90 DYNAMICAL GEOLOGY. 



iron carbonate (chalybeate springs). This is where we 

 took it up (page 75). Then, as was there shown, it again 

 comes in contact with air, gives up C0 2 and retakes oxy- 

 gen, and is reconverted into ferric oxide, which, being 

 insoluble, is deposited. 



The above is a complete explanation of the accumula- 

 tions of ferric oxide. In this case the organic matter is 

 consumed, i. e., changed into 00 2 and H 2 0, in doing the 

 work of reduction and solution, and there is nothing to 

 prevent the iron from returning to the condition of ferric 

 oxide. But, if there be an excess of organic matter, as 

 peat, for example, in the place where the deposit occurs, 

 then the iron will be deposited as ferrous carbonate, 

 because it can not exist in the form of ferric oxide in the 

 presence of decaying organic matter. This is a sufficient 

 explanation of deposits of iron-carbonate. 



Familiar Illustrations. We have gone so far into 

 this explanation because the effects of water containing 

 C0 3 in leaching out the coloring-matter of soils may be 

 observed on every hand, and thus, therefore, afford an 

 excellent field for cultivating the observing power of the 

 pupil. 



1. If a dead stump, with roots ramifying in red soil, be 

 examined, it will often be observed that the soil is bleached 

 immediately about each root. This is because water con- 

 taining organic matter, running down the root, leaches 

 out the red coloring-matter of the soil. 



2. In every railroad-cutting, or other excavation in red 

 soil, it will be observed that the walls of every fissure in 

 the soil, through which water from the surface descends, 

 will be bleached for a little distance on each side. 



3. Red clays exposed to view by excavations, natural 

 or artificial, are often variegated or marbled with irregu- 

 lar streaks and spots of deeper or lighter color. This is 

 produced by irregular percolations of water containing 

 organic matter. 



