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University of California Publications in Geology [Vol. ll 



activity of a peculiar type, which occurred during Franciscan time. 

 Part of the silica may be a direct contribution from the cooling 

 magmas. A part of it may be derived from preexisting rocks or pre- 

 viously deposited sediments, by the solution of silica in heated sub- 

 ocean water, set into circulation by the intrusion of molten magmas 

 into the water-saturated materials below the ocean floor. 



The only serious objection which has ever been raised against 

 this hypothesis of siliceous springs is one which was advanced by 

 Rothpletz. He regarded the rhythmic bedding of the cherts with 

 shaly material as due to alternations of deposition, and regarded it as 

 improbable that there should be such alternation in the supply of 

 siliceous springs. 



Several possibilities are considered in an attempt to explain the 

 rhythmic alternation of radiolarian cherts with shales. Of these the 

 most important are : 



1. Regular alternations of climatic conditions, due to seasonal 

 variations or to climatic cycles. 



2. Rhythm in deposition of silica due to periodic supersaturation 

 and resulting precipitation. 



3. Colloidal segregation with a regular separation of silica from 

 shaly material. 



The hypothesis of climatic variation encounters various objec- 

 tions. The most important are : 



a. The distribution of fine terrigenous material does not depend 

 upon the rivers, but upon the ocean currents which distribute the 

 fine sediment along the continental margin. 



b. The hypothesis of climatic variation will not explain the 

 wedge-bedding, and lenslike character of both the cherts and shales. 



The hypothesis of rhythmic precipitation of silica cannot account 

 for the common lenslike character of the shale partings, though it is 

 not inconsistent with the lenslike character of chert beds. 



In the absence of another hypothesis, one is forced to the assump- 

 tion of colloidal segregation of silica from intermixed shaly material. 

 This hypothesis will explain the lenslike nature of both the cherts and 

 shales. The objection that such a segregation would not produce the 

 regular banding is removed if one considers the results of certain 

 experiments on diffusion reactions in colloids. In these experiments 

 a very regular separation of precipitated material is produced. The 

 Liesegang rings show all the minor features of the banding of cherts 



