CARBOmFEROUS SYSTEM. 237 



but they are mostly too thin for profitable mining. It is true 

 that occasionally a bed of coal in the Lower coal-measnres is 

 also too thin to be profitably mined, but there are also beds 

 now being mined in this formation that reach from npward of 

 three feet to seven feet in thickness of solid coal. At the close 

 of this chapter, and in that upon the Geology of the Coal 

 Counties also, the subject of supplies of coal will be still 

 further discussed. 



The beds of clay that underlie almost every bed of coal 

 furnish a large amount of material suitable for potters' use, 

 some of which is excellent for the manufacture of common 

 stone ware. 



The sandstone of the Lower coal-measures is usually soft, 

 and much of it shaly and unfit for any practical use ; but at 

 some places it is quite a clear-grit sandstone, and has suffi- 

 cient firmness to serve as excellent building material, if 

 selected with care. This stone has some advantages not 

 possessed by ordinary limestone. It is often so massive in 

 its stratification, that blocks of large dimensions may be 

 obtained, and then its comparative softness enables the work- 

 men to fashion it into any desired shape with little labor. It 

 hardens considerably after being quarried and exposed to the 

 atmosphere for a few months. The best quarries at present 

 opened in this rock are those near the town of Eedrock, in 

 Marion county, which received its name from the stone. 

 Here the greater part of the stone is of a pleasant brick-red 

 color, produced by the per-oxyd of iron. Some of it has 

 been carried to the City of Des Moines for building purposes, 

 examples of which may be seen in the State Arsenal there. 

 It is well to repeat the caution to avoid carefully the softer 

 portions of this sandstone for use in important structures, 

 notwithstanding the fact that much of it is valuable. 



In Davis county there is a bed of dark bluish, impure lime- 

 stone, about four feet in thickness, being the heaviest deposit 

 of limestone yet found among the strata of the Lower coal- 

 measures in Iowa. It is of comparatively little value for the 

 ordinary purposes to which stone is applied, but several years 



