ORGANIC AGENCIES. 89 



form of iron is called bog iron-ore. It is by observing 

 such phenomena, and trying to find out how they are pro- 

 duced, that we may expect to throw light on the forma- 

 tion of the great iron-beds which are found in the strata 

 of earlier geological times. More commonly the iron is 

 in the form of hydrated ferric oxide (2Fe 3 3 ,3H 2 0) but 

 sometimes of ferrous carbonate (FeC0 3 ). 



Mode of Formation. Iron has a very strong affinity 

 for oxygen,, as is shown by the rapid rusting of iron when 

 exposed to the weather. But this is true, not only of 

 metallic iron, but also of ferrous oxide, and of ferrous 

 carbonate. In all cases it runs rapidly into the condition 

 of highest oxidation viz., ferric oxide. But, although 

 iron has so strong an affinity for oxygen, yet a, portion of 

 the oxygen of ferric oxide will be taken away from it by 

 the superior affinity of organic matter in a state of decom- 

 position. Thus, ferric oxide (Fe 2 3 ) in contact with 

 decomposing organic matter will be reduced to ferrous 

 oxide (FeO), which then readily unites with carbonic 

 acid (C0 2 ), always present in meteoric waters, and forms 

 ferrous carbonate (FeC0 3 ). Ferrous carbonate is feebly 

 soluble in water containing C0 2 . 



Now, iron is a very abundant substance, but, on account 

 of its affinity for oxygen, it exists most naturally only in 

 the form of ferric oxide ; in which state, therefore, it is 

 almost universally diffused as a red or yellow coloring- 

 matter of soil and rocks. In this state, though abundant, 

 it is unavailable to man. But organic matter, in a state 

 of decay, is everywhere on the surface of the ground. 

 This is dissolved by rain-water, and sinks into the earth. 

 Therefore, all subterranean water contains organic matter 

 in solution. Such water, percolating through red soils 

 or red rooks, first reduces the iron to ferrous oxide, then 

 to ferrous carbonate, then takes it into solution i. e., 

 washes it out of the soil or rock, leaving these decolor- 

 ized, then comes to the surface, as springs containing 



