1918] Davis: The Badiolarian Cherts of the Franciscan Group 377 



Possibly caution is necessary in the application of this particular 

 criterion to the Franciscan cherts, since it is found that in certain 

 cherts at igneous contacts there has been a crystallization of fibrous 

 chalcedony, giving rise to spherulitic cherts. It seems probable, how- 

 ever, that in these cases the silica was in the amorphous condition at 

 the time of intrusion of the igneous rocks. 



That a large part of the silica of the cherts was originally in the 

 gelatinous form is shown by the gradation in crystallization of the 

 matrix. This was first pointed out by Professor Lawson. The Fran- 

 ciscan cherts show all gradations between rocks whose matrix is 

 now composed of amorphous silica and those whose matrix is now all 

 chalcedonic silica. These represent various stages in the crystalliza- 

 tion of originally amorphous silica. 



Hypotheses op Chemical Precipitation 

 Certain hypotheses have been, or might be, advanced which regard 

 the silica of the cherts as a chemical precipitate, and look upon the 

 radiolarian skeletons as merely incidental to the accumulation of this 

 inorganic silica. 



HYPOTHESIS THAT THE SILICA OF THE CHERTS IS DERIVED PROM 

 RIVER WATERS AND PRECIPITATED BY 

 CHEMICAL PROCESSES 



It may be possible that the silica which comprises the cherts is 

 derived from river water coming clown into the basin of deposition, 

 precipitating its silica upon mixing with the saline waters of the ocean. 

 The hypothesis is somewhat similar to one suggested by R. A. Daly 140 

 for the jaspers of the Lake Superior District : 



Leibig states that the same alkaline carbonate lessens the solubility of 

 colloidal silica in water. It is also a familiar fact that this carbonate pre- 

 cipitates most of the silica from solutions of water glasses, silicate of sodium, 

 silicate of potassium, etc., substances which are present in river waters. The 

 suggestion lies near that we may have here a partial explanation of the puz- 

 zling cherts and jaspers so specially associated with the Lake Superior ores. 

 In this view they are due to the throwing down of silica from river-waters 

 relatively rich in dissolved silica or silicates. 



Lack of Silica in Ocean Water. — The hypothesis of precipitation 

 of silica by ocean water is based upon two facts, the presence of silica 



140 The Limeless Ocean of Pre-Cambrian Time, Am. Jour. Sci., vol. 173, 

 p. 93, 1907. 



