344 ILLINOIS STATE ACADEMY OF SCIENCE 



western Illinois, and in Cook County it is the first 

 stratum to be penetrated by the drill. 



The Maquoketa shale lies immediately below the Nia- 

 garan, and in northwestern Illinois some difficulty is en- 

 countered in differentiating it from the clays of the 

 glacial drift as the unindurated material above it caves 

 and glacial pebbles fall in and re-mix with the samples. 



The Galena and Platteville limestones are 170 to 450 

 feet thick. The Galena is dolomite; the Platteville is a 

 calcareous shale or limestone. These formations outcrop 

 in broad irregular belts extending north and south in 

 north central Illinois, in east and western parts of the 

 State. The Platteville is found only in drill records. It 

 lies uncomfortably on the St. Peter. This is of the same 

 general horizon if not identical with the Platten of south- 

 ern Illinois. The Galena dolomite is about equivalent to 

 the Kimmswick of Missouri. 



The St. Peter sandstone outcrops in a small area near 

 Ottawa, LaSalle, Sheridan, and Troy Grove. This is a 

 key horizon, and the sand grains are more rounded than 

 those of any other sandstone formation. They are per- 

 fectly sorted and are veiy uniform in size. The St. Peter 

 maintains the same characteristics over wide areas. 

 There is an unconformity at the base where it overlies 

 the Shakopee limestone. It ranges in thickness from 65 

 to 325 feet. 



The Lower Magnesian group is composed of three di- 

 visions: The Shakopee dolomite, New Richmond sand- 

 stone and the Oneota dolomite. Only the Shakopee out- 

 crops in this State. The outcrops are at Split Rock near 

 Utica and a few scattered inliers along the LaSalle anti- 

 cline. Nowhere is its entire thickness exposed. The 

 New Richmond sandstone is a medium to coarse sand- 

 stone about 20 to 70 feet thick. The Oneota dolomite is 

 the lower member of the lower Magnesian group. It is 

 known only in well cuttings, a light gray somewhat 

 cherty dolomite. The three members can be recognized 

 in the northwestern part of the State as far east as Dixon, 

 Amboy and LaSalle. Farther east the New Richmond 

 and probably the Shakopee are absent. If they were de- 

 posited there it is probable that they were eroded away 



