i82 DEPTHS OF THE OCEAN chap. 



sea-water sulphates. In all deep-sea muds there is a certain 

 amount of decaying animal and vegetable matter fallen from 

 the hydrosphere, the proteids of which leave their sulphur, so 

 far as it escapes oxidation, combined with the iron of the 

 surrounding mud. But apart from this rather insignificant item, 

 there are bacteria which, whilst living on sarcodic matter, seize 

 on the dissolved sulphates of sea-water and reduce them to 

 sulphides ; the latter react with whatever ferruginous material 

 is present, and produce the highly insoluble compound ferrous 

 sulphide. Free sulphur, when found, is to be accounted for by 

 the partial oxidation of sulphides, either by dissolved oxygen or 

 at the expense of ferric iron. The retention of sulphur in 

 bottom-deposits can only occur where there is plenty of decaying 

 organic matter, where the bottom-waters are stagnant, or nearly 

 so, and not well aerated, and where there is not a copious hail 

 of calcareous tests ; that is, mainly in the lower layers of muddy 

 bottoms at shallow and medium depths. The sea- water 

 imprisoned below the upper layer of mud becomes poorer in 

 sulphate and richer in carbonic acid,^ whilst the mud is darkened 

 in colour by very finely-divided and easily oxidizable ferrous 

 sulphide. Under suitable conditions the ferrous sulphide may, 

 as in Black Sea muds,-"' combine with free sulphur and attain a 

 condition of higher stability in the form of pyrites. The essential 

 chemical factor which renders possible the retention of sulphur 

 is the power of the colloidal ferric hydroxide in clay to react 

 with sulphides. A small quantity of ammonium sulphide added, 

 in the laboratory, to ordinary Red clay from the deep sea, at 

 once goes into reaction: the clay is darkened to a tint resembling 

 that of Blue mud ; the original tawny colour is restored by 

 atmospheric oxidation ; the darkened clay evolves sulphuretted 

 hydrogen with dilute acid. At the same time it is well to 

 remember that many Blue muds owe their colour to quite other 

 causes than the presence of sulphur. 



The reduction of sulphates occurs only where there is a 

 continuous deposition of detritus, and takes place, in the sub- 

 marine muds, in the deeper layers. Consequently under 

 normal conditions precipitated sulphur does not perform a cycle 

 between bottom and sea, but remains irrevocably buried, 

 accumulating as the deposit accumulates. No attempt seems 

 hitherto to have been made to determine the ferrous sulphide 

 in marine muds, but it is probably very minute in amount. 



' Murray and Irvine, Trans. Roy. Soc. Ediii., vol. xxxvii. p. 481, 1893. 

 - Murray, Scot/. Geogr. Mag., vol. xvi. p. 673, 1900. 



