84 F.M. Van Vuyl — Geodes or' the Keokuk Beds. 



Art. V. — The Geodes of the Keokuk Beds; by Francis 

 M. Van Tuyl. 



Introduction. 



Probably nowhere else in America do geodes attain such 

 an exceptional development as in the Keokuk beds of the Cen- 

 tral Mississippi A r alley, and representative specimens of geode6 

 from this region are now found in the mineral cabinets of 

 many of the museums of the world. Apart from Professor 

 Brush's preliminary examination and description of a few 

 select specimens submitted to him in 18G5 by A. IT. Worthen, 

 then director of the Geological Survey of Illinois, no study o,f 

 these remarkable geodes has ever been made in spite of the 

 fact that they bear a variety of metallic sulphides and promise 

 to throw some light upon the origin of more important deposits 

 of these minerals in sedimentary rocks showing no signs of 

 igneous influence. The following brief report on their charac- 

 teristics may therefore seem justified. 



Occurrence. 



The typical geode area is located in Southeastern Iowa and 

 adjacent parts of Northeastern Missouri and Western Illinois. 

 The most famous localities for geodes in this region are Keokuk 

 and Lowell in Iowa ; Way land and St. Francisville in Missouri ; 

 and Warsaw and Niota in Illinois. 



The age and stratigraphic relations of the geode-bearing 

 beds are shown in the accompanying table : 



System 



Name of Formation Thickness 



m feet 



Pennsylvanian 



Des Moines sandstone 



- .disconf ormity 



Pella limestone 



disconf ormity 



St. Louis limestone 



disconf ormity 



Salem limestone 



disconformity 



Warsaw shale and limestone 



0-50 



Mississippian 



- 30 



30 - 60 

 0-35 



40 



Keokuk 



Geode bed 



. 40 



Keokuk limestone 



50 



Burlington limestone 



75 



Kinderhook beds 150 



