250 TRANS. ST. LOUIS ACAD. SCIENCE. 



frequently contains disseminated crystals of zinc-blende, often so 

 fine and numerous that the lock appears impregnated with this 

 mineral. It contains also irregular cracks and pockets filled with 

 galena, and sufficiently large to be worked with great profit. 

 This quartzite must be of much later origin than the chert and 

 the limestone ; for it incloses and cements together numerous 

 chert fragments, with which it often forms a conglomeratic rock, 

 and it also incloses larger masses of softened limestone. In sev- 

 eral places the quartzite has evidently been infiltrated into loose 

 sandy beds of thoroughly dolomized and entirely disintegrated 

 limestone. In this case the quartz contains numerous rather irre- 

 gularly disseminated crystals of dolomite, as well as crystals of 

 blende, and occasionally of galena. Such crystals have often been 

 removed afterwards, leaving the quartzite as a porous mass, full 

 of impressions of dolomite and of blende crystals. The cavities 

 formerly occupied by these crystals are sometimes seen filled or 

 lined with carbonate of lead or with silicate of zinc. 



It seems evident from the descriptions just given that all these 

 various deposits are the results of certain chemical and geological 

 actions, weak and slow in themselves, but steady and prolonged 

 over many centuries. With the only exception of the valley 

 bottoms, where the action of running water has produced more 

 decided irregularities, the appearance of the lead-bearing rocks is 

 such as to indicate a very slow and gradual settling through the 

 metamorphic action of magnesian solutions and through the dis- 

 solving action of acid waters. The settling and breaking down 

 of the strata must have naturally taken place at intervals in many 

 instances, while the deposition of the ores appears to have been 

 continuous. 



The fact that thin seams of ore are sometimes found in unal- 

 tered limestone indicates that the deposition of the ores may have 

 begun before the metamorphic action. But there was at that time 

 comparatively but little room for ores to be deposited in, and it 

 was only after the pai'tial dolomization of the limestone beds 

 along certain vertical and horizontal fissures tiat richer ore de- 

 posits could be formed. These were at first ''runs" and "open- 

 ings" ; and afterwards, after the chert beds became more fissured 

 through the alteration of the interstratified limestone, more exten- 

 sive impregnations of such chert beds could be formed. The 



