SUGGESTED ORIGIN OF THE SANDSTONE 237 



near Saint Paul, Minnesota, and concludes from them that this formation 

 is much more nearly allied to the overlying Stones Kiver than to Shakopee 

 below. This is in accord with the existence of a break between the 

 Shakopee and the Saint Peter, indicated both by the unconformity be- 

 tween them and the cessation of sedimentation that has resulted in thin- 

 ner beds on this margin. 



Therefore the uppermost Saint Peter, especially in its northern mar- 

 gin, is Stones River in age ; but the whole formation is involved in a re- 

 treat and advance of the sea over the area, and has, as is usual in sand- 

 stones, no perfect time unity in its whole extent; and the break, except 

 where it has imprinted erosion features on the underlying Shakopee, is 

 swallowed up in the sands of the deposit. 



A few columns drawn to scale (see plate 24), with the lines of the 

 formations indicated, will serve to better show the relationship. 



Origin suggested by Writers 



There have been many different theories as to the origin of the Saint 

 Peter sands and their present purity. Its extreme purity and great 

 extent seem to have made ordinary sedimentation processes appear in- 

 sufficient to many observers. In accord with this idea, several early 

 writers have suggested* chemical precipitation as a probable origin. 

 Most observers, however, have agreed to the mechanical character of the 

 deposit and have suggested a variety of subordinate agencies. 



James Shaw, in the geological reports of Illinois, at one time advo- 

 cated the wind as the effective agent in producing the cross-lamination 

 and ripple-like marks, but later seems to have abandoned this idea. 

 Norton of Iowa also suggests the wind as a probable factor. Owen and 

 many others seem never to have considered any modification of the ordi- 

 nary sedimentary origin necessary. Sardeson,f on the other hand, while 

 maintaining the sedimentary origin, considers the unusual purity due 

 chiefly to the porosity of the rock, favoring free circulation of under- 

 ground water and the consequent leaching out in solution of all con- 

 stituents except the quartz grains. 



Any more definite statement than that the Saint Peter owes its exist- 

 ence and character to the usual processes of sedimentation depends for 

 support more on the variability of character than upon its reputed con- 



* Keating, Hall and Whitney, Winchell. 



f Sardeson : Bull. Minn. Acad. Nat. Sci., vol. iv, no. 1, 1896, p. 86. 



