1918] Davis: The Radiolarian Cherts of the Franciscan Group 379 



These writers show conclusively that diatoms obtain their silica 

 from clay. Experiments indicate that diatoms are able to live in 

 water free from dissolved silica, which contains suspended clay. 

 When diatoms are placed in water absolutely free from suspended 

 clay particles but containing dissolved silica, they do not appear to 

 be in a healthy condition, but as soon as clay in suspension is intro- 

 duced, the same diatoms become quite vigorous. It would appear 

 probable that radiolaria also obtain their silica from clay particles. 

 Murray and Irvine suggest that sponges derive their silica from the 

 clay in the sea bottom in which they are growing. 



The experimental work of Murray and Irvine would then indicate 

 that organisms do not play an important part in the precipitation of 

 the silica of ocean water. 



The general distribution of organisms in the ocean points to the 

 same thing. In the open oceans there is an abundance of these 

 siliceous organisms, yet here there is very little silica. If organisms 

 precipitated the silica brought down by rivers there should be a 

 swarming of these organisms near the points where the large rivers 

 come into the ocean, instead of in the open ocean where there is so 

 little silica. 



Coagulation of Colloidal Silica. — It is a well known fact that 

 electrolytes tend to cause the coagulation of colloids and their conse- 

 quent precipitation from solution. It is therefore possible that when 

 the river waters containing colloidal silica or colloidal alkaline sili- 

 cates come into the sea water, which is rich in strong electrolytes, that 

 there should be a precipitation of the silicic acid. This would be an 

 important source of silica, provided some mechanism existed capable 

 of concentrating it into definite areas of sedimentation. 



Quantitative Importance of Silica of Rivers. — Quantitatively, the 

 amount of silica, which might be thus precipitated, is important. 

 Clarke, in the Data of Geochemistry, gives the results of calculations 

 on amounts of material brought to the ocean by rivers. He finds that 

 319,170,000 metric tons of silicic acid are brought down into the ocean 

 each year by all the rivers of the world. This amount of silica is com- 

 pletely precipitated, since there is no accumulation of silica in the 

 ocean. 



Considering the amount of calcium which is precipitated annually, 

 the results of the calculations vary somewhat with the period which is 

 assumed for the age of the ocean. If an age of 100,000,000 years be 

 taken, then the amount of calcium which is precipitated annually 



