THE CHESTER SERIES IN ILLINOIS 283 



River bluffs of Randolph County which was commonly called the 

 "Ferruginous Sandstone." Worthen used the name Chester hme- 

 stone for the same beds which Hall called Kaskaskia, but included 

 this Chester limestone with the underlying sandstone in what he 

 called the "Chester Group." 



While both Hall and Worthen based their descriptions of the 

 upper Mississippian rocks upon observations made for the most 

 part in the second of the areas which have been mentioned, Henry 

 Engelmann carried on field studies in the more southern counties of 

 the state, under the direction of the Illinois Geological Survey. In 

 Johnson, and in the counties to the east and west, Engelmann 

 recognized an alternating succession of limestone and sandstone 

 members of the Chester Group, ten in all, which he designated by 

 the numbers i to 10, beginning the numbering at the top. The 

 sandstones in the series received the even numbers and the lime- 

 stone and shale members the odd numbers. The only one of these 

 members to which a distinct name was given was No. 8, which was 

 called the Cypress sandstone' from the good exposures in the bluffs 

 of Cypress Creek, but even this name was abandoned in the later 

 reports by Engelmann and was never used by Worthen. 



The real importance of the Chester series in the Mississippian 

 as a whole is well shown by its comparative thickness. The whole 

 of the lower Mississippian or Iowa series has a thickness of approxi- 

 mately 1,000 feet, which was subdivided at an early time, as has 

 been stated, into a succession of well-defined formations, but the 

 Chester series, with a maximum thickness of more than 1,200 feet, 

 commonly has been treated as a single formation by all geologists 

 up to a very recent date. 



The first serious attempt to subdivide the Chester was made by 

 Ulrich^ in 1905. He recognized four formations as follows: 4. Birds- 

 ville formation; 3. Tribune limestone; 2. Cypress sandstone; i. Ste. 

 Genevieve limestone. 



The observations which led to this division of the Chester into 

 definite formations were inadequate for the proper understanding 

 of the whole series, and mistakes of so serious a character were 



' Trans. St. Louis Acad. Sci., Vol. II, Part i (1863), p. 189. 

 ^ Prof. Paper, U.S. Geol. Survey, No. 36. 



