ce. The result is the leaching of
zinc and iron readily in the oxidized zone, thus differentially
enriching the lead which lags behind, and a further extension of
the lead horizon is provided by the early precipitation of such
lead as does migrate. Therefore, the lead often predominates in
the second and the upper portion of the third zone, with the zinc
and iron below. Although the action of all surface waters is toward
oxidation and carbonation of these metals, the carbonate development
of oxidized zones is more marked when the enclosing rocks are
calcareous.

In copper-iron deposits, the comparatively easy decomposition and
solubility and precipitation of the copper and some iron salts
generally result in more extensive impoverishment of these metals
near the surface, and more predominant enrichment at a lower horizon
than is the case with any other metals. The barren "iron hat" at the
first zone, the carbonates and oxides at the second, the enrichment
with secondary copper sulphides at the top of the third, and the
occurrence of secondary copper-iron sulphides below, are often
most clearly defined. In the easy recognition of the secondary
copper sulphides, chalcocite, bornite, etc., the engineer finds a
finger-post on the road to extension in depth; and the directions
upon this post are not to be disregarded. The number of copper
deposits enriched from unpayability in the first zone to a profitable
character in the next two, and unpayability again in the fourth,
is legion.

Silver occurs most abundantly in combination with either lead,
copper, iron, or gold. As it resists oxidation and solution more
strenuously than copper and iron, its tendency when in combination
with them is to lag behind in migration. There is thus a differential
enrichment of silver in the upper two zones, due to the reduction
in specific gravity of the ore by the removal of associated metals.
Silver does migrate somewhat, however, and as it precipitates more
readily than copper, lead, zinc, or iron, its tendency when in
combination with them is towards enrichment above the horizons of
enrichment of these metals. When it is in combination with lead
and zinc, its very ready precipitation from solution by the galena
leaves it in combination more predominantly with the lead. The
secondary enrichment of silver deposits at the top of the sulphide
zone is sometimes a most pronounced feature, and it seems to be
the explanation of the origin of man