GEOLOGY OF SOUTHWESTEKN IOWA. 325 



All the strata represented by the foregoing sections, and 

 all others observed in Decatur county, evidently come within 

 the verticle range of the section at Davis' mill in the southern 

 part of the county, so that there are no higher beds within its 

 limits than those of the Upper coal-measure formation. 



Economic Resources. Stone is the most valuable mineral 

 product yet known to exist within the limits of Decatur 

 county, or, at least, the only one at present accessible. By 

 turning again to the sections of Decatur county strata, it will 

 be seen that stone is comparatively plentiful in the valleys, 

 particularly in those of the west half of the county. All 

 these strata that are hard enough to receive the name of 

 stone, are limestone ; all of which, besides being suitable for 

 building purposes, furnish material for a good quality of 

 lime. Layers of it are not unfrequently found from which 

 the common forms of dressed stone for buildings may be 

 obtained. Such stone was observed at Mansfield's quarries 

 and also at several points in the valley of Elk creek and 

 those of its branches as well as elsewhere. It will be 

 observed that "black fissile carbonaceous shale" is men- 

 tioned as occurring among the strata represented in some 

 of the preceding sections. Some of this shale splits readily 

 into thin uniform sheets, closely resembling slate, which 

 name is often popularly applied to it. Sometimes it decom- 

 poses into a soft, coaly -looking mass, and being itself in a 

 slight degree combustible, it has been popularly believed to 

 indicate the existence and close proximity of coal ; or, that 

 if these beds of shale were followed by drifting into the 

 valley-sides, they would be found to have changed to coal. 

 It may seem hardly necessary to say that these opinions 

 are fallacious in all respects, and that the substance in 

 question is entirely valueless. It is true that similar car- 

 bonaceous shales exist in connection with coal, but they are 

 just as often found where no coal exists ; not only in the 

 unproductive portions of the coal-measures, but in other 

 formations also. 



All the rocks of this county, as before stated, belong to the 



