244 C. P. BERKEY PALEOGEOGRaPHY OF SAINT PETER TIME 



the separation* of this formation from the overlying Trenton (Stones 

 Eiver). This irregularity is greatest from Dodge county northward. 

 In Missouri an inequality of this type is credited to actual stream 

 erosion. 



Few localities exhibit the Shakopee-Saint Peter contact well enough 

 to furnish reliable data for most of the field. Hall and Sardeson have 

 described this contact in Minnesota, also as uneven as a rule, but have 

 considered it chiefly due to folding, f which dies out upward in the Saint 

 Peter. The overlying Trenton (Stones Eiver) is not affected. It 

 seems well supported then, by combining the observations of all these men 

 in their particular fields, that Saint Peter time was one of some consider- 

 able dynamic disturbance, that it resulted in both folding and faulting 

 of the preceding series locally, and, in the region as a whole, was accom- 

 panied by diastrophic movements by which much of the Upper Mississippi 

 valley and Great Lake regions became dry land for a part of the time. 



Some characters of the formation (noted briefly under the heading of 

 "Variability of the Saint Peter") bear on this problem and suggest 

 further modification of the general interpretation. 



Structural Character and Texture 



A study of the texture of the Saint Peter made on specimens X gathered 

 from localities of wide areal range furnishes a few suggestive facts. 



Microscopic examination shows that in typical Saint Peter there is a 

 total absence of large grains that are usually classed as gravel, and almost 

 as complete absence of extremely fine grains, such as fall within the 

 class of dust. In this respect there is no very marked difference among 

 the various samples. The sands from La Salle, Illinois, and one sample 

 from Wisconsin show most impurity. 



The range of sizes runs from 1 millimeter down to .01 millimeter in 

 a few cases, but the bulk of the rock is made up of grains from .4 milli- 

 meter to .05 millimeter in diameter. 



The diameter of grains in samples examined from typical localities 

 range prevailingly from .05 millimeter to .6 millimeter, an average for 

 by far the greatest number of grains being from .1 to .2 millimeter. 

 So far as different parts of the area or different horizons in the formation 

 have been subjected to scruthry, there is not any great variation. Hall 

 and Sardeson, however, note that there is considerable diversity of texture 



* Geology of Wisconsin, vol. ii, 1877, pp. 285-286. 



-j- Bull. Geol. Soc. Am., vol. 3, p. 353. 



J For most of this material I am indebted to Dr E. M. Buckley, of Missouri ; Superin- 

 tendent Thomas J. McCormick, of La Salle, Illinois ; Professor Ira A. Williams, of Ames, 

 Iowa, and Professor C. K. Leith, of Wisconsin. 



