Papers ox geography and geology 



343 



the same reason sandstones in Wisconsin may become 

 shales in Illinois. For similar reasons, formations under- 

 lying northeastern Illinois may differ from those under- 

 lying northwestern Illinois and vice versa. 



Formations often pinch out laterally, giving a different 

 stratigraphic succession of formations in northeastern 

 Illinois from that in northwestern Illinois. Under these 

 conditions it is necessary to have a large number of well 

 records to note the progressive changes taking place at 

 a distance from the out-crop. 



F. \V. Thwaites of the Wisconsin Survey gives the fol- 

 lowing succession of Cambrian, Ordovican, and Silurian 

 rock f onnations in southern Wisconsin : 

 Silurian — 



Feet 

 Niagaran limestone "Clinton" formation. .300-670 

 Ordovician — 



Maquoketa shale 100-200 



Galena-Platteville dolomite and limestone. 200-450 



St. Peter sandstone 325 



Prairie du Chien •. . . TOO 



Shakopee limestone 

 Xew Richmond sandstone 

 Oneota limestone 

 Cambrian — 



Madison limestone 50 



Jordan sandstone, grades to limestone 



towards the south 75 



St. Lawrence sandstone, grades to dolo- 

 mite below 35-150 



Mendota dolomite 0- 20 



Devils Lake sandstone, local depth unknown 



Franconia sandstone 90-175 



Mazomania sandstone, gray to red 100 = 



Dresbach sandstone -40-180 



Eau Claire sandstone and shale 70-410 



Mt Simon, coarse to medium as. 



Xot all of these formations are recognizable in northern 

 Illinois and probably all are not present. The Niagaran, 



a light gray dolomitic limestone, caps the hills in north- 



