University of California Publications in Geology [Vol. 11 



PAGE 



Possible change in rate of solution of calcareous shells 367 



Significance of Franciscan foraminiferal limestone 367 



Badiolarian cherts not radiolarian oozes 368 



Presence of excess silica in matrix 368 



Possible changes in ooze 369 



Evidence against ideas of change on sea floor 370 



Possibility of the alteration of a radiolarian earth to a chert 370 



Evidence opposed to the idea that matrix was derived from radiolaria .. 371 



Possibility that matrix may be derived from other organisms 372 



Other sources of silica of matrix 374 



Possible differences in radiolaria 374 



Hypothesis of later silicification _ 374 



Eythmic bedding not known in Present siliceous oozes 375 



Occurrence of chert in lenslike bodies 376 



Conclusion „ 376 



Evidence that a portion of the silica of the radiolarian cherts was chem- 

 ically precipitated 376 



Hypotheses of chemical precipitation 377 



Hypothesis that the silica of the cherts is derived from river waters and 



precipitated by chemical processes 377 



Lack of silica in ocean water 377 



Source of silica of siliceous organisms 378 



Coagulation of collodial silica 379 



Quantitative importance of silica of rivers 379 



Separation of silica from coarse mechanical sediment 380 



Objection that no sediments of this type are known 380 



Objection based on idea that silica is precipitated organically 381 



Chemical objections to the hypothesis .... — 381 



Occurrence of chert in lenslike bodies 383 



Hypothesis that the silica of the cherts is derived from submarine 



siliceous springs 383 



Hypothesis that the silica of the cherts is of magmatie origin and chem- 

 ically precipitated 384 



Hypothesis that the silica of the cherts is of magmatie origin precipitated 



by radiolaria - 384 



•igin of the rhythmic bedding of the radiolarian cherts - 386 



Variation in ocean currents 386 



Variation due to regular seasonal changes 387 



Other examples of rhythmic alternation 387 



Application of hypothesis to radiolarian cherts 388 



Hypothesis that cherts are radiolarian oozes 388 



Hypothesis of chemical precipitation 388 



Ehytlunie bedding due to cyclic changes of climate 390 



Statement of hypothesis 390 



Evidence in favor of idea of climatic cycles 390 



Origin of minute laminations of cherts 391 



