CARBONIFEKOTTS SYSTEM. 215 



Peotozoans. Of these low forms of animal life, a small 

 fossil related to the sponges is found in small numbers in 

 the Keokuk limestone. It is known by the generic name of 

 Sjplienojpoterium, (Meek and Worthen.) 



4. THE ST. LOUIS LIMESTONE. 



8yn —Concretionary Limestone (Owen) ; Warsaw Limestone in Part (Hall). 



Area and General Characters. Although the St. Louis 

 limestone is not the uppermost one of the Sub-carboniferous 

 formations, as the group is recognized in the valley of the 

 Mississippi river, yet it is the uppermost one of the group in 

 Iowa, where the Lower Coal-measures are usually found 

 resting directly upon it, forming, so to speak, a limestone 

 floor for the coal-bearing formations. There are exceptions to 

 this, but they will be more fully explained further on under 

 the head of Unconformability of the Coal-measures upon the 

 Older Rocks, &c. The lithological characters of the St. Louis 

 limestone are very constant wherever they are found in the 

 State ; and being principally limestone, it presents a marked 

 contrast with the coal -bearing strata which rest upon it. The 

 superficial area which this formation actually occupies in 

 Iowa is comparatively small, because it consists of long, 

 narrow strips; but its extent within the State is known to be 

 very great from the fact that it is found at points so distant 

 from each other. Commencing at Keokuk, where it is seen 

 resting upon the geode division of the Keokuk limestone, 

 and proceeding northward, it is found forming a narrow 

 border along the edge of the coal-field in Lee, Des Moines, 

 Henry, Jefferson, Washington, Keokuk, and Mahaska coun- 

 ties. It is then lost sight of beneath the Coal-measure strata 

 and overlying drift until we reach Hamilton county, where it 

 is found in the banks of Boone river with the Coal-measures 

 resting upon it as they do in the counties just named. The 

 exposures here are very limited, and the next seen of the 

 formation is in the banks of the Des Moines river at and near 

 Fort Dodge. These two last named localities are the most 



