GEOLOGICAL INTRODUCTION 17 



In the Monroe County exposures, the entire thickness of the formation is 

 made up largely of alternating thin bands of chert and limestone, the 

 chert constituting from 50 to 80 per cent of the total mass. In Union 

 County, the formation is more cherty below and contains thick beds of 

 pure limestone above. 



The fauna of the upper beds of the Burlington limestone is remarkably 

 uniform throughout the entire extent of the formation, and consists 

 largely of crinoids and brachiopods. The same species occur from Iowa to 

 southern Illinois, and to central and southwestern Missouri. The faunas 

 of the lower beds are more diverse in character, and in many localities 

 contain species with Kinderhook affinities. 



Keokuk limestone. The Keokuk limestone succeeds the Burlington with 

 perfect conformity in those sections where both formations are present, 

 and in places it is difficult to draw a sharp line of separation. In the 

 neighborhood of Keokuk, Iowa, where the formation is typically devel- 

 oped, it is about 75 feet thick, a measurement which perhaps is not ex- 

 ceeded elsewhere. The limestone beds of the formation are in many 

 places more or less argillaceous and are separated by shaly layers which 

 commonly become more and more conspicuous above, and locally, as in 

 Monroe County, Illinois, constitute the major portion of the entire forma- 

 tion. These limestone beds are prevailingly bluish, while those of the 

 Burlington are nearly white, or towards the base locally brownish, and 

 the chert associated with the Keokuk limestone is darker than that which 

 is commonly present in the Burlington. 



The fauna of the Keokuk, like that of the Burlington, is largely made 

 up of crinoids and brachiopods, but in addition to these there are many 

 bryozoans in the shaly beds, and some horn corals. South of St. Louis, 

 especially in Monroe and Randolph counties, Illinois, a limestone stratum 

 near the very top of the formation is made up of a crowded mass of the 

 shells of the large Productus magnus. A few feet above this Productus 

 layer is another limestone bed just as conspicuously filled with the shells 

 of a species of Spirifer. These two beds, in places quite separate, and 

 elsewhere combined in a single layer, mark the upper limit of the Keokuk 

 formation over a considerable area in southwestern Illinois and the 

 adjacent portion of Missouri. 



Wlarsaw formation. The Warsaw formation is typically developed at 

 Warsaw, Illinois, where it attains a thickness of approximately 40 feet, 1 

 the section at that locality being as follows: 



i Bull. Illinois State Geol. Survey, No. 8, p. 84 (1907). 



