PAPERS ON GEOLOGY AND GEOGRAPHY 223 



zone under discussion, for example in Kankakee County, 

 it is reached at about 500 feet above sea level where it 

 consists of the same three members. 



SILURIAN AND DEVONIAN LIMESTONE 



The State Geological Survey has been gathering sam- 

 ples, a study of which may give some good basis for differ- 

 entiating the Silurian from the Devonian beds. It is 

 usual for the Silurian to be dolomitic and the Devonian 

 to be calcareous. For the purpose of this paper the two 

 systems are considered together. The total thickness over 

 the area varies from 800 feet to 1,050 feet. Changes in 

 elevation of the top are from 300 feet above sea level to 

 2,600 feet below sea level. 



SO-CALLED UPPER DEVONIAN SHALE 



This shale varies in thickness from 100 to 175 feet over 

 the whole area. Tests have shown a variation in thick- 

 ness of 25 feet in half a mile. The overlying beds being 

 apparently regular, the loss in thickness of the shale sug- 

 gests the possibility of an underlying irregular limestone 

 surface due to erosion. The maximum change in thickness 

 occurs between the extreme north and south parts of area. 

 East and west off the crest of the fold the shale sometimes 

 thickens 25 feet. The top of the shale is found at elevations 

 varying from 400 feet above sea level to 2,500 feet below. 



LOV^ER MISSISSIPPIAN 



On account of a large unconformity at the top and the 

 rise of the anticline to the north, the Lower Mississippian 

 varies from very small thickness up to a maximum of 1,300 

 feet. The base of this formation occurs from elevations of 

 400 feet above sea level to 2,500 feet below sea level. 



CHESTER 



The base of the Chester formation is found from 100 

 feet to 1,600 feet below sea level. The Chester varies from 

 practically no thickness to a thickness of over 700 feet. 



PENNSYLVANIAN 



The base of the Pennsylvanian is found from 500 feet 

 above sea level to 800 feet below sea level. The Pennsyl- 

 vanian varies from practically no thickness to a thickness 

 of 1,400 feet. 



