ABSTRACTION OF OXYGEN FROM THE ATMOSPHERE. 951 



sediments as a whole is 1.77 : 3.025. The amount of ferrous oxide is 

 about one-half of that of the original igneous rocks, 3.52 per cent. The 

 amount of ferric oxide somewhat exceeds that in the original rocks, 2.63 per 

 cent, but this excess falls far short of the amount required to compensate 

 for the deficiency of ferrous iron in the secondary rocks, for the total 

 amount of iron in 1.77 per cent of ferrous oxide and 3.025 per cent of 

 ferric oxide is 3.49, whereas the total amount of metallic iron existing- 

 in the oxidized form in the original rocks is 4.58, the remainder of the 

 4.64 total iron, as given by Clarke, probably being largely accounted 

 for by the iron united with sulphur. It therefore appears that a large 

 amount of the iron of the original rocks is not accounted for in the 

 sediments which are ordinarily analyzed, and this difference amounts 

 to one-fourth of the total amount of iron in the original rocks. This vast 

 deficiency in iron oxide is probably largely accounted for by the segre- 

 gation of iron in the iron-bearing- formations and in the iron-ore deposits 

 such as those in the Lake Superior region. The iron in these formations is 

 dominantly in the ferric form. Certainly the ratio of ferric oxide to ferrous 

 oxide in them is as great as among the sediments. 



Supposing, therefore, that the ratio of ferrous oxide and ferric oxide in 

 the iron-bearing- formations is the same as in the sediments analyzed, and 

 increasing the amounts of ferrous oxide and ferric oxide in the shales, sand- 

 stones, and limestones together in the same ratio, so that they contain an 

 amount of iron equivalent to that in the original igneous rocks, thereby 

 including the iron-bearing formations, this would give 2.35 per cent of 

 ferrous oxide and 4.02 per cent of ferric oxide. Upon this hypothesis 

 the difference between the amount of ferrous oxide present hi the original 

 rocks, 2.35 per cent, and that now found upon the average in the secondary 

 rocks, 1.77 per cent, is 0.58 per cent. To change this percentage of fer- 

 rous oxide to ferric oxide for the 675,000,000,000,000,000 metric tons of 

 sediments of the zone of katamorphism would require 435,000,000,000,000 

 metric tons of oxygen, or 35.55 per cent of the oxygen now in the 

 atmosphere. (See p. 944.) This shows how enorrnous has been the draft 

 of oxygen from the atmosphere in consequence of the oxidation of ferrous 

 oxide alone. (See p. 1003.) 



Another very large demand upou oxygen is made by the oxidation of 

 the metals and the sulphur united as sidphides in the original rocks. Of the 



