THE CHESTER SERIES IN ILLINOIS 303 



In the Mississippi River counties the whole assemblage of fossil 

 forms which characterize the Glen Dean formation in the southern 

 counties has been found to be present in those beds of the Okaw 

 limestone which overlie the interrupted sandstone horizon in the 

 midst of that formation, and these upper Okaw beds may be corre- 

 lated directly with the Glen Dean and this name may be extended 

 to include these beds in the Randolph-Monroe County section. 



The thickness of the Glen Dean in the southern counties 

 exhibits some variation from a minimum of 40 feet to a maximum 

 of perhaps 75 feet. In the thinner sections it is apparently the 

 higher beds that are missing, due perhaps, to the erosion of the 

 upper surface of the formation before the deposition of the over- 

 lying sandstone. The thickness of the equivalent beds in Randolph 

 County is similar to that in the southern counties, the usual thick- 

 ness commonly being about 60 feet. 



[To he continued] 



