LEAD MITMAN. 



601 



tained in the above table but probably even more clearly portrayed 

 in the graph (fig. 5). They are the transitions brought about by 

 the changing character of the ores. Prior to 1870 lead was derived 

 from simple ores by a comparatively simple procedure, but after 

 that date, which marks the connecting of the East and West by rail, 

 the field of industrial opportunity was greatly enlarged, bringing 

 about increased demands, and subsequently the realization of the 

 limits of the ores then being used to meet the requirements. The 

 result was that ores wholly different in character had to be used, and 

 the industry, therefore, was compelled to progress from simple direct 

 methods to complex ones, requiring, more and more, the highest order 

 of industrial coordination. As a matter of fact, few industries in the 

 United States have escaped this transition, for it is a characteristic 

 of industrial progress and runs parallel with an advancing civiliza- 

 tion. 



Fig. 5. — Changing composition of the lead resources. Data from United States Geological 



Survey. 



LEAD ORES : OCCURRENCE, MINING, AND TREATMENT. 



Although no two deposits of the same type of lead ores occur 

 exactly alike, there is a similarity among types found in the same 

 general sections of country. Thus, in the Mississippi Valley where 

 the land is generally flat, the ore bodies occur in horizontal or gently 

 rolling rock strata, but in the Rocky Mountains they may occur in 

 all sorts of positions of rock strata from horizontal to vertical. In 

 the southeastern Missouri district the ore bodies lie in pitching 

 troughs and in a number of different ways: (1) Disseminations in 

 limestone and shale, (2) horizontal sheets along bedding planes, (3) 

 filling the walls of joints, and (4) in cavities in the country rock. 

 The first is of the greatest importance commercially. They are found 

 from 150 to 800 feet beneath the surface and vary in size from 5 to 

 100 feet in thickness ; 25 to 500 feet in width, and have been traced 

 for upwards of 2 miles. Being of such enormous extent and con- 



