1918] Davis: The Radiolarian Cherts of the Franciscan Group 393 



Hypotheses which Postulate the Cause op the Rhythmic 

 Bedding in the Nature op Gelatinous Silica 



The Monterey and Franciscan cherts both show regular alterna- 

 tions of chert and shale and yet there are many differences between 

 them. In spite of the differences in the characteristic fossils, and the 

 marked differences in the nature of the shaly partings, the chert beds 

 in both formations are about the same thickness and show almost 

 identical peculiarities of bedding. This is strongly suggestive of the 

 idea that the rhythmic alternation is due to some property of gelatin- 

 ous silica, since this seems to have been about the only common factor 

 in the two formations. 



A further fact is that this similarity in thickness of the chert beds 

 in the Franciscan and in the Monterey does not appear to depend 

 upon the amount of shale. In general the beds of chert are of about 

 the same thickness where the shale partings are thick, as where they 

 are thin. 



EHYTHMIC ALTERNATION DUE TO SUPEESATURATION AND 

 RHYTHMIC PRECIPITATION OF SILICA 



It may be possible to explain the rhythmic bedding of chert and 

 shale by means of a rhythmic precipitation of silica. One might 

 regard the silica as due to siliceous springs, as emanations from igne- 

 ous rocks, or as contributed by rivers coming into a basin of deposition. 



Silicic acid tends to form supersaturated solutions, and silica 

 might continuously increase in concentration in the water of the basin 

 of deposition, until it reached a state of supersaturation when pre- 

 cipitation would occur. When precipitation began, it would continue 

 until the concentration of silica reached that necessary for saturation. 

 After the precipitation took place the accumulation of silica in solu- 

 tion would begin again and the concentration would gradually in- 

 crease during a certain period of time when another precipitation 

 would occur. If during this process, fine shaly material was slowly 

 and continuously coming into the basin of deposition, the final result 

 would be alternations of shale with chert. If the conditions of super- 

 saturation and precipitation were uniform, there would be deposits of 

 chert of uniform thickness. 



On this hypothesis, we could account for the abundance of radio- 

 laria in the chert and their small number in the shale. At the time 

 of precipitation of the silica the water would be cleared of most of the 



