1918] Davis: The Radiolarian Cherts of the Franciscan Group 373 



were at first called diatomaceous oozes. Diatoms possess a finer and 

 much more delicate structure than do the radiolaria. In solution or 

 metamorphism these more delicate structures would tend to alter first 

 so that it is possible that they might entirely disappear from a rock in 

 which the radiolarians were still fairly well preserved. It might be 

 possible to regard the cherts as radiolarian oozes and explain all the 

 peculiarities of the preservation of radiolarian skeletons and their 

 relation to the siliceous matrix of the rock by this means. 



This seems to have been first put forward as a suggestion by 

 Hinde, 137 who wrote : 



No other microscopic organisms besides radiolaria can be seen in the 

 Angel Island or in the Buri-buri beds. It is quite possible that diatoms may 

 have been intermingled with radiolaria in these deposits, but the fossilization, 

 which has been sufficient to obliterate most of the radiolarian structure, would 

 completely destroy all traces of the smaller and more delicate diatoms. 



Crandall 138 later made a more definite statement regarding this 

 possibility : 



There is another possible origin for the large beds of jaspers in the Fran- 

 ciscan series and this is suggested by similar beds in later deposits. The 

 siliceous Miocene shales resemble very closely the jasper beds under discus- 

 sion except that they do not show so much folding and distortion. These Mio- 

 cene shales are diatomaceous, deep sea deposits, altered since the time of their 

 deposition. It is reasonable to assume that the older jasper beds may have 

 been formed similarly to those of the Miocene age whose origin is known. 



The fact that remains of diatoms do not show in slides of the jaspers does 

 not prove that they were not there originally. These jaspers are mostly amor- 

 phous silica and must have passed through a stage of solution and re-disposition 

 from their original form so that all traces of diatoms might easily be lost. 



This hypothesis is worthy of consideration because of the large deposits of 

 siliceous shale that we have in California; and it is more reasonable to con- 

 sider the older jaspers formed after the same method of later shales, than to 

 assume a different set of conditions and different organisms to have existed 

 without any definite proof. The presence of radiolaria does not affect this 

 either way, because small amounts of any organism might be present with the 

 diatoms and have their skeletons preserved. 



An objection to this last hypothesis, though it certainly is not a 

 very serious one, is found in the fact that in none of the slides of 

 Franciscan chert has anything resembling a diatom been found. It 

 might be expected that occasionally some diatom would escape com- 

 plete obliteration. 



is? Univ. of Calif. Publ., Bull. Dept. Geol., vol. 1, p. 238, 1894. 

 iss The Geology of the San Francisco Peninsula, Proc. Am. Philos. Soc, 

 vol. 46, p. 46, 1907. 



