86 The Saint Peter Sandstone. 



Such considerations may be used to explain other phe- 

 nomena as well. It has been shown that the folding of the 

 Shakopee took place at last partly after the sandstone was 

 deposited upon it. (See fig. 10, pi. 4.) The folding is similar 

 to that produced by lateral pressure, except that for some 

 reason the top and not the base of the Magnesian series is 

 involved in it. The Shakopee is very strongly arched here 

 and there and the top of the Oneota likewise, but before the 

 base of the latter is reached the undulation disappears, ex- 

 cept in one or two localities, near the city of Saint Peter, 

 Minnesota, for example, and there the underlying Jordan 

 sandstone is pure like typical Saint Peter. The Saint Law- 

 rence formation is not seen to be folded. 



In view of the facts as given above, I am constrained to 

 set forth a theory, without, however, considering it as con- 

 clusively proved, that the purity of the Saint Peter sat|dstone, 

 the paucity of its fossils and its variable depth are all due to 

 the effect of percolating waters and were not original char- 

 acters, and, further, that the supposed unconform ability at 

 its base may have been produced solely in this manner. That 

 is, that the Saint Peter has simply had all soluble material 

 washed out of it, that it is thus reduced in thickness, and 

 that the now dolomitic formations immediately beneath 

 have been attacked in a similar manner and reduced — the 

 whole process causing a shifting of the sand sufficient to pro- 

 duce inequalities in its thickness. 



* The great mass of Silurian and Devonian rocks resting 

 upon the Saint Peter have not been reduced in that manner, 

 nor to the same extent, but have preserved an unbending 

 crust, while the Middle Cambrian sandstones below the On- 

 eota formation remained likewise rigid. Inequalities pro- 

 duced during the reduction of the Saint Peter and Magnesian 

 series also must have caused shifting and readjustment, and 

 naturally these would culminate intermediately between the 

 firm overlying and underlying formations, that is, in the 

 formations that were being most reduced. This may explain 

 why the folding begins midway in the Oneota, culminates at 

 the base of the Saint Peter, and ends below the top of the 

 same. 



Heretofore geologists have considered the Saint Peter 



