LEAD MITMA1ST. 603 



As to the origin of these various deposits, they are all associated 

 directly with plutonic activitjr, hot waters originating deep down in 

 the earth and holding lead, silver, copper, gold, and zinc in solu- 

 tion, ascending and depositing their metallic burden as they en- 

 countered the right conditions of temperature, pressure, and reduc- 

 ing agencies. The Coeur d'Alene district of Idaho, for instance, is 

 an area composed originally of metamorphosed sandstones and shales 

 of one of the oldest geologic periods, which have been intruded by 

 masses of molten rock of varying compositions and then solidified. 

 Accompanying these intrusions were water solutions of subterranean 

 origin carrying metallic compounds from which the metals were 

 later precipitated, forming fillings in open spaces but largely replac- 

 ing the surrounding rocks along zones of Assuring or shearing. The 

 deposits, which have a vertical range of 4,000 feet, were probably 

 formed beneath several thousand feet of overburden, which has 

 since been removed by erosion, bringing the ore bodies to light. 



ORE DRESSING OR CONCENTRATION. 



The mined ore, no matter what its origin, on being brought to the 

 surface, is treated to separate the dirt from the valuable minerals 

 and further to separate the several minerals from each other. This 

 treatment is called ore dressing; the structure in which it is con- 

 ducted being called the mill. The treatment is almost wholly me- 

 chanical and involves " crushing " to unlock the waste and valuable 

 portions of the ore ; " screening " to grade the broken ore ; and " con- 

 centrating " to separate the valuable from the waste portions of 

 ore. Mills constitute the surface improvement of a mine, located 

 there for the reason that the lead content in a ton of ore is very small 

 and very heavy, and to transport such heavy ores just as they are 

 taken from the mines, directly to the smelters, would involve an ex- 

 pense greater than the value of the ore would warrant. By first 

 eliminating the waste portions, however, and concentrating the valu- 

 able minerals to such a degree that at least one-half the weight repre- 

 sents recoverable lead, shipping to the smelters becomes feasible. 



SMELTING AND REFINING. 



Smelting is the name applied to the practice of extracting a metal 

 from its ore by the aid of a flux and heat. Were the ores of lead 

 absolutely pure, smelting would be a simple operation, but the pres- 

 ence of impurities makes it an intricate and highly specialized art. 

 From the smelters' point of view there are two classes of lead ores 

 in the United States — high-grade nonsilver ores, such as those of 

 southeastern Missouri, and low-grade ores carrying silver, zinc, gold, 

 and copper in addition to lead, such as are found elsewhere. The 



