OAEBOXIFEKOUS SYSTEM. 265 



southern-central portion of the State, extending from the 

 southern boundary in a northwesterly direction, to the west 

 of and nearly parallel with the Des Moines river, into the 

 southwestern portion of Polk county, thence westward into 

 Guthrie county, embracing a superficial area about fourteen 

 hundred square miles. 



The eastern border of the formation presents an irregular 

 zigzag outline from the Missouri boundary to a point a few 

 miles northwest of the city of Des Moines, and then trends 

 nearly due west, passing through the central portions of 

 Dallas and Guthrie counties. The irregularity in the border 

 outcrops of the formation, may be, in part, attributed to the 

 undulations of its strata, and perhaps to a greater extent, to 

 the erosion it has undergone in the process of the streams 

 deepening their valleys. The line of demarkation between 

 this formation and the succeeding "Upper coal formation has a 

 corresponding irregular outline, which is also traced into 

 Guthrie county, in the western part of which the Middle 

 coal-measures are lost to view beneath the extensive accumu- 

 lations of drift and bluff materials in the great watershed 

 divide in that region. To the north and east the Lower 

 coal-measures define the marginal limits of this formation. 



The counties which are in greater or less part immediately 

 underlaid by the Middle coal-measures include the following: 

 Guthrie, Dallas, Polk, Madison, Warren, Clarke, Lucas, 

 Monroe, "Wayne, and Appanoose, and perhaps a narrow 

 strip along the western border of Marion county. However, 

 in Clarke and Monroe counties, no exposures of unequivocal 

 Middle coal strata have been observed, although the general 

 trend of the border outcrop passes across the northeastern 

 corner of the former and the southwestern portion of the 

 latter county. In the counties of Guthrie, Dallas, Warren, 

 and Lucas the formation is well developed, where it has 

 afforded an unusually interesting field for investigation. 



Surface Configuration. The surface features observed in 



the region of the Middle coal-measures, might not at first 



attract the attention of the casual observer as offering any 

 34 



