78 GEOLOGICAL FORMATIONS. 



In the case of the Kaskaskia the physical breaks are unusually 

 prominent both above and below, over its entire extent in the 

 upper Mississippi valley. What has just been said of the upper 

 member of the series is equally true of the one immediately 

 underlying, though in a less marked degree and over only a 

 part of its superficial extent. Between the lower two groups 

 the physical continuity is scarcely broken, and the separation 

 is chiefly upon faunal and lithological grounds. 



Lithologically the upper two members of the Mississippian 

 are more alike than any of the others ; yet, as a rule, they are 

 readily distinguishable everywhere. The Augusta limestone is 

 over all its range encrinital, and stands out in marked contrast 

 to the other three sections; while the lower subdivision is very 

 different again, both in its calcareous and argillaceous portions. 



In regard to the minor subdivisions of the four groups 

 above mentioned, much might be said. The several sectional 

 names proposed at various times have had wide values, and, 

 moreover, have been applied rather loosely. 



In the Kaskaskia the upper shales and the lower lime- 

 stones of Chester, Illinois, have been differentiated, while the 

 Aux Vases sandstone has been placed at the base of the group 

 provisionally. It has not had, as yet, sufficient study over its 

 entire exposure to satisfactorily consider its relationships in all 

 phases. Certain it is, however, that when the continental area 

 north of the present city of Saint Louis was being subjected 

 to denudation prior to the deposition of the Lower Coal Meas- 

 ures, the great sandstone was laid down south of that point in 

 the shallow littoral waters of the interior sea. 



The Saint Louis group has been divided into three lime- 

 stones. Of these the Ste. Genevieve has never come into gen- 

 eral usage, and practically has been forgotten. The Saint 

 Louis limestone proper has been widely recognized, and in 

 many places the lower portions have been correlated with the 

 Warsaw beds as developed at the mouth of the Des Moines 

 river. 



The Augusta group is now made to include all five of the 

 hitherto recognized beds — the Warsaw proper, the "geode'^ 



