512 T. E. Savage — Ordovioian and Silurian Formations. 



The outcrop of this formation is limited to a few small 

 patches in the bank and bed of the river, in the vicinity of 

 Thebes. The Kimmswick beds differ in their lithology and 

 fauna from those of any horizon in the Mohawkian series in 

 the northern portion of the state, with some part of which 

 they were doubtless contemporaneous. A barrier of some 

 kind probably separated the two areas during the time of 

 deposition of the respective beds. 



Correlation. — Out of thirty-five species of fossils listed from 

 the Mohawkian strata of Minnesota* which are also found, in 

 the Kimmswick limestone of Illinois, nine appear below the 

 Trenton, six of which persist into the lower Trenton beds ; 

 twenty occur in the lower division of the Trenton (Clitam- 

 bonites bed), nine of which continue upward into the over- 

 lying division ; twenty-four species occur in the middle division 

 (Fusispira bed) ; while only a single one of these is found in 

 the upper division (Maclurea bed). From these facts the 

 Kimmswick limestone is thought to correspond, in time, with 

 some part of the middle division of the Trenton (Fusispira bed) 

 of the upper Mississippi valley. 



The Post- Kimmswick Unconformity. 



An erosion interval succeeding the deposition of the 

 Kimmswick limestone is shown in the fact that the thickness of 

 this limestone varies from place to place, and the upper portion 

 of the formation is not a constant horizon. Land conditions 

 are also indicated in the presence, at the top of the formation 

 at Cape Girardeau and other points, of solution channels filled 

 with red colored, residual clay. The time involved in this 

 erosion period was long. Some of the upper part of the 

 Mohawkian and all of the Utica and Lorraine deposits are 

 wanting. 



Cincinna-tian Series. 



The rocks of the Cincinnatian series in Alexander county 

 are all embraced in the Richmond stage. They comprise three 

 distinct formations : 1, the Fernvale limestone at the base ; 2, 

 the Thebes sandstone, and 3, the Orchard Creek shale. 



The Fernvale Limestone. 



Overlying the Kimmswick strata is a thin bed of hard, gray 

 limestone, bearing Rhynchotrema capax, Dinoi-this subquad- 

 rata and other fossils characteristic of the lower portion of 

 the Richmond stage. Strata containing similar fossils have 



* Geol. and Nat. Hist. Surv. of Minn., vol. iii, pts. 1 and 2. 



