IV DEPTHS AND DEPOSITS OF THE OCEAN 187 



silica and alumina play the chief part, but ferric hydroxide and 

 even lime, magnesia, and alkalies are also represented. These 

 minor constituents are, at any rate, so combined as to resist 

 leaching out by dilute acids. Vast areas of the lowest depths of 

 the sea are covered by such a clay in a state of considerable 

 mechanical purity, a product of almost exclusively submarine 

 disintegration, known as Red clay. 



The chemical action by which pelagic clay is derived from 

 its volcanic mother- rocks must proceed, as compared with 

 subaerial weathering, with the utmost sluggishness. The 

 fundamental question, indeed, whether fresh or salt water exerts 

 the more powerful action upon rocks must be regarded as not 

 yet answered. Great experimental difficulties are encountered, 

 and we find the results of Thoulet, who concluded that fresh 

 water is a better disintegrant than salt, diametrically opposed to 

 those of Joly.^ But several other considerations must be taken 

 into account, and it cannot be doubted that rock silicates are 

 degraded more slowly in the sea than on land. For instance, 

 the clastic action of frost is never brought into play. There is 

 no comminution of the minerals by moving water. The soluble 

 by-products are removed, and the supply of oxygen and carbonic 

 acid maintained, by diffusion only. 



At this stage the state of rest of the deep-sea residuum is 

 not even yet necessarily final, but is capable of being disturbed 

 locally by organic agencies. Aluminous clay, indeed, is per- 

 manent once it is at the bottom, but, whilst floating, it is to 

 some extent decomposed, as we have seen, by siliceous algse for 

 purposes of nutrition. Iron and manganese oxides are suscept- 

 ible to reduction by purifying sarcodic matter, whence result the 

 ferrous iron of the Blue muds, and also many of the concretionary 

 forms of these oxides. 



The Blue mud areas, which are of vast extent, afford a 

 most important example of the reduction of submarine clay after 

 deposition. We may indeed divide the floor of the sea, accord- 

 ing to the relative abundance or paucity of dissolved oxygen 

 in the bottom-waters, into oxidizing and reducing areas. Re- 

 ducing conditions will prevail wherever there is a larger excess 

 of putrefiable organic matter than can be coped with by what- 

 ever supply of oxygen (depending on the circulation of the 

 area) may be available. In general, therefore, the coast-lines 

 of continents are girdled by reducing areas, and it is here that 



' It may be mentioned that the methods of leaching adopted by these experimenters are 

 somewhat dififererit, and that Thoulet measures his effects by loss in weight, whereas Joly deter- 

 mined the amounts taken up in solution. 



