OEIGIN OF FERROUS SILICATE ROCK. 827 



The material thrown down as a hydrated ferric oxide and basic ferric 

 sulphate is mingled with more or less of organic material, and a deposit of 

 considerable thickness may thus be built up. This deposit is below the 

 level of ground water, and is therefore in the zone of incomplete oxidation, or 

 is under the conditions of the belt of cementation. The oxygen required 

 for the partial oxidation of the organic material is derived in part from the 

 ferric oxide, and the iron is reduced to the ferrous form; but probably this 

 reaction does not take place on an important scale at the surface. The 

 reducing agent may be regarded as carbon, carbon monoxide, or some 

 of the carburetted hydrogens, such as methane. The result is the same in 

 any case. The oxygen and the carbon produce carbon dioxide, and thus 

 the conditions are reproduced for the production of iron carbonate. A 

 representative reaction may have been as follows : 



2Fe 2 3 .3H 2 0+3C0 2 +C=4FeC0 3 4-3H 2 0. 



In the Mesabi district of Minnesota Leith has shown that a compound, 

 which he has called greenalite, exists on an extensive scale as an important 

 original mineral of the iron-bearing- formation." (PI. VII, B.) The exact 

 composition of greenalite has not yet been determined, but according to 

 Clarke it appears to be either 



Fe" / 2 Fe" 3 (Si0 4 ) s .3H 2 or else FeSi0 3 .nH 2 0, 



with the probability somewhat in favor of the latter composition. 6 Where 

 silica in a colloidal form is especially abundant, with limonite produced as 

 explained above, and with organic matter to serve as a reducing agent, the 

 silica may unite with the ferrous oxide produced by the reduction of the 

 ferric oxide to ferrous oxide through the organic matter, and thus with 

 hydration produce hydrous ferrous silicate. The formation of the hydrous 

 silicate under such circumstances, rather than the carbonate, may have 

 been dependent upon the law of mass action. 



The principles are illustrated by the conditions under which the oxidized 

 compounds of zinc occur in the Wisconsin and southwestern Missouri 

 districts. In the Wisconsin district silica is not especially abundant, and 

 where zinc sulphide is oxidized the zinc oxide unites with carbon dioxide 



<*■ Leith, C. K., The Mesabi non-bearing district, of Minnesota: Mon. U. S. Geol. Survey, vol. 43, 

 1903, pp. 100-168. 



» Leith, cit., p. 246. 



