THE LOWER SILURIAN ROCKS. 543 



distinctly recognized throughout the Central Wisconsin district, 

 wherever the base of the Lower Magnesian can be inspected. The 

 northernmost point at which 1 have recognized them is at the north- 

 west corner of Marquette county, the southernmost on the south 

 shore of Lake Kegonsa in Dane county; the two points being about 

 70 miles apart. The most distant points east and west at which they 

 have been seen in Central Wisconsin are about as far from one 

 another. To the northeast, however, Prof. Chamberlin thinks he has 

 recognized the Mendota as far as the Michigan line, whilst Mr. Strong 

 carries the same layer westward to the Mississippi. 



The Mendota and Madison beds often have a marked effect upon 

 the topography, producing, by their different hardnesses, benches in 

 the sides of bluffs. Where the Mendota is at surface over any con- 

 siderable area, it produces generally an excellent clayey soil ; whilst 

 the Madison soils, as in a large part of the town of Otsego, Columbia 

 county, are as loose and sandy as those of the Potsdam proper. About 

 Madison, where the two layers were first distinctly recognized, the 

 Mendota has a thickness of 30 to 35 feet, of which the lower 20 feet 

 are of a heavily-bedded, dark -yellow and brown, jointed, conchoidal- 

 fracturing rock, which is stained in seams and patches by the red oxide 

 of iron, and leaves on solution 3 to 10 per cent, of an aluminous and 

 non-arenaceous residue. This rock quite closely resembles the lower 

 portions of the Lower Magnesian proper, having sometimes the con- 

 cretionary structure characterizing that formation. The upper part 

 of the Mendota about Madison resembles the lower, except in being 

 in thin, rough-surfaced, layers, and in carrying a somewhat larger 

 percentage of silicious matter. To show the close similarity in com- 

 position which this phase of the Mendota bears to the Lower Magne- 

 sian, the following analyses are given, I being the Mendota, from the 

 quarry near Greenbush, Madison, and II, Lower Magnesian from 

 Williams' quarry, on the south line of the town of Madison: 



I. IT. 



Silica 4.18 1.09 



Alumina 2.17 .44 



Iron sesquioxide 1 .45 .43 



Iron protoxide -63 



Lime cartxmate 55.68 66.82 



Magnesia carbonate 36 . 52 30.40 



Water 58 .35 



100.58 100.26 



It will be noticed that in both the proportion of lime to magnesia 

 carbonate is greater than in true dolomite (1.19: 1). Both yield, also, 



