Stratigraphtcal Relations. 



The General Section. — By reference to the general section of the Coal 

 Measures in central Iowa as illustrated in plate 9, it will be noticed that the 

 strata have a decided general dip toward the west, and that the inclinations 

 are more marked at the eastern part of the section than at the western. The 

 different inclinations of the various beds appear to be due largely to condi- 

 tions imposed by the original shore contours of the great Carboniferous seas 

 rather than to the subsequent operation of orographic forces. In support of 

 this supposition it is worthy of note that the greatest variation in the inclina- 

 tion of the beds is in the immediate vicinity of unconformities, such as are 

 shown at Elk cliif, Redrock bluff and Bennington. 



The stratigraphical importance of the coal seams is not so great as has 

 been generally supposed, since the bituminous beds are, with very few excep- 

 tions, quite limited. Only a single case is at present known in which the 

 geographic extent of a coal stratum is more than four or five miles, and for 

 the greater part of this distance the coal is but a few inches in thickness. It 

 follows that the coal seams of the region are not nearly so extensive as 

 commonly supposed, and that they possess little value in general correlations. 



There is an opinion prevalent among the miners of tlte district that there 

 are only three workable coal horizons. These are usually designated as the 

 "first," "second," and "third" seams. Should any subordinate seams be 

 encountered in the sinking of a shaft, they are not taken into considera- 

 tion. As a matter of fact, the " three " veins are not continuous over areas 

 of any great extent, and may have widely different stratigraphic values, even 

 within very short distances ; the " first," " second," and " third " seams of 

 one shaft may be entirely distinct from the similarly called seams of another 

 mine scarcely half a mile away. A noteworthy instance for citation in this 

 connection is a boring made near the city of Des Moines. It was two hun- 

 dred feet in depth. Twelve distinct coal horizons were met, giving a total 

 thickness of coal of thirteen and one-half feet ; yet none of the beds were 

 thick enough for profitable working. Only one-third of a mile away was 

 a mine removing coal from two seams, one of which was from four to five 

 feet in thickness. 



The basal coal seams in the Lower Coal Measures of Iowa appear to be 

 much more extensive than those toward the top, where they may be only a 

 few inches in vertical measurement and perhaps a hundred yards in extent — 

 too small for representation in the general section. The coal may, therefore, 

 be regarded as disposed in numerous basins of greater or less area, thickened 

 centrally, but gradually becoming attenuated toward the margins. These 



XLIII— Bull. Geol. Soc. Am., Vol. 2, 1890. (285) 



