with varying amounts of zinc carbonate, zinc silicate, lead
sulphide, and rarely lead carbonate. This zone is obviously developed
above water level, and is seldom as much as 100 feet thick. Zinc, and to
a less extent lead, have been taken into solution as sulphates, with the
aid of sulphuric acid resulting from the oxidation of the associated
pyrite. Zinc has been carried away from the weathered zone in solution
faster than lead, leaving the lead more or less concentrated near the
surface. Some of the zinc carried down has been redeposited secondarily
as zinc sulphide. Evidences of this secondary sulphide enrichment can be
seen in many places; yet certain broad quantitative considerations raise
a doubt as to whether this process has been responsible for the main
portion of the values of the sulphide zone. If downward secondary
enrichment had been a dominant process, it might be expected that the
ores would be richer in places where erosion had cut away more than
half the limestone formation carrying the ore, than in places where it
had barely cut into the formation. This is not the fact,--which suggests
that erosion in its downward progress has carried a large part of the
zinc completely out of the vicinity.

Zinc ores of this same general character are also found in Paleozoic
rocks (Knox dolomite) in Virginia and Tennessee. Their manner of
occurrence suggests the same problem of origin as in Missouri and
Wisconsin, but no decisive evidence of their source has been discovered.

Of the zinc ores associated with igneous intrusions, those of the Butte
and Coeur d'Alene districts are described in connection with copper
and lead ores on pp. 201-203, 208, and 212-213.

Zinc constitutes about 75 per cent by weight of the recoverable metals
of the Leadville district of Colorado. About two-thirds of the zinc
occurs as the sulphide and about one-third as the carbonate resulting
from weathering of the sulphide. The zinc sulphide is associated with
lead, iron, and copper sulphides and gold and silver minerals. In the
oxide zone the zinc carbonate is associated with oxides and carbonates
of various metals, including those of lead, copper, iron, and manganese.
The iron and manganese oxides are mined in considerable tonnage as a
flux. It is an interesting fact that, although mining has been carried
on in this district for upwards of forty years, only within the last
decade has the existence of zinc ores in the oxide zone been recogn