ORES OF LEAD, ZINC, AND SILVER 103 
porphyry but too remote from it to be contact ores. The 
leading example is Leadville in the Rocky Mountains in 
Colorado. The field (Fig. 32) consists of pre-Palaozoic 
granite covered by Cambrian quartzite, the Ordovician or 
White Limestone, Devonian quartzite, and the Carboni- 
ferous or Blue Limestone; this series has been invaded by 
sheets of Eocene quartz-porpyhry. After its intrusion the 
rocks were folded and greatly faulted, and large lenses and 
sheets of lead and zinc ores formed by replacement in the 
limestones and mainly in the Carboniferous. The ores were 
carbonates near the outcrop, and passed below into sulphides. 
wasH 
Fie. 32.—Tur Mine FIELD OF LeapviLLe, CoLORADO. 
Section across part of the mining field of Leadville, Colorado (after 
Emmons and Irving), Porphyry {(W.P.) Lower Kainozoic; Blue 
Limestone, B.L., Carboniferous; Parting Quartzite, P.Q., Devonian ; 
White Limestone, W.L., Silurian ; Lower Quartzite, L.Q., Cambrian; 
Granite, Gr., pre-Palzazoic: the ore-bodies solid black. 
The Leadville field was discovered in 1860, and worked till 
1874 for alluvial gold. The lead-silver ores were found 
during 1874 and 1877, when the first smelter was established. 
The ores used were secondary carbonates in which the silver 
was enriched to the grade of often 60 oz. to the ton. Emmons 
(U.S.G.S., Mon. No. 12, 1886) regarded the ores as leached 
from the porphyry and overlying rocks by descending solu- 
tions, and were therefore not expected to go deep or the 
field to have a long life. Better hopes were given in 1890 
by A. A. Blow (Tr. Amer. I.M.E., xviii, 1890, pp. 145-81), 
who concluded that the ores had risen up the faults. That