12 ECONOMIC GEOLOGY 



clearly defined as separating it from the surrounding rock. 

 Mineral veins fall into three distinct classes: 



(1) Fissure Veins or True Veins of Fracture. A fissure is 

 of indefinite length traversing strata independent of bedding, 

 generally nearly vertical and filled with mineral matter. The 

 fissure is not parallel to the bedding. The walls may or may 

 not coincide, and are nearly parallel with each other. The 

 fissure was in the rock prior to the filling. The fissure vein 

 then is the filled fissure, which is of indefinite length. It is 

 from the fissure veins that our largest supply of the precious 

 metals comes. (See Fig. 9.) 



(2) Gash Veins. Gash veins are represented by a metalli- 





FIG. 10. Gash vein in the magnesian limestone of Wisconsin. (After 



Chamberlain.) 



ferous deposit found only in limestones and confined to a single 

 layer or formation. They are the most common in the bedding 

 and the joint planes. (See Fig. 10.) 



(3) Segregated Veins. These correspond to the planes of 

 bedding or stratification and in many respects are not unlike 

 true fissure veins or gash veins. These veins vary in thickness 

 and direction. Their irregularities are many. They are often 

 pinched out as by forcing the walls together, or by the ex- 

 pansion due to tension of the rock masses, or solution of the 

 walls of the original channel. These veins also vary much in 

 richness. The hanging wall is that part of the country rock 

 lying geologically immediately above a vein or bed. The foot 



