6i8 NORTH AMERICAN INDEX FOSSILS. 



In northern Arkansas, only the lower and late upper Ordovicic 

 beds are represented. The series is : 



SUPERFORMATION. 



Siluric : St. Clair limestone. 

 Upper Ordovicic. 



Cason shale. 



Polk Bayou limestone. 



Izard limestone. 

 {Hiatus and disconformity.) 

 Lower Ordovicic. 



Key sandstone. 



Yellville limestone. 



SUBFORMATION. 



Upper Cambric. 



The Yellville limestone of this section represents lower Beekman- 



• town, while the Key sandstone holds the position occupied by the 



Saint Peter farther north. The Isard limestone is correlated by 



Ulrich with late Frankford, Lorraine, and early Richmond, and 



the Polk Bayou and Cason with later Richmond. 



In the northern Ozarks® the Ordovicic begins with the Roubidoux 

 formation which is in part Upper Cambric. It is succeeded by the 

 Jefferson City limestone and with it represents the lowest Beekman- 

 townian. The Crystal City sandstone, the extension of the St. 

 Peter, separates it from the Joachim limestone, which represents 

 the Middle Ordovicic. This in turn is succeeded after a hiatus and 

 disconformity, by the Plattin limestone of Upper Ordovicic age. 

 In southern Illinois, the Galena-Trenton limestone is succeeded 

 after a disconformity by beds of upper Cincinnati (Richmond) age. 

 The series comprises the Thebes sandstone and shales and the 

 Orchard Creek beds which are succeeded by the Cape Girardeau 

 limestone, which contains a fauna of Siluric affinities.^*' This is 

 followed by Niagaran. 



In northern Illinois, the following section occurs : 



SUPERFORMATION. 



Lower Siluric — Niagaran. 



» Bull. 237, U. S. G. S. 



^^Alexandrian Formation, T. E. Savage, A. J. S., XXV., 1908, p. 431. 



