246 TRAXS. ST. LOUIS ACAD. SCIEXCE. 



On the Forms and Origin of the Lead and Zinc Deposits 

 of Southwest Missouri. 



By Adolf Schmidt, Ph.D. 



The geological formation in which the lead and zinc deposits 

 of Southwest Missouri mainly occur is the Keokuk group, belong- 

 ing to the Lower Carboniferous system. This group consists, in 

 some places, of immense more or less thinly stratified chert beds ; 

 in others, of limestones, which are frequently bituminous ; in oth- 

 ers, of alternate layers of chert and limestone : and finally, in 

 places, of silico-calcite, which is an irregular and often very inti- 

 mate mixture of concretionary forms of both chert and limestone. 

 All these rocks are more or less fissured and broken in the vicinity 

 of the ore deposits, and the limestones are altered by dolomization 

 and by partial dissolution. These alterations seem to have cre- 

 ated the empty spaces in which the ores were principally depos- 

 ited ; for we find the best lead deposits in such districts where the 

 chert beds are the most fissured and broken, where the limestones 

 are the most cavernous and the most extensively metamorphosed 

 into dolomitic limestone and into dolomite rock. The lead and 

 zinc ores form in these rocks, principally, the following five kinds 

 of deposits : 



1. "Runs" (so called). 



2. "Openings" (so called). 



3. Impregnations of fissured chert beds. 



4. Irregular deposits in loose accumulations of broken chert. 



5. .Seams and impregnations in quartzite. 



I will give a general description of these various deposits. 



1. " Runs" is a designation given by the miners of Joplin to 

 such deposits which extend chiefly in one horizontal direction, 

 while limited to five or six feet only in width and height. They 

 are generally in a thick limestone layer, and have mostly a chert 

 layer as roof and another chert layer as bottom, which chert lay- 

 ers cut oil' the ore more or less completely. The ores are inva- 

 riably connected with dolomite, either fresh or rotten, and reach 

 sidewise into the limestone only as far as the latter is dolo- 

 mitic and crystalline. The outer portions of the run (in its cross 

 section) contain the galena merely disseminated in' the crystalline 



