LOWER SILURIAN SYSTEM. 181 



overlying formations, leave no doubt as to the propriety of 

 referring it to the same geological period as that in which the 

 rocks at Cincinnati, Ohio, were formed; but as a formation, 

 it is regarded as distinct from any other one of that group 

 hitherto defined. In the former report upon the geology of 

 Iowa, it received the name of "Hudson Kiver Group," in 

 consequence of its supposed equivalency to certain rocks 

 abundantly exposed along the Hudson river in New York. 

 But the designation "group" refers to a whole period in 

 geologic time, and when it is applied to any single formation, 

 its indefiniteness differs only in degree from a mere reference 

 of the formation to its proper system or age. Therefore, as 

 the strata of this formation, all referable without doubt to a 

 single epoch of its period, are well developed on the Little 

 Maquoketa river, where its characteristic fossils are also 

 abundant, the name Maquoketo shales is given to this par- 

 ticular formation of the group. 



Again Messrs. Meek and Worthen of the State Geological 

 Survey of Illinois, have shown that these Iowa shales, as well 

 as their equivalents and associated strata in their own and 

 other interior States, are not equivalent to those upon the 

 Hudson river to which the name of " Hudson River Shales" 

 was first applied. They therefore have very properly 

 dropped the name of Hudson River Group as applied to 

 these, and proposed the name Cincinnati Group, because the 

 group or its principal formations are well exposed at that 

 city, where also its abundant and characteristic fossils are 

 well known. Relying upon the determinations of these able 

 geologists, as well as upon my own observations, I also 

 reject the name " Hudson River Group " as applied to our 

 own strata and their equivalents, and apply the name " Cin- 

 cinnati Group" to the formations of the whole period as 

 they do, but use the name Maquoketa shales to designate 

 that particular epochal subdivision or formation of the group 

 which alone is found in Iowa.* 



* See " assification of Iowa Rocks/' in the introduction. 



