240 C. P. BERKEY PALEOGEOGRAPHY OF SAINT PETER TIME 



trated furnish some firmly bound rock, but this is in irregular bands 

 rather than a general constituent. 



STRUCTURE 



In many places the rock is massive, with but faint traces of bedding or 

 any other marking. At still others the bedding planes are strongly 

 marked and the beds comparatively thin, passing in numerous places 

 into shaly facies. Often there is cross-bedding, as noted particularly in 

 northern Illinois, with ripple marks well developed. These irregularities 

 are still further accentuated by color streaking. Variations are much 

 more notable in the central portion of the Saint Peter area than on its 

 margins, although they are not wholly lacking along the northern margin. 

 Chamberlin* refers to structures typical of ebb and flow and ripple marks 

 in Wisconsin. 



Nearly all the strong coloring described in the Saint Peter is confined 

 to structural inequalities, mostly bedding or cross-bedding lines. In so 

 porous a rock of so simple composition one would not expect enough 

 selective capacity to connect the color streakings with late infiltration. 

 Even the coloring seems to point back to the conditions under which tie 

 rock accumulated. 



Transition 



There are occasionally brecciated and conglomeritic Lower Magnesian 

 fragments in the marginal facies of the rock in eastern Wisconsin and 

 northern Michigan. Uusually the transition from Magnesian dolomite 

 to Saint Peter and from Saint Peter to Stones Eiver limestone again is 

 abrupt, but even this is not universal, sand mixed with calcareous matter 

 or clay mixed with the sand being an occasional variation. 



The overlying limestones and shales of the Stones Eiver group at aii 

 places are described as perfectly comformable to the Saint Peter. 



Unconformity 



The underlying dolomite and the succeeding Saint Peter are fre- 

 quently unconformable. This condition has been described in Michigan, 

 Wisconsin, Iowa, and Missouri, where occasionally the dolomite floor is 

 very uneven. In places the dolomite floor is so lmmmocky as to project 

 up through the whole thickness of the Saint Peter, so that the sandstone 

 only fills the hollows and valleys. 



In generalizing the isolated descriptions it appears that the greater 



* Geology of Wisconsin, vol. i, p. 146. 



