236 GENERAL GEOLOGY. 



shales ; the former often impure and shaly, and the latter 

 frequently clayey. In addition to these, there is occasionally 

 found in thin isolated layers, or, in somewhat concretionary 

 masses, impure compact bluish limestone. At intervals 

 among the strata are the beds of coal with their underlying 

 beds of so-called fire-clay, the thickness of other strata 

 between the beds of coal being very variable. 



It will thus be seen that all the materials composing the 

 formation are easily disintegrated and destroyed by exposure 

 to the atmosphere and frost, so that the valley-sides of the 

 whole region have gentle slopes and are nearly destitute of 

 those precipitous, rocky bluffs which characterize a limestone 

 region. In a few instances, however, the sandstone of this 

 formation is firm enough to present precipitous fronts in the 

 valley sides as may be seen in the valley of the Iowa near 

 Eldora, and that of the Des Moines near Ottumwa ; at Red- 

 rock in Marion county, etc. 



The fact that the natural exposures of the Lower coal- 

 measures are so few, makes it difficult to obtain a correct 

 knowledge of the order of superposition of the strata. They 

 are few because the edges of the strata, if ever exposed, are 

 now covered by the debris from the easily disintegrated 

 materials which compose them. It is believed, however, 

 that by careful study of the natural and artificial exposures 

 of its strata, we may yet arrive at a correct knowledge of the 

 full details of the formation. 



Economic Value. No formation in the whole State ' 

 possesses anything near the economic value that the Lower 

 coal-measures do, nor is there one which will have so great 

 an influence upon its future prosperity. These remarks, of 

 course, refer to the coal which the formation contains; for 

 although the Middle coal-measures will furnish no inconsider- 

 able quantities of coal, and the Upper coal-measures also 

 small quantities, far the greater part of that indispensable 

 element of material prosperity is contained in the strata of 

 the Lower coal-measures. The Middle coal-measures pro- 

 bably contain as many separate beds of coal as the Lower, 



