226 



GENERAL GEOLOGY. 



part of Washington county in its unmistakable characters, or 

 at most, no farther north than Marshall county. Then we hnd 

 in the northern part of Des Moines county the most northerly 

 recognized limit of the Keokuk limestone. In short, we 

 recognize the same gradual recedence of the borders of the 

 formations of this group, thus far mentioned, from each 

 other, and their strict conformability upon each other that we 

 do in all the other formations previously described. 



The border of the St. Louis limestone, however, does not 

 recede as the others have been seen to do, but on the contrary, 

 this formation extends much farther to the northward than 

 any of the preceding ones of the Sub-carboniferous group do, 

 except the Kinderhook beds, and nearly as far northward as 

 these. Consequently, the St. Louis limestone rests uncon- 

 formably upon all the other formations of the Sub -carbonifer- 

 ous group in Iowa, notwithstanding the fact that at Keokuk, 

 its most southerly point in the State, the Keokuk limestone, 

 through its Geode bed, can be traced, layer by layer, to a 

 strict conformability with and gradation into the St. Louis 

 limestone. 



Fig. 11, illustrates the relations of these formations to each 

 other, as before described, and also their relations to the 

 Lower coal-measures. 



Fig. 11. 



No. 1. Lower Silurian ; 2. Upper Silurian ; 3. Devonian ; 4. Kinderhook beds ; 

 5. Burlington limestone ; 6. Keokuk limestone ; 7. St. Louis limestone ; 9. Lower 

 coal-measures. 



The number 8 is omitted to indicate the absence of the Chester limestone, 

 which would, if present, occupy that relative position. 



This relation of the St. Louis limestone to the older 

 formations of the Sub-carboniferous group it will be seen is 

 true unconformability, and differs only in degree from the 



