CAEBOXTEEEOTTS SYSTEM. 235 



coal-bearing strata exist in that region at all, they are far 

 beneath the surface. 



Proceeding southward on the west side of the Des Moines 

 river, the next exposures of Lower coal-measnre strata, 

 except those along the banks and in the immediate valley 

 of that river, are found in the valle3 r s of the Xorth and South 

 Raccoon rivers in Greene, Guthrie, and Dallas counties. In 

 the two last named comities the Lower coal-measure strata 

 are seen to pass beneath those of the Middle formation. 

 These also are the most westerly points to which the dividing 

 line between those two lower coal formations can be traced, 

 and it is only southerly and southeasterly from here that 

 we find the Middle coal-measures occupying the surface. 

 The line of division between the Lower and Middle coal- 

 measures, so far as it can be traced, may be approximately 

 designated upon the map as follows: 



Commence at a point some five or six miles south of the 

 northeast corner of Guthrie county, and carry the line a little 

 south of eastward to the east boundary o'f Dallas county, 

 thence southeastwardly through Indianola to the southwest 

 corner of Marion county, thence to the northwest corner 

 of Appanoose county, thence along Chariton river to the 

 southern boundary of the State. 



The order of succession of the strata which constitute 

 the Lower coal-measures has not been completely made out 

 for want of time, but some of the more important practical facts 

 will appear in the chapter on the coal counties. The thick- 

 ness of this formation, summing up all the measurements 

 of accessible strata, is estimated at about two hundred feet. 



The lithological characters of the strata of the Lower coal- 

 measures, are greatly in contrast with those of the Sub-car- 

 boniferous formations upon which they rest. It has been 

 shown that the latter consist principally of limestones, and 

 that almost no carbonaceous matter is to be found among 

 them within the limits of Iowa ; but the Lower coal-measures 

 from their very beginning is composed of very different mate- 

 rials. The great bulk of this formation is sandstone and 



