118 TWENTY-SEVENTH EEPOET ON THE STATE MUSEUM. 



geologists are agreed in considering these dolomites to be the 

 stratigraphical equivalents, of the Niagara group of New 

 York. 



"In southern Illinois we find these dolomites replaced by a 

 series of silicious and argillaceous limestones, forming a group 

 two hundred and fifty feet or more in thickness, which, like 

 the dolomites of northern Illinois, rest directly upon the Cin- 

 cinnati group, and are immediately succeeded by Devonian 

 strata. At the base of this group of silicious limestones there 

 are some reddish mottled beds, from ten to twenty feet in 

 thickness, that in color bear considerable resemblance to the 

 Medina sandstone of New York ; and these mottled limestones 

 pass gradually into the buff and gray silicious beds that con- 

 stitute the upper and main portion of the group. Fossils are 

 rare in the lower portion of the group here ; but the mottled 

 limestones contain some Orthoceratites, and joints of large 

 Crinoidea, while the middle and upper portions are locally 

 quite fossiliferous, and have afforded many of the character- 

 istic species of the so-called Lower Helderberg group, among 

 which are the following : Orthis subcarinata, 0. oblata, Ccelos- 

 pira subcarinata, [Sic]* C. imbricata, Bpirifer perlamellosus, 

 and Platyeeras spirale of Hall, and Acidaspis hamatus of 

 Conrad, together with species closely resembling, if not iden- 

 tical with, Merista princeps. Platyeeras pyramidatum, P. 

 unguiforme, P. incite, and P. multistriatum of Hall. 



" In the first volume of the ' Report on the Geological Sur- 

 vey of Illinois,' these silicious limestones of the southern por- 

 tion of the State, and the dolomites of northern Illinois, were 

 regarded as the stratigraphical equivalents of the Niagara 

 group, and' were included together as representing a single 

 division of the Upper Silurian series ; but, subsequently, in a 

 corrected section of the Illinois strata, published in the intro- 

 duction to the second volume, we were induced, from the dis- 

 similarity of the fossils from the different sections of the State, 

 to regard the silicious limestones of southern Illinois as the 

 representatives of a higher geological horizon, and therefore 

 placed them above the dolomites of the northern part of the 

 State, as the equivalents of the so-called Lower Helderberg 

 group. We are now, however, fully satisfied, from a further 

 examination of these Upper Silurian strata, over a more ex- 

 tended region, that our first conclusion was correct, and that 

 these silicious limestones and dolomites represent the same 

 geological horizon, and that the difference in the specific char- 

 acter of their fossil contents is entirely due to the changes in 

 the oceanic conditions under which they were deposited, and 

 not to the different ages of the sediments themselves. 



" South of the Ohio river, these Upper Silurian strata are 

 found well exposed in Tennessee, in the counties of Wayne, 



["* Probably this should be 0. concava.] 



