140 



University of California. 



[Vol. 2. 



In the vicinity of San Luis Obispo, 1 according to Fairbanks, the 

 Miocene series has a thickness of 6,000 to 8,000 feet, in which occur 

 beds of volcanic ash several hundred feet in thickness. Thinner 

 beds of the same material were found in the lower portion of the 

 Miocene series of Point Sal. 2 It appears, therefore, to have been 

 satisfactorily shown that the material composing these deposits has 

 been derived in part from both these sources. 



One of the difficulties met with in the study of the bituminous 

 shales of this series, has been to account for the absence of siliceous 

 organic structures, and the presence of the silica in an amorphous 

 or pulverent condition. 



Why should there not be a greater abundance of the casts of 

 diatomacese, radiolaria, or sponge spicules, if these have been the 

 chief contributors to the silica of the deposits? 



But whatever its sburce, it is evident, as Professor Lawson has 

 shown, that the silica has undergone a secondary solution and 

 redisposition as a pulverulent or gelatinous silica. This action has 

 been attributed to sea water by both Lawson and Fairbanks. 



Another agent of solution, however, has been shown to be 

 both more efficient for this purpose and more closely connected 

 with the mass of accumulating organic remains even than the sea 

 water itself. That is the humus acids resulting from the slow 

 decomposition of the soft organic tissues. 



In a paper by Alexis A. Julien 3 it is shown that not only is this 

 a possible explanation of the segregation or concentration of silica 

 into the flinty layers and nodules of chalk, but it is even probably 

 the correct interpretation to be attached to many of the masses of 

 bedded cherts and jaspers in both Mesozoic and Paleozoic rocks. 

 In this connection the author says: — 



" The established solubility of silica in solutions of the azohumic 

 acids therefore, suggests that, during the consolidation of the deep- 

 sea sediment, such a solution of disseminated silica by albuminoids 

 and by acids of this character is constantly in progress, etc." 



1 Jour. Geol. 1898, Vol. 6, No. 6, p. 562. 



2 Bull. Dept. Geol. Univ. Cal., Vol. 2, p. 16. 



3 Proc. Am. Ass. Adv. Sci., Vol. 28, pp. 311-410. 



