i88 DEPTHS OF THE OCEAN 



Blue muds characteristically occur. Oxidation of the organic 

 matter is here effected at the expense of ferric iron, probably 

 by bacteria] agency. A special case of this, viz, the bacterial 

 production of ferrous sulphide and free sulphur, has already 

 been referred to. It may be that sulphur plays an inter- 

 mediate part in the formation of Blue muds, but the end- 

 product is simply a clay, in which some or most of the iron 

 has been reduced to the ferrous state, containing i or 2 per 

 cent of amorphous black organic substance. To these two 

 factors the distinctive dark colour is due. The organic sub- 

 stance is associated with but little nitrogen and hydrogen, and 

 it no doubt represents the final refuse of bacterial and higher 

 forms of life. Blue muds are produced out of the deposit from 

 the top downwards, as is evidenced by the reddish unreduced 

 layer overlying the deeper Blue ones. Since Blue mud is of 

 terrigenous origin, the undegraded silicate which it contains 

 consists of continental minerals. 



From the general conditions obtaining in reducing areas it 

 follows that carbonic acid must be unusually plentiful in the 

 mud-waters. A consequence of this is that calcium carbonate, 

 if deposited, is readily redissolved. Hence the Blue muds are 

 on the whole poor in lime. It further follows that lime is 

 tending to accumulate in the deposits of the moderate depths 

 of the ocean, between the reducing areas and the abysses where 

 it is dissolved before reaching the bottom. 



Doubtless the decay of minerals on the floor of the sea 

 follows much the same course as subaerial weathering. Inter- 

 mediate products, however, are comparatively rare, since the 

 general conditions are not (as on land) subject to variation. 

 The only substances of this category which form in any pro- 

 fusion are zeolites, especially the one known as phillipsite. 

 Here and there intermediate products are arrested by being 

 surrounded with concretions. A notable instance is the mineral 

 palagonite, which is frequently found at the centre of ferro- 

 manganic nodules. Basic volcanic glass (an amorphous fused 

 silicate of calcium, magnesium, and ferrous iron) has the 

 property of combining with water continuously from the peri- 

 phery inwards without crumbling, giving what is virtually a 

 hydrated aluminium-iron silicate in a medium of opal. A 

 coating of concretionary matter prevents the gelatinous silica 

 from breaking away and dissolving, but offers no resistance to 

 the diffusion of calcium and magnesium, which are leached out. 

 Meanwhile the colloidal silica exerts its absorbing power on 



