\ 



The Saint Peter Sandstone. 81 



scribes this transition as abrupt. Chamberlin (21) says 

 that breccia?ted pieces of dolomite occur in places in the 

 lowest stratum of Saint Peter. Irving (22) writes that 

 "between the periods of deposition of the lower Magnesian 

 and the Saint Peter there was a long gap whose record 

 is in the eroded surfaces of the first named formation." 

 Moses Strong (23) held a similar view. Chamberlin, who 

 wrote later (26) corecting the theory that the great inequal- 

 ity at the top of the "Magnesian" was due to erosion, and 

 showing that the undulating surface was really due to fold- 

 ing of the upper part of the "Magnesian" i. e. Shakopee, 

 still does not contradict Hall's statement that the transi- 

 tion is abrupt. I have not observed a real unconform ability 

 between the tw^o formations but such may exist, and the 

 contact has been found an abrupt one as often as observed 

 at all. In Iowa no exposed contact has yet been found by 

 me. The Shakopee consists not only of dolomite but of 

 more or less sandy strata and sometimes clay, and it may 

 be that these sandy strata have been considered as a con- 

 tinuation dow^nwards of the Saint Peter, a view that would 

 require first other evidence to prove. It is contradicted 

 really by the paleontological and stratigraphic evidence 

 that we now have. 



The Saint Peter folds conformably with the Shakopee, 

 and the contact can be sometimes at least exactly marked 

 because it is an abrupt transition. It is also conformable to 

 the Trenton with a transition of alternating strata or of 

 mixed materials. The Shakopee and Trenton are not how- 

 ever parallel to each other, for as well known the former is 

 much folded while the latter is almost exactly horizontal, 

 the Saint Peter betw^een them being consequently of unequal 

 depth. It was naturally supposed by geologists that the 

 folding of the Shakopee was completed before the deposition 

 of the Saint Peter began and that may still be supposed cor- 

 rect in part but not entirely. 



Figure 10, plate 4 is a sketch of an exposed contact of 

 the last member of the Magnesian series (i. e. Shakopee 

 proper) and the base of the Saint Peter formation, as seen 

 along the Illinois Central Ry., at the crossing of the Chicago 

 Milwauli:ee and Saint Paul Ry., in LaFayette county, Wis- 



