IV DEPTHS AND DEPOSITS OF THE OCEAN 185 



land by means of rivers. Since the ocean cannot retain in 

 solution more than a trace, all this silica must eventuate as 

 organic deposits, especially Radiolarian and Diatom oozes. 

 Furthermore, a certain quantity of suspended terrigenous clay 

 is being continually converted into the hydrated silica of these 

 deposits. Neglecting the latter source of biological silica and 

 the comparatively inconsiderable radiolarian areas, we can say 

 that the dissolved silica yielded by the continents is tending to 

 accumulate on the frontiers of the temperate and polar zones, 

 especially in the Antarctic Ocean. 



The amount of phosphorus in sea-water is comparable in its Phosphc 

 tenuity to that of silica, Raben's determinations for North Sea 

 and Baltic waters show a seasonal variation ranging from 

 0.14 to 1.46 parts of PgOj; per million. Phosphorus originates 

 as calcium phosphate in the form of apatite, passes through the 

 ionized condition, and is deposited on the bottom of the sea as 

 calcium phosphate. In the deposits this compound is of 

 universal distribution ; all samples of whatever character contain 

 from a trace to about 3 per cent of tricalcium orthophosphate. 

 The clays and muds no doubt retain traces of undecomposed 

 mineral phosphate. On the other hand, calcium phosphate is 

 secreted to a greater or less extent by the living denizens of 

 the sea, whence its presence in calcareous and siliceous deposits ; 

 here we have the phosphorus withdrawn from aqueous solution 

 and partly going through a cycle between the sea and the 

 bottom, like lime and silica. 



If there were no organic life in the ocean, the deposit every- pecompos 

 where would consist of a mud or clay, composed of mineral m^JJerais. 

 detritus. As it is, this detritus is nowhere wholly absent, and 

 large areas consist of little else. Whether the mud be brought 

 into'the sea by rivers or through the agency of tidal erosion, or 

 whether it be formed in sittt at the bottom, it is always of a 

 dual nature. The one ingredient is more or less finely powdered 

 original mineral matter produced by mechanical comminu- 

 tion ; the other is a mixture of substances resulting from the 

 chemical decomposition of rocks. It has not been found 

 possible to disentangle these components quite satisfactorily by 

 chemical analysis, but it is safe to state that the proportion of 

 one to the other ranges from one quarter to three quarters. 



In chemically-produced mud we have the result of the action 

 of water on crystalline silicates without the intervention of any 

 solute except dissolved gases. Qualitatively, therefore, it is of 



