Forma Hon of Central Oceanic Plateaux. ■ 131 



with their volume than the latter, and will therefore be deposited 

 sooner than organic particles, which can only fall to the bottom 

 and form strata under peculiarly favourable conditions. The 

 most favourable of these conditions is the absence or nearly 

 complete absence of currents; and this conclusion is remarkably 

 confirmed by observation. A large proportion, if not by far the 

 largest proportion, of the particles suspended in the waters 

 of the ocean consists of the bodies of the myriads of animal 

 and plant organisms which there live and die, and no doubt 

 derive their sustenance from the still finer oro-anic and inorganic 

 particles dissolved in the surrounding fluid. A teaspoonful of salt 

 water examined under the microscope reveals the presence of 

 hundreds and thousands of these minute organisms. Their 

 distribution in the ocean, no less than that of the larger animals, 

 depends, among other conditions, upon the nature and abun- 

 dance of the food they require ; and hence we can distinguish 

 between a marginal and a central oceanic fauna, and between 

 a surface and a bottom fauna. Although the dredge has brought 

 to light sufficient proofs of the presence of animal life at great 

 depths — two or three miles from the surface — yet a considerable 

 diminution has been observed beyond these limits, ending with 

 an almost complete absence of living organisms as we attain a 

 depth of 4000 fathoms. Most of the minute organisms seem to 

 have their home in the upper strata, being especially abundant 

 at or near the surface, and their bodies reach the bottom only 

 after death, and after having floated for a considerable time with 

 the currents. During this time they undergo a process of 

 decomposition which reduces them to a mere skeleton, and the 

 latter, being heavier in proportion to its bulk than the living 

 body, ultimately sinks to the bottom. 



The deposit of these ■ light remains of organic life can 

 only take place, as already mentioned, over areas of minimum 

 circulation, and these areas are confined to the centre of 



