282 STUART WELLER 



Valmeyer in Monroe County, are any formations other than the 

 Mississippian exposed. The greatest width of this area is in 

 Monroe County, where the Mississippian formations form the sur- 

 face rocks for a distance of about fifteen miles back from the river 

 bluffs, where they pass beneath the Pennsylvanian strata. Within 

 this area both the lower and upper series of Mississippian forma- 

 tions are present, and it includes the typical area of the Chester 

 series, as these rocks were described by Hall and Worthen more 

 than half a century ago. 



The third area of Mississippian rocks in Ilhnois is in the extreme 

 southern portion of the state, where these formations constitute 

 the surface rocks throughout a belt ranging from fifteen to thirty or 

 more miles in width, across Union, Johnson, Pope, and Hardin 

 counties. The greater portion of this area is occupied by the upper 

 Mississippian formations of Chester age, although the lower forma- 

 tions do occupy considerable areas in Union and Hardin counties. 

 The northwestern corner of this southern belt is separated from the 

 central area by the valley of the Big Muddy River in Jackson 

 County. 



The main portion of this paper will be devoted to a discussion 

 of the Upper' Mississippian or Chester series, although the Lower 

 Mississippian, or Iowa series as it may be called for want of any 

 comprehensive name already in use, will be given some considera- 

 tion in the discussion of the geological history. 



The Iowa series was subdivided into a number of well-recognized 

 formations more than half a century ago, mainly through the work 

 of James HalP in Iowa, although some attempt at subdivision had 

 been made before Hall's time, and the subdivisions and classifica- 

 tion has been somewhat elaborated in later years. In the main, 

 however, the divisions estabhshed by Hall constitute the formations 

 that are recognized at this time. Not so with the upper Missis- 

 sippian. The early workers generally recognized in this series a 

 more or less confused succession of limestones, shales, and sand- 

 stones, and but Uttle attempt was made to subdivide the series. 

 Hall gave the name Kaskaskia limestone to the whole of the suc- 

 cession above a conspicuous sandstone formation in the Mississippi 



^ Report on the Geological Survey of Iowa (1858). 



