190 GENERAL GEOLOGY. 



western boundary of the area is to a considerable extent 

 the same as that which separates it from the coalfield, but 

 northward from the southern part of Pocahontas county we 

 know little about it with accuracy. From this point it passes 

 southeastward to Fort Dodge, thence to Webster City, thence 

 to a point three or four miles northeast of Eldora, in Hardin 

 county, thence southward to the middle of the north line of 

 Jasper county, thence southeastward to Sigourney, in Keokuk 

 county, thence to the northeast corner of Jefferson county, 

 and thence, by sweeping a few miles eastward, to the southeast 

 corner of Yan Buren county. The area, as thus defined, is 

 nearly two hundred and fifty miles long and from twenty to 

 forty miles wide. 



The general southerly and westerly dip has carried the 

 strata of the group beneath the Lower coal-measure along 

 the line last designated, but after passing beneath the latter 

 strata for a distance of from fifteen to thirty miles, they 

 appear again in the valley of the Des Moines river, where 

 they have been bared by the erosion of that valley. 



The following is a full list* of the formations of the Sub- 

 carboniferous group as developed in the Mississippi valley. 

 They are given in their natural order of superposition, com- 

 mencing with the uppermost. 



1. Chester limestone. 



2. St. Louis limestone. 



3. Keokuk limestone. 



4. Upper and Lower Burlington limestones. 



5. Kinderhook beds. 



Of these formations all but one are found in Iowa. This 

 missing formation is the Chester limestone of the Illinois 

 geologists, so named from the town of Chester, where its 



* The beds referred to a separate epoch, in the former Geological Report under the 

 name Warsaw limestone, are in this report, included among those referred to the 

 epoch of the St. Louis limestone, to which they properly belong, and the use of the 

 name of Warsaw limestone, as indicating a separate formation of the Sub-carbon- 

 iferous group discontinued. The " Geode bed " and " Ferruginous sandstone " are also 

 regarded as only subordinate divisions respectively, of the Keokuk and Cheater 

 limestones. 



