Dean — Formation of Missouri Cherts. 415 



density of the Fe 2 3 xH 2 which replaces the pyrite must 

 be given by 



120 80 -f- 18a; 



and hence d = 3-33 -\- %x 



if x= 1 (gothite) d = 4-08 actual density 4-0 to 4-3 



x = 2 (xanthosiderite) d = 4-83 " " less than 40 



x = y 2 (turgite) d = 3-70 " " 4-2 to 4-6 



From this it is seen that in order to fulfill the conditions 

 the oxide must be very near gothite in composition. 

 Analyses of pseudomorphs show that such is the case : 



(1) Pseudomorph from Germany, analysis from Eammelsberg 

 Fe 2 3 9002%, H 2 10-19% 



(2) Pseudomorph from Saxony, analysis from Kobell 

 Fe 2 3 86-34%, H 2 11-60% 



(3) Pseudomorph from Rolla. Mo., analysis by the Author 

 Fe 2 3 90-5%, H 2 9-5% 



(4) Pseudomorph from Missouri 

 Fe 2 3 90-1%, H 2 9-9% 



In these cases no pressure may have been actually 

 exerted, the latent pressure, so to speak, being sufficient 

 to prevent further hydration of the iron oxide. 



In the case of silica replacing limestone no such calcu- 

 lations can be made since the amount of colloidal silica 

 thrown down by a given volume of limestone varies 

 greatly with the conditions. It is safe to say however 

 that the volume of the silica would be very large in com- 

 parison to the limestone replaced and hence the replace- 

 ment would take place under pressure and the silica 

 would be largely dehydrated. That chert nodules have 

 been formed under pressure is evident from the bowing 

 up of the beds previously spoken of. 



The formation of tripoli has been assumed to be due 

 to the weathering of the chert and to the leaching out of 

 some constituent of the chert. It seems more probable 

 that it represents the deposition of silica in open spaces 

 and not under pressure and hence in a hydrated form 

 which on drying gives tripoli. This is confirmed by the 

 presence of tripoli in the shrinkage cracks in the center of 

 chert nodules where the pressure has been removed by 

 arch action. Colloidal silica may then be thought of as 

 capable of undergoing two modifications, the one to 

 tripoli and the other to chert. 



Missouri School of Mines, Eolla, Mo. 



