22 ILLINOIS STATE ACADEMY OF SCIENCE 



Mississippian formations, for in large areas these strata 

 are now covered by younger sediments, and elsewhere they 

 may have been removed by the processes of erosion since 

 the time of their deposition. With the more or less rhyth- 

 mic fluctuations of the ocean level during the Mississippian 

 period, the shore line of the basin was constantly under- 

 going change. When the relative level of the ocean was 

 raised, the shore-line advanced inland, just as the shore 

 line of the present Gulf of Mexico would advance if the 

 present ocean level were raised in relation to the land sur- 

 face. At other times the waters of the basin were with- 

 drawn so that it became much contracted in size, and not 

 a few times the waters retreated entirely and the basin be- 

 came a part of the continental dry land. The records of all 

 these changes have been preserved in the rock strata, and 

 from a study of the characters of the rocks themselves, 

 their faunal contents, and the geographic distribution of the 

 several formations, the geologist is able to arrive at some 

 conclusions concerning the geological history. 



A cursoiy examination of the Mississippian formations 

 of southern Illinois brings clearly into view two divisions of 

 the system as a whole. The lower portion of the section is 

 made up of a succession of limestone formations, containing 

 some shale or clay layers, but with almost no sandstone 

 strata. The upper portion of the section is made up of a 

 succession of alternating sandstone and limestone-shale 

 formations, giving to the upper and lower divisions of the 

 System distinctly different characteristics. On the basis of 

 the lithologic characters alone the Mississippian System of 

 this basin may be rather sharply differentiated into a Lower 

 Mississippian and an Upper Mississippian Series, and when 

 the evidence of the fossil faunas and the geographic distri- 

 bution of the several formations are considered, such ? dif- 

 ferentiation is clearly established. In some recent literature 

 these two divisions of the Mississippian have been called the 

 Iowa Series and the Chester Series. 



As has been indicated already, the Iowa or Lower Miss- 

 issippian Series is made up almost entirely of a succession 

 of limestone formations, there being but few thin sandstone 

 beds. This succession of formations was largely differen- 



