378 University of California Publications in Geology [Vol. 11 



in river waters, and its almost complete absence in ocean water. All 

 rivers contain dissolved silica. The absolute amount carried by them 

 depends upon the climate and general nature of the rocks exposed in 

 their hydrographic basin, so that it is variable for different rivers. 

 Important amounts of silica are thus carried to the ocean by rivers, 

 but the waters of the ocean contain only very minute amounts of 

 silica. As shown by Murray and Irvine, 141 the amounts are so small 

 that it is improbable that siliceous organisms are able to obtain their 

 silica from the dissolved silica of the ocean. 



Murray and Irvine showed that in filtered sea water the quantity 

 of silica in solution was extremely small, ranging from one part in 

 220,000 to oue part in 460,000. E. Raben obtained results which 

 were lower than these. He found that the ocean contained somewhat 

 varying amounts of dissolved silica which ranged from 0.2 milligrams 

 to .4 milligrams per liter. Ordinary sea water seldom contains more 

 than one or two parts of dissolved silicic acid per million. It is 

 apparent that the silica which is brought down to the ocean by rivers 

 must be eliminated in some way, since it does not accumulate in the 

 ocean. It may be precipitated by the intervention of organisms, or 

 by some chemical process. The statement is often made that this 

 silica is precipitated through the intervention of siliceous organisms, 

 but there is evidence that the inorganic methods are important. 



Source of Silica of Siliceous Organisms. — Murray and Irvine 

 show that, with the extremely minute quantities of silica present in 

 sea-water, it is improbable that siliceous organisms would be able to 

 pass through their bodies enough sea water to supply them with the 

 amount of silica necessary to build their skeletons. They suggest that 

 these organisms derive their silica by some process of decomposition 

 of clay. While the greater part of the suspended colloidal clay of 

 rivers is precipitated upon coming into the ocean, by the flocculating 

 power of the dissolved salts, there is a considerable amount of clay in 

 suspension even at great distances from the land and in waters of 

 high salinity. Sea water may contain as much as 2000 tons of clay 

 per cubic mile. Sea water contains about 17,000 tons of soluble silica 

 per cubic mile, an amount considerably greater than the suspended 

 clay, but the clay occurs in discrete particles, and it is possible in this 

 way for an organism to obtain, at one time, a comparatively large 

 supply of silica. It would be impossible for an organism to pass 

 enough water through its body to get the same amount of silica. 



1*1 Murray and Irvine, On Silica and the Siliceous Remains of Organisms in 

 Modern Seas, Proc. Eoy. Soc. Edin., vol. 18, p. 229, 1891. 



