USEFUL METALS 117 



In the Doe Run mine in southeast Missouri, the ore is galenite 

 in limestone. The ore body is sometimes from 50 to 90 ft. in 

 diameter. It is generally in layers parallel to the stratification, 

 but sometimes in vertical or inclined seams and occasionally dis- 

 seminated through the limestone with calcite and nickeliferous 

 pyrite, bearing pyrite or pyrrhotite as accessory minerals. The 

 sedimentaries rest unconformably upon Archean granite and 

 gneiss. Mine La Motte, Bonne Terre, and Doe Run are the 

 most important mines in this district. 



In southern Illinois and Kentucky the gangue mineral is 

 fluorite, which is occasionally mined and sold to iron blast- 

 furnace operators. 



(3) The Cordilleran District: In Colorado, Leadville is the 

 most important locality. Here the galenite is oxidized at the 

 surface and is argentiferous. The associated rocks are Lower 

 Carboniferous limestones and dolomites, Ordovician limestones 

 and dolomites, and Cambrian quartzites resting on Archean 

 granite. These terranes are all traversed by sheets and dikes of 

 Cretaceous and post-Cretaceous age. The white and gray por- 

 phyries are older and more important. The main ore body lies 

 in the limestone of the Lower Carboniferous age. The blue lime- 

 stone at or near its contact with the Leadville porphyry is the 

 most important horizon. According to S. F. Emmons, they con- 

 stitute a contact sheet, whose upper surface, formed by the base 

 of the porphyry sheet, is comparatively regular and well defined. 

 The lower surface is irregular and ill defined. There is a gradual 

 transition from ore into unaltered limestone. The ore sometimes 

 occupies the entire thickness of the blue limestone. (See Fig. 76.) 



The ore also occurs in irregularly shaped bodies or in transverse 

 sheets not always connected with the upper or contact surface of 

 the ore-bearing bed or rock. It also occurs sometimes at or near 

 the contact of sheets of gray or other porphyries with the blue 

 limestone. Less frequently it occurs in both the calcareous and 

 siliceous sedimentaries. According to Hancock the main mass 

 of the argentiferous galenite lies in the limestones and dolomites 

 while the ores containing gold and copper are more pronounced 

 in the siliceous beds, in porphyries and in the crystalline rocks. 



From an economic standpoint, the most important mineral 

 is the argentiferous galenite with its secondary cerussite and 

 cerargyrite. Lead also occurs here as the sulphate, anglesite, 

 as the phosphate, pyromorphite, and in the form of the oxides 



