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University of California Publications in Geology [Vol. 11 



as very locally, through the agency of siliceous waters, the older portions of the 

 formation, and possibly the more disturbed portions, having been most generally 

 subjected to the change. The limestone has in places been altered after a fashion 

 somewhat similar to that of the siliceous shales, being changed to marble, prob- 

 ably as the result of solution and redeposition. 



Again in 1910, Arnold and Johnson 29 discuss the origin of the 

 cherts in the following words : 



The Monterey shale has been subjected to a varying degree of alteration, none 

 of it pronounced enough, excef»t in certain restricted localities, to greatly change 

 the appearance of the formation. Silicification has taken place, however, espe- 

 cially in the lower and middle portion, but the process by which the silica was 

 so intimately introduced into the shales is not fully understood. A suggestion is 

 at hand in sees. 13 and 14, T. 30 S., E. 21 E., where an intense local silicification 

 has taken place along fractures in a much folded and possibly faulted zone of the 

 Monterey. The rock has been so altered as to closely resemble both in hardness 

 and color the grayish-green phases of jasper and chert seen in the pre-Cretaceous 

 Franciscan formation. It is conceivable that solutions percolating along similar, 

 though perhaps smaller, fractures resulting from the folding and faulting to which 

 the Monterey shale has been subjected have produced a less intense but none the 

 less definite silicification in other parts of the series. 



While the idea of the origin of the cherts by the introduction of 

 silica into previously existing rocks has certain points in its favor, 

 it seems impossible to reconcile this hypothesis with many other facts 

 concerning these cherts. 



The rhythmic bedding seen in much of the chert is opposed to the 

 hypothesis. "Whenever the bedded cherts occur, they are character- 

 istically separated by argillaceous partings. The chert layers vary 

 in thickness from one inch to six inches. The shale partings vary 

 from one-eighth inch to two inches. On the other hand most of the 

 diatomaceous earth is rather massive. Throughout considerable thick- 

 nesses, it consists largely of siliceous material, showing little admix- 

 ture of shale. Where bedding is apparent in the diatomaceous earth, 

 the beds are generally much thicker than the beds of chert and shale 

 seen in the cherts. The bedding of the diatomaceous earth is usually 

 due, not to clean cut, regular alternations of chert and shale, but to 

 minor variations in the diatomaceous earth itself. It seems improb- 

 able for this reason, that such thin bedded material as the cherts could 

 be derived from ordinary diatomaceous earth. 



One might imagine the cherts to come from the introduction of 

 silica into ordinary shale. They might be due to the introduction of 



29 Preliminary Report on McKittrick Sunset Oil Eegion, California, U. S. 

 Geol. Surv. Bull. 406, p. 57, 1910. 



