OCEANIC DEPOSITS AND BOTTOM FAUNA 27 



capacity for solids of waters of varying degrees of salinity, 

 notably by the Americans, Hilgard* and Boliver.t 



Blue Mud is the commonest deposit met with in the deeper 

 waters surrounding continental land, and in all enclosed or 

 partly enclosed seas more or less cut off from the open ocean. 

 When collected it is blue or slate coloured, with an upper red 

 or brown layer in immediate contact with the water. The blue 

 colour is due to iron sulphide and organic matter. These 

 muds have, as a rule, a small quantity of sulphuretted 

 hydrogen. The reddish tint, of the uppermost layer is due 

 to ferric oxide or ferric hydrate ; as the deposit accumulates the 

 oxide is transformed into sulphide. When dried the deposit 

 becomes grey or brown, owing to the oxidation of the sulphide 

 of iron. When wet, this mud is plastic and behaves like a 

 true clay, but it is really more earthy than clayey. The per- 

 centage of carbonate of lime varies from nil to 35. Blue mud 

 is principally made up of land detritus (quartz being the 

 characteristic mineral), which becomes less abundant with 

 increasing depth, until finally the blue mud passes gradually 

 into one of the types of pelagic deposits. 



Blue mud areas afford an important example of the 

 reduction of submarine clay after deposition. Reducing 

 conditions obtain where there is an excess of putrefiable 

 organic matter which cannot be dealt with by the supply of 

 oxygen available. Oxidation of the ferric iron is effected 

 probably by bacterial agency. It is known that bacterial 

 production of ferrous sulphide and free sulphur takes place. 

 It may be that sulphur plays an important part in the formation 

 of blue muds, the final product being a clay in which most of 

 the iron has been reduced to the ferrous state, containing i or 

 2 per cent, of amorphous black organic substances. Murray 

 and Renard give 58 examples of blue mud, of which 12 are 

 from depths less than 500 fathoms, 9 from over 2,500 fathoms. 



The average percentage of CaCO 3 is 12^, ranging from 



* Amer. Jonrn. ScL, vol. xvii., 1879. 



t Mem. Nat. Acad. Sci., Washington, vol. ii., 1883. 



