472 A TREATISE ON METAMORPHISM. 



with carbon dioxide, as explained under "Carbonation;" and thus may be 

 produced a great quantity of iron carbonate, which may join the sea of 

 ground water and furnish the material for chalybeate springs and for 

 siderite deposits, as explained on pages 824-829. The clay ironstones and 

 the siderite, often associated with the coal beds and frequently in the coal 

 itself, are illustrations of such siderite deposits. Thus the association of 

 deposits of iron carbonate with coal deposits is explained, and also — as 

 developed on pages 842-846 — there is in this iron carbonate thus formed 

 the source from which other iron-ore bodies are concentrated. 



Another example of deoxidation is furnished by the nitrates, which may 

 be reduced to nitrites, or even to free nitrogen when the conditions of the 

 belt of weathering are those of abundant vegetation and very high humid- 

 ity." The reduction of the nitrites is effected by definite bacteria. It is 

 highly probable that the oxygen abstracted in the deoxidation of nitrates, 

 like that abstracted from ferric iron, is used hj the oxidizing bacteria in 

 decomposing the organic matter. 



Under conditions similar to those in which ferric iron is reduced to 

 the ferrous form and nitrates are reduced, sulphates may also be reduced 

 to sulphides. This is usually accomplished mainly by organic material 

 where abundant to serve as a reducing agent; but also in this reduction, as 

 in various other processes, bacteria may play a part. 6 



Finally, where the moisture is too abundant the oxidation of the plants 

 themselves is greatly delayed, and may be permanently stayed. The best 

 illustration of the lag in decay of plants and deoxidation of the ferric salts, 

 nitrates, and sulphates is found in the marshes where water entirely covers 

 the soil, and thus makes the oxidation very slow indeed. Where the plants 

 fall below the surface of the water the plant decay may be very partial, 

 and hence there may be produced peat or coal beds. But in this connec- 

 tion it should be recalled that where, as a result of great humidity, the 

 decay of plants is slow and deoxidation takes place the conditions approach 

 those of the belt of cementation. In swamp areas the thickness of the belt 

 of weathering is practically reduced to zero; the roots of vegetation reach 

 below the level of ground water, and under these circumstances the reactions 

 which take place in connection with organic matter are those of the belt of 

 cementation rather than those of the belt of weathering. 



t'Aikman, cit., pp. 177-178. *>Lafar, cit., pp. 363 et seq. 



