BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 2, pp. 277-292, PLS. 9, 10 MaRCH 5, 1891 



STRATIGRAPHY OF THE CARBONIFEROUS IN CENTRAL 



IOWA. 



BY CHARLES R. KEYES. 



[Rend before the Society December 31, 1890.) 



CONTENTS. 



Introduction • ^*fJ: 



277 



Description of Sections __ ~" " 2-o 



Lithological Features of the Strata __ 282 



General Characters _• 282 



Argillaceous Materials _ __' 282 



Arenaceous Deposits __ _ 283 



Calcareous Beds __ _ ^g. 



Coal :3:::::::::::::::::::::::::: ^u 



Stratigraphical Relations _ 285 



The General Section _ _ __ 285 



Thickness of the Lower Coal Measures '_ '~~ 286 



Unconformities _ ----- 



Faunal Considerations __ _ oqq 



-T) ^ , '' ZOO 



Kesume 



291 



Introduction. 



The exposed stratified rocks of central Iowa are made up chiefly of Lower 

 Coal Measure clays, shales, aud saudstones. ' In the southeastern portion of 

 the area the upper member (for Iowa) of the Sub-Carboniferous— the St. Louis 

 limestone— is exposed* along the Des Moines river. To the westward the so- 

 called Middle Coal Measures and the Upper Coal Measures are represented. 

 Hitherto it has been supposed that the three recognized divisions of the upper 

 Carboniferous rocks in the state have each a maximum thickness of about 

 two hundred feet. Lately, however, the Upper Coal Measures alone have 

 been discovered to have at least double this estimate; and at a still later 

 date the vertical extent of the other two formations has been found to differ 

 very much from the limit usually assigned : the Middle Coal Measures being 

 considerably thinner than was supposed, and the Lower Coal Measures very 

 much thicker. 



XLtr— Rui,L. GEor-. Soc. Am., Vor,. 2, 1890. (277) 



