276 IDEXTITV OF THE WESTERN FORMATIONS 



have been chiefly relied upon. All the specimens which I saw, 

 however, are of dilTercnt species from those of Trenton. So that 

 although certain species of this genus do occur in the Trenton 

 limestone, and are characteristic of that formation, others are not 

 necessarily so, and unless we take wide ranges in our groupings, 

 we cannot depend on generic types. In this case the amount 

 of evidence appears to be about equally divided between the 

 Trenton and Hudson river groups ; but since there are fossils 

 decidedly typical of the latter, and we know that in New York 

 they never occur in a lower position, we are compelled to admit 

 that this formation is of the same geological age. 



Besides the fossils enumerated, are many which do not occur 

 in New York ; among these a beautiful crinoid and several 

 species of Delthyris, Atnjpa and Orthis. It should not be omit- 

 ted, that in the hill-side at Cincinnati, we find, attached to the 

 limestone beds, numerous thin wedge-form layers of sandstone, 

 which usually contain a species of fucoid similar to one in the 

 Hudson river group, and the same as that noticed at Maysville. 

 Besides the fucoid, this sandstone contains a species of Stropho- 

 mena similar to one of the same gi-oup in New York. 



From the evidence here adduced, it appears that in the West 

 there is not so great a transition from the Black river and Tren- 

 ton rocks to those above, as in New York ; and that, from the 

 fact of the greater similarity of lithological character, and the 

 occurrence of many important fossils, specifically and generically 

 similar, throughout the mass, we may yet be inclined to consider 

 tlie whole as one great natural group, exhibiting well defined lines 

 of minor subdivisions. The termination of the Hudson river 

 group, in New York, is the first point of marked and unequivo- 

 cal change in the fossil characters. Below this point there are 

 many forms which pass from one rock to another upward, often 

 rendering it almost impossible to decide what are to be consid- 

 ered as characteristic. In every case, however, certain species 

 are entirely limited to the mass they occupy. The great range 

 of some of these species through the lower rocks, with their total 

 extinction at the termination, indicate a great change in the condi- 

 tion of the ocean. Such a change is further corroborated by the 



