536 GEOLOGY OF CENTRAL WISCONSIN. 



sand from the Lower Magnesian limestone of Minnesota, at Eed Bird, 

 on the Mississippi, which is beyond doubt the same materiaL This 



analysis (I), as also another, by the same gentleman, of the Cretace- 

 ous greensand of ISTew Jersey (II), is given below:' , 



L IL 



Silica 46.58 50.70 



Alumina 11-45 8.03 



Iron protoxide 20.61 22.50 



Magnesia 1-27 2.16 



Lime 2.49 . 1.11 



Soda 0.98 0.75 



Potash 6.96 5.80 



Water 9 . 66 8 . 95 



100.00 100.00 



The green grains of both Cretaceous and Silurian greensands, as 

 also of similar deposits in existing seas, are often found as casts of 

 the shells of rhizopods. So far as the writer's knowledge goes no 

 such observation has ever been made with regard to the Wisconsin 

 greensand. Tlie greensand layers are by no means restricted to the 

 Potsdam series; they occur in both the Lower Magnesian and St. 

 Peters. Grreensand grains occur also apart from the regular green 

 layers. The thin, yellow and brown, rough-textured, calcareous 

 bands, that characterize the layers immediately beneath the Mendota, 

 are often dotted with coarse grains of glauconite, which are not in suf- 

 ficient quantity to impart their color to the rock. 



The generalized section given above for the Potsdam series, below 

 the Mendota base, holds true for a large part of the Central Wiscon- 

 sin district, and would be satisfactory for all of it, but for the facts 

 next to be stated. Proceeding northward from the valley of the Wis- 

 consin, we encounter, traversing Sauk and eastern Columbia counties 

 for 25 miles from east to west, the ArchsEan quartzite ranges described 

 in the last chapter. The Mendota horizon continues well marked di- 

 rectly up to the ranges, whilst in the country west and east, it extends 

 much further to the northward. Everywhere about the quartzite, 

 however, we find, lying unconformably upon it, layers of sandstone 

 and bowlder-eongloraerate, which, as regards altitude, appear to oc- 

 cupy the entire distance between a horizon considerably below the 

 Mendota, and one nearly as high as, if not higher than, the summit of the 

 Lower Magnesian. These layers of sandstone and conglomerate can- 

 not, in all, be less than 400 feet in thickness, being nearly always 

 without calcareous admixture. Single cliffs occur showing 225 feet 



' Geology of Canada, p. 488. 



