ABSTRACTION OF OXYGEN BY IRON 323 



To determine the amount of oxygen combined with iron in the 

 sediments, Joly's 1 estimate of the mass of silicious sediments, 64 X to' 6 

 tons (of 2,240 pounds) is taken as the amount of shales, sandstones, 

 and ferruginous rocks. 



This gives 256 X10 14 tons of iron, which, occurring as Fe 2 3 

 and FeO in the ratio 2.64:1.52, requires 96X10 14 tons of oxygen. 

 The same amount of iron occurring as Fe 2 3 and FeO in the ratio 

 of the old crust, 2.63:3.52, requires 8,765 Xio 12 tons of oxygen. 



The difference between these two amounts, 835 Xio 12 tons, is a 

 measure of the quantity of oxygen taken from the atmosphere and 

 fixed in the silicious sedimentary rocks through the agency of iron. 

 Calculating, from Woodward's 2 statement as to the mass of the 

 atmosphere, the total amount of oxygen as 1,213 Xio 12 tons, the 

 quantity abstracted by iron is equal to 68.8 per cent, of that now pres- 

 ent in the atmosphere. 



While this estimate manifestly can lay no claim to even approxi- 

 mate accuracy, it suffices to show that the abstraction of oxygen by 

 iron is a factor that cannot be disregarded in any attempt to work 

 out the geological history of the atmosphere. 



1 Scientific Transactions of the Royal Dublin Society, Vol. VII, Ser. II, p. 46. 



2 Bulletin No. 78, U. S. Geological Survey, p. 34. 



