F. M. Van Tuyl — Geodes of the Keokuk Beds. 35 



The geodes attain their maximum development in the 

 Geode bed but some layers of the Keokuk limestone are geo- 

 diferous locally. 



The Geode bed consists in its typical development of an 

 impure, siliceous, dolomitic limestone at the base, usually con- 

 taining large and well-developed geodes, followed by an argil- 

 laceous shale with more numerous but less perfectly developed 

 geodes. Each subdivision is about twenty feet in thickness. 



The composition of the lower subdivision of the Geode bed 

 where it contains large and well-formed geodes at Keokuk, 

 Iowa, is as follows : 



Insoluble matter (largely free silica) 33 # 



Fe,0, + Al a G 3 2-80 



CaCO, . . 39-99 



MgC0 3 12-50 



Moisture and carbonaceous matter Y - 70 



Undetermined 3-21 



Total 100-00 



In size the geodes range from about -2 cm up to 75 cms in 

 diameter. But well-developed geodes of either extreme are 

 rarely found. In general, the geodes of a given layer do not 

 vary greatly in size at a given locality, but there may be con- 

 siderable variation in this respect at different levels in the 

 same exposure. Moreover, there may be marked changes in 

 their dimensions at the same level at different localities. Often 

 geodes of similar size are arranged roughly in bands parallel 

 to the stratification. They usually lie with their longest diam- 

 eter parallel to the bedding-planes, and at some localities they 

 are closely associated with calcareous concretions of similar 

 shape and size. 



The abundance of the geodes in the geodiferous phase of 

 the Keokuk formation is quite variable both laterally and 

 vertically. At times they are so numerous, in a given layer 

 that their freedom of growth has been interfered with, and 

 they are thus of very irregular shape. At other times, they 

 may be so sparsely distributed through the rock that none may 

 appear in an outcrop embracing several square yards. Again 

 they may be absolutely wanting at some localities. The pro- 

 portion of well-developed geodes in the beds varies greatly at 

 different localities ranging from less than ten per cent at some 

 places to more than ninety per cent at others. 



As to the mineralogical relationship of the geodes to the 

 containing rock, it is found that at any given locality each 

 geodiferous layer as a rule bears geodes which are closely 

 related among themselves, but which may be mineralogically 



