ORES OF LEAD, ZINC, AND SILVER 107 
ore particles (A. Winslow, Missouri G.S., vii, pt. 2, 1804, 
PP. 477-87). The process was most active in the Carboni. 
terous, but is still going on (Winslow, 4bid., p. 487); and 
C. E. Siebenthal has shown (U.S.G.S., Bull. 606, 1915, pp. 
124, 125, 128-9, 131-3) that the well waters of Missouri 
tontain lead, zinc, and copper. The distribution of the ore 
was determined by the descending solutions. Where the 
Potosi Dolomite has been exposed by removal of the Davis 
Shale the ore is in compact shallow deposits; where the 
dolomite was covered by shale the galena is dispersed in 
fissures and over the surface of beds of shale. 
Buckley holds that instead of the lead having originally 
come up fault planes, it was scattered sparsely through the 
Pre-Palzozoic rocks of the St. Francois Mountains, which 
formed islands in the Cambrian Sea. Analyses record 
‘002 per cent. of lead and +002 per cent. of zinc in the granite ; 
‘005 per cent. of lead and -017 per cent. of zinc in the Archean 
rhyolite ; and -006 per cent. of lead and 015 per cent. of 
Znc in the diabase. Buckley calculated that ‘002 per cent. 
of lead in the Archean rocks would amount to 68,000 short 
tons per square mile for every 1400 feet in thickness. The 
lead would have been carried to the sea and deposited in the 
Paleozoic rocks; during their denudation the scattered 
Particles would have been dissolved and carried into the lime- 
Stones, and there concentrated and precipitated as the ore. 
The low Proportion of zinc in the ores is probably due to the 
greater solubility of its salts. 
(0) “Frars ” anp Ore-Bopies Due to DESCENDING 
SOLUTIONS— Some bodies of lead ores are due to replace- 
ent by descending solutions, as in the deeper parts of 
Jnclinal troughs in the Muschelkalk (Middle Trias) of 
Silesia, and the lead ores in the Devonian and Carboniferous 
Limestone of Aachen, which were apparently formed during 
the Hercynian movements and enlarged during the Alpine 
Movements. The shallow ore-bodies in the Mendip Hills 
are doubtless also of secondary origin; they occur as ir- 
regular lenticles of galena in the upper part of the Carboni- 
ferous Limestone, and similar zinc ores occur in the adjacent 
Permian Dolomitic Conglomerate. The two metals have 
apparently been separated as they were carried down from 
former lead-zinc deposits above the present surface.