als.
2. A lower horizon, still in the zone of oxidation, where the
predominant feature is the deposition of metals as native,
oxides, and carbonates.
3. The upper horizon of the sulphide zone, where the special
feature is the enrichment due to secondary deposition
as sulphides.
4. The region below these zones of secondary alteration, where
the deposit is in its primary state.

These zones are seldom sharply defined, nor are they always all
in evidence. How far they are in evidence will depend, among other
things, upon the amount and rapidity of erosion, the structure and
mineralogical character of the deposit, and upon the enclosing
rock.

If erosion is extremely rapid, as in cold, wet climates, and rough
topography, or as in the case of glaciation of the Lake copper
deposits, denudation follows close on the heels of alteration,
and the surface is so rapidly removed that we may have the primary
ore practically at the surface. Flat, arid regions present the
other extreme, for denudation is much slower, and conditions are
most perfect for deep penetration of oxidizing agencies, and the
consequent alteration and concentration of the metals.

The migration of metals from the top of the oxidized zone leaves
but a barren cap for erosion. The consequent effect of denudation
that lags behind alteration is to raise slowly the concentrated
metals toward the surface, and thus subject them to renewed attack
and repeated migration. In this manner we can account for the enormous
concentration of values in the lower oxidized and upper sulphide
zones overlying very lean sulphides in depth.

Some minerals are more freely soluble and more readily precipitated
than others. From this cause there is in complex metal deposits a
rearrangement of horizontal sequence, in addition to enrichment at
certain horizons and impoverishment at others. The whole subject
is one of too great complexity for adequate consideration in this
discussion. No engineer is properly equipped to give judgment on
extension in depth without a thorough grasp of the great principles
laid down by Van Hise, Emmons, Lindgren, Weed, and others. We may,
however, briefly examine some of the theoretical effects of such
alteration.

Zinc, iron, and lead sulphides are a common primary combination.
These metals are rendered soluble from their usual primary forms
by oxidizing agencies, in the order given. They reprecipitate as
sulphides in the reverse sequen