386 University of California Publications in Geology [Vol. 11 



later stages of the crystallization of these rocks, vapors or solutions 

 were given off which contained considerable amounts of soda. 



Dewey and Flett point to the frequent occurrence of radiolarian 

 cherts with pillow lavas. They call attention to the fact that thin 

 beds of radiolarian cherts 'are interbanded with some of the lava 

 flows. Also, in certain localities, 150 radiolarian cherts occur in masses 

 among the pillows, filling the spaces between them. 151 Assuming the 

 submarine extrusion of such basaltic lavas, they suggest that when the 

 igneous rocks of this group cooled they exhaled vapors or solutions 

 rich in sodium silicate. In the sea water thus enriched in silica, radio- 

 laria would multiply and give rise to deposits of radiolarian skeletons. 



In this hypothesis the organic precipitation of silica is stressed, 

 while in the hypothesis of Van Hise and Leith the emphasis is placed 

 on the inorganic precipitation. The evidence presented above indi- 

 cates that the silica of the Franciscan cherts is in part organic and in 

 part, inorganic. 



ORIGIN OF THE RHYTHMIC BEDDING OF THE 

 RADIOLARIAN CHERTS 



One of the most characteristic features of the radiolarian cherts 

 of the Franciscan group, as well as of the cherts of the Monterey 

 group, is the rhythmic alternation of cherts and shales. Since this 

 peculiar feature is undoubtedly connected in some way with their 

 origin, a discussion of the origin of radiolarian chert must include 

 a consideration of the possible causes of the alternation of such differ- 

 ent lithologic types. 



Variation in Ocean Currents 



The first explanation which comes to mind, perhaps, is that the 

 alternation of chert and shale was due to some variation in ocean 

 currents. It is not possible to discuss this question fully because of 

 insufficient knowledge regarding the causes of variation in such cur- 

 rents, or of the manner in which they change. It seems extremely 

 improbable, however, that such currents could change rapidly enough 

 or in such a regular alternation as to produce the bedding. 



150 E.g., Scotland. 



151 Jaspers which were probably once radiolarian cherts occur in Anglesey. 

 Thev are often found here filling spaces between pillows in pillow basalt. Greenley, 

 Quar. Jour. Geol. Soc, vol. 58, p. 425, 1902. 



