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University of California Publications in Geology [Vol. 11 



It is possible also that there may have been a fine mixture of the 

 type just postulated and that some substance diffused into this mix- 

 ture (possibly ammonium carbonate from decaying organic material) 

 which possessed the property of changing the surface tension of one 

 of the colloids in the mixture. Due to this change in surface tension 

 the colloid may have tended to aggregate rather than to remain in 

 the dispersed condition. In such a segregation there may have been 

 a purification of silica in a rhythmic manner. 



The last cases postulated are not like the conditions produced in 

 the experiments performed by the writer. There the silica was in 

 solution at the time of diffusion of the ammonium carbonate and 

 during the diffusion there resulted the floeculation of silica jelly in 

 such a way as to expel the clay in regular bands — a different state of 

 affairs from one where the silica is already in the gelatinous condition. 



To summarize the above : no other hypothesis explains the peculiar- 

 ities of bedding of the cherts and shales. The hypothesis of colloidal 

 segregation will explain these peculiarities. All the features of bed- 

 ding may be duplicated in the bandings resulting from diffusion reac- 

 tions in colloids. Experiments show that a rhythmic segregation of 

 silica from mechanical impurities can occur. While the exact method 

 by which the segregation was brought about cannot be specified, at 

 the present time, it seems clear that the banding of the cherts and 

 shales did result from some sort of rhythmic segregation of colloidal 

 silica. 



CONCLUSIONS 



In the foregoing pages, various hypotheses regarding the source 

 of the silica of the cherts are discussed. It is shown that there are 

 serious objections to any hypothesis which regards all the silica of the 

 cherts as due to radiolaria or other siliceous organisms, and the prob- 

 ability of an additional inorganic source of silica is shown. The 

 hypothesis that silicic acid would be precipitated from the dilute 

 solutions carried in river waters is shown to be an impossible explana- 

 tion of the source of the additional silica. 



Only two other sources of silica then remain, in the absence of 

 other possibilities. These are the emanations from igneous rocks and 

 the supply of silica from siliceous springs. 



The hypothesis of siliceous springs explains the local occurrence 



