186 GENEBAL GEOLOGY. 



Economic Value. Although a large part of the material 

 of this formation is practically quite worthless, yet other 

 portions are very valuable for several economic purposes; 

 and having a very large geographical extent in the State, 

 it constitutes one of its most important formations in a 

 practical point of view. 



The magnesian limestones of this formation, are not so 

 near a pure dolomite in any case as those are which con- 

 stitute the greater part of the Upper and Lower Silurian 

 formations in Iowa, but the admixture of other substances 

 seems beneficial to them for economic purposes. For 

 example, the admixture in a finely divided condition of five 

 or six per cent each of silica and alumina, to the proportions 

 of the carbonates of lime and magnesia that form dolomite, 

 is found to give the rock very decided hydraulic properties. 

 Rock of this kind occurs in considerable quantities at 

 Waverly, in Bremer county, and its value for the production 

 of hydraulic lime has been practically demonstrated there by 

 Mr. Robert D. Brown, an account of which is given 

 Volume two, besides the analysis of the rock in Prof. 

 Emery's report. It is not improbable that the same forma- 

 tion will furnish material equally good for the same purpose 

 in other parts of the State, but thus far the rock at Waverly • 

 alone has been practically tested. 



Wherever any part of this formation is exposed, the 

 common limestone portions exist in sufficient quantity to 

 furnish abundant material for common lime of excellent 

 quality, as well as good stone for ordinary masonry. The 

 heavier and more uniform magnesian beds furnish in many 

 places excellent material for dressed stone, and for bridge 

 piers, as well as for other works requiring strength and dura- 

 bility. 



The clayey portions are usually too calcareous and impure 

 for any economic use, but some indications were observed in 

 Bremer, Floyd, and Cerro Gordo counties, that lead to the 

 supposition that its clayey portions there may possibly fur- 

 nish a clay suitable for the manufacture of light colored brick. 



