POTSDAM SANDSTONE. 259 



have examined the other and much more prevalent, which consists 

 of a mixture of ordinary white quartz grains, and deep-green grains 

 of glauconite, or a closely allied mineral. 



The ferruginous varieties embrace at one extreme, those in which 

 the amount of iron oxide is barely sufficient to color or cement the 

 mass, and at the other, those in which it becomes so great as to justi- 

 fy calling the mass an iron ore. Neither of these classes is abun- 

 dant in this portion of the state, though important features in other 

 parts. The sandstone of this region is usually very light colored, and 

 nodules, seams, or layers of iron, though present, attain to no signifi- 

 cant development. 



As has been already remarked, the great mass of this sandstone is 

 more or less calcareous. The limy portions become so great in some 

 layers that they are more properly limestones than sandstones. The 

 lime in this case, as is usual in this state, is associated with mag- 

 nesia, so that these layers become really arenaceous dolomites. In 

 some portions, the calcareous matter, instead of being dissemina- 

 ted through the rock, forms concretions, by binding lumps of sand 

 into hard spherical masses, giving the beds a nodular structure. 



The foregoing general statements may perhaps suffice to indicate 

 the prevailing chemical and molecular nature, and the minute struc- 

 ture of this formation. More specific facts will be found in connec- 

 tion with the local sections and descriptions. We may now consider 

 its more massive characters. Where bedding is distinctly marked 

 the layers are not usually more than three or four feet in thickness. 

 From this thickness they are to be found of all lower dimensions down 

 to layers of slate-like thinness. The beds show oblique lamination, 

 ripple marks, and other evidences of. shallow water deposit. 



Owing to the prevalence of heavy deposits of drift, exposures of 

 this formation in eastern Wisconsin are quite unfrequent, and very 

 limited in extent; a very fortunate circumstance for the agriculturist, 

 though quite the reverse for the geologist. However, by diligent 

 search and careful collating of data, it appears that the formation 

 consists of six subdivisions, as follows, beginning at the top: 



Sandstone (Madison) 35 feet. 



Limestone, shale and sandstone (Mendota) , . 60 " 



Sandstone, calcareous 155 " 



Bluish shale, calcareous, 80 " 



Sandstone, slightly calcareous, 160 " 



Very coarse sandstone, non-calcareous, 280 " 



The thicknesses given are subject to considerable variation. As a 

 general rule they grow less toward the northeast. Where the total 



