1918] Davis: The Radiolarian Cherts of the Franciscan Group 355 



and shales — wedging out of bands, etc. Certain experiments by 

 Bradford produced bands of nodules and lenses. 



An objection might be raised, however, that all the above diffusion 

 reactions represent reactions occurring in a colloidal medium by the 

 interaction of two dissolved salts. These bands are formed at the 

 time of precipitation of the material. They involve no segregation of 

 already precipitated material, such as the hypothesis above set out 

 would require. 



The writer has shown that if a solution of ammonium carbonate 

 be allowed to diffuse into a suspension of clay, finely divided quartz, 

 or other solid material, in a solution of sodium silicate, that a 

 rhythmic segregation of silicic acid from the solid material will be 

 brought about. These experiments prove that under certain condi- 

 tions gelatinous silicic acid possesses the power to expel mechanical 

 impurities from itself, and show that this segregation may occur in a 

 rhythmic way. 



The acceptance of the hypothesis of colloidal segregation as a 

 means of explaining the rhythmic bedding, removes the objection to 

 the idea that the silica of the cherts is, in large part, derived from 

 siliceous springs. 



Interpretation op Cherts as Volcanic or Metamorphic Rocks 

 Some of the earlier workers in Europe thought that the radio- 

 larian cherts were volcanic rocks. Many others regarded them as due 

 to metamorphism or silicifieation of ordinary shales. These ideas 

 were abandoned about 1880, when radiolaria were discovered by 

 Rothpletz 117 in the kieselschiefer of Saxony, and by Pantelli, 118 in the 

 jaspers of Tuscany. 



In California, certain workers, notably Dana, Blake, Whitney and 

 Becker, regarded the cherts as due to the silicifieation of Cretaceous 

 shales. This interpretation was generally accepted by geologists in 

 California until Ransome and Lawson showed that they contained 

 numerous radiolaria and were identical with radiolarian cherts in 

 England and on the continent of Europe. 



Interpretation of the Cherts as Fossil Radiolarian Oozes 

 There are many investigators who have regarded these rocks as 



the equivalents of the radiolarian oozes. Among those who hold that 



idea are two distinctly separated classes. 



"7 Op. cit. 

 "8 Op. cit. 



