INTRODUCTION. 
IS 
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 Crinoidese and Cystidem com¬ 
paratively few speciqs 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 
