414 Dean — Formation of Missouri Cherts. 



the solid rock results from the pores becoming filled with 

 a silica hydrosol containing insufficient carbon dioxide to 

 cause precipitation of the silica on coming in contact with 

 the limestone. Precipitation may then start at a number 

 of centers and nodules be built up around these neuclei. 

 Why precipitation should start at these points cannot be 

 said but neither can we say why crystallization starts at a 

 given number of centers. A number of possible explana- 

 tions can be thought of, e. g., a small piece of pyrite might 

 furnish enough sulphuric acid to start the precipitation 

 at a given point and it is not uncommon to find particles 

 of sulphides in chert nodules. The formation of chert 

 after the exposure of a fresh face which has been noted 

 by Lee 2 may be a similar phenomena; the rocks being- 

 saturated with silica hydrosol, which is actually sta- 

 bilized by the calcium carbonate if no carbon dioxide is 

 present, form chert nodules by taking up carbon dioxide 

 from the air. 



The dehydration of the silica to form chert has been 

 tacitly evaded by all those who have previously consid- 

 ered the chert problem. Pressure has frequently been 

 assigned to this difficult task but without reason or 

 experimental evidence of the efficacy or presence of such 

 pressure. 0. Ruff 3 has shown that if colloidal iron oxide 

 be subjected to high pressure (6000 to 10,000 atmos- 

 pheres) it may be dehydrated to a very considerable 

 extent if not entirely. This dehydration follows from 

 the rule of LeChatelier since the volume of the iron oxide 

 hydrates is less than the volume of the iron oxide and 

 water in the gel. Similarly we would expect colloidal 

 silica to be dehydrated since the volume of chert is less 

 than the volume of the silica and water combined to form 

 a gel. But how are we to obtain these pressures in 

 nature? If we take the case of iron oxide we find that 

 most of the dehydrated forms are results of alteration of 

 sulphides and pseudomorphs of iron oxide after pyrite 

 and marcasite are very common. That these pseudo- 

 morphs actually go down to near molecular dimensions 

 can be seen from the preservation of very fine pyrite 

 striae. If then the replacement is molecule by molecule 

 as we have reason to suppose it is, 120 grams of pyrite or 

 marcasite must give 80 grams of iron oxide and the 



2 Geology of the Eolla Quadrangle, Wallace Lee. Mo. Bureau of Geology 

 and Mines. 



3 Berichte, 34, 3420, 1901. 



