Geological History of the Atmosphere. 437 



quantity of ferric oxide (sesquioxide of iron) in known rocks 

 is 2*65 per cent. If this were all originally protoxide of iron 

 it would require to effect its peroxidation for each half-mile 

 thickness of the earth's crust belonging to the continental 

 and transitional areas, or for each ^-mile thickness of the 

 whole of the earth's crust, a quantity of oxygen equal to the 

 whole quantity in our present atmosphere. Also it may be 

 noted that the oxygen required to produce 2*65 per cent, 

 sesquioxide of iron from protoxide is very nearly equivalent 

 to O'l per cent, of carbon, that is, there would require to be 

 this amount of carbon or carbonaceous matter in the ground 

 to balance the oxygen that has been removed from the 

 atmosphere in the oxidation of the protoxide of iron. 



Besides peroxide of iron, the binoxide of manganese found 

 in the ground has probably been peroxidized by atmospheric 

 free oxygen, but it may be passed over on account of its 

 comparatively small quantity. But besides peroxides there 

 is a very considerable quantity of sulphates found on the 

 earth, and these have very probably been formed through the 

 oxidation of sulphides by free oxygen. Clarke and Hille- 

 brand estimate the quantity of anhydrous sulphuric acid 

 (S0 3 ) which is found combined with lime and other bases in 

 the crust of the earth at 0*06 per cent, of the total rocks 

 present. The quantity of oxygen (viz. 0*036 per cent.) 

 represented in this percentage, being only about one ninth of 

 that required to produce the sesquioxide of iron (referred to 

 above) from protoxide, may be passed over, but besides the 

 sulphates found in the solid crust of the earth, there is a very 

 considerable quantity found in the sea. Prof. Dittmar (see 

 Encyclopedia Britt., 9th ed.) estimated the sea to contain 

 2192 xlO 12 tons MgS0 4 , 1666 xlO 12 tons CaS0 4 , and 

 1141 x 10 12 tons K 2 S0 4 , which are equivalent to a total 

 quantity of 2994 x 10 12 tons S0 3 (anhydrous sulphuric acid). 

 The oxygen present in this, viz. 1776 XlO 12 tons, is con- 

 siderably greater in amount than the total free oxygen of the 

 atmosphere (viz. 1200 x 10 12 tons). It is no doubt possible 

 that the sulphur present in the above sulphates may have 

 always existed in combination with oxygen, but this is not 

 very probable if there was originally a considerable excess of 

 oxidisable elements relatively to oxygen itself on the earth. 

 For the same reason we may infer that the sulphur of the 

 sulphide of iron which has been formed from sulphates by 

 the reducing action of organic matter has in the course of 

 such a reaction only returned to its original form of com- 

 bination, and should therefore be left out of account in 

 calculations regarding the composition of the primitive 



