24S TRANS. ST. LOUIS ACAD. SCIENCE. 



quite similar to the "runs" just described : but they differ in their 

 extent and somewhat in their genesis. Their vertical extent, al- 

 though often as large as that of the runs, does not as a rule reach 

 from one chert layer to another. The openings have generally a 

 chert layer'as roof, but the bottom is often an uneven, wavy sur- 

 face of limestone, somewhat softened and sandy near the opening, 

 and turning rapidly into an intact, hard limestone a few feet below 

 the bottom of the opening. 



Horizontally the openings extend not in one direction princi- 

 pally like the runs, but they form continuous beds extending over 

 small districts of irregular outlines, generally several hundred feet 

 in diameter. 



The above description throws a light on the genesis of these 

 deposits, showing that the processes of alteration have not started 

 in vertical fissures, in this case, and proceeded sidewise, but that 

 they were started, in horizontal partings of strata, mostly below a 

 chert layer, and have proceeded downward into the underlying 

 limestone layer to a greater or less distance according to circum- 

 stances. The alterations themselves are similar to those that pro- 

 duced the runs, and the deposition of the ores seems to have taken 

 place in the same manner. 



Numerous such deposits generally occur together, forming 

 larger ore districts, as e.g. that of Granby, which covers a whole 

 section or square mile, as far as known at present. The single 

 deposits are separated from each other bv the rising surfaces of 

 the underlying limestone. 



We also find in many places two or three such deposits, one 

 below the other, being separated from each other by alternate 

 layers of limestone and chert. 



The zinc ores in these u openings" are not uniformly sulphurets 

 like the lead ores, but they are mostly oxidized ores, either sili- 

 cates or carbonates, and often occur in wavy layers, several feet 

 in thickness. 



3. Impregnations of fissured chert beds with lead and zinc ores 

 frequently occur in the vicinity of the runs and openings above 

 described, as well as sometimes alone. The ores are mostlv sul- 

 phurets. and fill all the larger and smaller vertical and horizontal 

 fissures in the chert beds. They fill them exclusive of any other 

 substance, and in most instances fill them completely. The limit 



