GENERAL STRATIGRAPHIC RELATIONS 357 



era Illinois. The rocks that overlie the Alexandrian strata in Jersey and 

 Calhoun counties, Illinois, and across the river in the northeast part of 

 Lincoln County, Missouri, are Devonian limestones of Hamilton age be- 

 longing to the Iowa or northwest (Dakota) province. A few miles still 

 farther north, in the vicinity of Pleasant Hill, Illinois, and Louisiana, 

 Missouri, they are immediately succeeded by a bed of dark shale of Upper 

 Devonian age. 



In the northeastern Illinois area the Alexandrian strata rest on the 

 Maquoketa shale and, where exposed beneath superjacent strata, they are 

 followed by dolomites of Niagaran age. 



Detailed Stratigraphy of the Alexandrian Series 



thickness and formations of the series 



The strata comprising the Alexandrian series, as defined above, have 

 a maximum aggregate thickness of about 175 feet. The series is divisible 

 into four formations, which, with the possible exception of the Essex 

 limestone, are unconformable among themselves, but their faunas are 

 clearly related. The sequence of the formations from below upward is 

 as follows : Girardeau limestone, Edgewood limestone, Essex limestone, 

 and Sexton Creek (= Brassfield) limestone. 



THE GIRARDEAU LIMESTONE 



Occurrence and stratigraphic relations. — The name Cape Girardean 

 limestone was given to the formation by Shumard in 1855, from Cape 

 Girardeau, Missouri, near which town the strata are well exposed. The 

 shortened form of the name has been adopted for this formation. 



The Girardeau limestone is present only in the south part of the area 

 under consideration and is not known farther north than a few miles 

 above Cape Girardeau, Missouri. It is well exposed in Illinois near the 

 mouth of Orchard Creek about two miles south of Thebes, and in the 

 bluff of the Mississippi River and in some cuts along the New Cairo and 

 Thebes Railroad for some distance farther south. It also outcrops in 

 the bank of Mississippi River and along the Chicago and Eastern Illi- 

 nois Railroad P/i* miles north of Thebes. In Missouri those strata orrui 

 over a small area in Cape Girardeau County, They are well exposed at 

 the type locality, aboul two miles north of ("ape Girardeau. 



In some places the Girardeau limestone rests unconformable on the 



Thebes sandstone (Richmond), and in other places it overlies different 

 horizons of Orchard Creek (Richmond) shale. The rocks of this for- 

 mation consist of dark, line-grained, hard, brittle limestone, in layers 



