IV DEPTHS AND DEPOSITS OF THE OCEAN 183 



Free sulphur has been found in a maximum of 0.003 P^^" cent 

 in oceanic deposits,^ although inland and estuarine deposits may 

 contain rather more. We may therefore take it that the 

 aggregate influx of oxidized sulphur into the ocean greatly 

 exceeds the fixation of reduced sulphur at the bottom. 



The elements silicon (as hydrated silica) and phosphorus 

 (as calcium phosphate) are transported by biological agencies 

 from the sea to the bottom, the former in large, the latter in 

 small, quantities. The compounds referred to are capable of 

 existing in solution in sea-water only to an infinitesimal extent, 

 so that all the silicic and phosphoric acids carried into the 

 ocean must eventually find their way to the bottom. 



The silica of organic origin in deep-sea deposits, which of sm 

 course represents but a tiny fraction of the total silica present, is 

 peculiar in having been derived not only from dissolved, but also 

 from suspended, silicates.^ It takes the form of tests and skeletons 

 characterising the important Diatom ooze and Radiolarian 

 ooze areas, and of sponge spicules, which are ubiquitous but 

 nowhere concentrated enough to give rise to a definite deposit. 

 Chemically, this silica is in the hydrated colloidal condition 

 not unlike opal. By what process the siliceous organisms 

 convert their intake of dissolved silica and floating clay into 

 structural silica is not clearly known ; as regards the former, it 

 is evident that the organisms possess some means of coagulating 

 to a hydrogel the silica which they receive either as SiO^'' ions 

 or as a hydrosol of silicic acid ; whilst their argillaceous food is 

 probably decomposed by some acid juice with elimination of 

 alumina in solution and eventual deposition of coagulated silica. 

 During life, siliceous tests are protected from dissolution by 

 an admixture of albuminoid matter, which rots away after death. 

 The hydrogel of silica then undergoes peptisation (that is, so 

 much of it as does not fall to the bottom), probably by virtue of 

 the free alkali in sea-water, and returns to the dissolved state. 

 The conditions of dissolution of silica and, for instance, calcium 

 carbonate are very different. Silica, as being a colloid, has not 

 a definite solubility ; its existence as a hydrosol is limited only 

 by the coagulating action of the electrolyte solutes of sea- water 

 or by its precipitation in combination with a base. As to the 

 former effect, we have no data except that sodium chloride is 

 comparatively feeble as a coagulant. It is remarkable that no 

 silica seems ever to reach the bottom as a chemical precipitate 



^ Buchanan, Proc. Roy. Soc. Ediii., vol. xviii. p. 17, 1891. 

 - Murray and Irvine, Froc. Roy. Soc. Edin., vol. xviii. p. 229, 189 1. 



