742 MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 



organisms, we would expect them to be more abundant where marine organisms are 

 especially numerous ; the reverse is, however, the case. On coral reefs, Blue Muds, and 

 Globigerina Oozes, manganese nodules are usually rare or entirely absent, although 

 animal life is especially abundant in these areas. On the other hand, where living 

 organisms are relatively rare or least numerous, as for instance around some volcanic 

 islands and in the deepest recesses of the ocean, there we meet with depositions of dioxide 

 of manganese and manganese nodules in the greatest abundance. It follows, then, 

 that the mineralogical nature of the deposit has most to do with the rarity or abund- 

 ance of manganese nodules, and not the greater or less abundance of living organisms. 



Iii a recent publication Professor Judd,* misled apparently by some supposed analo- 

 gies, has argued that the manganese, the iron, and the other rarer metals, found in man- 

 ganese nodules " must have been separated from their state of diffusion in sea-water " by 

 organic agency. In the decomposition of all organic structures, it may be admitted that 

 the chemical changes are initiated by bacteria, but with this exception it does not appear 

 that organisms play any part in those changes which result in the formation of the 

 manganese-iron nodules in marine deposits. In the foregoing pages we have shown 

 satisfactorily that the formation of manganese nodules is due to purely chemical reactions 

 taking place in marine deposits on the floor of the ocean, and not to secretion by 

 organisms. Professor Judd's views on this matter are rejected as inadmissible by all 

 investigators who have devoted their attention to the problem. 



In his first preliminary paper on Manganese in Deep-Sea Deposits, published in 

 1877, Murray! pointed out the association of large numbers of manganese nodules with 

 the great abundance of basic volcanic debris in the deep-sea deposits at many localities. 

 He held that the manganese dioxide originated in the decomposition of the manganiferous 

 volcanic materials in the deposits through the action of carbonic acid, the carbonate of 

 manganese formed passing gradually in the presence of the oxygen of the sea-water to 

 the higher oxides of manganese. All the investigations described in this paper, indeed 

 al] subsequent researches on the subject, seem to confirm this view, which may now be 

 regarded as firmly established. Bischoff long ago showed that the rocks which furnish 

 iron and manganese ores contain both these metals as silicates of the protoxides, and that 

 water which had permeated these rocks held carbonates of these metals in solution. 

 " There can be," he says, " no doubt that the sesquioxide of manganese that occurs in 

 manganese ores originates from carbonate of manganese. "J More recently Boussingault § 

 discussed the formation of coatings of manganese dioxide in various regions, and 

 arrived at conclusions similar to those of Bischoff and Murray. The distribution 

 and localisation of manganese dioxide on the floor of the ocean at the present time is 

 thus shown to be strictly comparable with the corresponding subaerial phenomena, 

 modified by the peculiar conditions which obtain on the floor of the ocean. 



* Fortnightly Review, January 1894, p. 73. 



t On the Distribution of Volcanic Debris over the Floor of the Ocean, Proc. Roy. Soc. Edin., vol. ix. p. 255, 1877. 



I BischolT, Chemical Geology, vol. iii. p. 508, English edition. 



§ Annates de Chemic et de Physique, ser. 5, torn, xxvii. pp. 289-311, 1882. 



