USEFUL METALS 



261 



According to E. T. Hancock, the ore deposits fall into two 



groups: Firt, runs ;ui<l their modifications; and second, blanket 

 veins, or sheet ground deposits. The runs are irregular, usually 

 elongated, sometimes tabular und inclined bodies of ore uniformly 

 associated with disturbed strata which are bn-cciatrd, slickm- 

 sided and faulted. The runs are generally a few hundred feet 



FIG. 128. Depression in the limestone extending down to the Grand 

 Falls chert member, filled with Cherokee shale, Joplin, Missouri, (After 

 W. S. Tangier-Smith and C. E. Siebenthal, U. S. Geological Survey.) 



in length but the Arkansas run exceeds 1000 ft. The runs also 

 have an average width of about 50 ft. and a maximum width of 

 300 ft. The roughly elliptical closed runs constitute one of the 

 most distinctive and constantly recurring types of ore bodies in 

 the Joplin District. 



The blanket veins are nearly horizontal, tabular ore bodies 

 extending parallel with the bedding planes of the limestones and 



FIG. 129. Depression in the limestone extending down to the Grand 

 Falls chert member, filled with shale and compressed. Joplin, Missouri. 

 (After U. S. Tangier-Smith and C. E. Siebenthal, U. S. Geological Survey.) 



cherts. The typical sheet ground appears to be developed inva- 

 riably in the Grand Falls chert. 



In the vertical distribution of the ores the sulphide of lead is the 

 most abundant in the upper portions of the ore deposits and the 

 sulphide of zinc in the lower portions. This distinction, however, 

 is not universal. The most profitable mining is confined to the 



