1918] Davis: The Badiolarian Cherts of the Franciscan Group 381 



counters the objection that we know of no red mud which shows such 

 a high proportion of siliceous organisms as this would require. This 

 objection has little force, since the presence of radiolaria in a red mud 

 would be very largely a matter of a special combination of circum- 

 stances. Since only a few occurrences of red mud are known at the 

 present time it might well be that none of them was exactly like that 

 required by the theory. 



A very serious objection to this hypothesis is to be found in that 

 absolutely no evidence of accumulations of gelatinous silica has ever 

 been found in oceanic investigations. 



Objection Based on Idea that Silica is Precipitated Organically. — 

 The idea of chemical precipitation of silica contributed by rivers is 

 based, in part, on the work of Murray and Irvine. They concluded 

 that siliceous organisms did not utilize the dissolved silica of the ocean. 



Certain biologists appear to differ with them. For example, John- 

 stone 145 claims that it is not necessary that the organisms shall pass 

 all the water through their bodies in order to utilize the dissolved 

 material. He states that the lower organisms can probably absorb 

 material over their entire surface. By this means they could extract 

 silica from very dilute solutions. 



Johnstone refers to the work of Raben, who made very careful 

 determinations of the silica in Kiel Bay at different periods of the 

 year. Raben showed that twice a year the amount of dissolved silica 

 reached a maximum. Immediately after each of these maxima there 

 was a maximum in the number of diatoms in the bay. This might be 

 taken to indicate that the diatoms were dependent upon the dissolved 

 silica. 



Chemical Objections to the Hypothesis. — Even if one disregards 

 the work of Raben and assumes that the silica from river waters is all 

 eliminated by inorganic precipitation, serious objections may still be 

 raised against the hypothesis above outlined. 



On examining it further, it is seen that there are other possibili- 

 ties for the precipitation of silica beside the flocculation by electro- 

 lytes. In fact, it appears from experiments that the flocculating 

 power of even such a strong electrolyte as sodium chloride is rather 

 low. Strong solutions of sodium chloride will fail to precipitate com- 

 pletely the silicic acid in dilute solutions of sodium silicate. 



Beside the experimental evidence of the inefficiency of sodium 



145 Johnstone, James, Conditions of Life in the Sea, Cambridge University 

 Press, 1908. 



