1918] Davis: The Radiolarian Cherts of the Franciscan Group 397 



common in other sections of Monterey cherts and they are also fre- 

 quent in the Franciscan cherts. They are altogether too numerous to 

 be explainable as due to the gradual feathering out of layers of 

 sediment. Another fact which is opposed to this interpretation is the 

 abrupt termination of some of the shale beds. Shale beds, deposited 

 by gentle currents, should taper down gradually. Many of the ter- 

 minations shown in the photographs are very abrupt. 



It seems certain that the idea of mechanical production of these 

 lens shaped terminations is unfounded. It may be possible to explain 

 the lenticular character of the cherts by supposing them to be chem- 

 ical precipitates, but the explanation will not hold for the shales. 

 The only other possibility is to believe that the shaly material and 

 siliceous material were deposited together and later underwent a 

 segregation. 



In volume 55 of the Quarterly Journal of the Geological Society 

 of London, David and Pitman present a section showing details of 

 interbedding of radiolarian cherts and submarine tuffs. These show 

 nodules, stubby lenses, and branching sheets of radiolarian cherts 

 with similar features in the interstratified tuffs. The relations are 

 almost identical with those observed in Franciscan cherts and shales. 

 In the New South "Wales occurrence, the details of the boundaries 

 are extremely irregular due to minute indentations. In the Fran- 

 ciscan the boundaries of lenses and nodules are smoothly rounded. 



Other remarkable features of the bedding are what might be ex- 

 pected on the idea of segregation in colloidal mixtures. These are : 



1. Pinching and swelling of beds of chert and shale, not due to 

 thickening and thinning produced by folding. 



2. Bounded knobs in chert without any lithologic difference be- 

 tween the knob and the rest of the bed. 



3. Manner in which the irregularities on one bed are compensated 

 by corresponding variations in surrounding beds. 



4. Convergence of lamination shown in many cherts of the Mon- 

 terey group. 



All of these last mentioned facts would be difficult to explain on 

 other hypotheses, but they fit in very well with the idea that the 

 rhythmic alternation is due to some sort of colloidal segregation. 



It appears therefore that no hypothesis yet presented will explain 

 all the facts observed with regard to the rhythmic bedding, save the 

 one of colloidal segregation. This hypothesis is in agreement with all 

 the observations on the features of this rhythmic bedding. 



