XII. ] ORIGIN OF METALLIFEROUS DEPOSITS. 231 



the ancient wreck of a treasure-ship in the Spanish Main were 

 found to be deeply incrusted with sulphide of silver, formed in 

 the ocean's depths by the process just explained, which is one 

 that must go on wherever organic matters and sea-water are 

 present, and atmospheric oxygen excluded. 



The chemical history of iron is peculiar ; since it requires 

 reducing matters to bring it into solution, and since it may be 

 precipitated alike by oxidation, and by further reduction 

 provided sulphates are present. The metals, copper, lead, and 

 silver, on the contrary, form compounds more or less soluble 

 in water, from which they are not precipitated by oxygen, but 

 only by reducing agents, which may separate them in some 

 cases in a metallic state, but more frequently as sulphides. 

 The solubility of the salts and oxides of these metals in water 

 is such that they are found in many mineral springs, in the 

 waters that flow from certain mines, and in the ocean itself, 

 the waters of which have been found to contain copper, silver, 

 and lead. Why, then, do not these metals accumulate in the 

 sea, as the salts of soda have done during long ages? The 

 direct agency of organic life comes again into play, precisely as 

 in the case of phosphorus, iodine, and potash. . Marine plants, 

 which absorb these from the sea- water, take up at the same 

 time the metals just named, traces of all of which are found 

 in the ashes of sea-weeds. Copper, moreover, is met with in 

 notable quantities in the blood of many marine molluscous 

 animals, to which it may be as necessary as iron is to our own 

 bodies. Indeed, the blood of man, and of the higher animals, 

 appears never to be without traces of copper as well as of 

 iron. 



In the open ocean the waters are constantly aerated, so that 

 soluble sulphides are never formed, and the only way in which 

 these dissolved metals can be removed and converted into 

 sulphides is by fixing them in organisms, either vegetable or 

 animal. These, by their decay in the mud of the bottom, or 

 the lagoons of the shore, generate the sulphides which fix their 

 contained metals in an insoluble form, and thus remove them 

 from the terrestrial circulation. 



