THE LOWER SILURIAN ROCKS. 525 



CHAPTER IT. 

 THE LOWER SILURIAN ROCKS. 



I. In General. 



THE LOWER, OR POTSDAM, SANDSTONE SERIES. 



Forming the base of the pile of Silurian strata everywhere in the 

 states bordering the Upper Mississippi, but having a much greater 

 surface development in Wisconsin than elsewhere, and resting di- 

 rectly upon the irregular surface and upturned edges of the older 

 crystalline rocks, is a great thickness of sandstone, which, through the 

 larger part of its mass, is made up of rolled grains of quartz, of vary- 

 ing size, cemented together by a minute quantity of hydrous iron 

 oxide. Towards the upper part of the formation, in Central Wiscon- 

 sin, this sand becomes mingled with more or less dolomitic and cal- 

 careous material, which further up tends to aggregate into thin bands 

 of limestone, finally forming, at 35 to 50 feet below the base of the 

 next great formation, the Lower Magnesian limestone, a well marked 

 and very persistent yellow limestone layer, which has a thickness of 

 30 feet, and is so well marked and important a horizon in Central 

 Wisconsin, that I have given it the specific name of Mendota lime- 

 stone, from a large exposure at MacBride's point on the north shore 

 of lake Mendota. Above the Mendota horizon, sandstone, 35 to 50 feet 

 in thickness, again comes in, the larger part of which is either nearly 

 pure white quartz sand, or sand turned brown by oxide of iron, thus 

 approaching more nearly in character to the Upper or St. Peters 

 sandstone than to that immediately beneath the Mendota beds. To- 

 wards its upper portions, however, just beneath the overlying lime- 

 stone, it generally becomes again somewhat dolomitic, the upper limit 

 being frequently marked by layers of greensand and oolitic chert. 

 To this layer I have given the name of Madison sandstone, it yield- 

 ing large quantities of a very excellent building sandstone at Madi- 

 son. These names are not meant to be of anything more than local 

 importance. 



For some distance above the Madison horizon the Lower Magnesian 



