J. D. Whitney on changes in Mineral. Veins. 55 
ferruginous aspect of the lode disappears; the gangue becomes 
more solid, and the walls are better defined. 
These changes in the upper portion of the sulphuret-bearing 
lodes are usually conceived to be the result of the action of air 
and water introduced from the surface and penetrating gradually 
downwards. Through their joint influence the sulphuret of cop- 
per and iron is gradually decomposed and while the latter metal 
remains behind in the form of an impure hydrous oxyd, or gos- 
san, the copper is also converted into an oxyd and may remain in 
that state, or combine with the sulphuric acid furnished by the 
oxydatiou of the sulphur of the original ore, or with any other 
acid which may chance to be present, thus giving rise to the nu- 
thus far, from which copper has been obtained in any quantity 
Worthy of notice. i3 
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