MARINE DEPOSITS 203 



contact with the superincumbent water. 

 The colour of the upper layer is due to the 

 presence of hydrated ferric oxide which, as 

 the deposit accumulates, is partially trans- 

 formed into ferrous sulphide and oxide in the 

 presence of organic matter in the underlying 

 layers ; when dried the blue colour changes 

 to grey or brown owing to oxidation of the 

 iron sulphide. Sometimes the blue muds 

 appear to be homogeneous, and may have the 

 plasticity of true clay, but as a rule they 

 are heterogeneous from the admixture of 

 larger or smaller rock and shell fragments, 

 and are rather earthy than clayey. Cal- 

 careous and siliceous remains belonging to 

 plankton organisms vary greatly in amount 

 according to position. Rock fragments and 

 mineral particles may make up as much 

 as 75 per cent, in some cases, the most 

 characteristic species being quartz ; the usual 

 proportion of mineral particles is about one- 

 fourth of the whole deposit. Amorphous 

 clayey and muddy matters are always abun- 

 dant, the average percentage being about 60, 

 generally increasing in amount with greater 

 distance from the land. 



2. Red Mud. — This type is merely a local 

 variety of blue mud found off the coast of 

 Brazil in the Atlantic and off the coast of 

 China in the Yellow Sea, the red-brown colour, 



