1140 A TREATISE ON METAMORPHISM. 



and carbon dioxide. As denudation continues the belt of weathering 

 continues to migrate downward. As a consequence of the reactions of the 

 belt of weathering, combined with denudation, the character of the upper 

 part of the lode is greatly changed, there being produced in the upper part 

 of it a second concentration. If this process continues long enough under 

 favorable conditions, the deposits formed may be divided into three belts: 

 (1) Above the level of ground water, and in some instances extending 

 somewhat below it, is a belt largely composed of oxides of manganese, iron, 

 and copper; carbonates of lead, zinc, and copper; hydrated silicates of 

 nickel and zinc; chloride of silver; metallic gold, silver, and copper; and 

 residual enriched sulphides and tellurides; all with the associated gangue 

 minerals. (2) At and below the level of ground water, and in some cases 

 extending somewhat above it, is a transition belt composed of rich 

 sulphides of silver, mercury, copper, lead, zinc, nickel, etc.; tellurides 

 of gold and silver; and free gold, silver, and copper; with these are 

 subordinate amounts of oxidized products and of course the associated 

 gangue minerals. (3) Deeper down is a belt in which iron sulphides, 

 pyrite, and pyrrhotite preponderate; but in which are considerable amounts 

 of the more valuable sulphurets, the tellurides of gold and silver, and free 

 gold, silver, and' copper. Between the three classes of material there are 

 gradations. The oxidized belt passes gradually into the rich sulphide and 

 telhiride belt; the rich sulphide and telluride belt passes gradually into 

 the lean sulphide belt. It is not to be supposed that all of the above 

 products are to be found in a single lode. The development of the belts 

 is due to a complicated set of reactions caused by descending waters. 

 These reactions will be more fully appreciated when individual combina- 

 tions of metals are considered. Here only a brief general statement will 

 be made. 



In the belt of weathering the reactions which transform the original 

 products — sulphides, tellurides, and metals — are oxidation, hydration, and 

 carbonation, the fundamental reactions of the belt. In this belt the forma- 

 tion of the materials mentioned is but the result of the application of these 

 general processes to the exceptional geological products, ore deposits. 

 Before hydration and carbonation' can take place oxidation must occur. If 

 the sulphides be equally abundant, the sulphide which is most easily oxidized 

 is the first to disappear. The order of disappearance for the metals in 



