THE CHESTER SERIES IN ILLINOIS 415 



present belt of outcrop of the formations, the unconformity might 

 be looked for over part of the area and be entirely absent else- 

 where. 



With each readvance of the Chester sea in the IlHnois basin, 

 prolific invertebrate faunas occupied the waters, and their fossil 

 remains have been preserved in the Kmestones and calcareous 

 shales. These successive faunas were much alike in many respects, 

 but as they are critically studied, it is found that each one of them 

 possesses certain characteristics which serve to differentiate it 

 from the others. Farther to the south or southwest, beyond the 

 area of alternating land and sea conditions which obtained within 

 the IlHnois basin, the Chester fauna was doubtless undergoing a 

 continuous, normal, evolutionary development, and the successive 

 stages of this evolution, modified more or less by the local environ- 

 mental conditions, are mirrored in the successive faunas of the 

 several calcareous formations in the Chester section of the Illinois 

 basin. 



The succession of events that has been outhned has an impor- 

 tant bearing upon the interpretation of the Mississippian period of 

 North America. In his "Revision of the Paleozoic Systems," 

 Ulrich^ has split the Mississippian into two so-called systems, the 

 Waverlyan below and the Tennesseean above, the line of cleavage 

 between the two being placed between the Warsaw and Keokuk 

 formations. From the evidence afforded by the Mississippi 

 Valley section of the Mississippian, which is the type section of 

 these strata, there is less reason for placing a major dividing line 

 at this horizon than at almost any other position in the entire 

 succession of formations, and there is no basis whatsoever for the 

 recognition of the so-called Waverlyan and Tennesseean as systems. 

 There are, however, many excellent reasons for making a lower and 

 upper division of the Mississippian, the line of separation being 

 at the base of the Chester series. This position is approximately 

 at the horizon where Ulrich has subdivided his Tennesseean into 

 the Meramecian and Chesterian, but even here he has made a grave 

 error in including the Ste. Genevieve hmestone in the Chesterian. 

 This error was introduced by his failure to separate the Renault 



^ Bull. Geol. Soc. Amer., Vol. XXII (1910), Plate XXIX, opp. p. 609. 



