526 GEOLOGY OF CENTRAL WISCONSIN. 



itself often shows alternations of nearly purely dolomitic, and distinctly 

 sandy layers, even including thin seams of white sand, whilst some 

 sandy material occurs at horizons well towards the summit of this for- 

 mation. The two series thus evidently graduate into one another, 

 and in many parts of the northwest it is undoubtedly difficult to fix 

 upon the dividing line. In Central Wisconsin, however, the alter- 

 nating beds are well defined, and two hori.zons are well marked by 

 beds of greensand. These are the base of the Lower Magnesian prop- 

 er, and the base of the Mendota beds, 65 to 100 feet below. During 

 the writer's earlier work in this field, the lower of these limits was 

 adopted, in. mapping, as the line of division between the two forma- 

 tions, whilst subsequently both horizons were mapped, the upper one 

 being' taken as the base of the Lower Magnesian, in order that that 

 name might cover the same beds as included under it by other mem- 

 bers of the geological corps. 



The facts which led at first to the use of the lower limit may be 

 briefly given here. In Dane and Columbia counties, where the Men- 

 dota and Madison horizons are very prominent, and were first made 

 out, the entire thickness of the Lower Magnesian, between the upper 

 surface of the Madison beds and the base of the St. Peters or Upper 

 sandstone, was found to be only 50 to 80 feet. It M-as well known 

 that not far to the westward, along the Wisconsin, this formation 

 attains a thickness of 250 feet. It was not thought so great a thick- 

 euing as this could exist, and to explain the difference, it was supposed 

 that the Mendota and Madison beds were a local modification of the 

 lower portion of the Lower Magnesian formation. This supposition 

 was encouraged by the great similarity between the Mendota, as seen 

 in the typical localities in Dane county, and the limestone beds im- 

 mediately overlying the Madison sandstone, the former here being al- 

 most a pure dolomite, with only two or three percent, of insoluble resi- 

 due. Subsequently, however, it was ascertained beyond a doubt that the 

 Mendota beds are to be recognized occupying the same position below 

 the Lower Magnesian, even where that formation attains its greatest 

 thickness, its irregularity in this regard being proved to be due to its 

 having suffered a surface erosion prior to the deposition upon it of the 

 St. Peters sandstone. The Madison and Mendota beds were therefore 

 thrown back into the Potsdam series as its uppermost layers, and arc 

 so mapped on all of the Atlas maps. 



For all those portions of the maps, however, which represent the 

 Central Wisconsin district, except southwestern Jimeau county, on 

 Area II, the base of the Mendota beds is indicated by a brown line 

 within the color for the Potsdam series, the space between this line 



