REDISTRIBUTION OF IRON. 987 



the amount of ferric oxide is 3.94 per cent and of ferrous oxide is 3.48 per 

 cent, and in his estimate of 1900 the ferric oxide is 2.63 per cent and the 

 ferrous oxide 3.52 per cent. Reckoned as an oxide, iron thus has the third 

 place in abundance, being surpassed only by silica and alumina. 



Iron occurs as an original constituent of the igneous rocks in perhaps 

 more numerous forms than an}?- other element. It is found as a sulphide as 

 pyrite, pyrrhotite, etc.; and as an oxide as hematite, magnetite, and ilmenite. 

 It occurs in many silicates. Of these, the pyroxenes and amphiboles, the 

 olivines, and the micas are the more important. In the meteorites iron is 

 the most abundant constituent, occurring alloyed with nickel and cobalt, 

 and in sulphides, oxides, silicates, etc. 



In 78 shales the ferric oxide is 4.03 per cent of the rock, the ferrous 

 oxide 2.46 per cent; in 624 sandstones the ferric oxide is 1.24 per cent, and 

 the ferrous oxide 0.57 per cent; in 843 limestones the ferric oxide is 0.66 

 per cent, the ferrous oxide being undetermined." 



The 4.03 per cent of ferric oxide and the 2.46 per cent of ferrous oxide 

 in the shales is equivalent to 4.73 per cent metallic iron. The 1.24 per 

 cent of ferric oxide and the 0.57 per cent of ferrous oxide in the sand- 

 stones is equivalent to 1.31 per cent metallic iron. The 0.66 per cent of 

 ferric oxide in the limestones is equivalent to 0.462 per cent metallic iron. 

 It thus appears that there is a slight increase, less than 0.1 per cent, in the 

 amount of iron in the shales as compared with the original rocks. There is 

 great depletion of iron in the sandstones — which contain between one-third 

 and one-fourth of the amount in the original rocks. The depletion of the 

 iron in the limestones is very great — there being about one-tenth the 

 amount in the original rocks. The slight increase in the shales is by no 

 means sufficient to account for the depletion of the sandstones and lime- 

 stones. If we multiply the percentage of iron present in the different kinds 

 of sediments by the estimated quantity of those sediments, and add the 

 three together, we have the following equation: 



4.73 X .65 + 1.31 X .30 + .462 X .05 = 3.491 per cent. 



Since the amount of iron present in the original rocks is 4.64 per cent, 

 this shows a deficiency of 1.149 per cent for the entire mass of sediments. 

 This difference seems small, but it amounts to 7,155,750,000,000,000 



«Clarke, fit., Bull. 168, pp. 16, 17. 



