452 GEOLOGY OF CENTRAL WISCONSIN. 



nearness to the surface of certain clayey layers included within the 

 sandstone, as in the town of Eeedsburg, Sauk county; or from the 

 presence of' considerable dolomitic material in the sandstone, as in 

 the town of Honey Creek, Sauk county. All of these, are, however, 

 but exceptions to the general rule, that, for the most part, the Pots- 

 dam area, on both sides of the drift limit, is a barren sandy region. 



Above the lowest sandstone, we find, first, a layer thirty feet in 

 thickness of clayey or sandy dolomitic limestone, to which I have 

 given the name of Mendota limestone. "Where it is the surface 

 rock, as in portions of the towns of Scott and Marcellon, Co- 

 lumbia county, a good clay soil usually results. Next above the 

 Mendota is again a layer of sandstone, which I will designate as the 

 Madison sandstone, having a thickness of 30 to 40 feet. Where it is 

 the surface rock, as in a large part of the town of Otsego, Columbia 

 county, the soil is a loose sand, and of little value. These rapid al- 

 ternations explain the change of soil noticed with a slight change of 

 elevation in many places along the borders of the areas occupied by 

 the Lower Magnesian limestone, which is the next formation in 

 order. 



The Lower Magnesian is a great mass of dolomitic limestone, 80 

 to 200 feet in thickness, often carrying much silicious and clayey 

 matter in its composition. The country occupied by it is hence 

 nearly always one of most excellent soils. Such is the soil of the high 

 prairie belt of northern Dane and eastern Columbia. These prairies 

 are often much covered with drift, which occasionally lends a sandy 

 character to the soil, but not frequently, since the drift of this partic- 

 ular district itself is commonly calcareous and clayey. 



The next rock above the Lower Magnesian, the St. Peters sand- 

 stone, but i-arely forms the surface rock, being generally concealed, 

 when present, by overlying beds, and showing only on hill-sides and 

 steep slopes. Where, however, it comes to the surface, as in the val- 

 ley of Sugar river in western Dane county, and the drift is absent, 

 the same sandy soil is observed as that produced by the disintegra- 

 tion of the Lower sandstone. 



The Trenton limestone soils are usually of excellent quality, and 

 frequently somewhat more argillaceous and less dolomitic than those 

 from the Lower Magnesian. The Galena is so unimportant a forma- 

 tion in Central "Wisconsin that it need not be especially alluded to in 

 this connection. 



