MKTM.S 



57 



drift ing along the line of the vein must be carried for thousands of 

 t 1 1 in ore that does not pay for the extraction of the metal before 

 such a pocket is encountered. Again, the line carrying the gold 

 may be simply a thin seam or film, so that all the ore workable 

 with profit may be deposited from solution on a film so thin as 

 to be almost invisible to the naked eye. Sometimes the material 

 of t he entire vein is rich enough to pay for the profitable extrac- 

 tion of the metal. This is especially true in some gold mines of 

 British Columbia and Alaska. The ores are more likely to be 

 spotted than otherwise, that is, gold occurs in small pockets. 





FIG. 42. Section in the Elkton mine, Cripple Creek district, Colorado, 

 showing the relation of the vein A to 'the dike B and to the country rock C. 

 (After Penrose.) 



i 



rather than with a uniform distribution throughout the entire 

 vein. The vast majority of gold deposits within the United 

 States occur in true fissure veins. (See Fig. 41.) 



The second type is known as the propylitic in which through 

 the metasomatic alterations of the wall rock there is developed 

 secondary minerals such as chlorite and epidote, the former re- 

 sulting from the metamorphism of various micas, the latter from 

 the alterations of feldspars. These propylitic types are in close 

 proximity to veins of sericite and kaolin. The first type occurring 

 with quartz gangue in true fissure veins is seldom highly argenti- 

 ferous, the second type in which kaolinization has taken place is 



