356 University of California Publications in Geology [Vol. ll 



Certain writers, as for example, Molengraaff, Hinde and Hahn, 

 are impressed by the fact that radiolarian oozes are not known to be 

 accumulating in depths of water less than 2000 fathoms at the pres- 

 ent time, and that they occur in the deepest portions of the ocean. 

 They believe that the radiolarian cherts are the equivalent of radio- 

 larian oozes, which accumulated in the abyssal depths of the ocean, 

 and believe that they were later uplifted to form part of the present 

 continents. 



Other writers hold that the radiolarian cherts are fossil radio- 

 larian oozes, but were not deposited at extreme depths. Among 

 these are probably to be included Ransome and Fairbanks, who did 

 not insist upon the deposition of these rocks in excessive depths of 

 water. Similar views are held by many other writers with regard to 

 chert occurrences elsewhere. Dixon and Vaughan adopt an extreme 

 view and regard these cherts as deposits formed in very shallow 

 waters. Their views have been set out in preceding pages. 



HYPOTHESIS OF ABYSSAL ORIGIN 



We may first consider the view that the radiolarian cherts repre- 

 sent abyssal radiolarian oozes. Our knowledge of the present day, 

 deep sea deposits favors this explanation. The only deposits forming 

 at the present day, which contain abundant radiolarian skeletons 

 are the radiolarian oozes. They are unknown in depths of water less 

 than 2000 fathoms, and usually occur at much greater depths. They 

 are known only in the tropics since only in the tropics are the radio- 

 laria numerous enough in the surface waters to give rise to notable 

 deposits on the sea bottom. If it be granted that the cherts are for- 

 mer radiolarian oozes, the distribution of present day deposits of this 

 type would indicate that they were formed under tropical conditions 

 in great depths of water. 



Calcium Carbonate. — In favor of the idea of a deep sea origin for 

 the Franciscan cherts, is the fact that they contain no lime carbon- 

 ate. This is true for many of the occurrences of radiolarian cherts in 

 other parts of the world. It is not, however, a general property of all 

 the occurrences. Many cherts which appear to be identical with the 

 Franciscan cherts contain lime carbonate, and must, therefore, have 

 accumulated at depths such that all lime carbonate was not dissolved 

 from the bottom deposits. The cherts of the Culm measures in Eng- 

 land contain calcareous fossils. In the Alps calcareous layers occur 

 in the radiolarian cherts. Limestones are rather closely associated 



