102 ILLINOIS ACADEMY OF SCIENCE. 



Ohio Clinton fauna in the more southern locality, and the two 

 faunas occur so distinctly separated further north in Illinois. This 

 difficulty was called to the attention of Dr. Foerste,^ who replied 

 that he was now inclined to consider the strata at Clifton, Tennes- 

 see, that has been referred to the Clinton, as consisting of two 

 distinct horizons ; the upper part being coarser grained, and carry- 

 ing the Ohio Clinton fauna, while the lower part is more cherty 

 and contains the fossils Rhynchotrcta simplex, Camarotoechia ? 

 cliftonensis, Pentamerella ? manniensis and their associates, but 

 not bearing the typical Ohio Clinton fauna. If this is the true 

 interpretation of the so-called Clinton strata in western Tennessee 

 it would indicate that the Essex limestone fauna was to be corre- 

 lated with that of the strata immediately below the layers 

 representing the normal Ohio Clinton at Clifton, Tennes- 

 see. The presence of the Edge wood species, Atrypa putilla, 

 Dalmanella elegantida var., and Rhynchotreta thebesensis in the 

 Essex limestone is further evidence that this fauna is older than 

 that of the Ohio Clinton, which, in Alexander County, Illinois, 

 overlies the Edgewood formation. 



The above facts indicate that neither the Niagara limestone, 

 nor even the Chnton, represents the earliest Silurian strata that 

 were deposited in northern Illinois ; that the Channahon and Essex 

 limestones represent pre-Clinton beds of Silurian age that were 

 probably spread over considerable areas in the Mississippi valley, 

 but were later mostly removed by erosion prior to the deposition 

 of the overlying Niagara strata. 



The term Alexandrian* has been proposed as a geological series 

 to receive all of the early Silurian strata in the IMississippi 

 valley that are older than the Ohio Clinton beds and younger 

 than the Richmond. The Girardeau limestone and the Edgewood 

 formation of southwestern Illinois were assigned to this series. 

 If the correlations of the Channahon and Essex limestones, sug- 

 gested above, are correct, the Essex limestone would belong above 

 the Edgewood, near the top of the Alexandrian series. The 

 Channahon limestone should probably be referred to a horizon 

 corresponding in general with that of the Edgewood formation, 

 while the Girardeau limestone, in southwestern Illinois and south- 

 eastern Missouri, continues to hold a place in the lower part of 

 the series. 



'Savage, T. £., Am. Jour, of Sci., Vol. 25, p. 434, 1908. 



