the epochal, occurs in vertical 

 and almost independent succession. Each of the palaeozoic sections 

 of New York and Wales has its own typical or epochal fossils, 

 besides those which are recurrent. The genera often survive, species 

 more rarely. A new epoch is usually produced by a change in 

 depth, the result of insensible oscillations — a movement of depres- 

 sion or elevation, which is almost always going on||, and which ne- 

 cessarily occasions new currents, new sediments, and new animal 

 occupants. It is very interesting to notice the frequent commixture 

 of fossils about the outer or terminal layers of two adjacent sections 

 or epochs, and to witness their gradual replacement in the deeper 

 situations by a new assemblage of life. Geologists now agreed 

 that this proves that new epochal centres of life were introduced 

 quietly and with little disturbance. D'Orbigny has a strong opinion 

 to the contrary ** ; but facts are clearly against him. These pro- 

 vinces, universal and indisputable, are particularly well defined in 

 New York. They owe their origin to an agency very different from 

 that which gives rise to geographic centres ; and so occult is that 



* Geol. of Russia, vol. ii. p. 396. t Portlock, Greol. Londonderry, p. 476. 



X Pal. Foss. Dev. p. 178. § Murckison, G-eol. of Russia, vol. i. p. 302. 



ji Dana, Address Amer. Assoc. 1855, p. 315. 



% Barrande, Syst. Sil. Boheme, p. 72 5 ; Hall, &c. 



^* Cours Paleont. vol. i. p. 93, and toI. ii. p. 252. 



VOL. XV. PART I. Y 



