314 T. E. SAVAGE ALEXANDRIAN ROCKS OP ILLINOIS AND WISCONSIN 



common in the limestone of Sexton Creek age in northern Illinois have 

 been found in the Cataract formation. The fauna of the limestone of 

 Sexton Creek age in Illinois is more closely related to that of the Gun 

 Eiver formation in the Anticosti region than to that of the Cataract and 

 seems to indicate during this time a more direct sea connection of the 

 upper Mississippi Valley basin with the Anticosti embayment than with 

 that portion of the epicontinental sea in which the rocks of the Cataract 

 formation were laid down. 



History of the Alexandrian Epoch in the Mississippi Valley 



The earliest deposits of the Alexandrian series in the Mississippi Valley 

 comprise the Girardeau limestone which accumulated in an arm of the 

 sea that advanced from the Gulf of Mexico region, reaching a few miles 

 north of Cape Girardeau, Missouri. Deposition of this limestone was 

 closed by the more or less complete withdrawal of the sea, and the sedi- 

 ments of the succeeding Edgewood formation were laid down during the 

 next advance of this southern sea, which extended north as far as Will 

 and Kendall counties, Illinois. That portion of the Mayville beds in 

 Wisconsin that corresponds in age to the Edgewood limestone in Illinois 

 is thought to have been deposited in a basin that had a northern sea con- 

 nection and was separated from the Illinois basin by a land area or other 

 barrier across southern Wisconsin or northern Illinois. This assumption 

 seems justified on account of the difference in the fossils associated with 

 the characteristic Edgewood species in Wisconsin compared with those 

 in Illinois. The presence of the Virgiana zone near the top of the May- 

 ville limestone appears also to indicate a connection of this province with 

 the Anticosti region during the time the Upper Mayville beds were de- 

 posited. If this assumption is correct, these strata of Edgewood age in 

 Wisconsin belong to a different province from that in which the Edgewood 

 strata in Illinois and Missouri were laid down, and should very appro- 

 priately continue to bear the name "Mayville limestone/' 



In northeastern Illinois the zone of Platymerella manniensis imme- 

 diately follows the Edgewood limestone without clear evidence of a sedi- 

 mentary break, although in places in Calhoun County, in southwest Illi- 

 nois, and in Pike County, Missouri, a distinct unconformity separates 

 these horizons. Platymerella manniensis occurs also in rocks • belonging 

 to a corresponding horizon in western Tennessee, and this species is 

 thought to have entered the Mississippi Valley from the Gulf of Mexico 

 region, as did the earlier Edgewood fauna. The geological provinces of 

 Edgewood time in this part of the Mississippi Valley appear to have re- 



