182 GENERAL GEOLOGY. 



Fossils. Several species of fossils which characterize the 

 Cincinnati group are fonnd in the Maquoketa shales, such as 

 Orthis testudinaria, 0. occidentalism Strophomena alternata, 

 S. sericea, etc., but they contain a large number of species 

 that have been found nowhere else than in these shales in 

 Iowa. They belong to the genera Orthoceras, Murcliisonia, 

 Pleurotomaria, ScTiizodus (?) Discina, Graptolithus, etc. The 

 distinct faunal characteristics presented by these fossils last 

 referred to, seem to warrant the separation of the Maquoketa 

 shales as a distinct formation from any others of the group. 

 Its true position is probably at the base of the group. 



3. UPPEK SILUPIAJNT SYSTEM. 



NIAGARA GROUP. 

 THE NIAGARA LIMESTONE. 



Syn— Le Claire Limestone, in Part, of Hall. 



Area and General Characters. In the former report upon 

 the geology of Iowa, two distinct formations of the Niagara 

 group were recognized and described, namely, the Niagara 

 limestone and the Le Claire limestone; the latter resting upon 

 the former. Subsequent stratigraphical and palseontological 

 examinations fail to show any good reasons for referring 

 any of the Upper Silurian rocks of Iowa to any other than 

 the epoch of the Niagara limestone; therefore, they are all 

 regarded as belonging to that formation alone. Messrs. 

 Worthen and Meek, of the » State Geological Survey of 

 Illinois, long since came to the same conclusion, and in the 

 recently published State reports, they re-affirm it. 



The Magara limestone, as thus understood, occupies a 

 very large area of the surface of the State, much larger in 

 fact than that of all the formations of Lower Silurian age 

 together. Its area has also a shape quite different from those 

 occupied by the preceding formations, being somewhat in 

 the form of a scalene triangle, with its longest side to the 

 southward, and its widest portion nearest the southern end. 



