238 C. P. BERKEY PALEOGEOGRAPHY OF SAINT PETER TIME 



stancy. The views of the writer as to origin are necessarily involved 

 in a discussion of these lines of evidence. 



Variability of the Saint Peter 



variation more prominent in minor details 



A minute study makes it clear that the reported constancy of this 

 formation is somewhat overdrawn. In the broader characteristics of 

 sandstone formations, it is fairly constant, but in minor detail it is very 

 variable, and these usually neglected variations, it is thought, are in this 

 case a key to the bit of history that the Saint Peter represents. 



THICKNESS 



This formation varies in thickness from a mere film, a layer of sand 

 grains, as in certain cases in eastern Wisconsin, to 225 feet in Illinois.* 

 It occasionally exhibits a range of from 1 foot to 100 feet within so short 

 a distance as a quarter of a mile.t 



This rapid change is described as due to the undulating magnesian 

 limestone floor. The normal thickness in Iowa is about 100 feet, and 

 this may be taken as a fair estimate of its average thickness over the 

 greater part of the whole region of its distribution. 



PURITY 



Although more than 99 per cent pure quartz in some localities, this 

 condition is by no means universal. An analysis given by Hall and 

 Sardeson shows for the fossiliferous rock occurring at South Saint Paul: J 



Si0 2 99.75 per cent 



Fe 2 3 Trace 



MgO Trace 



A Minneapolis sample gave 98.50 per cent of Si0 2 . 



The saccharoidal sandstone (Saint Peter) of eastern Missouri, in the 

 vicinity of Saint Louis, so important for many years past in the glass 

 industry, is also remarkably pure. It is given as more than 99 per cent 

 Si0 2 . 



The Saint Peter, however, is not always free from clastic impurity. 

 Hall and Sardeson note the presence of fine white kaolin in the Saint 

 Anthony area. 



* Geological Survey of Illinois, vol. vii, p. 49. 



t T. C. Chamberlin : Geology of Wisconsin, vol. ii, 1887, pp. 285-286. 



jBull. Geol. Soc. Am., vol. 3, p. 351. 



