KEVIEW OF GEOLOGICAL STRUCTURE. 3 



The name Cincinnati Group is retained for the rocks under considera- 

 tion in our report, since they are not the exact equivalents of any strata 

 described under other names elsewhere, and are the typical series to which 

 this name was first applied by Messrs. Meek and Worthen; but it should 

 be distinctly understood that the term is not, as is represented by these 

 authors and others, synonymous with the " Hudson River Group." 



If it were so, it would have been much better to retain the older name, as 

 the argument advanced for the change, viz. : " that as the ' Hudson River 

 Group ' does not reach the Hudson River, it was, therefore, a misnomer " 

 — has no foundation in fact. As it is, the law of priority, as well as the 

 interests of science, require the retention of the name Hudson River 

 Group, for the rocks to which it was applied, but it should not be con- 

 founded with the Cincinnati Group. 



In the State of New York, the differences between the Trenton and 

 Hudson Groups are chiefly local and lithological ; nearly all the fossils of 

 the Hu 'son being found in the Trenton. The only fossils characteristic 

 of the Utica shale, are graptolites which seem to have grown in great 

 profusion in certain shallow and quiet parts of the Lower Silurian Sea. 

 Subsequently, with the progressive shallowing of the portions of this 

 sea adjacent to the shores of the Adirondack and Canadian highlands, 

 the sediments deposited became more purely mechanical and coarser, and 

 the Oswego sandstone, the Lorraine and Pulaski shales were laid down. 

 These changing local conditions produced different groupings of mollus- 

 cous life in the subdivisions of the Trenton group; but further south 

 and west, where an open sea prevailed, the physical conditions were 

 much more constant, and there were few changes in the fauna through- 

 out the entire period of the deposition of the Lower Silurian limestones. 



With all the facts brought out by a careful study of the Cincinnati 

 Group and its fossils, we were compelled to qualify, in our first volume, 

 as has been specified, the definition before given to it. Subsequent ob- 

 servations have strengthened the arguments then used, and no conflict- 

 ing evidence has been advanced by others. Hence, we protest against 

 the course pursued by those who represent the Cincinnati Group of Ohio 

 as the equivalent of the Hudson River Group of New York, and call it 

 by the latter name ; and that of those others who, committing the same 

 error of identification, employ the name Cincinnati Group, and represent 

 it to be only the upper portion of the Lower Silurian system, and as 

 overlying all the Trenton series of New York. In the light of all the 

 facts cited in our reports, we cannot but regard adherence to these errors 

 as a willful perversion of the truth. 



Since the publication of the description of the Cincinnati Group, in 



