3 2 ° 



C. H. SMYTH, JR. 



degree tentative, it is hoped that the mere formulation of the problem 

 may be sufficiently suggestive to prove of some value. 



Under atmospheric conditions, with abundant oxygen, iron is 

 stable in the ferric condition, and during the circulation of mineral 

 matter involved in the processes of denudation and sedimentation 

 there would be, in the absence of opposing agents, a strong tendency 

 toward the conversion of ferrous into ferric compounds, resulting, 

 in the case of thoroughly disintegrated and decomposed materials, 

 in a complete oxidation of the iron. The younger sediments, in so 

 far as they are derived from other sediments, implying a thorough 

 working over of their materials, would contain little or no ferrous 

 iron. But opposed to this process we have the very potent reducing 

 agent, organic matter. Not only does this agent, when present, 

 take up oxygen that might otherwise combine with ferrous com- 

 pounds, but it is also able to take oxygen from ferric compounds, 

 reducing them to the ferrous state, thus counteracting the tendency 

 toward oxidation above referred to. 



According as the one or the other of these processes has predom- 

 inated during geologic time, there has been a progressive oxidation 

 or reduction of iron, and the rocks of the crust contain more or less 

 ferric iron, while oxygen has been taken from, or added to, the 

 atmosphere. If the processes balance, the ratio between ferrous and 

 ferric iron remains constant, and the atmosphere is unaffected. 



It is manifest that a solution of the problem would be afforded 

 by analyses representing, on the one hand, the average composition, 

 so far as ferrous and ferric iron are concerned, of the crust of the 

 earth before denudation and sedimentation began; and, on the other, 

 of the sedimentary rocks. 



An approximation to the former is afforded by Dr. F. W. Clarke's 1 

 estimate of the composition of the "older crust," based upon 880 

 analyses of crystalline rocks. 



For obvious reasons, a reliable estimate of the bulk composition 

 of the sedimentary rocks is much more difficult to obtain, but the 

 figures given by Stoke's analyses 2 are doubtless the best now available, 

 and they are used as the basis of the present discussion. The samples 

 for these analyses were prepared by Mr. G. K. Gilbert, in an effort 



1 Bulletin No. 168, U. S. Geological Survey, p. 14. 2 Ibid., p. 17. 



