107 



upon the saturation of phosphate in sea water, which in turn 

 is a function of the pH. Apparently slight changes in the 

 physical -chemical or biological conditions near the bottom 

 may cause a saturation of phosphate causing precipitation. 

 The formation of phosphorite is apparently a slow process ajid 

 consequently its presence indicates slow or negligible detrital 

 deposition in the region where it occurs. Since some of the 

 nodules in the bay area have grown to considerable size, they 

 must represent a reasonably long period of nondeposition. The 

 phosphorite generally occurs on the surface, not buried so 

 that its occurrence here is indicative of either nondeposition 

 or very slow sedimentation 



Most of the nodules collected by Dietz, Emery, and 

 Shepard contained a predominance of Foraminifera of Miocene 

 age, and at one station a mixed fauna of Miocene and Pliocene 

 ages. In several other samples, separate nodules and phos- 

 phatized cement of conglomerates contained Foraminifera of 

 Quaternary age. These authors concluded that while a small 

 proportion of the phosphorite must have a Quaternary age, the 

 bulk of the phosphorite was formed dviring the Miocene period. 



There are at least two ways in which the phosphorite 

 could have formed during Miocene time and still be abundant 

 on the sea floor: (1) the phosphorite formed during the 

 Miocene on submarine highs which have been neither eroded nor 

 covered by later deposition, (2) the nodules on the present 

 sea floor may have undergone residual concentration from 

 Miocene rocks. Dietz, Emery, and Shepard (1942, p. 841) point 

 out some of the difficulties of both hypotheses. In the first 



