;'. s /•'. .)/. Van Tuy I— Qeodes qf the Keokuk /ieds. 



niillerite. chalcopyrite or pyrite. Intervening between this 

 calcite and that of younger age, crystals of dolomite orankeritc 

 are also sometimes found. 



Origin of the Oeodes. 



The origin of the geodes of the Keokuk beds has long been 

 a disputed question, and, although there has been considerable 

 speculation npon the subject, no one theory of their develop- 

 ment has, as yet, been widely held. 



The existence of perfectly developed geodes in strata often 

 very impervious to underground circulation furnishes a problem 

 which is exceedingly difficult to solve. The containing rock 

 in the Keokuk region is often highly argillaceous and no 

 structures which might serve as passage ways for mineralizing 

 solutions are to be seen. 



It was formerly believed that the geodes were formed by the 

 deposition of mineral matter on the walls of cavities formed 

 by the solution of sponges imbedded in the rocks. Thus, Dana 

 states:* 



" 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 S. J. 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 differ- 

 ence in size, shape, and surface markings of the geodes. The 

 sponge hypothesis, however, is not now widely held. No 

 evidence of sponges capable of giving rise to geodes have ever 

 been found in the Keokuk beds. Moreover, the geodes vary 

 widely in size and shape, a fact which argues strongly against 

 any theory which presupposes such an origin. Many speci- 

 mens are nodular and irregularities of the greatest variety char- 

 acterize their exterior form. It may safely be said that no 

 two of them assume exactly the same proportions. 



Professor Shaler, in a paper entitled "Formation of Dikes 

 and Veins,":]: also devotes some space to the development of 

 geodes and, although his studies were based upon geodes known 

 to be of fossil origin which occur in the Knobstone shales 

 of Kentucky, his conclusions may well be considered at this 

 point : 



♦Manual of Geology, 4th ed., pp. 97, 98, 1895. 

 fThis Journal (3), vol. xv, p. 366 ff., 1878. 

 {Bull. Geol. Soc. Am., vol. x, p. 253 ff., 1899. 



