GEOLOGY OF SOUTHWESTERN IOWA. 373 



Geology. The stratified rocks of Cass county, which 

 immediately underlie the surface deposits, are of Upper coal- 

 measure and Cretaceous age. Those of the latter age are the 

 lower strata of Nishnabotany sandstone, and rest uncon- 

 formably upon the Upper coal-measure strata. Their 

 exposures are isolated, comparatively small, and have the 

 general appearance of outliers separated by denudation from 

 the body of the formation as they doubtless are. The most 

 important of these exposures is at the the town of Lewis, in 

 the east valley-side of the Mshnabotany, where it reaches a 

 known thickness of about sixteen feet, but is doubtless much 

 thicker there further beneath the drift. Besides several 

 smaller exposures of it within a few miles of Lewis, it also 

 appears on section 5, township 76, range 35; section 33, town- 

 ship 75, range 36; section 22, township 74, range 35, and on 

 section 3, township 74, range 37. This sandstone, like that of 

 the same formation in Mills and Montgomery counties, is 

 usually too soft for any practical use, but at Lewis it is 

 bedded with considerable regularity, and possesses a good 

 degree of firmness. At the last named locality it is darker 

 than usual, being mostly of a dark brown color. At Lewis, 

 and also at other localities, the Mshnabotany sandstone 

 contains occasional isolated lumps of clayey shale imbedded 

 in the stone. They were evidently rounded by attrition in 

 water, and imbedded in the mass, while the whole was in the 

 form of moving sand. It is suspected that these clayey 

 lumps were detached from coal-measure strata, but no 

 fossils were found in them which might have proved it. 



Fig. 39. The most important exposures 



fojk^Ez^^^Sz of the Upper coal-measure lime- 



<Mgf=^^=L' stone are also found in the 



x r^ — .-^r— ^ \ /■ 6- immediate vicinity of Lewis. The 



accompanying section, illustrated 



by Fig. 39, was measured just 



r below the mill at that place, in 



the right bank of the Mshnabo- 



^^ri^zzTrr^rE^^^} 2 tany. 



