598 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



resent very closely the general growth of most of the metal industries 

 in this country, date for date, and period for period. 



LEAD ORES: TYPES AND DISTRIBUTION. 



The mineral galena, a compound of lead and sulphur, is the im- 

 portant constituent of the majority of lead ores. It occurs in groups 

 of cubical crystals which are " as heavy as lead " and whose faces 

 have the appearance both in color and luster of highly polished 

 steel. Some lead is derived from ores containing the minerals cerus- 

 ite and anglesite, formed by the oxidation of galena. They, too, 

 are " as heavy as lead " but are not metallic in appearance. 



When these minerals occur concentrated in rock in quantities suf- 

 ficient to make the extraction of their lead content profitable, they 



mm^, •:•■>■? -■■■ : ,n •:; \-^~ 



i'-'-^'r , ' > i li ' Al ™ 



Fig. 2. — Development of the lead Industry in the United States. Data from United States 



Geological Survey. 



constitute true lead ores. In the whole of the United States there 

 is only one section where such ores are found and mined, namely, 

 southeastern Missouri, mainly within St. Francis, Washington, and 

 Madison Counties. A more widespread mode of occurrence of the 

 lead minerals is that in which they are mechanically combined with 

 the minerals of other metals, and while the extraction of the lead 

 alone in such ores would not be profitable, the extraction of all the 

 metals in the combination is profitable, one metal aiding another. 



Quite a number of such mineral associations of lead and other 

 metals exist, but four only may be said to constitute the major sources 

 from which the country's lead supply is derived. They are found 

 and exploited in many of the Rocky Mountain States and yield ap- 

 proximately two-thirds of the lead used annually, while the true 

 lead ores of southeastern Missouri yield one-third. In summary, 



