606 A TREATISE ON METAMORPHISM. 



Under ordinary circumstances oxygen is practically exhausted in the 

 early part of the journey, so that it is rather unusual for marked oxidation 

 to extend more than a few meters below the level of ground water. But 

 where, because of marked relief, there are large and strong downward- 

 moving currents through open and porous rocks, the process of oxidation 

 may extend to a very considerable depth into the belt of cementation. 

 This has been esjjecially noticed in mining regions, where ore deposits are 

 commonly located along joints, faults, or other trunk channels where there 

 are numerous and large openings. One of the best illustrations of oxidation 

 extending to a considerable depth is that furnished by the iron ores of the 

 Lake Superior region. Here, as fully explained in the chapter on ore 

 deposits, the process of oxidation has extended on a very great scale to the 

 depth of 100 meters, in mines has produced great ore bodies to a depth 

 of 300 meters, and exceptionally has gone on to au important extent to a 

 depth of 500 to 700 meters. As an instance in which oxidation has 

 extended to a great depth may be mentioned the San Juan district of Colo- 

 rado. Here in some of the mines oxidation is marked to a depth of 600 

 meters, and is occasionally noticeable to a depth of 1,000 meters. In 

 contrast with such regions as these are others in which the water ascends 

 almost or cpiite to the surface. In such instances oxidation scarcely extends 

 below the level of ground water. An excellent illustration of this is fur- 

 nished by the Missouri-Kansas lead and zinc district. 



The oxygen is chiefly utilized in the oxidation of iron, sulphur, and 

 organic material. In the upper part of the belt, where oxygen is abundant, 

 large amounts of hematite and limonite may be produced by the oxidation 

 of the ferrous particles, but for the greater part of the belt of cementation, 

 where oxygen is somewhat deficient, the ferrous oxide is oxidized only to 

 the form of magnetite, since this requires, per unit of iron, only two-thirds 

 as much oxygen as to produce hematite and limonite. 



The sulphur is mainly united with the iron as pyrite and marcasite. At 

 the same time the sulphur is oxidized the iron also is oxidized. Where the 

 oxygen is abundant there may be produced ferrous sulphate, or ferric oxide 

 and sulphuric acid; but where oxygen is not abundant, as in the major 

 portion of the belt, magnetite and sulphurous acid are more likely to be 

 formed. 



Where the oxygen is utilized in the oxidation of organic material 

 carbon dioxide and water are produced, and these compounds join the 



