THE ORIGIN OF SOIL. 63 



three well-known genera, Crcnothrix, Lcptothrix and Cla- 

 dothrix (Fig. 10), quite a number of species of each being 



FIG. 10. 



Iron bacteria, Cladothrix. A shows a bit of thread more highly magnified. (Zopf.) 



known. These bacteria make use of iron compounds as 

 sources of energy just as the last group makes use of sulphur 

 compounds. Their chief source of iron is from iron carbonate 

 which comes from at least two sources. Some of it is origi- 

 nally from the soil \vhere it occurs in considerable quantity, 

 especially at some depths. Some of the iron comes from the 

 decomposition of organic compounds, the liberated iron com- 

 bining with the carbon dioxide to form carbonates. This iron 

 furnishes these bacteria with practically all the food they need, 

 for they will grow readily upon the surface of bricks if moist- 

 ened with ferrous sulphates. 



The iron carbonates are oxidized by the bacteria as follows : 



2FeCO 3 + 3H 2 O -f O = Fe 2 (OH) 6 -\- CO 2 



The iron hydroxide thus formed is deposited in the cell of 

 the bacterium causing it to assume a reddish brown color, or 



