1918] Davis: The Radiolarian Cherts of the Franciscan Group 369 



Possible Changes in Ooze. — The solvent effect of sea water under 

 pressure was suggested by Professor Lawson as a possible mode of 

 change. In the deep sea the pressure may so increase the solvent 

 power of water as to dissolve radio] arian skeletons and reprecipitate 

 the silica in the gelatinous form. This idea meets with the objection 

 that the Franciscan cherts by their intimate association with the 

 Franciscan sandstone, must have been deposited in shallow water. 

 Similar objections would meet the application of this idea to other 

 occurrences of radiolarian chert. 



One might imagine, also, that a change occurs in the siliceous oozes 

 on the sea bottom, analogous to that observed in certain foraminiferal 

 oozes. In the dredging and sampling of bottom deposits it is found 

 that the upper layers of foraminiferal oozes are made up of unbroken 

 shells of foraminifera, but in the deeper layers there remain only the 

 coarser and more resistant fossils in a fine paste composed of broken 

 and disintegrated shells. In this way there finally results a rock 

 which is all organic, but in which there are preserved only a few 

 fossil forms. 



However, it does not seem permissible to argue from analogy with 

 limestone. In the limestone the fossils preserved are those of the 

 coarser and heavier shells, which are able to resist solution. In the 

 radiolarian oozes there are not such differences in the organisms. 

 They are all much alike and there is no apparent reason why one skele- 

 ton should be preserved in preference to another. Yet the chert con- 

 tains excellently preserved skeletons embedded in a matrix which 

 shows no sign of organic derivation. 



Another idea is due to Julien, 133 who believes that organic matter, 

 in its decomposition, gives rise to complex organic compounds which 

 possess the property of dissolving the skeletons of siliceous organisms. 

 These substances combine with the silica, in the process of decomposi- 

 tion, forming compounds of silica with organic acids. On oxidation, 

 silica is liberated from these compounds in the gelatinous condition. 

 This hypothesis is opposed by the red color of the cherts and shales, 

 which indicates oxiding conditions during their deposition, and the 

 absence of any large quantity of decaying organic matter. 



The fact that the white diatomaceous earths of the Monterey group 

 are almost unconsolidated and show no evidence of solution would 

 indicate that such action of organic material was not very powerful. 



135 Julien, A. A., On the Geological Action of the Humus Acids, Proe. Am. 

 Assoc. Adv. Sci., vol. 28, p. 311, 1879. 



