F. M. Van Tuyl—Geodes of the Keokuk Beds. 41 



by the presence of this mineral so related to impurities in some 

 of the nodules that its derivation cannot be questioned. The 

 more common occurrence of kaolin in the geodes from the 

 more argillaceous portion of the beds is significant in this con- 

 nection. Moreover, the great majority of the geodes which 

 contain kaolin are imperfectly developed and the calcite of 

 such specimens invariably includes the white powder of this 

 mineral. These facts strongly support the idea that the kao- 

 lin must be a residual product. 



Concerning the time of formation of the geodes, little is 

 definitely known. The removal of the calcareous nodules 

 which, it is assumed, preceded the geodes, implies an interval 

 of solvent action during which the Keokuk beds were above 

 ground-water level. Such a condition must have obtained dur- 

 ing the period of denudation which succeeded the deposition 

 of the St. Louis limestone. Some solvent action must also 

 have been inaugurated during the pre-Salem and post-Salem 

 emergences but these were of limited duration. The growth 

 of geodes, on the other hand, undoubtedly took place below 

 ground-water level. 



In the development of the geodes at least two periods of 

 mineralization are involved. The first period of development 

 was by far the most important. During this period of growth 

 the quartz, chalcedony dolomite, and a considerable amount of 

 the calcite together with almost all of the metallic sulphides 

 were deposited. This period of mineralization possibly took 

 place during the interval which just preceded the Pennsylvanian 

 inundation. The region was certainly near base level at this 

 time and the Keokuk beds must have been below ground-water 

 level. The occurrence of geodes, supposedly derived from the 

 Keokuk beds, in the basal Pennsylvanian conglomerate in 

 Indiana, where similar conditions probably prevailed, supports 

 this view. 



Of the minerals of the second period of growth, transparent 

 crystals of calcite and slender, untarnished flakes of pyrite are 

 by far the most important. The minerals of this class are 

 doubtless much younger than those of the former as suggested 

 by the fact that in the same geode the pyrite associated with 

 the newer calcite is often perfectly fresh while the earlier 

 pyrite is badly decomposed. 



The secondary minerals of the geodes such as limonite, gyp- 

 sum, smithsonite, and malachite are for the most part of much 

 more recent origin. They have resulted from the alteration of 

 the primary sulphides as shown by their association with the 

 partially decomposed members of this group. 



The bitumen which occurs in some of the geodes must have 

 been introduced sometime after their formation, since it has 

 not interfered with the normal geode development. 



