286 IDENTITY OF THE WESTERN FORMATIONS 



a ragged and irregular outline, in place of the uniform cliff, formed 

 by the higher limestone. In tracing this limestone up the river 

 as far as Galena, its lithological character is the same as that of 

 the middle and upper portions of the Niagara limestone, and all 

 its associations are the same. The cliffs in many places, par- 

 ticularly on the river above Dubuque, have all the appearance of 

 those at the falls of Niagara, so far as regards lithological charac- 

 ter, weathering, &c. Above the town of Dubuque, this rock 

 (cliff limestone) rests upon the blue limestone, which, according 

 to Dr. Owen's statement, is much thinner here than in Ohio ; but 

 not having traced it to its termination below, I am unable to speak 

 from personal observation. Specimens which I saw from the 

 lower part of this rock about Prairie du Chien, indicate the pres- 

 ence of the Trenton and Black river limestones, by the large 

 Orthocerata and other fossils. From a section made by Dr. Owen, 

 it appears that there are beds of sandstone, interstratified with, and 

 underlying the blue limestone. 



It is in the cliff formation as before described, that the lead ores 

 of Illinois, Wisconsin, and Iowa are found, a part only of the rock 

 yielding these ores. This portion appears to correspond to the Ni- 

 agara limestone ; the upper part of the " cliff rocks," which repre- 

 sents the Helderberg series, being destitute of metallic veins. Al- 

 though the rock contains few fossils, these indicate its position to 

 be the same. The Catenipora, which in New York occurs far 

 more abundantly in the Niagara limestone than in any other rock,* 

 is found here in great abundance, with Aulopora and a few other 

 fossils. The next fossiliferous group below the lead-bearing rock 

 on the Mississippi river, is that corresponding to the Hudson river 

 group. If the Clinton group exists, it has there become so incor- 

 porated with the rocks above, as to be overlooked as a distinct 

 formation. This point requires examination, and it is possible 

 that some representative of the group may be found there. Being 

 a very variable assemblage in New York, it would not be surpris- 

 ing to find it under another aspect or even incorporated with the 

 Niagara group as far west as the Mississippi. Its last appear- 

 ance in New York on the Niagara river, is in the form of a mass 



• See Note. 



