ORE DEPOSITS 13 



wall is the lower boundary of a lode. The selvage is the zone of 

 clay or decomposed rock, or both, separating the vein material 

 fn>m its walls (Fig. 11). 



The Occurence of Metalliferous Veins. (1) Metalliferous 

 veins occur mostly in disturbed and highly metamorphosed 

 regions. The tilting and the folding causes fissures that may be 

 subsequently filled with mineral matter. Mineral veins therefore 

 occur most frequently in mountainous regions or in the regions 

 dis turbed by igneous activities. The lead deposits of Missouri 

 are an exception to the rule for these occur in undisturbed and 

 fossiliferous Paleozoic limestones. 



FIG. 11. A vein with its ores extending into the altered country rock. 



(2) These veins are more abundant in the older geological 

 formations. There is no relation between the occurrence of min- 

 eral veins and age alone. The connection is with metamorphism, 

 which is more common in the older terranes. In the Pacific 

 coast belt metalliferous veins often occur in Jurassic, Cretaceous 

 and Tertiary formations. But these terranes have been sub- 

 jected to folding and metamorphism. 



(3) Parallel veins usually have the same metallic content. 

 (Veins at right angles may in some cases be exceptions.) The 

 parallel fissures were formed by the same causes, at the same time 

 and filled with similar material. Fissure veins not parallel with 



