IOWA ACADEMY OP SCIENCE 
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grass would furnish hollow balls, which in the presence of water bearing silica 
in solution might develop into geodes. 
Professor Shaler^ has also contributed to the literature on geodes His 
studies were based on geodes, known to be of fossil origin, whicn occur in the 
Knobstone shales of Kentucky. His conclusion is that these geodes are spher- 
oidal veins and that they have been formed by a process of vein building at 
the junction of the plates of criiioids. 
More recently BassleP has published on the formation of geodes. He, too, 
limited his study to the geodes of the Knobstone group of Indiana and Ken- 
tucky. He agrees that these geodes are of fossil origin but his ideas concern- 
ing the details of their method of formation are somewhat at variance with 
those of Shaler. He attributes the same method of origin to the Keokuk geodes, 
although he admits he has been unable to secure a full set of these geodes to 
illustrate this method of formation. I quote from his article: 
“Returning to the suggestion in Dana’s Manual of geology that the 
Keokuk geodes are hollowed out sponges lined with crystals, it seems 
more reasonable, in view of the absence of such sponges in that forma- 
tion and the presence of numerous specimens indicating the origin 
described above, that the latter is nearer the truth.” 
This generalization has no facts to substantiate it. Specimens which are in 
any way related to fossils are extremely rare. Several thousand geodes were 
examined from the Keokuk beds yet only two showed evidence of fossil remains. 
These came from fossiliferous portions of the Upper Keokuk and the relation- 
ship was such as to indicate clearly that the fossil fragments had been merely 
accidently enveloped by the siliceous shell when it was formed. It should also 
be recalled at this point that the typical geodiferous phase of the Keokuk beds 
is non-fossiliferous, and that the most productive layers are utterly devoid of 
life remains. 
The writer’s investigation of the geodes of the Keokuk beds has led him to 
conclude that they are closely related to the calcareous nodules which occur 
in the strata at some localities. Such masses consist at times of almost pure 
calcium carbonate, but frequently a considerable amount of argillaceous material 
is intermixed. These nodules have exactly the same relationship to the con- 
taining rocks as the geodes and possess analagous shapes. They were obviously 
formed on the sea bottom while the beds were being deposited since lines of 
stratification are very rarely found passing through them and no evidence of 
expansion is encountered. 
Between well formed geodes and such nodular masses a fairly complete grad- 
ation has been found. Starting with rounded calcareous nodules a whole series 
of intermediate specimens consisting of solid nodules inclosed with siliceous 
shells identical with those of perfect geodes, and of inclosed nodules bearing 
geodiC cavities in their centers, to ordinary geodes may be found. 
The steps involved in the formation of the geodes, therefore, must have been 
as follows: (1) the growth of the nodules by the segregation of calcareous 
material on the sea bottom; (2) the development of siliceous shells upon these 
nodules; (3) the removal of the enveloped nodules by solution; (4) the deposi- 
tion of mineral matter upon the inner walls of the empty siliceous shells. 
The solution of the nodules evidently took place when the Keokuk beds were 
above ground water level, but the deposition of the minerals which now line 
