i84 DEPTHS OF THE OCEAN 



of calcium or magnesium silicate, although magnesium silicate 

 is known to be soluble to only i part in 100,000 of sea-water.^ 

 This perhaps indicates that the silica in solution in the sea is 

 always below saturation-point, so that a local concentration large 

 enough to determine precipitation never occurs. Or again, 

 excess silica perhaps combines with what little alumina there is 

 in sea-water and is deposited as clay ; if that were the case, the 

 limit of dissolved silica would be set by the solubility of this 

 substance, which may well be less than that of magnesium 

 silicate. At any rate, the quantity of silica really dissolved in 

 sea-water is extremely small. According to the most recent 

 and trustworthy determinations," there is on the average about 

 one part, and never more than two parts, per million in North 

 Sea and Baltic waters. 



Although for obvious reasons vastly less silica is produced, 

 by biological agencies, in the waters of the sea than calcium 

 carbonate, the former, like the latter, is found in almost all 

 submarine deposits. When siliceous remains fall into a calcareous 

 deposit, the silica has little tendency to combine with lime, 

 since silicic at low temperatures is an even weaker acid than 

 carbonic ; but, the process of peptisation being accelerated by 

 the higher alkalinity of the superjacent waters, we should expect 

 the predominance of lime to favour the dissolution of silica. 

 This seems to be borne out by the fact that silica is least 

 abundant in the most calcareous bottoms of the open sea, and 

 also by the almost total absence of silica in coral reefs and 

 muds.^ Again, essentially siliceous accumulations (Radiolarian 

 ooze) are characteristic of the very deepest parts of the ocean, 

 where calcareous remains have such enormous columns of sea 

 to fall through that they may fail to reach the bottom. There 

 is thus a tendency for silica and calcium carbonate to be 

 mutually exclusive. In terrestrial calcareous deposits (chalk) 

 we find imprisoned silica going into solution, migrating to 

 centres of coagulation and forming nodular segregations (flint). 

 No such phenomenon is observed at the bottom of the sea, 

 where the silica brought into solution has probably no difficulty 

 in diffusing into the hydrosphere out of the comparatively loose 

 deposit. 



The soluble silica of the sea is derived ultimately from 

 felspathic minerals, and the greater bulk is introduced from 



1 Murray and Irvine, Proc. Roy. Soc. Edin., vol. xviii. p. 238, 1891. 

 ^ Raben, Wissensch. Meeresuntersiichiingeii, Kiel, vol. viii. pp. 99 and 277, 1905. 

 ^ The Atoll of Funafuti : Report of Coral Reef Committee of the Royal Society ; Chemical 

 Examination of the Materials from Funafuti, by J. W. Judd, p. 370, 1904. 



