OAEBOOTFEROUS SYSTEM. 233 



IoAva has been fully and minutely studied. The very nature 

 of the work has made it impossible to do this before other 

 and more general portions of it were accomplished. The 

 Upper and Middle coal-measures have been more fully 

 studied than the Lower, the results of which will be found 

 upon other pages. ' 



It was shown in the preceding chapter, that the area at 

 present fully occupied by the Coal-measure strata is 

 considerably smaller than it was when the deposit was 

 first formed; that from subsequent causes a wide portion of 

 its thin border was removed, leaving only outliers or patches 

 of that portion of the formation, resting within .depressions in 

 the older strata; many of these outliers now being found at 

 considerable distances beyond the unbroken border of the 

 field, with which they were doubtless once continuously 

 connected. For this reason, and also because all the strata 

 are so generally covered by the Drift Deposit, it is difficult 

 to draw a line which shall indicate the exact limit of the area 

 occupied by the coal-bearing strata. However, a line drawn 

 upon the map of Iowa, as follows, will represent the eastern 

 and northern borders of the coal-field of the State with 

 sufficient distinctness for ordinary purposes, but the line, as 

 now understood, may be found more definitely drawn upon 

 the geological map. 



Commencing at the southeast corner of Yan Buren county, 



carry the line to the northeast corner of Jefferson county by a 



slight easterly curve through the western portions of Lee and 



Henry counties. Produce this line until it reaches a point six 



or eight miles northward from the one last named, and then 



carry it northwestward, keeping it at about the same distance 



to the northward of Skunk river and its north branch that 



it had at first, until it reaches the southern boundary of 



Marshall county a little west of its centre. Then carry it to a 



point three or four miles northeast from Eldora, in Hardin 



county; thence westward to a point a little north of Webster 



City, in Hamilton county, and thence still further westward 



to a point a little north of Fort Dodge, in Webster county. 

 30 



