1918] Davis: The Radiolarian Cherts of the Franciscan Group 395 



nodules of impure limestone. He concluded, as a result of his inves- 

 tigations, that a calcareous ooze had originally been deposited, and 

 that at a later stage the lime carbonate had aggregated itself into 

 nodules and bands. 



At first view, this idea might appear to be improbable, when ap- 

 plied to the radiolarian cherts, on account of the fact that the contacts 

 of chert and shale are quite sharp and distinct, and a perfect plane 

 of cleavage exists between chert and shale. It might be supposed that 

 such results could not be due to segregation. However, concretions 

 very often show abrupt endings and sharp boundaries against the 

 surrounding material and there is no reason why such sharp distinc- 

 tions should not be found in segregated shale and silica. 



Also there are certain minor features of the bedding which render 

 the notion of segregation very probable. These peculiar minor fea- 

 tures of the bedding of the cherts and shales in both the Franciscan 

 and Monterey cherts have been described in detail in preceding pages 

 and may be summarized : 



1. Lenticular and nodular character of the beds of chert. These 

 might be explained also by rhythmic chemical precipitation of the 

 silica of the cherts and the irregular settling of gelatinous silica. 

 They cannot be explained by the shearing out of chert layers, either 

 before or after solidification, since the shale partings on either side 

 show absolutely no evidence of mechanical disturbance. 



2. Lenticular character of the shale partings. This can be ex- 

 plained by the assumption that the shales are chemical precipitates. 

 The shales of the Franciscan cherts are, in appearance, very unlike 

 ordinary shales and it is conceivable, though unlikely, that they might 

 have been produced in some way by chemical action. However, the 

 shales of the Monterey cherts are shales of the ordinary type. They 

 could not be distinguished in any way from ordinary terrigenous 

 shale. Since the shale of the Monterey chert is clearly a mechanical 

 sediment, the notion of chemical precipitation cannot be used as a 

 means of explaining the lenticular character of the shale partings. 



These two features of the bedding of cherts have been noticed be- 

 fore. Dana speaks of the coalescence and subdivision of beds of jas- 

 per, in his description of Franciscan cherts. Fox and Teall 1 ' 12 noticed 

 these features in the cherts of Mullion Island and considered the pos- 

 sibility of "concretionary action," but abandoned it because of the 

 presence of radiolaria in the cherts. They explained these features as 



i«2 Quar. Jour. Geol. Soc, vol. 49, p. 213, 1893. 



