MANGANESE IN GOLD DEPOSITS 35 



in the character of the drainage or of the more constant conditions 

 controlling the water-circulation, the chemical composition of the 

 solutions affecting this zone may change from season to season. 

 They may at one time be ferric sulphate or oxidizing waters, and 

 at another time ferrous sulphate or reducing waters, since, after 

 a wet season, the ferrous sulphate waters from below would tend 

 to rise, after dilution with fresh water added by the rains. Conse- 

 quently, the minerals of this zone may include, besides the residual 

 primary and secondary sulphides, the oxides, native metals, chlo- 

 rides, etc. Between the top of this zone and the surface or the 

 apex of the deposit chemical activity is probably slow, because 

 there is a scarcity of sulphides and other easily altered minerals 

 to supply the salts upon which the chemical activity of ground- 

 water in a large measure depends. As the country is eroded, this 

 zone also descends; and if a mineral or metal persists long enough, 

 the upper limit of the zone of active change passes below it, and 

 may ultimately be exposed at the outcrop. 



3. The several successive zones in depth. — -As shown by S. F. 

 Emmons, W. H. Weed, and others, many lodes, when followed from 

 the surface down the dip, show characteristic changes. Below the 

 outcrop, the upper part of the oxidized portion of the lode may be 

 poor. Below this there may be rich oxidized ores; still farther 

 down, rich sulphide ores; and below the rich sulphides, ore of 

 relatively low grade. Such ore is commonly assumed to be the 

 primary ore, from which the various kinds of ore above have been 

 derived. The several types of ore have a rude zonal arrangement, 

 the so-called "zones" being, like the water-table, undulatory. 

 They are related broadly to the surface and to the hydrostatic 

 level, but are often much more irregular than either; for they 

 depend in large measure on the local fracturing in the lode which 

 controls the circulation of underground waters. Any zone may 

 be thick at one place and thin, or absent, at another. If these 

 zones are platted on a longitudinal vertical projection, it is seen 

 that the primary sulphide ore may project upward far into the 

 zone of secondary sulphides, or into the zone of enriched oxides, 

 or into the zone of leached oxides, or may even be exposed at the 

 surface. The zone of secondary sulphide enrichment (which is 



