312 T. E. SAVAGE ALEXANDRIAN ROCKS OF ILLINOIS AND WISCONSIN 



jSTo older Silurian strata corresponding to the Girardeau limestone of 

 southwest Illinois and eastern Missouri are known in Wisconsin. 



Correlation of the Alexandrian Rocks of the Mississippi Valley 



WITH THE EARLY SlLURIAN STRATA OF THE ANTICOSTI EmBAYMEXT 



There are two fossil zones in the Alexandrian series of the Mississippi 

 A" alley whose equivalents can be recognized in the section of strata ex- 

 posed on Anticosti Island, recently described by Twenhofel."' One of 

 these is the Stricklandinia horizon, which, in Illinois, occurs 18 to 25 

 feet above the base of the Sexton Creek formation, and contains shells 

 of a number of species of Stricklandinia, among which 8, piriformis, 8. 

 pyriformis var. elongata, 8. breviuscula, and 8. circularis respectively 

 resemble Billings species, 8. davidsoni, 8. salteri, 8. brevis, and #. lirata. 

 which occur in the upper part of the Gun Eiver 6 formation of Anticosti 

 Island as defined by Schuchert and Twenhofel. 



Associated with the species of Stricklandinia in the limestone of Sexton 

 Creek age in northern Illinois are Diphyphyllum ccespitosum. Schucher- 

 tella pecten var. Plectambpnites transver salts, Orthis flabellites, Plaly- 

 strophia daytonensis, Pentameriis oblongus, and Triplecia aff. insularis 

 var. antic osiiensis. These species are also found in association with the 

 shells of Stricklandinia in the upper part of the Gun Eiver formation of 

 the Anticosti region. Prom the resemblance of the species of Strick- 

 landinia in variety and in the number of individuals, and from the sim- 

 ilarity of their associates, in the middle part of the limestone of Sexton 

 Creek age of northern Illinois, to those in the upper part of the Gun 

 Eiver formation of Anticosti Island, it seems certain that the Strick- 

 landinia zone in these widely separated regions represents about the same 

 period of time, and that there was a sea connection between the Missis- 

 sippi Valley basin and the Anticosti embayment during the time the 

 rocks of this zone in the two areas were laid down. 



Another horizon in the Alexandrian rocks of Wisconsin that can be 

 confidently correlated with a definite portion of the Anticosti section is 

 the Virgiana zone. In Wisconsin shells of Virgiana barrandei var. are 

 found only in the upper part of the Mayville beds. In the Anticosti 

 Island section Virgiana barrandei is a guide to the upper part of the 

 Becsie Eiver 7 formation, where the shells of this species are numerous 

 and exhibit a multiplicity of variation, as they do in Wisconsin. 



5 W. H. Twenhofel : The Anticosti Island faunas. Canada Geol. Survey, Mns. Bull, 

 no. 3. Geol. series no. 19, Oct., 1914. 



6 Charles Schuchert and W. H. Twenhofel : Ordovicie-Siluric section of the Mingan 

 and Anticosti islands. Gulf of St. Lawrence. Bull. Geol. Soc. Am., vol. 21, no. 4, 1910, 

 pp. 708-713. 



7 Schuchert and Twenhofel : Bull, Geol. Soc. Am., vol. 21, 1910, pp. 705-708. 



