VARIATION IN PURITY . 239 



There are shale intermixtures at Boone, Iowa, calcareous and shale 

 intermixtures in Wisconsin, and iron oxide heavily developed in a few 

 places. The formation in Missouri, Indiana, Ohio, and Kentucky car- 

 ries a marked percentage of lime and argillaceous matter in contrast with 

 the average northern outcrops. 



Specimens at hand from La Salle county, Illinois, show a considerable 

 percentage of earthy impurity, while one from Wisconsin, probably a 

 transition phase, is very heavily charged with carbonates, though on the 

 whole the Saint Peter is a remarkably pure sandstone formation. Its 

 greatest variability is marginal and throws some light on the question of 

 origin and conditions prevailing during its accumulation. This variabil- 

 ity, however, is markedly different along opposite margins — that is, the 

 landward as opposed to the seaward side — and is well within the range 

 of characters in every respect that one should expect. Thus on the 

 northern margin there are developed occasionally breccias and marginal 

 conglomerates and various foreign intermixtures, while far southward 

 the Saint Peter and its equivalents become shaly sandstones, calcareous 

 shales, and even siliceous limestones.* 



BINDING 



Throughout most of its extent the individual grains of the Saint Peter 

 have almost no binding. At Minneapolis a freshly exposed fragment may 

 be crushed in the hand or its grains readily rubbed off by the fingers. 

 In all cases exposure increases the strength of the bond, so that occasion- 

 ally it has been made use of in light structures. In Ogle county, Illi- 

 nois,! it is recorded that the outcrops, although apparently of so friable 

 material, resist weathering with great success. 



Occasionally in other localities there is strong lime or iron oxide 

 binding, while local induration to a quartzite is less common. A sample 

 from Missouri sent by the kindness of Dr E. M. Buckley exhibits sec- 

 ondary growth of almost every grain, the pyramidal faces being especially 

 sharply developed and occasionally developing almost a complete doubly 

 terminated quartz crystal; yet the rock is not well cemented; it is not 

 much more firm and resistant than the average specimen. 



In all cases there is either a striking lack of cementation or if well 

 developed it is extremely local. The places where iron oxide is infil- 



* Marginal character may have prevailed also in Pennsylvania and West Virginia, as 

 well as along some continental border in the west, but if so the type of rock is different 

 and is not recognized as Saint Peter. 



t James Shaw : Geology of Illinois, vol. v, p. 116. 



