CAEBONIFEEOTJS SYSTEM. 255 



formation, instead of being diminished in aggregate thickness 

 further southwestward, by the absence there of a few of the 

 upper strata found near Winterset, is doubtless thicker in the 

 southwestern part of the State than it is near that last named 

 town. Besides the exposures near the town where this 

 section was measured, Madison county also contains many 

 other fine exposures of Upper coal-measure rocks, principally 

 along the valley of Middle river, but so rapidly do the six or 

 seven upper members represented in that section disappear, 

 (by thinning out, or by having been eroded), in all directions 

 from the centre of the county that they have not been recog- 

 nized outside its boundaries, except in a limited region a few 

 miles from its southeastern corner. This disappearance of 

 the upper members of the section is of course most rapid to 

 the eastward and northward, because the drainage, as well 

 as the extreme borders of the formation are in those direc- 

 tions, the dip of the strata being in opposite directions. 



Going westward from Madison county, the first exposures 

 to be seen are west of the Great Watershed, in the valley of 

 the East Mshnabotany, near Lewis, in Cass county, the deep 

 drift deposit of the intervening space having covered all 

 strata from view. The last named exposures are referred to 

 about the horizon of Nos. 6 to 9, inclusive, of the Winterset 

 section. So deep are the Bluff and Drift Deposits in south- 

 western Iowa, that the only other exposures found in a 

 westerly direction from Madison county are: 1, in the valley 

 of the West Mshnabotany, in the southern part of Pottawat- 

 tamie county; 2, three miles east of Council Bluffs, in the 

 valley of Mosquito creek: 3, six miles north of that city, at 

 the base of the Missouri river bluffs; 4, in the valley of Boyer 

 river, a few miles from Magnolia, in Harrison county. The 

 horizon of all these exposures is not far from the same as 

 that to which those near Lewis, in Cass county, belong. The 

 third and fourth, however, are probably a little higher in the 

 series, as would also be suggested by the slight west-north- 

 westerly dip which the strata there are known to have. 



To the southward and southwestward, the exposures are 



