MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 737 



deposits doubtless arise from the oxidation of the bicarbonate of manganese, which oozes 

 or seeps out of the dark-coloured reducing layers beneath. 



The blue-coloured layers of those muds situated in deep water near shore gradually 

 disappear towards the central parts of the ocean basins, the deposit passing into Globi- 

 gerina, Pteropod, and Eadiolarian Oozes and Red Clays. These latter deposits have gene- 

 rally one uniform colour throughout, of being usually red, chocolate, rose, or dull grey. 

 No trace of a blue layer beneath a red-coloured surface one can be detected in these 

 abysmal deposits. Apparently the organic matter, which reaches the bottom in these 



Fig. 6. — Tooth of Carcharodon megalodon (natural size). This is the 

 largest specimen taken during the cruise of the "Challenger." 

 Station 281 ; 2385 fathoms. South Pacific. 



Fig. 7. — Manganese nodule with 

 Scalpcllum darwinii growing on 

 it. Station 299 ; 2160 fathoms. 

 South Pacific. 



abysmal regions, is less abundant than nearer shore, and probably the rate of accumulation 

 is so slow that the decomposing organic matter is never covered up, as in the case of the 

 Blue Muds. At all events, when deoxidation of the sulphates in sea-water does take place 

 in the Red Clays or Globigerina Oozes, it never results in the formation and permanent 

 addition of any large quantity of sulphide of iron to the deposit, as in the case of the 

 Blue Muds, the iron being nearly all in the form of sesquioxide, and hence the whole 

 deposit, at least to the depth of 18 inches, is of a red colour. In the case of the Blue 

 Muds, the thin red-coloured watery layer on the surface becomes gradually converted 

 into the blue layers beneath with the growth of the deposit. 



VOL. XXXVII. PART IV. (NO. 32). 5 T 



