1918] Davis: The Radiolarian Cherts of the Franciscan Group 353 



of silica locally present in the sea, or simply to the favorable conditions for 

 preservation afforded by this kind of rock. If the springs were strong, the cur- 

 rents engendered might in some places have been sufficient to deflect sediment- 

 laden countereurrents, and this may serve to explain the general absence of clastic 

 materials in the chert. 



It is entirely probable that radiolarian remains will be found in rocks which 

 represent sediments deposited in the same sea as that in which the cherts were 

 locally developed. 



The alternation of beds of chert with partings of shale may perhaps be 

 aseribable to a rhythmical or intermittent action of the springs. But in any 

 theoretical consideration of the cherts, the stratification is their most obscure 

 feature. . . . 



ORIGIN OF THE RADIOLARIAN CHERTS OF THE 

 FRANCISCAN GROUP 



Statement op Thesis 



As a result of a consideration of the various lines of evidence, 

 discussed in the succeeding pages, the following conclusions have 

 been reached : 



The radiolarian cherts and their associated shale partings do not 

 represent abyssal radiolarian ooze and Red Clay, but were deposited 

 in shallow water or in water of moderate depth. 



Four possible sources of the silica of the cherts are considered : 



1. Radiolarian skeletons or siliceous parts of other organisms. 



2. Silica contributed to the ocean by river waters and flocculated 

 by the salts of the ocean. 



3. Silica in emanations from igneous rocks. 



4. Silica from submarine siliceous springs. 



The petrographic study of the cherts together with certain 

 peculiar features of their bedding indicates that they are not pure 

 radiolarian oozes, nor are they siliceous oozes composed in part of 

 radiolaria, and in part of the remains of other siliceous organisms. 

 A portion of their silica is chemically precipitated and the radiolaria 

 are simply incidental fossils, entombed in the precipitated silica. 



It appears that the silica contributed to the ocean by the rivers 

 may be precipitated entirely through the intervention of organisms. 

 If it is chemically precipitated, it probably comes down in the form 

 of magnesium silicate and not as silicic acid. 



The last two hypotheses as to the source of the silica are regarded 

 as interdependent, on account of the usual association of siliceous 

 springs with vulcanism. It is concluded that the silica of the cherts 

 arises from submarine siliceous springs associated with igneous 



