MODE OF ACCUMULATION 495 



sulphide, and silicate, mostly the first, in the Ordovician and Cambrian 

 limestones and the intercalated slates. 



Mode of Accumulation of the Iron 



The form of the ore fragments and the mode of their occurrence, as ex- 

 plained above, necessitate the conclusion that the ores are secondary or 

 derived products. It yet remains to show the manner in which the iron 

 oxide was collected from its diffused condition in the original rock into 

 the deposits of commercial ore in which it now occurs. To bring this 

 about, (1) solution, (2) transportation, and (3) precipitation are neces- 

 sary. The atmospheric waters are the agents which have produced the 

 change. 



Oxygenated waters acting on the pyrite would take it into solution by 

 oxidizing the sulphur into sulphuric acid, part of which combines with 

 the iron as ferrous sulphate and part remains free or combines with any 

 other base that may be present. The reaction is FeS 2 + 70 + H 2 = 

 FeS0 4 + H 2 S0 4 . The H 2 S0 4 would readily combine with any lime or 

 magnesia carbonate present, forming the corresponding sulphates. The 

 FeS0 4 will rapidly oxidize to basic ferric sulphates and finally to 

 hydroxide. 



The iron carbonate is soluble in all acidulated waters — in sulphuric 

 acid as the sulphate, in carbonic acid as the bicarbonate, in organic acids 

 as the corresponding organic salt. As one or more of these acids is nearly 

 always present in meteoric waters near the surface, the solution of any 

 iron carbonate necessarily follows a contact with them. In the presence 

 of oxygen and moisture both the sulphide and carbonate change readily 

 to ferric oxide, which change may take place in the original position of 

 the iron, and later it may be again taken into solution and be added 

 to the iron supply for the ore deposits. 



The ferric oxide is much less soluble than the ferrous oxide, and is 

 the most stable form of iron in nature. It is not appreciably soluble in 

 carbonic acid or ordinarily as ferric oxide in the organic acids, although 

 Bischof* and Julien t state that ferric oxide is soluble in apocrenic acid 

 in the presence of ammonia. The organic acids, however, are active 

 reducing agents, and change the ferric oxide to ferrous, in which form it 

 is soluble, as above stated, in any of the acids. Illustrations of this 

 action may be seen at almost any point where roots penetrate yellow 

 clay. A zone of white or light colored clay from which the iron has 

 been leached may be seen surrounding the decayed or decaying roots. 



* Elements of Chem. and Phys. Geology. 



fProc. Amer. Assoc. Adv. Sci., 1879, vol. 28, p. 401. 



