CAKBOISTIFEKOUS SYSTEM. 263 



thickness. This want of uniformity consists first, of what 

 may be termed an undulatory thickening and thinning of the 

 beds; and secondly, of a more general and gradual diminu- 

 tion and increase of thickness, at points more or less distant 

 from each other. In the first place a bed may, if not very 

 thick, so thin out within a few rods, as to be unprofitable for 

 working, or it may unexpectedly increase a foot or two in 

 thickness, forming local basins, or what the miners term 

 " pockets." This is particularly the case with many of the 

 mines east of the Des Moines river. 



In the other case, we find that where there are more than 

 one bed in the series, one of them may be more important at 

 one locality, and another more important at another locality. 

 It is of course not impossible that the thin place of each bed 

 may come at the same locality, and the thinning may possibly 

 even amount to an entire obliteration of the coal-bed. This 

 hazard must of course be encountered in sinking shafts for 

 coal in southwestern Iowa, as well as everywhere else. 



