1918] Davis: The Radiolarian Cherts of the Franciscan Group 295 



silica, along lines of fracturing in an originally massive argillite, 

 resulting in bands of chert in shale. In such a case, one would not 

 expect the bands to possess the remarkable regularity seen in the 

 bedded cherts, nor expect them to be parallel to the original bedding 

 of the formation, as revealed in intercalated sandstones and limestones. 



The other two possibilities that remain, under the silicification 

 hypothesis, are : 



1. Some substance, easily replaced by silica, was originally inter- 

 bedded with ordinary shale, thin layers of this material alternating 

 in a rhythmic way with thin layers of shale. The only possibility 

 that seems reasonable here, is that this original material, in alterna- 

 tion with shale, was limestone. This possibility is barred at once by 

 the finding of lime carbonate in shells of foraminifera embedded in 

 the chert. It would be impossible for them to remain unchanged 

 while a limestone matrix became silicified. 



2. Thin beds of diatomaceous earth may have been laid down in 

 regular alternation with thin beds of shale. Later these layers of 

 diatomaceous earth may have been compacted into chert by the later 

 introduction of silica that filled the pores ; or the silica of the diato- 

 maceous earth may have been aggregated into compact jaspery layers 

 by the solution and redeposition of the skeletons of diatoms. 



FACTS OPPOSING THE HYPOTHESIS THAT CHEETS A EE 

 ALTEEED DIATOMACEOUS SHALES 



The evidence presented in the following paragraphs seems con- 

 clusive in indicating that the cherts are not due to an alteration of 

 diatomaceous shales. 



1. If the cherts are due to the introduction of silica into beds of 

 diatomaceous earth in alternation with beds of shale, it is hard to 

 understand why the intervening shale beds were not silicified also. 

 Yet they present all the characters of ordinary unaltered shale, and 

 show no sign of silicification. 



2. The cherts may be found, in many cases, resting on, or inter- 

 bedded with, the peculiar soft sandstones of the Monterey group. 

 If there had been an introduction of silica in solution, these porous 

 rocks should certainly have become completely silicified and altered 

 to quartzite. On the contrary, they are usually soft — often so soft 

 that they may be easily broken in the fingers — and are very porous. 

 They are certainly as permeable to solution as the diatomaceous 

 shales. 



