DEFORMATION OP THE SAINT PETER. 355 



of the Saint Peter sandstone and the Trenton limestone. Nowhere is 

 there any indication, however slight, of an unconformity. The transi- 

 tion zone of a green shaly calcareous sandstone shows the steady oncom- 

 ing of that Lower Silurian sea which, if it did not submerge the whole 

 Northwest, at least extended so far that the dry land was reduced to 

 islands or narrow peninsular stretches of very uncertain connection with 

 a mainland lying somewhere. For a considerable distance below this 

 contact zone the sandstone shows no such faulting or jostling of the 

 strata as can be seen in the spot already mentioned, estimated to lie from 

 75 to 90 feet from its base. The same may be said of the exposures of 

 the Saint Peter in the southern area, notably at Cannon falls, Faribault 

 and Fountain, where the beds are exposed for a considerable distance 

 from the top downward. 



_L 



' , ' , I I , I I I ,1 , 1 ' , I , l , ,1 , ! 



I II I II II 1 I II 



i 



i. 



Figure 4.— Diagrammatic Sketch showing the Relations of the Magnesian, Saint Peter and Trenton. 



1 = Magnesian with gentle fo.lding; 2 = Saint Peter folded with the Magnesian in its bottom layers 

 and displaced by faults, which extend upward but disappear before the topis reached; 3 = Trenton 

 i Bun i limestone conformable with the Saint Peter. 



From the three considerations pointed out we conclude that this sand- 

 stone which geologically occupies so important a place in Michigan (?) * 

 Wisconsin, Minnesota, Iowa, Missouri and Illinois, represents, for Min- 

 nesota at least, a great transition epoch between Cambrian and Lower 

 Silurian time. It stands for the interval between the close of the depo- 

 sition of those rocks which are now dolomites, whatever they once might 

 have been — an interval which in eastern Wisconsin was one of dry land 

 and erosion, — and that succeeding period of long-time permanent Silu- 

 rian seas with their varied fauna and well defined flora. 



These physical conditions and the fauna! characters recently discov- 

 ered seem to us to place beyond all question the Saint Peter sandstone 

 of the northwestern states in the column of bower Silurian epochs, and 



Geologj "i Wisconsin, vol. i, 1877, p. L49. 

 XLVIU I'.i i.i . Geol. Soc. A.m., Vol. 3, 1891. 



