CHAPTER II. 



CARBONIFEROUS SYSTEM. 



Of the three groups of formations that constitute the 

 Carboniferous System, namely, the Sub-carboniferous, Coal- 

 Measures, and Permian, only the two first named are found 

 within the limits of Iowa. Both of these are well developed 

 here in all their distinctive characteristics. 



THE SUB-CARBONIFEROUS GROUP. 



Syn —Carboniferous Limestone, Sub-carboniferous Limestone, Mountain 



Limestone. 



Area and General Characters. The area of surface occu- 

 pied by this group is very large, and is shown as accurately 

 as practicable upon the geological map. Considerable diffi- 

 culty exists in determining the boundaries of the area in its 

 northern part, particularly along its western border; because 

 the exposures of strata are so very rare there, and we have 

 been obliged to rely to a great extent upon its general trend 

 for this purpose rather than upon actual observations of 

 outcrops. 



Its eastern border passes from the northeastern part of 

 Winnebago county with considerable directness in a south- 

 easterly direction, to the northern part of Washington county. 

 Here it makes a broad and direct bend nearly eastward, 

 striking the Mississippi river at the city of Muscatine ; this 

 of course being the western and southern boundary of the 



area occupied by the Devonian rocks. The southern and 



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