322 C. H. SMYTH, JR. 



cent. Fe 2 3 and 0.57 per cent. FeO. These figures represent the 

 results as stated in five distinct columns, in each of which the direction 

 of the change is the same — an increase of the ratio of ferric to ferrous 

 oxide as compared with the corresponding ratio in the old crust. 



By combining the figures for shales and sandstones, we have, as 

 an expression of the iron contents of the sedimentary rocks, excluding 

 limestones, 2.64 per cent. Fe 2 3 and 1.52 per cent. FeO. 



Comparing these figures with those given above for the older crust, 

 the contrast is pronounced. In the older crust the ferrous oxide is in 

 excess, while in the sediments the ferric oxide is markedly preponder- 

 ant. This relation is expressed by the following ratios: 



In older crust, FeO :Fe 2 3 : 13.52 : 2.63, or about 1:0.75. 

 In sediments, FeO : Fe 2 3 :: 1.52:2.64, or about 1:1.75. 



In other words, in the old crust there is about three-fourths as much 

 ferric as ferrous oxide, while in the sediments there is about one and' 

 three-fourths times as much. 



Corresponding ratios for the two chief groups of sediments are as 



follows : 



Shales, FeO : Fe 2 3 : : 2.46 : 4.03, or about 1 : 1 .64. 



Sandstones, FeO:Fe 2 3 : : 0.5 7: 1.24, or about 1:2.17. 



Thus, as stated above, the magnitude, but not the sign, of the 

 results for the sediments will vary with the weights given to shales 

 and to sandstones. But even were the figures for shales taken alone, 

 throwing out the sandstones entirely, it is evident that the ratio of 

 ferric to ferrous oxide is double that found in the older crust. 



As the shales, sandstones, and limestones represent, in altered 

 form, the materials of the old crust, and as the two latter show, as 

 compared with the old crust, a decrease of iron, while the shale 

 shows no increase, it is clear that, if the analyses are to be used as a 

 basis of calculation, allowance must be made for concentration of iron 

 in ores and other highly ferruginous rocks. Moreover, in the change 

 from old crust to silicious sediments, there must be a loss of calcium 

 and magnesium to form limestones, and of sodium held in sea- water,, 

 while there is a gain of carbon dioxide and water. 



In view of these facts, it is probable that 4 per cent, is a moderate 

 estimate of the total oxidized iron of sediments, excluding limestones, 

 and it is assumed that the ratio between ferrous and ferric oxides is- 

 that above derived from Stokes's analyses. 



