3(! F. Jf. Van Tuyl — Geodes of the Keokuk Beds. 



unlike those from other layers. But sometimes closely placed 

 specimens in the same layer may bear very different minerals. 



The contact relations of the geodes with the containing 

 rock are not such as to indicate appreciable expansion during 

 their formation. At no place is the inclosing rock found to 

 be under any strain, nor is there any evidence of deformation 

 of the layers at the contact. "Rather the layers end abruptly 

 where they abut into the geodes or exhibit a thinning where 

 they pass immediately over or under them. The calcareous 

 concretions which occur in the beds at some localities exhibit a 

 similar relationship. 



The extent of the geodes in the rock back from the outcrop 

 is worthy of consideration. Bassler,* in his discussion of t^ie 

 geodes of the Knobstone shales of Kentucky and Indiana, 

 calls attention to the impervious nature of shale and inclines 

 to the view that the geodes of that formation are confined to 

 the surface or the immediate neighborhood of joint planes or 

 rifts in the strata through which water had easy access. Such 

 a relationship does not seem to hold for the geodes of the 

 Keokuk beds. 



Mineralogy of the Geodes. 



Mineralogically, the geodes are almost invariably siliceous 

 but a few calcareous geodes have been found. The siliceous 

 types are characterized without exception by a thin outer si i ell 

 of chalcedony and this is usually followed inwardly by crystal- 

 line quartz, but calcite may succeed the chalcedony. In some 

 instances, however, the interior is lined with botryoidal chal- 

 cedony and no crystalline quartz nor calcite appears. At other 

 times these minerals may all occur in a single geode, but usually 

 only quartz and calcite or chalcedony and calcite are present. 

 In addition the interior linings of the geodes are frequently 

 studied with dolomite or ankerite, and one or more metallic 

 sulphides are often represented. Moreover, some hollow 

 siliceous geodes contain water, and in the vicinity of Niota, 

 Illinois, many specimens are filled with black viscous bitumen. 

 Finally others contain kaolin in the form of flocculent, white 

 powder. 



The primary minerals found in the geodes are: quartz, chal- 

 cedony, calcite, aragonite, dolomite, ankerite, magnetite, 

 hematite, pyrite, millerite, chalcopyrite, sphalerite, kaolin, and 

 bitumen. The alteration products represented are : limonite, 

 emithsonite, malachite and gypsum. 



With reference to the paragenesis, or order of deposition of 

 the primary minerals, no constant order of succession holds for 



*Proc. U. S. Nat. Mus., vol. xxxv, p. 133 ff. 



