82 The Saint Peter Sandstone. 



consin, a few miles north of the Illinois-Wisconsin state line. 

 It represests one of the extreme cases of folding* seen in the 

 Peccatonica valley and is chiefly interesting in that the two 

 feet of shale which is always fotmd at the top of the Shako- 

 pee in that region is preserved beneath the sandstone undim- 

 inished except by compression. Had the folding taken place 

 before a considerable depth, 20 feet or more, of sand was de- 

 posited, the shale would have fallen from the eminence by 

 its own weight or at least been washed down by the water. 

 The sandstone in this exposure is somewhat confused by 

 compression yet it can be seen that here as at other expos- 

 ures in the Peccatonica valley the stratification of the Saint 

 Peter is conformable to the Shakopee. The upper surface of 

 the Shakopee i. e. at the contact, is ripple marked. 



There are small faults as deacribed in a former paper 

 (see 30., p. 354. fig. 4.) which extend upwards from the base 

 of the Saint Peter formation through the sandstone, about 

 halfway up to the Trenton. 



These phenomena were considered to be evidence that 

 the Saint Peter was deposited partly before and partly after 

 the folding of the Shakopee. Of this I shall speak again in 

 discussing the origin of the sandstone. 



STRATIGRAPHIC POSITION. 



The position of the Saint Peter is of course clearly, 

 between the Lower Buff limestone of the Galena (Trenton) 

 series above, and the Shakopee dolomite of the Magnesian 

 series below. It is conformable to both. Paleontologically 

 it is closest united to the Galena (Trenton) series above it. 

 Lithologically it belongs to the "Magnesian" series with 

 the Shakopee, for it stands as the last of that series of sand- 

 stones and dolomites, and does not unite with the Galena 

 (Trenton) series which consists of limestones and clays 

 without sandstone. A time break seems to separate the 

 Saint Peter from the Shakopee, while a physical revolution 

 separates it from the Galena series. It thus appears to be a 

 transition formation between the Magnesian series or Upper 

 Cambrian and the Lower Silurian, and it has been so con- 

 sidered (30). 



But that the Saint Peter represents the lowest member 



