[06 ECONOMIC GEOLOGY 
the Davis Shale, the Potosi Dolomite, and a series of sand- 
stones and shales which pass up into the Ordovician. The 
Bonneterre and Potosi Dolomites contain most of the ores. 
The rocks have been greatly faulted, but show no evidence of 
the action of hot waters or high temperatures. 
The ores were at first attributed to solutions rising up 
fault planes. This view was modified by Foster Bain, who, 
although accepting the primary introduction of the metals 
by ascending solutions, explained the ores as secondary and 
concentrated by descending water. The evidence for the 
descending water is convincing. The ore in the weathered 
zone of Potosi Limestone is in vertical channels and pipes, 
which are richest near the surface. and some ore was deposited 
F16. 34.—LEAD AND Zinc ORES oF 
MissourL 
Diagram of the lead and zinc ores of Mis 
souri (after Buckley). The rocks con- 
sist of limestone and. shale with the 
ore deposits descending sometimes 
in funnel-shaped concentrations and 
spreading out over the upper side of 
the bands of shale. Some veins of 
solid ore. 
in cavities as stalactites. The galena is associated with 
pyrites, barite, blende, smithsonite, anglesite, cerussite, and 
calcite. The barite of this field supplies a large part of that 
worked in the United States. The amount of zinc is small. 
The ores in the Bonneterre Dolomite were also concentrated 
by descending solutions, and the mines first worked shallow 
deposits in cavities, caverns, and veins. The ore in the 
upper workings was mostly the carbonate, cerussite, and 
coarsely crystalline galena; the deeper galena is in widely 
disseminated particles. The ore is in places collected in 
veins, which cut steeply across the bedding ; the disseminated 
ores occur along the upper and lower sides of a band of shale, 
and in carbonaceous or bituminous layers, and in seams of 
limestone between shales. 
The condition and distribution of the ore shows that it 
was formed by descending solutions concentrating scattered