SUBMARINE DEPOSITS 183 



and other hard parts, and these after death add to the 

 deposits at the bottom. This takes place in shallow water 

 as well as in the open ocean, but where there is much sedi- 

 ment from the land these organic deposits may be swamped 

 and masked by the terrigenous gravels, sands and muds. 



Chemical action may also take place in the sea-water, 

 and so produce changes in the deposits in some locahties 

 or under some conditions, giving rise, for example, to glau- 

 conite, phosphatic concretions and manganese nodules. 



Finally, there are contributions to the deposits made by 

 submarine volcanic action, by the disintegration and 

 decomposition of floating pumice into clay, by volcanic 

 dust carried by the wind from the land, and by meteoric 

 particles falhng from space upon the oceans. 



The leading authority on submarine deposits, and es- 

 pecially upon those of the deep sea, is the late Sir John 

 Murray, who commenced the detailed study of the subject 

 during the " Challenger " expedition, and continued it to 

 the end of his life. It is safe to say that he has examined, 

 classified and described more deep-sea deposits than any 

 other man. The most comprehensive and authoritative 

 work on the subject is the " Challenger " report by Murray 

 and Renard on the deep-sea deposits of the Expedition, 

 published in 1891. 



Sir John Murray's primary classification of all deposits 

 is into (1) Terrigenous, the gravels, sands and muds derived 

 from adjacent land ; and (2) Pelagic, the deep-sea '' oozes " 

 far removed from land and largely made up of the cal- 

 careous and siliceous remains of organisms which once lived 

 in the surface waters of the open ocean, and after death 

 sank to the bottom. It is convenient, however, to recognize 

 and add a third category, which has been named Neritic, 

 for those deposits, mainly calcareous, which are found in 

 many shallow waters amongst terrigenous sands and muds, 

 but are not themselves terrigenous in origin, being formed 

 of the shells and other remains of Molluscs, Echinoderms, 



