USEFUL METALS 



115 



(2) The Mississippi River belt comprises the following states: 

 Minnesota, Missouri, Illinois, Iowa, Kansas, Kentucky, and Ar- 

 kansas. The Missouri-Kansas district is the most important of 

 them all. It is further divided into three distinct fields: (a) 

 The southeast; (b) the central; and (c) the southwest. In 

 the first two fields the ore is distinctively galenite. In the third 

 district the ore is associated with zinc. 



Joplin is in the southwest district. The ore occurs in lime- 

 stones and chert of Mississipian age intimately associated 





FIG. 74. Open cut in barren ground near Tola, Marion County, 

 Arkansas, showing jointed dolomite. (After G. I. Adams, U, S. Geological 

 Survey.) 



with the slates and shales of the Coal Measures. The ore 

 bodies are often massive and sometimes hundreds of feet in 

 diameter. The ore occurs massive and granular and in crystals 

 of the isometric system. The associated minerals are chert, 

 calcite, dolomite, barite and pyrite. These are all of secondary 

 origin. The country rock is sometimes massive and sometimes 

 fragmental (Fig. 75). In this district the ore is associated with 

 sphalerite, ZnS, which sometimes has a resinous luster; sometimes 

 it'is the ferriferous sphalerite, containing 10 per cent, or more of 



