MINING GEOLOGY 
349 
materials now seems not unlikely. It is probable that most of the 
diffused metallic content of the sedimentary rocks is in reality 
immediately derived from meteoric sources; for its derivation 
from the country-rock of mining districts, especially in tracts far 
removed from volcanic activity, has never been a very satisfactory 
explanation. 
As commonly regarded a mineral vein consists of (1) the deep¬ 
er primary portion below groundwater level, and (2) above the 
latter a limited weathered part known as the gossan. The lower 
part is composed of sulphides and the upper portion of oxides. 
Of late, between the two, at groundwater level, there has come 
to be recognized a third zone, that of secondary sulphide enrich¬ 
ment. According to this simple conception the movement of ore- 
