ABSTRACTION OF OXYGEN FROM THE ATMOSPHERE. 955 



Besides the compounds already considered there are various rarer 

 substances which have been oxidized, such as manganous oxide. Doubt- 

 less, however, the consumption of oxygen by the rarer compounds is incon- 

 siderable. 



The above calculations may be only roughly approximate, but they 

 seem to show that the amount of oxygen which has been abstracted from 

 the atmosphere during geological time in the oxidation of iron and sulphur 

 is enormous — apparently about twice that now free in the atmosphere. 

 If these results be correct, omitting any consideration of the amount which 

 has been required for the oxidation of hydrogen, nitrogen, and other sub- 

 stances, the original atmosphere must have been much more voluminous 

 than at present, or else during geological time there has been some source 

 of oxygen from which the atmosphere has been replenished. 



In the previous pages only the sediments of the zone of katamorphism 

 are considered. While in the zone of katamorphism oxygen has been added 

 to the lithosphere and abstracted from the atmosphere, it is equally certain 

 that in the zone of anamorphism and in the belt of cementation deoxidation 

 takes place to some extent. Reduction is usually accomplished through 

 vegetation, carbon being oxidized at the same time and largely returned 

 to the atmosphere as carbon dioxide. So far as this is the case it involves 

 no correction beyond that already considered, since the end result is the 

 same as if this carbon had been oxidized by the process of decomposition 

 at the surface of the earth, and thus to that extent balanced the process of 

 liberation of oxygen when the organic carbon compounds were originally 

 formed. On the whole there is no reason to believe that the sedimentary 

 rocks metamorphosed in the zone of anamorphism contain more ferric oxide 

 than the original rocks. Until the relative amounts of ferrous and ferric 

 oxides in the sedimentary rocks of the zone of anamorphism are compared 

 with the amounts of these compounds in the original rocks, it can not be 

 asserted whether the sum total of the reactions of the sedimentary rocks in 

 the zone of anamorphism is in the direction of the consumption or liberation 

 of oxygen. 



The oxygen added to the zone of katamorphism is not uniformly 

 distributed, but is segregated to a very large extent. The most important 

 segregation is that occurring in connection with iron. There are vast 

 quantities of ferric oxide in the red sandstones, and the added oxygen is 



