80 The Saint Peter Sandstone. 



upon, since it is drawn from too few specimens and these 

 not all of the very best kind. But the evidence as we have 

 it agrees essentially with that found by stratigraphical and 

 lithological comparison. 



PHYSICAL RELATIONS. 



There is in general, an upper and lower division of the 

 Saint Peter sandstone, as has often been suggested by 

 writers who have studied the lithologic characters of the 

 formation, and I can see also a faunal difference, although 

 not a wide one. No one has ever found an exact division, 

 however, even in a single locality and a division of the whole 

 formation is not at all practicable. It must be considered 

 for the present, at least, as an undivided unit. The extreme 

 upper portion of the Saint Peter has been described some- 

 times as a transitional zone, which it really is, but whether 

 it is clearly distinguishable as such from the main formation 

 is not certain. At Fountain, there are such strata which 

 could be arbitrarily separated where the shales begin, (see 

 fig. 9, pi. iv). But these strata are not constant for any 

 considerable distance. There are also transitional shaly 

 strata with intermingled sand grains at the top of the Saint 

 Peter at Minneapolis, which contain only Trenton fossils 

 and belong to the Trenton. Transition strata are usually 

 found but they are not necessarily equivalent those in one 

 locality with those of another. 



The Trenton has been everywhere conformably laid 

 upon the Saint Peter sandstone, with usually a short tran- 

 sitional zone between them. Where the Trenton has been 

 eroded away, the Drift, the Cretaceous, or the Carbonifer- 

 ous rests unconformably upon the Saint Peter, as already 

 described above under the head of Geological distribution. 



Upon the relation between the Saint Peter and the Shak- 

 opee below it geologists are not agreed as they are upon the 

 transition between Saint Peter and Trenton. McGee (31) 

 Keyes (32) and Norton (33) agree in pronouncing the Shak- 

 opee inseparable from the Saint Peter and include the former 

 in the latter. They say that the former really passes by 

 degrees into the latter. This theory does not agree with 

 observations made by other writers. Hall in 1852 (9) dc-r 



