40 /•'. .)/. Van Tuyl Geodes of the Keokuk Heels. 



fossil is exempt from replacement by silica when the proper con- 

 ditions obtain." 



Bassler is of the opinion that the Keokuk geodes may have 

 the same mode of origin as those of the Knobstone. But he 

 disagrees with Shaler as to the details of geode development. 

 Thus : 



"Returning to the suggestion in Dana's Manual of Geology 

 that the Keokuk geodes are hollowed out sponges lined with crys- 

 tals it seems more reasonable, in view of the absence of such 

 sponges in that formation and tlie presence of numerous speci- 

 mens indicating the origin described above, that the latter is 

 nearer the truth. Prof. Shaler's idea that this class of yeodes is 

 formed when deeply buried is not in accord with the facts, nor 

 does there appear to be any necessity for the water of formation 

 to be under a considerable though variable pressure. Ordinary 

 surface waters charged with silica seem to be sufficient." 



This generalization in so far as it relates to the geodes of the 

 Keokuk beds in the region studied, would seem to be too broad. 

 Out of several thousand geodes examined from the Keokuk 

 beds only one, which had plainly been formed by the enlarge- 

 ment of a specimen of the crinoid jBaryorinus, showed evi- 

 dence of this method of geodization. 



The origin of the Keokuk geodes in the region studied is 

 believed by the writer to be related to the calcareous concre- 

 tions which originally must have been very abundant in the 

 beds and which are still preserved at some localities. These 

 nodules, being more soluble than the inclosing rocks, have been 

 in large part removed, thus affording cavities in which the 

 geodes could be formed. Where still preserved, the concre- 

 tions have exactly the same relationship to the containing rock 

 as the geodes and possess analogous shapes. They were 

 obviously formed on the sea-bottom while the strata were being 

 deposited, since lines of stratification do not pass through them 

 and no evidence of expansion is encountered about their 

 borders. The process of solution seems to have started in the 

 interior and proceeded outwards. That this was the method 

 of removal is indicated by the occurrence, in the beds, of some 

 geodic nodules whose interiors were only partially hollowed 

 out when deposition began. Carbonic acid and sulphuric acid, 

 of which the latter must have been generated by the decompo- 

 sition of the pyrite so common in the beds, were probably the 

 most active solvents. 



The white powder of kaolin found in some of the geodes is 

 thought to represent, at least in part, a residual product result- 

 ing from the leaching of the original argillaceous content of 

 the nodules. That kaolin can be 60 formed is clearly indicated 



