HAMILTON CEMENT ROCK. 395 



OHAPTEE IX. 



DEVONIAN. 



HAMILTON CEMENT ROCK. 



The uppermost and newest of the indurated formations of Wiscon- 

 sin, and the only representative of the Devonian age, is the Hamilton 

 cement rock, near Milwaukee. It occupies a limited area, lying adja- 

 cent to the lake, immediately north of the city, and rests in part upon 

 the shaly limestone above described, and apparently upon the Niagara 

 limestone in other portions. In general lithological characteristics, 

 it consists of a bluish-gray or ash-colored, impure dolomite, which 

 weathers, upon exposure, to a yellowish or buff color, owing to the 

 oxidation of -the iron which constitutes one of its ingredients. The 

 impurities consist chiefly of silica and alumina. The rock is char- 

 acterized in certain portions by the occasional presence of cavities, in 

 which occur crystals of iron pyrites and calcite, and, very rarely, zinc 

 blende. Crystals of the two former minerals- are disseminated more 

 or less through certain portions of the rock. In texture, it is some- 

 what varying, being quite homogeneous in some layers, and quite ir- 

 regular and lumpy in others, while the chemical composition changes 

 much less markedly, though sufficiently to affect the hydraulic prop- 

 erties of the rock. In degree of induration, it ranges from rather soft 

 to moderately hard. The beds are usually thick, with the exception 

 of some portions, which are somewhat shaly. The general aspect of 

 the bedding and stratification is well shown in the accompanying 

 chrom o-lithograph. 



TI1& chemical composition of the typical portion is very fully indi- 

 cated by the accompanying table of analyses of cement rocks, in which 

 will be found eight analyses of the rock from the Milwaukee river, 

 the selections and determinations being by different parties. 



