y38 GEOLOGY OF EASTERN WISCONSIN. 



some cases, rendering it liable to disintegrate to a calcareous sand. 

 It occurs in heavy beds, is easily quarried, cuts with the greatest fa- 

 cility, and is much used at Fond du Lac for cappings, etc. It is 

 probable that this and the preceding rock are to be referred to the 

 same horizon, as they seem to graduate into each other at some points, 

 and to replace each other, mutually, at others. 



Upon this rests a stratum similar to the second in being charac- 

 terized by thick beds and vertical fissures, but differing from it in 

 being less hard, and in possessing a highly brecciated structure in 

 some localities. To the position just above this is to be referred a 

 layer containing many obscure casts of a Pentamerus {Gypidula), 

 very similar to the species occidentalis. This stratum is succeeded 

 above by the white, even-textured limestone of the Byron and Wau- 

 kesha beds. 



As a whole, the Mayville beds may be readily recognized by their 

 thick bedding, uneven structure, and the rough, craggy, pitted sur- 

 face of the weathered ledges, when taken in connection with thoir 

 position. It is the best exposed member of the Niagara group, as 

 it forms a chain of craggy cliifs, upwards of a hundred m.iles in 

 length, though broken down and concealed at frequent intervals. 

 The greatest observed thickness is one hundred feet. The aver- 

 age thickness is probably not more than sixty feet. The chemical 

 composition of some of the more important of these beds is shown 

 in the following table: 



I. 



Carbonate of lime 50.52 



(^'arbonate of magnesia 40.97 



Sesquioxide of iron 0.77 



Sesquioxicle of alumina 3.49 



Silica 3.57 4.52 



Insoluble residue 1.23 0.26 1.35 1.73 



Water 0.48 0.70 trace. 0.29 0.26 0.45 



Total 99.80 99.82 99.86 100.23 100.18 99.81 



Per cent, of impurities 7.83 8.21 1.63 0.57 2.24 2.48 



The rock for the first analysis was from the slialy beds, at the base 

 of the formation, that are used for making waterlime in "Williams- 

 town (N. W. I of B. E. I of Sec. 27, T. 16 N., E. 12 E.), and the sec- 

 ond was from similar beds found in Stockbridge (IST. E. \ of Sec. 11, 

 T. 19 N., E. 18 E.). The third was from the limestone used at the 

 Appleton iron furnaces for flux, and obtained from the lower 20 feet 



