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IOWA ACADEMY OP SCIENCE 
Furthermore, calcareous nodules without geodic cavities and sometimes 
inclosed with siliceous shells identical with those on perfect geodes are 
sometimes found in the Upper Keokuk. These have exactly the same rela- 
tionship to the containing rock as the geodes and have analagous shapes. 
The most popular conception as to the origin of the geodes is that they were 
formed when the rocks were in a heated condition and that their genesis 
is related to that of the “niggerhead” boulders of the region. Those enter- 
taining this view believe that there were once molten masses containing 
gas in the rocks and that the expansion of this gas, followed by . infiltration 
of mineral matter, gave rise to the geodes. 
Still another idea, and one even more preposterous, is that the geodes are of 
meteoric origin. 
Of the theories which must be considered more seriously, that adopted 
by Dana® in his Manual of Geology is probably the most unique. In referring 
to the geodes from the Keokuk beds, he says: 
“They have been supposed to occupy the centers of sponges that 
were at some time hollowed out by siliceous solutions, like the hollowed 
corals of Florida, and then lined with crystals by deposition from the 
same or some other mineral solution.” 
This theory has had many followers and Wallace® has even gone so far 
as to coin a generic name for the sponge whose solution is supposed to have 
afforded the cavities in which the geodes were developed. To this genus, 
called Biopalla, eight species were referred upon the basis of difference in size, 
shape, and surface markings of the geodes. The sponge hypothesis, however, is 
not now widely held and the investigations of the writer convince him that Wal- 
lace’s conclusions are not in accord with the facts. No evidence of sponges 
capable of giving rise to geodes has ever been found in the Keokuk beds. A 
few depauperate specimens have been collected from the lower limestone member 
of the formation, but these are not of the shape of geodes and have nothing 
in common with them. 
Moreover, the geodes vary widely in size and shape, a fact which argues 
strongly against any theory which pre-supposes an organic origin. Many speci- 
mens are nodular and irregularities of the greatest variety characterize their 
exterior form. It may safely be said that no two of them assume exactly the 
same proportions. Furthermore, evidence of the symmetry so characteristic 
of organisms is noticeably lacking. 
Quite a different theory to account for the Keokuk geodes is that proposed 
by Benge®. His assumption is that, following the deposition of the Coal 
Measure, strata of the region, water surcharged with carbonic acid migrated 
down into the Keokuk beds, and that solution and re-deposition of the lime- 
stone took place. It is his thought that during the process of re-deposiion 
bubbles of carbon dioxide were included, and that after the hardening of the 
limestone the cavities resulting furnished conditions favorable to the forma- 
tion of geodes. Unfortunately this ingenious theory will not account for the 
occurrence of geodes in shale, and it is certain that such a process has not 
operated in the limestone. 
Mooers® has further attempted to outline the origin of geodes, using as a 
basis the occurrence of wads of grass inclosed in clay balls along the beds of 
certain creeks in the region. It was his thought that the decay of this included 
