THE CHESTER SERIES IN ILLINOIS 299 



Okaw limestone of Monroe and Randolph counties. Such a 

 correlation, however, is not supported by the evidence of the 

 fossils and the faunal studies of the Chester have established 

 beyond question the exact equivalence of the Golconda limestone 

 of the southern counties with the Lower Okaw in Randolph 

 County. 



Golconda limestone. — When the Chester section in the Missis- 

 sippi River counties was first elaborated by the writer, the name 

 Okaw limestone was given to a thick series of limestones with shale 

 partings overlying the so-called " Ruma " formation. It was recog- 

 nized that this was probably a composite formation, and an attempt 

 was made to map the higher beds as a separate unit from the lower 

 ones, but this was finally abandoned because the heavy covering of 

 drift seemed to make such a procedure impracticable. When the 

 studies were carried into the more southern counties, it developed 

 that the limestone beds equivalent to the Okaw were divided into 

 two distinct units separated by an important sandstone formation. 

 The lower of these two units has been named Golconda limestone 

 from the excellent exposures in the Ohio River bluffs just above 

 Golconda, in Pope County. 



The Golconda limestone is constituted of a succession of lime- 

 stone and shale beds, the details of which are commonly obscured 

 by surficial material, and it is not known whether the details of the 

 succession of beds are uniform throughout the areal extent of the 

 formation. The limestone beds vary considerably in character, 

 but in general they are of a light- or dark-gray color, and more or 

 less crystalline in texture, with some layers oolitic. The shales are 

 fully as variable and perhaps more variable than the limestones. 

 Some of them are highly calcareous, while others are quite purely 

 argillaceous; many of the beds are gray or buff, but others are 

 dark and even black, and at a number of localities a layer of 

 reddish shale has been observed. In the basal part of the forma- 

 tion there are shale beds with a considerable content of sand, and 

 even some thin sandstone layers, but beds of this character are not 

 present higher up in the formation. 



In tracing the Golconda limestone into the Mississippi River 

 counties, where it is represented by the lower and main portion of 



