14 



ECONOMIC GEOLOGY 



each other (save perhaps at right angles) were formed at different 

 times and filled under different conditions. The east and west 

 veins at Cornwall, England, carry tin and copper and are pre- 

 Triassic. The northeast and southeast veins are post-Triassic, 

 but also contain tin and copper. The north and south veins are 

 Cretaceous and contain lead and iron. 



The Richness of Veins. (1) If two mineral veins intersect 

 each other one or both are generally richer at the point of inter- 

 section. This increased value may be due to the reaction of 

 waters bearing different bases in solution in the two fissures. 



FIG. 12. Section showing the form of the ore body in the, Victor, 

 Smuggler Lee and Buena Vista miners, Cripple Creek district, Colorado. 

 A, Ore. (After Penrose.) 



(2) Mineral veins are likely to be richer near their intersection 

 with either acid or basic intrusives. This is especially true in 

 regions that have suffered much metamorphism. It shows the 

 influence of heat upon the metallic contents of the veins. 



(3) A change in the character of the county rock which a vein 

 traverses may determine a change either in the contents or in the 

 richness of the vein material. A vein may be well defined in the 

 sedimentaries in close proximity to an acid or basic intrusive but 

 upon invasion of the igneous rocks the vein is often subdivided 

 into numerous branches. 



Irregularities in Veins. (1) Fissure veins are often irregular, 

 as shown in Fig. 12. The vein often divides into numerous 



