IOWA ACADEMY OP SCIENCE 
169 
THE ORIGIN OP THE GEODES OP THE KEOKUK BEDS. 
By Prancis M. Van Tuyl. 
The presence of great numbers of geodes of varying size and degree of de- 
velopment in the Keokuk beds of the Central Mississippi Valley has long 
been known to geologists, but their origin has been, from the earliest times, 
a disputed question, and, although there has been considerable speculte,- 
tion upon the subject, no theory of their formation has been, as yet, widely 
held. 
The geodes attain their greatest development in the Upper Keokuk, that 
division of the Keokuk beds formerly known as the Geode bed or Geode 
shales. But geodes also occur locally in the Lower Keokuk or Keokuk lime- 
stone. 
The rocks in which the geodes appear vary in composition from an argil- 
laceous shale, through carcareous shale, to an impure magnesian limestone. 
Such material constitutes almost entirely the strata of the Upper Keokuk; 
in the Lower Keokuk it appears only in the form of layers interbedded 
with bluish, fossiliferous crystalline limestone. 
In size the geodes range from about 1-16 of an inch up to 30 inches in 
diameter. Well developed geodes, however, of either extreme are very rarely 
found. 
Mineralogically the geodes are almost invariably siliceous, but a few carcar- 
eous geodes and geodic calcareous nodules have been found. The siliceous 
types are characterized, without exception, by an outer shell of chalcedony 
and this is usually followed by crystalline quartz, but calcite sometimes suc- 
ceeds the chalcedony. In some instances, however, the interior is lined with 
botryoidal chalcedony and no crystalline calcite nor quartz appears. These 
chalcedonic types frequently bear small cubes of pyrite and one specimen 
was found which contained sphalerite partly decomposed to smithsonite. 
This geode bore, also, a slight incrustation of gypsum. The quartz geodes are 
often solid, but when they are hollow the quartz crystals of the interior may 
be studded with crystals of calcite, dolomite, ankerite, sphalerite, pyrite, 
or magnetite, and occasionally they are stained with the powder of limonite 
or hematite. Sometimes an incrustation of aragonite or gypsum is found. 
Moreover, hollow siliceous geodes from the vicinity of Niota. 111., are often 
filled with a black bitumen, and those from the more shaly portions of the Upper 
Keokuk, particularly at Keokuk, la., and Warsaw, 111., commonly contain 
kaolin in the form of a fiocculent white powder. 
Calcite geodes and geodic calcareous nodules with, or without, siliceous 
shells are much less common. A few calcite geodes have been found to bear 
small sphenoids of chalcopyrite. The geodic calcareous nodules, however, 
are characterized, in general, by calcite of two periods of growth and fre- 
quently contain crystals of pyrite. 
