1918] Davis: The Radiolarian Cherts of the Franciscan Group 401 



The segregation occurred independently of whether the impurity 

 in the silica was a colloid or a crystalline substance. The experiments 

 show conclusively that silicic acid possesses the ability to free itself 

 from mechanical impurities and that it can do so in a rhythmic 

 manner. 



While the evidence presented indicates that the bedding of the 

 cherts and shales is due to some sort of segregation in a colloid mass 

 of mixed silica and shaly material, and the experiments cited are proof 

 that such rhythmic segregation can occur, it is not possible at the 

 present writing to state the exact manner in which the banding is 

 brought about. 



We might think of an ooze consisting of two substances, one of 

 which tended to dissolve and reprecipitate about definite centers. 

 Due to this process of recrystallization there might result a segrega- 

 tion into definite bands, as in the concretionary limestones mentioned 

 above. This type of process is uot applicable to the cherts, since the 

 silica there is in the amorphous condition. 



The ultimate physical structure of the colloidal gels is believed 

 to be that of a honeycombed framework through which liquids may 

 diffuse. There is a slight analogy here with crystal structure. Crys- 

 tals in their formation are often able to exclude from their substance 

 certain foreign materials. It is barely possible that colloids in form- 

 ing gels may tend in a similar way to exclude foreign material. 



It is possible that a solution of silicic acid might diffuse through 

 a fine mud on the sea floor and that the silicic acid should be precipi- 

 tated throughout the mud in regular layers, thus producing the 

 rhythmic bedding. 



Perhaps a fine mud was permeated with a solution of silicic acid 

 or some silicate, and by the diffusion of some other substance into it, 

 as in the above experiments, a regular rhythmic banding was brought 

 about. 



We might think, also, of an ooze which consisted of a mixture of 

 fine shaly material with minutely divided particles of gelatinous 

 silica. In such a substance, given sufficient time, there might be a 

 tendency for the minute particles of gelatinous silica to aggregate 

 themselves into larger masses, and during their assemblage it "might 

 be possible that shaly material would be excluded in the form of 

 definite bands. Experiments were tried with a view of testing this 

 idea. Finely divided gelatinous silica was mixed with fine clay and 

 the mixture allowed to stand for several months. In that time, how- 

 ever, no evidence of tendency to segregate was observed. 



