IAIN.] ORIGINAL SOURCE OF ORE MATERIAL. 63 



erally true thtit the galena of the older rocks in this region is non- 

 argentiferous, this would seem to emphasize the unusual character of 

 the conditions under which such deposits as those of southern Illinois 

 originated and would point to the improbability of the whole of the 

 material entering into the com])osition of the ore bodies being derived 

 from the normal rocks of the region. 



(3) The nuirked development of fluorite in connection with the 

 lead and zinc is wholly exceptional for this region. In none of the 

 other lead and zinc districts has this mineral ever been noted, despite 

 its showy character; and in the Joplin district the country rock, so 

 far as Mr. Steiger's analyses go, fails to show a trace of fluorine, 

 ^Vhile any negative evidence derived from country rock is not con- 

 sidered strong evidence, it is at least suggestive. 



(4) The presence of numerous and important fault planes and the 

 association of the ores with them are peculiarly characteristic of this 

 district. \Vhile faulting is not absent elsewhere, and, indeed, is 

 believed to have a high significance as regards the genesis of the Jop- 

 lin ores, it is nowhere as important as here, and in none of the other 

 districts do the ores so persistently follow the fault planes. In this 

 particular the ore bodies foimd here are more like those of the West- 

 ern States than like those of the Mississippi Valley generally. 



(5) The presence of igneous rock in the vicinity of the ore bodies 

 is, so far as known, wholly restricted to this particular district. 



These various facts point to the conclusion that, while it would 

 perhaps be possible to consider the lead and zinc found here as being 

 in the same category with that found elsewhere in the Mississippi 

 Valley, the probability is that either its source was unusual or the con- 

 ditions under which it was concentrated were entirely different from 

 those prevailing elsewhere. The most notable peculiarities in each 

 particular seem connected with the presence in the district of igneous 

 rock. 



GANGUE MINERALS. 



The gangue minerals associated with the ore bodies of this district 

 are those connnon in the Mississippi Valley, with the exception of the 

 fluorite. Calcite, quartz, and barite are found in all the districts, 

 though not in equal amounts. There is a striking and peculiar 

 absence of dolomite, which elsewhere is a constant associate of galena 

 and blende. Whether this absence is due to the derivation of the 

 ores from the immediately surrounding rocks, which are nonmag- 

 nesian, or from some entirelv new source, or wdiether the Cambrian 

 and Silurian rocks underlying the district are here nonmagnesian, is 

 uncertain. The greatest peculiarity of the ore bodies, however, is the 

 presence of large quantities of fluorite. This mineral is somewhat 



