PAPERS ON GEOLOGY AND GEOGRAPHY 225 



lowed Maquoketa time. (Contact between the upper Ma- 

 quoketa shale and the overlying limestone has been taken 

 as the contact of the Ordovician and the Silurian. This 

 contact seems very regular and if it represents 

 the division between the Silurian and the Or- 

 dovician, the uncomformity must have been slight.) The 

 total thickness of the Silurian and Devonian limestone 

 phase varies about 250 feet, being 800 feet thick to the 

 north in Douglas County, and 1,050 feet south in Crawford 

 County. It also thickens in Clark County to about 950 

 feet off the arch of the fold. The tests north in Coles and 

 Douglas Counties prove unconformable relations with the 

 overlying chocolate shale, showing Onondaga limestone im- 

 mediately below the shale at one location and Hamilton as 

 a top formation elsewhere. As erosion is proven to the 

 north, undoubtedly part of the increase of 100 feet found 

 locally and 250 feet found regionally must have been caused 

 by erosion. Were this whole difference due to a pre-exist- 

 ing fold, it would be but slight, and since it is in part due 

 to erosion, any folding which there may have been must 

 have been insignificant. The fact that there was only a 

 250-foot decrease in the entire area from Crawford County 

 to Douglas County with present variations of 3,000 feet in 

 elevation proves that the land surface must have been 

 practically level at the close of that part of Devonian time, 

 or that any folding that took place caused practically negli- 

 gible effects both north and south, and east and west. 



LATER DEVONIAN TIME 



The so-called Devonian or chocolate shale varies from 

 100 to 175 feet in thickness. The maximum difference in 

 thickness occurs between the north and south parts of the 

 area. Locally, in places off the structure, the shale in- 

 creased 25 to 30 feet in thickness. At present the top of 

 the shale occurs at elevations varying up to 3,000 feet, with 

 a maximum variation in thickness of 75 feet. At the close 

 of this time this whole area must have been practically 

 flat. 



LOWER MISSISSIPPIAN TIME 



The Lower Mississippian is about 1,300 feet thick in 

 Lawrence County, where it is capped by the Chester for- 



