MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 739 



dioxide of manganese found on terrestrial rocks may be carried to the ocean, and widely 

 distributed over the sea-bed along with other detrital matters. In marine deposits, 

 mineral particles, derived from the disintegration of the crystalline rocks of the earth's : 

 crust, are everywhere present. In some regions the fragments derived from the disinte- 

 gration of the acid series of rocks predominate, in others the fragments from the basic 

 series. These minerals and rock fragments undergo alteration in the soft oozy deposit, 

 in the same way as the rocks on the terrestrial surfaces. The manganese in the silicates 

 is converted into bicarbonate of manganese, which is deposited as manganese dioxide 

 wherever there is a sufficient supply of oxygen. Experience has shown that nodules of 

 manganese are much more abundant in all those areas where the basic series of rocks pre- 

 dominate, and this is evidently connected with the greater abundance of iron and 

 manganese in these rocks. 



Manganese dioxide is a very stable and insoluble substance, and it might be supposed 



Fig. 9. — Manganese nodule with two 

 Tunicates (Sty da squamosa and Sty da 

 bythia) and a Brachiopod attached. 

 Station 160 ; 2600 fathoms. Southern 

 Indian Ocean. 



Fig. 10. — Petrous and tympanic bones of Meso- 

 plodon (species allied to layarrli), outer surfaces 

 covered with manganese (natural size). 

 Station 286 ; 2335 fathoms. South Pacific. 



that when once formed in marine deposits it would be permanent. We have shown that 

 there is, however, sufficient evidence, that, owing to repeated deoxidation and reoxidation, 

 the manganese in marine deposits is continually being transferred from one position oh 

 the floor of the ocean to another. Wherever decomposing organic matter is present in the 

 muds, deoxidation of the sulphates of the sea- water and of the manganese dioxide in the 

 deposits takes place with the formation of sulphides of iron and manganese. The sulphide 

 of iron is stable and remains in the deposit, but the sulphide of manganese, being unstable 

 in the presence of carbonic acid, passes to bicarbonate of manganese and on meeting with 

 a supply of oxygen is once more deposited as dioxide of manganese at some not very 

 distant spot. It appears, then, that the manganese dioxide in marine deposits, whether 

 originally derived from the decomposition of the rocks of the land surfaces or from the 

 minerals forming part of the marine deposits, is continual]}^ shifting its position. The 

 chemical processes in operation tend to favour its accumulation towards the surface of 

 marine deposits, or in those areas on the ocean floor where there is a relatively small 



