956 A TREATISE ON METAMORPHISM. 



therefore distributed through, these. A much greater concentration of the 

 added oxygen is, however, found in the rich and lean iron-ore formations, 

 from the Archean to the Pleistocene. Such segregation is well illustrated by 

 the Clinton iron ores, and is shown on a magnificent scale by the ferruginous 

 formations of the Lake Superior region. The gypsum and nitrate deposits, 

 aside from the iron, are those in which the oxygen abstracted from the 

 atmosphere is segregated to the greatest extent. 



In summary it appears that the chief certain source of oxygen for 

 the atmosphere is the reduction of carbon dioxide by vegetation and the 

 burial of a part of this vegetation in the earth. This source is vast in 

 amount. 



On the other hand, oxygen has been consumed by the oxidation of 

 the carbon compounds of various kinds in the original igneous rocks and in 

 the meteorites; by the oxidation of ferrous oxide to ferric oxide in the 

 zone of katamorphism ; by the oxidation of iron in the metallic form and 

 as a sulphide; by the oxidation of sulphur, nitrogen, and hydrogen; and 

 in some small measure by the oxidation of rarer compounds, such as 

 manganous oxide. The sum of these gives the amounts of oxygen 

 consumed during geological time. Until estimates have been made of the 

 various amounts of the oxidized compounds, it is wholly out of the question 

 to make any quantitative estimate of the total amount of oxygen which has 

 been abstracted from the atmosphere by the process of oxidation since the 

 beginning of geological time, but it is certain that the amount of oxygen 

 thus consumed has been enormous. It probably vastly exceeds the amount 

 of oxygen which has been liberated to the atmosphere by the reduction of 

 carbonic acid through plants. 



If this conclusion be correct such wild guesses as those of Koene and 

 Phipson (see p. 949), that the carbon dioxide of the original atmosphere 

 greatly exceeded the oxygen and that the proportions of these elements 

 have been reversed in consequence of the reduction of carbon dioxide by 

 organic matter, are wholly unwarranted. This case illustrates the danger 

 of drawing a conclusion from the consideration of only one factor of a 

 complex problem. 



