G02 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



tained in strong rock strata, the ores can be and are mined in great 

 rooms or " stopes " where, by the aid of mechanical devices both in 

 breaking down the ore and in handling it, large tonnages are ob- 

 tained. These large scale mining operations are an economic neces- 

 sity, for the reason that hardly more than 70 pounds of lead are 

 obtained from each 2,000 pounds of ore brought to the surface, so 

 that only by extensive and efficient mining operations can the low 

 price of lead be maintained. 



The formation of these vast ore bodies required millions of years 

 for the accomplishment. They had their beginning at a time when 

 the greater portion of the middle section of the United States was 

 an inland sea banked by land composed of rock formations of plu- 

 tonic origin and containing galena as well as other minerals. During 

 successive periods of erosion of these land areas by surface and under- 

 ground waters in circulation, the metallic compounds resulting were 

 carried by these waters into the sea, where they became incorporated 

 in the sediments then forming. At a subsequent geologic period the 

 inland sea receded southward, leaving these sediments, which were 

 gradually consolidated into the rock masses of limestone and shales 

 that to-day form the land areas of the central portion of the United 

 States. The galena in these rocks was at first widely dispersed in sev- 

 eral successive beds, but, due to the activities, in part of waters de- 

 scending, and in part ascending through them, the galena was again 

 taken into solution and was moved about in the fractures in the rock 

 beds through which these waters traveled. Finally, however, on 

 coming in contact with reducing agents, such as organic matter in 

 the limestone or shale, the galena was precipitated from the solutions 

 resulting in the accumulation of the masses of ore as they are found 

 to-day. 



The lead deposits of the Rocky Mountain States occur in limestones, 

 sandstones, shales, and also in rock formations of volcanic origin, 

 all of which, due to orogenic activity, have been twisted, folded, 

 cracked, and faulted. The ore bodies are irregular in dimensions and 

 extent and, literally speaking, one day will find the miners picking in 

 ore and the next day in barren rock. The combination of these condi- 

 tions requires, first, extensive timbering of the mine to prevent the 

 falling and sliding of rock both when in the ore and when in barren 

 rock, and, second, conducting mining operations on a small scale with 

 a preponderance of hand labor. The cost of this method of mining 

 is considerably higher than that in the Mississippi Valley and would 

 be reflected in the price of lead except for the fact that the presence of 

 silver, copper, or zinc in these ores enhances their value to such an 

 extent that the lead can be marketed in competition with that from 

 other sources. 



