286 C. R. KEYES — CARBONIFEROUS STRATA OF CENTRAL IOWA. 



are arranged in various horizons, interlocking with one another, but sepa- 

 rated by varying thicknesses of shale or sandstone. Thus, at any one point 

 a dozen or more seams may be passed through in sinking a shaft, only two 

 or three perhaps being workable. 



Thickness of the Lower Coal Pleasures. — In connection with this brief 

 review of the leading geological features of central Iowa, as brought out by 

 an examination of some of the natural exposures, allusion should be made 

 to the information pertaining to the Carboniferous rocks below the datum 

 line of the general section. While the notes already taken are quite volumi- 

 nous, they are not at present in a shape suitable for presentation. All 

 attempts to secure reliable accounts of the strata passed through in borings 

 and the sinking of mine shafts have availed but little, since such information 

 is almost invariably withheld by the parties in charge of the operations. 

 For this reason the difficulties of working out the structural details of this 

 part of the Carboniferous group were somewhat greater than they otherwise 

 would have been ; and the final results are thus considerably delayed. 



As already stated, the general dip of the strata along the present line of 

 investigation is southwestward. The mean thickness of the Lower Coal 

 Measures, as shown by careful measurement of the various members, must 

 originally have been considerably more than seven hundred feet. This 

 determination was arrived at in the following way : At the most easterly 

 exposure of the section, the distance from the St. Louis limestone to an easily 

 recognizable bed jiear the top of the bluff was perhaps fifty feet in a direction 

 normal to the dip. This particular layer was then traped to the point where 

 it disappeared below the datum line and the measurement was repeated in the 

 same manner as before. Of course it is not to be supposed that the present 

 thickness of the Lower Coal Measures in central Iowa is nearly so great as 

 the figures above given would suggest ; for in reality the maximum vertical 

 measurement of the beds is probably somewhat less than one-half that esti- 

 mate. Erosion has largely removed the coal-bearing strata of the district, 

 and therefore the original thickness of these rocks is not preserved in any one 

 place. 



Unconformities. — Perhaps the most notable feature in the consideration of 

 the Lower Coal Measures of central Iowa is the existence of several well- 

 marked unconformities. More than twenty years ago White* called atten- 

 tion to the unconformity of the Coal Measures of the state upon the St. Louis 

 limestone, and of the latter upon the other members of the Lower Carbonif- 

 erous. While the correctness of these views is very apparent in the first 

 case, in the latter it is by no means so certain. The present investigation 

 shows that the unconformity of the Lower Coal Measures upon the St. Louis 

 limestone is much more pronounced than was at first suspected, and that in 



* Geology of Iowa, vol. 1, 1870, p. 225. 



