950 A TREATISE ON METAMORPHISM. 



But the oxidation of carbon and deoxidation through life take into 

 account only one aspect of the question. The inorganic carbon compounds 

 of the original earth stuff should also be considered, and Koene and Phipson 

 ignore this side of the question. It will be seen, under carbon, that this 

 element in various forms is a constituent of the original igneous rocks, and it 

 has been seen that carbon componds are also constituents of meteorites. So 

 far as elemental carbon, carbon monoxide, carbureted hydrogen, or other 

 carbon compounds are present in the original igneous rocks and have 

 been oxidized to carbon dioxide, demands have been made upon the oxygen 

 of the atmosphere. These demands have certainly been large. No data 

 are at hand to make even an approximate estimate as to how large a 

 quantity of oxygen has thus been abstracted from the atmosphere during 

 geological ages. But, as explained under carbon, the oxidation of carbon 

 compounds of the original igneous rocks is believed to be one of the chief 

 sources of carbon dioxide. In so far as this reaction has taken place it 

 is a reversal of that of vegetation. Which of these opposing processes is 

 preponderant is a question of great importance, but one upon which not 

 even a qualitative guess is warranted. Therefore no statement can be 

 made as to whether the atmosphere has permanently gained or lost oxygen 

 as a consequence of the oxidation and deoxidation of carbon compounds 

 throughout geological time. 



A second very important reaction which depletes the atmosphere in 

 oxygen is the oxidation of the ferrous iron of the original rocks. It has 

 been pointed out (Chapters VI and VIII) that oxidation of the ferrous 

 oxide to the ferric oxide is characteristic of the belt of weathering and 

 that the reduction of the ferric, oxide to the ferrous oxide may occur in 

 the belt of cementation and is characteristic of the zone of anamorphism. 

 But the oxidation of ferrous iron to the ferric form clearly preponderates 

 over the reduction of ferric oxide to ferrous oxide, and hence oxygen is 

 consumed. This is shown by the following facts: 



In 78 shales the percentage ratio between the ferrous oxide and the 

 ferric oxide is 2.46 : 4.03; in 624 sandstones the ratio is 0.57 : 1.24; in 843 

 limestones the ferrous oxide is so small that it is undetermined, but the 

 ferric oxide is 0.66. Giving these sets of numbers the respective weights of 

 0.65, 0.30, and 0.05 (the estimated proportions of these different kinds of 

 sediments), the ratio between the ferro^^s oxide and ferric oxide in the 



