1918] Davis: The Radiolarian Cherts of the Franciscan Group 293 



The change which the shale has undergone is not so much the introduction of new 

 silica as the transformation of that which it already contained. The non-polar- 

 izing amorphous silica of the unaltered shale has given place to the polarizing 

 chalcedonic variety. 



Fairbanks believed that this silicification occurred in large part 

 before the deposition of the Upper Miocene San Pablo formation, 

 since the base of the latter formation contains boulders of Monterey- 

 chert. 



Arnold and Anderson, 28 in describing the Santa Maria Oil Dis- 

 trict, discuss the origin of the cherts in the following words : 



The soft shale has been described in the preceding pages as ' ' unaltered, ' ' and 

 in referring to the harder varieties different degrees of ' ' alteration ' ' have been 

 mentioned, for the reason that the best explanation of the origin of the harder 

 rocks appears to be that they are products of metamorphism of the soft variety. 

 It is believed that the soft white and chocolate-colored organic shale represents 

 the original state of the beds of the whole formation, and that a process of silicifi- 

 cation and crystallization has caused the changes, this process having been aided 

 possibly by structural disturbances and pressure. The beds of soft shale are 

 usually found in attitudes only gently disturbed, whereas the harder shale is most 

 commonly much folded and is invariably the component rock of folds where the 

 forces have been especially intense. This fact may throw light on the problem of 

 the alteration of the shale, and yet it may be simply the outcome of the removal 

 of the softer portion of the formation in the regions of greatest uplift and dis- 

 turbance. The chief agent in causing the change was probably infiltrating water 

 carrying silica in solution. In some places the process may have been simply 

 or largely infiltration in the extremely porous original shale and deposition of 

 silica in the interspaces, thus giving rise to hardened and compacted irregular 

 granular aggregates of the original amorphous silica and the new crystalline silica 

 combined, the result being an increase in the total percentage of silica. In more 

 extreme cases the original material was probably partly taken in solution and 

 redeposited, being replaced almost entirely along bands or in spots, and the 

 change being carried to a less extent along other layers and in other areas, or else 

 the replacement was almost complete throughout. As the rock was rendered more 

 compact in this process a shrinkage may have been the result, or the same volume 

 may have been retained and the pores filled. That solution took place along with 

 the deposition seems to be shown by the almost complete destruction of the forms 

 of organisms. 



It is possible that the differences in the shales may be original, the result of 

 variation in the material deposited. Whole series of beds of different material 

 might have been deposited, giving rise to harder, more siliceous rocks than the 

 softer varieties, and the same material might have been locally deposited in thin 

 beds or in lenses and nodules, or have been intermingled with the others to form 

 the intermediate varieties. But it would be difficult to say what this material 

 might have been, and the more or less completely crystalline character of the 

 harder shales shows that metamorphism has taken place. The most plausible 

 theory, therefore, is that the Monterey shale as originally laid down was fairly 

 constant in character and that it has undergone alteration extensively, as well 



2» U. S. Geol. Surv. Bull. 322, p. 46, 1907. 



