292 University of California Pxiblications in Geology [Vol.11 



Similar beds are intercalated in the organic shale at many points. 

 The diatomaceous shales frequently contain a large admixture of vol- 

 canic material. The tuffs are acidic, and usually rather soft and un- 

 consolidated. 



Other Formations in California Similar to the 

 Monterey Group 



Diatomaceous earth, bituminous shales, and cherts are known in 

 other formations in the California Coast Ranges. 



The Upper Cretaceous Moreno formation contains diatomaceous 

 shales and is regarded by Anderson and Pack 25 as a source of 

 petroleum. 



The Oligocene Krayenhagen shale contains considerable amounts 

 of diatomaceous shale and some chert. 



Similar shales have been described from the upper Miocene Santa 

 Margarita (San Pablo) formation in the Sunset-McKittrick field. 20 



Origin of the Monterey Cherts 



With regard to the origin of the cherts in the Monterey group, 

 there has been some difference of opinion among geologists. 



HYPOTHESIS OF SECONDARY SILICTFICATION 



One idea, which has been accepted by several geologists, is that the 

 cherts are formed by the silicification of diatomaceous shales. This 

 idea seems to have been first advanced by Fairbanks 27 in the San 

 Luis Folio : 



Over large areas it lias undergone silicification, which has so changed its 

 appearance that, were it not for numerous transition phases, the origin of the 

 silicified beds would often be difficult to recognize. The different degrees of 

 change can be traced from the dark bituminous shale through the light porcelain- 

 like varieties to the flinty forms. Some of the flints are opaline, while others have 

 a waxy appearance and still others are jet black. The metamorphism has affected 

 the rock so irregularly that often considerable variation can be seen in the same 

 hand specimen. 



Analyses show that the unaltered shale generally contains 80 to 90 per cent 

 of silica, and the flints as high as 98 per cent. In those areas of shale which 

 have undergone the most metamorphism the bands are generally sharply folded 

 and contorted and are filled with a network of veinlets of chaleedonic quartz. 



25 TJ. S. Geol. Surv. Bull. 603, 1915. 



26 TJ. S. Geol. Surv. Bull. 406, p. 67, 1910. 



27 Op. ext., p. 4. 



