368 University of California Publications in Geology [Vol. ll 



tion of radiolarian ooze would be masked by the accumulation of cal- 

 careous organisms. It is not possible at the present time to get a 

 radiolarian ooze at moderate or shallow depths, and it appears that it 

 was not possible in Franciscan time. 



RADIOLARIAN CHERTS NOT RADIOLARIAN OOZES 



While the fineness of grain, the color, and the characteristic fossils 

 of these rocks are in accord with their interpretation as radiolarian 

 oozes, there are certain facts which are strongly opposed to this idea. 



Presence of Excess Silica in Matrix. — The principal objection to 

 the interpretation of the cherts as radiolarian oozes is based on the 

 study of thin sections. The cherts contain much more silica than can 

 be accounted for by the radiolarian skeletons which are now present in 

 them. 



A similar fact with regard to the cherts in New South Wales has 

 already been mentioned. It was noted there that the radiolarian clay- 

 stones contained as many radiolaria as the flinty cherts which were 

 associated with them. Yet the silica content of the cherts was con- 

 siderably higher than that of the claystones. 



In the microscopic descriptions of many radiolarian cherts from 

 other parts of the world, frequent reference is made to the presence of 

 a siliceous matrix in which the radiolaria are embedded. 



If the original material consisted entirely of the skeletons of 

 radiolaria, it would not, without extensive alteration, have produced 

 such a rock as chert. Even if the skeletons were so numerous that 

 they were closely packed together they could not give enough silica 

 to produce chert. The skeletons are hot solid but consist of a delicate, 

 open lattice work surrounding a central chamber. Such an ooze 

 woidd give a porous, slightly consolidated rock like the radiolarian 

 earths of Barbados, or similar in texture to the diatomaceous earths of 

 the Monterey group in California. It is certain that a radiolarian 

 ooze would not give rise to a chert until there had been a considerable 

 amount of reworking of the silicic acid in the original skeletons. This 

 alteration might have taken place in various ways. It may have 

 occurred: (1) In the ooze on the sea bottom before consolidation; 

 («) by the solvent action of sea water; (b) by the solvent action of 

 organic acids produced in the decomposition of organic matter; (2) 

 after the radiolarian ooze had been consolidated into a rock like the 

 Barbados radiolarian earth. 



