38 KEMP'S ORE DEPOSITS. 



the expressive term " melon vein," thus comparing them with a 

 vine and its melons. 



1.05.05. Changes in Character of Vein Filling. In discuss- 

 ing the influence of wall rock the changes that occur in veins 

 were briefly mentioned. But even where the walls remain uniform 

 there is always variation in contents, and of course in value, from 

 point to point. Ore, gangue, horses, and walls alternate both 

 longitudinally and in depth, and such changes must be allowed for 

 and averaged by keeping exploration well in advance of excava- 

 tion. Even a series of parallel veins may all prove fickle. In 

 illustration of the above the Marshall tunnel of Georgetown, Colo., 

 may be cited. It cut twelve veins below their actual workings, 

 and every one was barren at the tunnel though productive above. 

 (J. J. Stevenson, Wheeler's Survey, Geology, Vol. III., p. 351.) 



1.05.06. Secondary Alteration of the Minerals in Veins. It 

 has already been stated that the chief ore minerals in vein fill- 

 ings are sulphides. Where these lie above the line of permanent 

 subterranean water they are exposed to the oxidizing and hydrat- 

 ing action of atmospheric waters, which, falling on the surface, 

 percolate downward. The ores are thus subjected to alternating 

 soakings and dryings which encourage alteration. The sulphides 

 change to sulphates, carbonates, oxides, or hydrous forms of the 

 same, and the metallic contents are in part removed in the acid 

 waters which are also formed. Pyrite, which is the most wide- 

 spread of the sulphides, becomes limonite, staining everything with 

 its characteristic color. Galena becomes cerusite or anglesite. 

 Blende affords calamine and smithsonite. Copper ores, of which 

 the usual" one is chalcopyrite, change to malachite, azurite, chryso- 

 colla, cuprite, and melaconite, and to the sulphide chalcocite. The 

 silver sulphides afford cerargerite. The rarer metals alter to cor- 

 responding compounds of less frequency. These upper portions 

 are also more cellular and porous, being at times even earthy. The 

 rusty color from the presence of limonite often marks the outcrop 

 and is of great aid to the prospector. It has been called the iron 

 hat, or gossan. This feature has important economic bearings. 

 The character of ores may entirely change at a definite point in 

 depth, and the later products, if not lower in grade, as is often the 

 case, may demand different, perhaps more difficult, modes of treat- 

 ment. Oxidized ores are the easiest to smelt, and the benefit of 

 careful exploration before indulging in too confident expectations 

 may be emphasized. As examples, the Ducktown copper deposits 



