42 F. M. Van Tuyl—Geodes of the Keokuk Reds. 



The process of gcodization evidently consisted of the inward 

 growth of crystals upon the inside walls of cavities left by the 

 solution of the imbedded concretions. The growth was neces- 

 sarily accomplished by deposition from a solution which filled 

 the interior completely. As this solution became depleted in 

 its mineral content, more was introduced by some process of 

 diffusion and a continuous deposition resulted. In some 

 instances a very impervious wall was developed and growth 

 must have been extremely slow. But in the majority of 

 geodes numerous feeding channels in the walls afforded ready 

 passage to the solutions after they penetrated the siliceous 

 shells. 



The mineralogical variation of geodes which may occur in 

 close proximity to each other is difficult to account for. It 

 must either be assumed that the process of geodization was a 

 very local one and that each individual geode possessed only a 

 small sphere of attraction, or that a peculiar localization of con- 

 ditions favored in some instances the deposition of mineral 

 matter more widely diffused through the mineralizing solu- 

 tions. 



University of Illinois, Urbana, 111. 



