1918] Davis: The Radiolarian Cherts of the Franciscan Group 375 



cherts by the introduction of silica at some later time. Absolutely no 

 evidence in favor of this view was observed anywhere. By analogy 

 with the Monterey cherts this explanation is not admissible ; many of 

 the arguments which oppose the hypothesis of later silicification of 

 those cherts hold good for the Franciscan cherts. The hypothesis is 

 opposed entirely to certain facts regarding the bedding of the cherts. 



Rhythmic Bedding not known in Present Siliceous Oozes. — One 

 fact which opposes the idea that the cherts are radiolarian oozes or 

 mixed oozes is the regular rhythmic bedding. There is nothing known 

 from the records of investigations of marine deposits which corre- 

 sponds to the peculiar rhythmic bedding of the Monterey and Fran- 

 ciscan cherts. There are a few instances of alternation of diverse 

 types of deep sea sediments, but the radiolarian oozes and diatoma- 

 ceous oozes never show any regular alternation with shaly material 

 in the manner now to be found in the radiolarian cherts. It seems 

 certain that if radiolarian oozes ever showed such a regular alterna- 

 tion it would have been detected in the core samples taken from bottom 

 deposits. 



The minor irregularities of bedding, such as the occurrence of 

 bands of nodules, and short irregular lenses, of the sort previously 

 described and shown in photographs, are incomprehensible on the 

 idea that the cherts are radiolarian oozes. Such deposits of skeletons 

 of pelagic organisms should be in wide spread sheets covering wide 

 areas of the ocean floor. 



There is, however, the possibility of segregation of some sort in 

 the radiolarian ooze on the sea bottom, so that, before the consolida- 

 tion of the material, the radiolarian skeletons were separated in large 

 part from the shaly constituents of the ooze. This segregation might 

 conceivably be in a regular manner and result in the rhythmic bed- 

 ding now seen. While this is a bare possibility, facts are known 

 which render it very improbable. The lower portions of the cores 

 brought up in sounding tubes from the radiolarian oozes, occasionally 

 show a slight amount of lamination, but nothing comparable with 

 the regular alternation of chert and shale is seen. Moreover, the core 

 samples show that the deposit, a few inches below the surface has 

 already progressed far toward a consolidated condition. This fact 

 renders it impossible to believe in such a segregation in radiolarian 

 oozes as has been suggested. 



The Barbados earth also gives information on this question. Here 

 we have a deposit which represents a radiolarian ooze, and is the only 

 undisputed occurrence of such material. The Barbados earth shows 



