SCHMIDT LEAD AND ZINC DEPOSITS OF S.W. MISSOURI. 247 



limestone, while the central portion of the run is much richer, 

 and often shows very thick seams and pockets of galena, as well 

 as a more completely dolomized, and in places leached, rotten, 

 and disintegrated rock, with layers of dolomite crystals. The 

 length or longest horizontal extent of a run varies from twenty to 

 several hundred feet. 



It can hardly be doubled that these runs were formerly vertical 

 fissures in a layer of limestone, limited above and below by chert 

 layers ; that solutions of bicarbonate of magnesia attacked the 

 limestone on both sides along the fissure, and converted it, to a 

 greater or less extent, into cracky and porous dolomitic limestone, 

 and often completely into dolomite. A diminution of volume in 

 the mass of the rock is always connected with such dolomization, 

 and the pores, cracks and cavities thus produced gave the mineral 

 solutions an opportunity to deposit the ores in them. The occa- 

 sional occurrence of disseminated ores with well developed crys- 

 tals entirelv inclosed bv the dolomitic rock, indicates that the 

 dolomization and the deposition of the ores have taken place, in 

 most localities, simultaneously or nearly so, and that the galena 

 has been deposited in the rock while the latter was in a soft or 

 magmatic condition. The absence of galena in the mass of en- 

 tirely unaltered limestone forbids to think that the galena had 

 been an accessory constituent of the limestone as formed origin- 

 ally. The galena seams are frequently found disturbed and 

 broken in the runs, and are then always accompanied by loose 

 disintegrated masses of coarsely crystalline dolomite, of a rotten 

 appearance. In such places a leaching process has undoubtedly 

 taken place after the deposition of the ore. Waters containing 

 carbonic acid dissolved the purely calcareous portions of the do- 

 lomitic rock, leaving behind the crystallized dolomite, which is 

 less soluble. The downward movements in the rock masses, caus- 

 ed by this leaching process, broke and disturbed the ore seams. 



Several runs often occur together, and are then mostly parallel 

 to each other and separated by altered limestone. Two, three, 

 and four runs are also occasionally found one below the other, 

 and separated by mostly broken, yet sometimes solid, layers of 

 chert and of limestone. 



2. "Openings" are deposits which in their interior construction, 

 and in the character of the materials which compose them, ai-e 



