PAPERS ON GEOGRAPHY AND GEOLOGY 345 



in the post Prairie du Chien erosion period. The base of 

 the Oneota is glauconitic and in some wells there are 

 about 15 to 25 feet of glauconitic sandstone at the base. 



Underlying the Oneota sediments are the rocks of the 

 Cambrian period. These in Wisconsin are about 1000 

 feet thick, and in Illinois they have been penetrated to 

 nearly that depth. It is from these strata that most of 

 the artesian water in northern Illinois is obtained. 



The Madison limestone is absent in the Illinois section. 



The Jordan formation which outcrops as a sandstone 

 in "Wisconsin become more calcareous southward, becom- 

 ing a dolomite in Illinois where it can not be distin- 

 guished from the underlying St. Lawrence limestone. 



The St. Lawrence is a dolomite, either pinkish, purp- 

 lish or dark gray in color. The combined thickness of 

 the St. Lawrence and Jordan dolomites is about 150 to 

 175 feet; one or both of these formations are known from 

 Dixon westward, but neither one has been recognized in 

 the eastern part of the State. 



The Mendota dolomite may be present near Chicago, 

 but can not be distinguished from the Oneota. Its pres- 

 ence is indicated by the thickness of the Oneota dolomite, 

 which is greater than that of the Oneota formation north- 

 ward, and part of this increased thickness may be the 

 St. Lawrence dolomite. 



In Illinois the Devils Lake and Franconia formations 

 are either absent or indistinguishable from the Mazo- 

 manie formation. The Mazomanie formation is a sand- 

 stone, containing some limestone and shale lenses. The 

 sandstone is angular or subrounded, and poorly sorted. 

 It has often been confused with the Jordan sandstone in 

 this State. It is the first thick sandstone below the 

 Oneota. This immediately underlies the Oneota in Kane 

 and Dupage counties and probably in Cook and Lake 

 counties. 



The Dresbach sandstone is in most places a medium 

 white sand. However, in Cook and Lake counties it may 

 contain dolomite and shale. There is the possibility that 

 the Mazomanie formation does not extend as far east as 

 Cook County, and that the first Cambrian sandstone pene- 

 trated in Cook and Lake counties is the Dresbach, and 



