UNCONFORMITY OF SAINT PETER AND M AON ESI AX. 



353 



and Endoceras, sp. undet., are among the specimens obtained. They 

 show the distinctively Lower Silurian character of the Saint Peter sand- 

 stone. 



Physical Relations. — There are some points in the structural characters 

 of this formation which lead the authors to regard it as a transition lied 

 between the Cambrian and Silurian periods. 



1. In the first place, the Wisconsin geologists have proved that for 

 many localities the Lower Magnesian is an eroded formation. This ero- 

 sion represents within their area a period of cessation in the deposition 

 of rock material. Several of them have described the conditions observed 

 and have in several figures represented the unconformity of the Saint 

 Peter upon the Lower Magnesian — i. e., Cambrian. T. C. Chamberlin, 

 in Ins report on the geology of eastern Wisconsin, mentions places where 



Figure 2.— Unconformity of the Saint Peter on the Magnesian and the Conformity of the Trenton on 

 the Saint Peter * 



1 = Magnesian ; 2 = Saint Peter; 3 = Trenton. 



the former lies upon the eroded edges of the latter, and in instances cited 

 its upper surface is many feet below the crests of the bower Magnesian 

 ridges.f He also cites localities where the Saint Peter is wholly wanting 

 and the Trenton, winch has been preserved throughout the erosion 

 which this region has subsequently undergone, lies directly noon the 

 bower Magnesian.J No such evidence as this has been found in Minne- 

 sota thus to establish the boundary between the Cambrian and Lower 

 Silurian at the base of the Saint Peter. < )n the contrary, this formation 

 is everywhere found in thickness varying from 75 S to 16 1 feel beneath the 

 Trenton. 



* Diagrammatic section from Chamberlin. Geol. Wis., \ ol. i. 1882, p i 15. 



; i leology of Wisconsin, vol. ii, 1*77, |> 27 I. 



| [bid., p 285. 



gN. II. Winchell : Geology of Minnesota, Final Report, vol, i. 1884, p 219. 



