584 REPORT— 1881. 



of the available oxygen, the resulting formula varies between MnOj.5 and MnOj.g.. 

 Taking the soft kernel alone, separated from the more sandy rind, the formula is 

 MnOj.,5. The rind alone had the formula MnO,.5. In some oceanic nodules 

 which were examined, the formida varied from MnOi-gg to MnOj.gj. 



Some of the mud which contained the Loch Fyne nodules was examined. It 

 contained 0-71 per cent, of MnO, and a hardly distinguishable trace of extra oxygen. 



If we express the available "oxygen in terms of its equivalent Mn02 we have in 

 oceanic nodules from 22 to 32 per cent., in Loch Fyne ones from 13 to 22, and in 

 the kernel treated separately 33 per cent, of MnOj. 



The insoluble residue varies in oceanic nodules from 16 to 30 per cent., and in. 

 the Loch Fyne ones from 28 to 33 per cent., both containing from 83 to 88 per 

 cent, of silica. 



Oceanic nodules appear to contain more nickel and cobalt than the Ijoch Fyne 

 ones, while the latter contain distinct traces of copper, which does not appear to be 

 so common in the oceanic ones. 



With regard to the method of formation of these nodules, and the deposition of 

 higher oxides, not only of manganese but of iron, on the sea-bottom, it is perhaps 

 unwise at the present moment to speak positively ; but I think there is enough 

 known to enable us to indicate a probable process of production. 



All the nodules without exception contain residues of organic matter ; where 

 the oxide of manganese is not aggregated into nodules, it is found encrusting 

 animal remains, as shells, worm-tubes, &c. Whether in the form of nodules or 

 incrustations, the black matter contains all the recognisable sand and other matter 

 of the surrounding mud ; moreover, they are certainly formed in situ, and not 

 brought from a distance. The mud in Loch Fyne, at any rate, contains manga- 

 nese in the form of protoxide, and no doubt as silicates ; it also contains oxides of 

 iron, which are present in all manganese nodules and incrustations ; indeed in the 

 same dredgeful we can pick out all gradations of nodules, from those which con- 

 tain almost exclusively MnO^, with only traces of Fe^Oj, to those which contain 

 FegOj, without any INInO,,. 



The formation of the sesquioxide of iron nodules has gone on under my own 

 eyes. In July 1879 I dredged from the bottom of the Sound of Jura, in 70 to 80 

 fathoms, a large mass of veiy stifl" blue clay, as stiif as the best brick-making clay. 

 Throughout the mass there were disseminated patches and pockets of blackish blue 

 matter which proved to be sulphide of iron ; hydrochloric acid liberatmg sul- 

 phuretted hydrogen in abundance. This was dried in the usual way, in plates on 

 the cylinder covers of the engine, and when dry not a trace of sulphide was to be 

 found, every black patch being now represented by a reddisli-brown one, even in 

 the centre of large and compact masses. Now, what has here taken place in a very 

 short time, under the combined action of air, moisture, and heat, takes place as 

 surely, but more slowly, in the surface layer of the mud at the bottom .; and, in 

 point of fact, not only in our shallow seas, but even in the deepest water of the 

 ocean, we find the surface layer of the mud almost invariably of a different colour 

 from the underlying mass, the top being red while below the mud is grey. I believe, 

 then, that for the formation of the nodules we have to look to the organic world as 

 a necessary though indirect assistant. The decomposition of animal matter in 

 presence of the sulphates of the sea-water, reduces these to sulphides, which in theu- 

 turn react on the iron and manganese minerals (principally silicates) in the mud,, 

 forming sidphides of these metals. As the organic matter gets exhausted, these 

 sulphides are oxidised to oxides by the oxygen of the water, forming concretions or 

 incrustations of the ochreous oxides, which naturally also enclose the other and 

 unaltered constituents of the mud. 



In conclusion, I cannot help thinking that the shell-producing animals of the 

 sea obtain the lime from the sea-water in this way, the hme being assimilated, 

 from the dissolved sulphate, in the form of sulphide in the interior of the animal, 

 and transformed into carbonate on the outside. 



