GEOLOGICAL INTRODUCTION 



The Mississippian section in Illinois and the adjacent portions of the 

 Mississippi Valley Basin constitutes the typical section of that system of 

 strata in America. The entire series of formations is prevailingly cal- 

 careous from its base to above the middle, but the upper portion includes 

 conspicuous sandstone strata interbedded with important shales and lime- 

 stones. The name "Subcarboniferous limestone" was first used by Owen 

 for this series of formations and was the common designation for these 

 rocks until 1891, at which time Williams 1 proposed to subdivide the Car- 

 boniferous into the Mississippian and the Pennsylvanian, the former name 

 being applied to the so-called Subcarboniferous rocks of earlier authors, 

 and the latter to the series of formation which had previously been desig- 

 nated as Coal Measures. These two names were at once generally adopted 

 by American geologists. 



The subdivision of the entire Mississippian system, with the application 

 of geographic formation names to the several units, in accordance with 

 modern usage, was first accomplished by James Hall in his Iowa Report 2 

 in 1858, although formation names, mostly of lithological or paleontologi- 

 cal derivation, had been applied to the several formations by earlier 

 authors. In his study of the section Hall had as an assistant Mr. A. H. 

 Worthen, who later became the state geologist of Illinois, and who used 

 Hall's nomenclature, with some minor changes, in his reports on the 

 geology and paleontology of Illinois. These names, used by Hall and by 

 Worthen, have become standard in the typical Mississippian section, al- 

 though later, more detailed and more critical studies have necessitated 

 certain important changes in the interpretation of the section, and the 

 introduction of additional formation names. Future studies may necessi- 

 tate other changes in the classification and nomenclature of the beds. 



The classification of the Mississippian system here recognized is as 

 follows : 



CLASSIFICATION OF THE MISSISSIPPIAN SYSTEM 



V. Chester group. 



Clore formation. 

 Palestine formation. 

 Menard formation. 

 Okaw formation. 

 Ruma formation. 

 Paint Creek formation. 



1 Bull. U. S. Geol. Survey No. 80. (1891.) 



2 Report on the Geological Survey of the State of Iowa, vol. 1, Pts. 1 and 2. (1858.) 



