smith. I LEAD AND ZINC OF JOPLIN DISTRICT. 203 
north and south of it; (3) the Webb City belt, including the mining 
zones from south of Duenweg northwesterly between Webb City and 
Carterville to Oronogo and beyond. Of these the Joplin belt is char- 
acterized b}^ the common association of the sphalerite with dolomitized 
limestone (as already noted), while in the Galena and Webb City belts 
dolomite is of comparatively rare occurrence. The latter belt is char- 
acterized by extensive sheet deposits which are either entirely absent 
or unimportant in the other belts. Around Joplin itself the breccias 
on the east side of the town are cemented mainly with calcite, and 
black secondary chert is not common, while on the west side of the 
city black chert is common, and in places forms the far larger propor- 
tion of the cement of the brecciated cherts. 
Deposition of ores. — In this district as elsewhere, the conditions 
governing ore deposition are partly physical and partly chemical. 
The physical conditions are those governing the circulation of under- 
ground waters in general; the chemical conditions are more complex, 
and as yet are only partly understood. Lead and zinc sulphides are 
known to be widely distributed in minute quantities in both the Car- 
boniferous and Cambro-Silurian rocks. The underground circula- 
tion probably takes lead and zinc sulphide from all the rocks where 
conditions are favorable to oxidation and solution, tending to concen- 
trate the ores as sulphides wherever favorable conditions for this 
process are met. The lead- and zinc-bearing waters come from both 
the Carboniferous and Cambro-Silurian rocks, and entering at the 
surface east of the Joplin district, they have flowed and still flow 
down the dip of the rocks. Reaching the Joplin district they meet 
conditions favorable to primary concentration, and the ores are 
deposited. It is believed that such deposition is still taking place, as 
it has done in the past. 
As the waters which give rise to the primary concentration flow 
down the dip of the rocks their motion is partly descending until the 
Joplin district is reached. There the waters coming from the Car- 
boniferous rocks have, on the whole, a lateral motion, while those 
bringing lead and zinc from the Cambro-Silurian into the Carbonifer- 
ous rocks are, on the whole, ascending. The direction of flow of the 
underground waters is not in general a fixed factor in ore deposition, 
but solution and redeposition of ores may take place in waters having 
either an ascending, descending, or lateral motion, provided other 
conditions are favorable to oxidation and solution at one point and to 
reduction and precipitation at another. 
Although this i>rocess of primary concentration is still effective, it 
was far more important when the entire district was covered with the 
comparatively impervious Coal Measure shales, and the conditions were 
more favorable than now to an artesian circulation. When erosion had 
largely removed this covering from the district, and had brought many 
of the ore deposits near or quite to the surface, within the reach of the 
