THE LOWER SILURIAN ROCKS. 557 



The St. Peters does not usually much affect the soil, since it forms 

 only steep side-hills, or else is buried beneath the drift. Occasion- 

 ally, however, where it comes near the surface over small level areas, 

 as in part of the Sugar river valley, in the town of Yerona, it pro- 

 duces a loose sandy soil. 



In lithological characters the St. Peters is remarkably uniform. 

 So far as my observation extends, it is invariably formed of a fine, 

 purely silicious, sand, whose constituent grains are much rolled. No 

 sign of crystalline surfaces to the grains has been observed in the 

 many specimens examined with the microscope. The only foreign 

 materials in the St. Peters are the hydrous and anhydrous iron oxides, 

 which occur in all parts of the formation, banding it, or staining it 

 for great thicknesses, with yellow, brown, or red. The iron oxide 

 acts as a cementing material, but is not commonly in sufficient quan- 

 tity to give the rock any considerable coherence. In the more west- 

 ern development of the St. Peters, it is de-scribed as often wholly with- 

 out the iron oxides, and made up of pure white, entirely incoherent, 

 sand, but this is not common in Central Wisconsin. Greensand lay- 

 ers, like those that occur in the Lower sandstone, are found also in 

 the St. Peters, but none have come under my observation. No gra- 

 dation downwards or upwards into the adjacent limestones by mingling 

 with calcareous material has ever been noticed. No subordinate divi- 

 sion of the St. Peters sandstone exists. It is quite uniform in char- 

 acter throughout. The bedding, however, is usually distinct, the lay- 

 ers being ordinarily very heavy, though sometimes quite thin and 

 shaly. The lines of lamination are often marked by a red and white 

 banding when no planes of separation can be detected. The surfaces 

 of large exposures frequently show the hard, vitrified crust so charac- 

 teristic of the Lower sandstone. To this induration is evidently due 

 the maintenance of tower-like forms and cliffs in so friable a material. 



The older geologists describe the St. Peters sandstone as very uni- 

 form in thickness placing it at from 70 to 100 feet, with a nearly 

 constant thickness of 80 to 90 feet. According to the results of the 

 present survey, although such constancy probably holds true for south- 

 western Wisconsin, elsewhere the formation is exceedingly variable 

 in this regard. On the east side of the Catfish valley it is 50, 40 and 

 20 feet in thickness. Further northeast along its line of outcrop, 

 Prof. Chamberlin has found it but a few inches in thickness, and then 

 suddenly expanding to 80 or 100 feet. The same irregularity is ob- 

 served along its line of outcrop to the Michigan line. 



The St. Peters has been reported hitherto as entirely barren in fos- 

 sils, but recently a few have been found in the Eastern Wisconsin 



