192 DEPTHS OF THE OCEAN 



admixtures of lower oxides of manganese when laid down in 

 landward waters/ where the supply of oxygen is competed for by 

 much organic matter. The hydration MnOg-^H^O is assumed 

 by Murray and Renard, and FCgOg. i^Hfi (limonite) for the 

 accompanying ferric oxide. Deep-sea nodules are never purely 

 manganic, but contain inclusions of clayey and other matters, 

 and always a considerable proportion of iron. The mean of 

 forty "Challenger" analyses works out at 29.0 per cent of 

 MnO.2 and 21.5 per cent of Fe.^Og, soluble in hydrochloric acid. 

 As a rule, then, surcharged waters hold both iron and manganese 

 ready to be deposited simultaneously. The mode of formation 

 of these nodules and the origin of the manganese from volcanic 

 minerals have been thoroughly elucidated by Murray and Irvine.^ 



It should be noted that these oxides need by no means 

 necessarily assume a concretionary form. They are very 

 commonly found as thin incrustations on granular and frag- 

 mentary objects. Furthermore many, if not most, of the pelagic 

 clays contain intimate admixtures of finely-divided brown 

 manganese and occasionally of limonitic iron. Here the super- 

 saturation would seem to have been so high as to transgress the 

 metastable limit, whereupon the oxides have precipitated them- 

 selves without the intervention of nuclei ; they certainly must 

 have been precipitated from solution. 



Manganese originates in the form of silicates and comes to 

 rest exclusively in the form of peroxide. It is imported, on the 

 one hand, from land as detritus or in solution ; but in the 

 terrigenous areas of the bottom, where reducing conditions 

 prevail, as a rule, it tends to exist in the suboxidized, i.e. soluble, 

 form. Hence much of the terrigenous manganese will be 

 carried on to the deeper oxidizing waters before it can deposit. 

 There is thus a constant accession of manganese from land to 

 the pelagic deposits. In the second place, manganese comes 

 into the floor of the ocean from certain basic volcanic minerals 

 of vitreous habit, and these are to be regarded as the principal 

 source of ferromanganic nodules. These basic glasses are the 

 only primary minerals in the deep sea which contain important 

 amounts of manganese. It so happens that they are common 

 in the Pacific, less common in the Indian Ocean, and rare in 

 the Atlantic. Consequently the greatest abundance of manganese 

 peroxide, pulverulent and nodular, is met with in mid-Pacific. 



Phosphatic concretions are of very localised occurrence and 



^ Buchanan, Trans. Roy. Soc. Edin., vol. xxxvi. p. 459, 1892. 

 - Trans. Roy. Soc. Edin., vol. xxxvii. p. 721, 1894. 



