1918] Davis: The Badiolarian Cherts of the Franciscan Group 303 



It appears, then, that some of the white shales are really weathered 

 cherts ; but this does not imply that all the white shales are formed 

 in this way. The writer has not had opportunity of investigating this 

 particular question in the field and of reaching any conclusion with 

 regard to the relative amounts of this sort of white shale. While he 

 is convinced that this is the origin of much of the non-diatomaceous 

 white shale, he is also aware that there are great thicknesses of diato- 

 maceous earths which undoubtedly represent former diatomaceous 

 oozes, accumulated on the sea bottom. Much of the white shale which 

 does not now show organic remains may be due to the alteration of 

 these diatomaceous earths ; but this is not established. All the non- 

 diatomaceous white shales may be due to the weathering of chert. 



In one specimen of white shale in the collection of the University 

 of California there are found streaks of black cherty material. These 

 when examined closely may be seen to be due to the silicification of 

 white shale along cracks. A specimen of this kind if seen alone might 

 easily give rise to the opinion that the cherts were due to the intro- 

 duction of silica along cracks. The evidence presented before shows 

 that this cannot be the origin of the cherts. It is believed that 

 instances of this sort are due to the local deposition of silica during 

 the process of leaching silica from a mass of chert in the manner 

 above suggested. 



Comparison op the Bedded Cherts op the Monterey and 

 Franciscan Groups 



While the bedded cherts of the Monterey and the cherts of the 

 Franciscan are similar in many ways, yet they also show a great many 

 points of difference. 



Their most striking common feature is probably their rhythmic 

 bedding — the alternation of chert and shale, repeated hundreds of 

 times. Due probably to the incompetence resulting from the thin 

 bedding of such greatly dissimilar rocks the Monterey cherts are 

 locally contorted like the Franciscan cherts. 



The Monterey cherts are not cut by so many cross veins as are 

 found in the Franciscan cherts ; in fact veining is uncommon in them. 

 In the Franciscan the veins are often filled with white quartz. In 

 the Monterey cherts the vein filling is chalcedonic. 



Witli regard to the rocks themselves, the most noticeable difference 

 is probably in the color. The bedded Monterey cherts are usually 



