826 A TREATISE ON METAMORPHISM. 



possibly in part as basic feme sulphate. Supposing the iron salt to be car- 

 bonate, it would be precipitated according- to the following reaction: 



4FeC0 3 +3H 2 0+20=2Pe 2 3 .3H 2 0+4CO ! . 



Where this process goes on, on an extensive scale, limonite bodies (considered 

 on pages 842-843) are built up. 



It was formerly supposed that this reaction took place as a result of 

 tne work of oxygen and moisture alone, and this is true to some extent. 

 But recent observation has shown that where in lagoons iron carbonate is 

 abundant the oxidation is largely performed through the agency of a class 

 of bacteria called the iron bacteria. It has been found that these bacteria 

 are unable to exist without the presence of iron carbonate or manganese 

 carbonate, but the iron carbonate is the chief compound used. This material 

 they absorb into their cells. There the iron carbonate is oxidized and the 

 limonite is precipitated. Says Lafar: 



The decomposing power of these organisms is verj r great, the amount of fer- 

 rous oxide oxidized by the cells being a high multiple of their own weight. This 

 high chemical energy on the one hand, and the inexacting demands in the shape of 

 food on the other, secure to these bacteria an important part in the economy of 

 Nature; the enormous deposits of ferruginous ochre and bog iron ore, and prob- 

 ably certain manganese ores as well, being the result of the activity of the iron 

 bacteria. 3 



Evidence is furnished of the precipitation of the limonite of bog iron- 

 ore deposits in this manner by the discovery in some of them of large 

 numbers of the sheaths of the iron bacteria. 6 Further evidence of the 

 importance and activity of these bacteria is furnished , by their partly or 

 completely closing water pipes of cities where the water contains a consid- 

 erable amount of iron carbonate/ 



The iron part of the salts carried down to the sea as a sulphate would 

 be likely to be thrown down as basic ferric sulphate, d according to the 

 following reaction : 



12FeS0 4 +60+(x+9)H 2 0=Fe 2 (S0 4 ) s .5FeA.xH 2 0+9H 2 S0 1 . 



« Lafar, F., Technical mycology, Lippincott & Co., 1898, vol. 1, p. 361. 



6 Fischer, A., The structure and functions of bacteria, trans, by A. Coppen Jones, Clarendon 

 Press, Oxford, 1900, p. 69. 



"Lafar, cit., p. 361. 



<"' Pickering, S. P. U., On the constitution of molecular compounds; the molecular weight of basic 

 ferric sulphate: Jour. Chem. Soc. London, vol. 43, 1883, p. 182. 



