542 F. T. THWAITES 



where there is no Lower Magnesian.^ An example of this phe- 

 nomenon occurs between Green Bay and De Pere, Wisconsin. 

 Over an area of several thousand square miles in eastern Wisconsin 

 and in portions of northern Illinois, the St. Peter rests upon the 

 Cambrian; formations ranging from Jordan to Eau Claire have 

 been distinguished at different places as underlying the St. Peter. 

 The bottom of the St. Peter is best studied from well records, since 

 the softness of the rocks causes this horizon to be concealed quite 

 generally at the surface. At some exposures the beds of the 

 underlying dolomite dip with the irregularity of the bottom of the 

 sandstone; the writer is not prepared to state the significance 

 of this phenomenon. Where the St. Peter rests upon the Cambrian 

 sandstones, it is at places almost impossible to make out the exact 

 plane of contact; such is the case near Milwaukee. 



CANADIAN AND OZARKIAN SYSTEMS^ 

 Lower Magnesian group (Prairie du Chien formation) 



Distribution. — The Lower Magnesian group has been termed the 

 Prairie du Chien formation in some reports; however, Ulrich urges 

 that the old name be retained for the present. He divides the group 

 in ascending order into the Oneota dolomite, the New Richmond 

 sandstone, and the Shakopee dolomite; of these he places the first 

 in his Ozarkian system and the others in his Canadian system. 

 The group caps a broad, well defined cuesta whose U-shaped es- 

 carpment surrounds the central plain of Wisconsin. In Illinois only 

 a few small inliers of the Shakopee are associated with the areas of 

 exposed St. Peter sandstone. 



Character. — The exact discrimination of the several formations 

 of the Lower Magnesian group has thus far been made only in the 

 outcrops of the western part of Wisconsin. These formations can 

 be followed in well records for some distance into Iowa and Illinois. 

 The two dolomites are rather similar in character, both are gray 

 and carry gray, yellow, and pink chert, some of which is oolitic. 

 The presence of oolitic chert is a certain marker of the Lower 



^ According to Ulrich all this is good evidence tending to establish this unconformity 

 as marking the base of the Ordovician in the Mississippi Valley. 



2 These systems have been proposed by Ulrich and have not as yet been generally 

 recognized; others place most of the formations concerned in the Ordovician. 



