1918] Davis: The Radiolarian Cherts of the Franciscan Group 367 



careous ooze, like the foraminiferal oozes of today, were produced, 

 and suggests that in earlier times the foraminifera were not as 

 abundant in the open ocean as they are today. While at present it 

 would be impossible to get a pure radiolarian ooze in shallow water, 

 even at a distance from land, Hill believes that in pre-Tertiary time 

 foraminifera were not adapted to life in the open ocean, while radio- 

 laria could thrive there. 



If this idea should find acceptance it would be possible to under- 

 stand how the older deposits could contain fossil radiolarian oozes laid 

 down in comparatively shallow water. However, the chalk of England 

 is of Cretaceous age. It would seem also that the immense thicknesses 

 of Paleozoic limestones owed their existence to some organic deposition, 

 probably being largely foraminiferal. Also there is little in the 

 hypothesis to interfere with the development of calcareous oozes in 

 lagoons near shore, and it is hard to see why they would not form in 

 association with radiolarian oozes in such places. 



Possible Change in Rate of Solution of Calcareous Shells. — It 

 might also be possible that present day conditions of solution of cal- 

 careous deposits were not duplicated in Franciscan time. As Cham- 

 berlin 134 has pointed out, the present temperatures, salinities, and gas 

 content of the ocean water are largely due to the recent glacial 

 period. The present conditions are in large part the result of absorp- 

 tion of gases and the circulation caused by the sinking of colder water 

 at the poles. Possibly some change in general conditions of circula- 

 tion in past time would permit the disintegration of calcareous re- 

 mains at shallower depths than is now possible. 



Significance of Franciscan Foraminiferal Limestone. — We have, 

 however, a very effective check on all these ideas in the presence of a 

 foraminiferal limestone laid down in Franciscan time. The Fran- 

 ciscan foraminiferal limestone shows the same relations to the sand- 

 stone as the radiolarian cherts. Its presence indicates that ideas of 

 different habits of foraminifera, or of markedly different conditions 

 of temperature, or chemical composition of the ocean as a whole are 

 not applicable to the case of the Franciscan cherts. By analogy, such 

 ideas are probably inapplicable to similar radiolarian cherts elsewhere. 

 It then becomes exceedingly difficult to see how a true radiolarian 

 ooze could be formed under the moderately shallow water which pre- 

 vailed in the Franciscan basin of deposition. In shallow water, with 

 freedom from mechanical sedimentation, it seems that the accumula- 



134 Jour. Geol., vol. 24, p. 363, 1916. 



