144: ORGANIC AGENCIES. 



this iron-ore accumulates is very interesting, and in a geological point 

 of view very important. 



Peroxide of iron exists very generally diffused as the red coloring- 

 matter of soils and rocks. In this form, however, it is insoluble, and 

 therefore can not be washed out by percolating waters. For this pur- 

 pose the agency of decomposing organic matter, present in all percolat- 

 ing waters, is necessary. Decomposition of organic matter is a process 

 of oxidation. In contact with peroxide of iron (ferric oxide) it deoxi- 

 dizes, and reduces it to protoxide (ferrous oxide). The acids, especially 

 carbonic acid, produced by decomposition of the organic matter, then 

 unite with the protoxide, forming carbonate of iron. The carbonate, 

 being soluble in water containing excess of carbonic acid, is. washed 

 out, leaving the soils or rocks decolorized, and the iron-charged waters 

 come up as chalybeate springs. But the ferrous carbonate rapidly 

 oxidizes again in the presence of air, by exchanging its carbonic acid 

 for oxygen, and returns to its former condition of ferric oxide, and is 

 deposited. Thus all about iron-springs, and in the course of the 

 streams which flow from them, and in low places where their waters 

 accumulate, we find reddish deposits of hydrated ferric oxide. This 

 is the most common but not the only form. For if the iron-waters 

 accumulate, and the iron be deposited in the presence of excess of 

 organic matter, as peat, then the iron is not (for in the presence of 

 this reducing agent it can not be) reoxidized, but remains in the form 

 of ferrous carbonate. 



Thus there are two forms in which iron leached out from the soils 

 and rocks may accumulate, viz., ferric oxide and ferrous carbonate : the 

 former is accumulated where the organic matter is in small quantities, 

 and consumes itself in doing the work of dissolving and carrying ; the 

 latter where the organic matter is in excess. 



Many familiar phenomena may be explained by the principles given 

 above : 1. Clay containing both iron and organic matter is never red, 

 but always blue or slate-colored, because the iron is in the form of fer- 

 rous carbonate ; but the same clay will make good red brick, because 

 by burning the organic matter is destroyed and the iron peroxidized. 

 2. In red-clay soils, such as those of our primary regions, the surface- 

 soil, especially in forests, is always decolorized, the coloring of peroxide 

 of iron being washed out and carried deeper by water containing or- 

 ganic matter derived from the vegetable mold. 3. In sections of red 

 clay, at the sides of gullies or railroad-cuttings, along every fissure or 

 crevice through which superficial waters percolate, the clay is bleached. 

 The marbled appearance of red clays is also probably due, in a great 

 measure, to the irregular percolation of superficial waters containing 

 organic matter. 4. The under clay or sand of peat-bogs is usually 

 decolorized. 



