i86 DEPTHS OF THE OCEAN 



the same composition whether formed in fresh water or in the 

 sea. Quantitatively, it might be expected to show a difference 

 for terrigenous and pelagic origin respectively, since the mother- 

 rocks are in general not the same. Nevertheless, a remarkably 

 close similarity is revealed by analyses, such as the " Chal- 

 lenger" analyses of Blue muds and Red clays, or still better, of 

 Clarke's ultimate analyses of averaged "Challenger" deposits.^ 

 One notable point of difference is brought out, viz. the greater 

 manganese-content of pelagic deposits. 



The action of unlimited water, oxygen, and carbonic acid 

 on the earth's crust tends to lead to certain definite end-products, 

 the nature of which is dictated by the abundance and the 

 affinities of the elements concerned, and by their habit as 

 regards solubility. All minerals, given time, succumb to these 

 agencies. Reviewing the chief elements, we find the final con- 

 ditions of stability under subaqueous influences to be as follows. 

 The alkalies, being of a highly soluble tendency, go into 

 solution and accumulate in the hydrosphere. Calcium and 

 magnesium are rendered soluble by the presence of carbonic 

 acid and become sea- water constituents, the former being 

 ultimately redeposited by organic processes. Phosphorus 

 behaves similarly. Ferric iron is very feebly basic, and therefore 

 tends to the condition not of a salt but of a hydrated oxide 

 (FeoOg.Aq) which, being very insoluble, remains in the 

 residuum. Ferrous iron, which is a much stronger base, is 

 leached out by the aid of carbonic acid, but is soon oxidized to 

 ferric iron and rendered insoluble. Much the same holds good 

 of manganese, which exists in minerals almost exclusively in the 

 manganous state : it is dissolved as bicarbonate, undergoes 

 oxidation, and comes to rest as hydrated peroxide (MnO^. Aq). 

 Aluminium forms only one base, which is very weak, but has 

 the property of combining with silica to form a highly insoluble 

 substance, ideal clay (AI0O3. 2510,. 2H2O), which represents its 

 final stable condition. Silicon exists as a weak acid (SiOo) of 

 insoluble tendencies, which, after having been brought into 

 solution, partly joins the residuum as clay and is partly re- 

 deposited as hydrated silica through organic agency. 



The ultimate mineral residuum, then, consists, if we pass 

 over the rarer elements, of aluminous clay, hydrated ferric oxide, 

 and hydrated manganese peroxide. In all probability the two 

 former substances should be considered together and submarine 

 clay regarded as an ill-defined colloidal compound in which 



^ P)Oc. Roy. Soc. Edin., vol. xxxvii. pp. 167 and 269, 1907. 



