1918 J Davis: The Radiolarian Cherts of the Franciscan Group 239 



PAGE 



Objections to both hypotheses of climatic variation 391 



Bhythm in growth of radiolaria 392 



Rhythm in siliceous springs H 392 



Hypotheses which postulate the cause of the rhythmic bedding in the nature 



of gelatinous silica 393 



Rhythmic alternation due to supersaturation and rhythmic precipitation 



of silica 393 



Possibility that the rhythmic bedding may be due to a segregation in a 



colloidal ooze 394 



Conclusions 402 



INTRODUCTION 



The Franciscan group contains several remarkable lithologic types, 

 of which the most interesting are probably the radiolarian cherts. 

 These rocks are seen throughout the Coast Ranges of California and 

 have aroused the interest and excited the curiosity of geological work- 

 ers from the earliest explorations of the state down to the present 

 time. The opportunities for the study of these rocks in the region 

 around the Bay of San Francisco are excellent. Not only are they 

 exposed in many road cuttings and quarry sections on the San Fran- 

 cisco and Marin Peninsulas, but they may also be examined in numer- 

 ous sea cliffs. The visitor to San Francisco may inspect these rocks 

 with little effort at several places within the city, notably in Golden 

 Gate Park and on the Twin Peaks Boulevard. 



The present paper presents the results of an investigation under- 

 taken primarily for the purpose of determining the origin of the 

 radiolarian cherts. During the progress of the work a study was made 

 of the bedded cherts of the Monterey group, which are, in certain 

 respects, similar to the Franciscan cherts. 



The writer would make grateful acknowledgement of his obliga- 

 tion to Professor Andrew C. Lawson for numerous suggestions and 

 much friendly criticism during the progress of the investigation and 

 the preparation of the manuscript. He is similarly indebted to Pro- 

 fessor George D. Louderback, who originally suggested the problem 

 to him. 



