COMPOUND KEGRESSIVE AND TRANSGRESSIVE OVERLAP 617 



and the Dakota sandstone, have so far been worked out in any detail. 

 These will be sufficient, however, to illustrate the foregoing principle. 



The Saint Peter formation is typically developed in the upper Mis- 

 sissippian region. In Minnesota it is a friable quartz sandstone of ex- 

 treme purity in most cases. An analysis of material south of Saint Paul 

 give:* 



SiO, 99.78 



Fe 2 8 trace 



MgO trace 



Sometimes, however, impurities in the form of kaolin or iron stain 

 occur. The sandstone is mostly of a white color. "This white color is 

 due to the condition of the surfaces of the grains ; they are worn simply 

 to a dead finish — not polished, as can readily be seen by immersing them 

 in water, when they become limpid." f In texture the sandstone is some- 

 what coarser in the bottom than in the middle and upper beds, but no 

 conglomeratic texture is known. In Wisconsin, however, dolomitic pebbles 

 from the underlying rock are included in its base, this being also more 

 or less eroded. J The occurrence in the upper portion of Hormotoma 

 gracilis (Hall) and Lophospira perangulata (Hall) shows its close rela- 

 tion to the overlying Stones Eiver beds, with which it is perfectly con- 

 formable. 



One of the most striking features of this formation in Minnesota is 

 the fact that its base is perfectly conformable with the underlying Lower 

 Magnesian limestone (Shakopee), while its top is also perfectly con- 

 formable with the overlying Stones Eiver formation. "Nowhere," say 

 Hall and Sardeson,§ "is there any indication, however slight, of an 

 unconformity [between the Saint Peter and the overlying rock]. The 

 transition zone of a green shaly calcareous sandstone shows the steady 

 oncoming of the Lower Silurian [Ordovicic] sea. . . . The Saint 

 Peter has a thickness varying from 75 to 164 feet in Minnesota. It 

 rests, as noted, conformably on the Lower Magnesian or Shakopee 

 dolomite, which, with the New Eichmond and Oneota, is, as shown by 

 Berkey and others, a normal depositional successor of the late Cambric. 

 The thickness of the lower Magnesian (Oneota to Shakopee) varies from 

 105 to 260 feet, || and the fossils show it to be of basal Ordovicic age. 

 The beds overlying the Saint Peter are 32 feet thick^f and are conform- 



* Hall and Sardeson : Bull. Geol. Soc. Am., vol. 3, 1892, p. 351. 



t Hall and Sardeson : Loc. cit., p. 351. 



t T. C. Chamberlin : Geology of Wisconsin, vol. ii, 1877, p. 287. 



§ Loc. cit., p. 355. 



|| Hall and Sardeson : Loc. cit., p. 368. 



If Winchell and Ulrica : Paleontology of Minnesota, vol. ii, introduction. 



