INTRODUCTION. 13 



Lake Michigan ; in Wisconsin ; in Minnesota, at the Falls of St.Anthony ; 

 and at numerous points along the Mississippi both in Iowa and Wisconsin, 

 as far south as Dubuque. It reappears along the river in Illinois above 

 Point au Gris with even more of its characteristic features, both in fossils 

 and in lithological characters, than in the more northern localities in this 

 valley. Still farther to the south, along the Mississippi valley, this rock 

 assumes more and more the aspect which it possesses in New- York : its 

 thickness increases ; and in all its characteristics it assimilates to the rock 

 in its normal and best developed condition. In numerous localities in 

 Missouri, the Trenton limestone presents an important development, and 

 everywhere contains characteristic fossils. Both in Kentucky and Tennessee 

 this limestone has comparatively a very extensive development ; more 

 particularly in the latter State, where it contains many of its characteristic 

 fossils. 



With all this additional information in regard to the extension and 

 development of the Trenton limestone, we have no knowledge of any 

 localities of equal perfection with those of Central New- York. No others 

 have yet shown the rock so perfectly developed in all its phases of litho- 

 logical character, and from no other point have we obtained so great a 

 number of fossil species. Still, among the Crinoideae and Cystideag com- 

 paratively few species have been found in the rocks of this period in 

 New- York, or in the Western States. So far as I know, not three species 

 have been added to those described in the first volume of the Palaeontology 

 of New- York ; and from the numerous collectors now at work upon this 

 rock in different localities, we should expect them to be discovered, if 

 really existing. In the mean time, a single locality on the Ottawa river 

 has yielded to the investigations of Mr. Billings, now of the Canada 

 Geological Survey, more species of crinoids and cystidians than all the 

 Lower Silurian rocks of the American continent besides, and even more 

 than all from strata of the same age in Europe and America together. It 

 is probable that other families of fossils may prove equally abundant in 



