oxidation has not proceeded uniformly to the present, but has depended
upon a fortunate combination of factors which has not been often
repeated during geologic time. As illustrative of this, the principal
oxidation of the Bisbee copper ores of Arizona (p. 204) occurred before
Tertiary time, with reference to a place that has since been covered by
later sediments. The conditions in the Ray, Miami, and Jerome copper
camps of Arizona (pp. 203-205) likewise indicate maximum oxidation at an
early period. The Lake Superior iron ore deposits (pp. 167-170) were
mainly concentrated before Cambrian time, during the base-leveling of a
mountainous country in an arid or semi-arid climate. The oxide zone of
these deposits has no close relation to the present topography or to the
present ground-water level. In the Kennecott (Alaska) copper deposits
all oxidation has been stopped since glacial time by the freezing of the
aqueous solutions. At Butte and at Bingham the main concentration of the
ores is believed to have occurred in an earlier physiographic cycle than
the present one. The _cyclic_ nature of the formation of oxide zones is
of comparatively recent recognition, and much more will doubtless be
found out about it in the comparatively near future. Its practical
bearing on exploration is obvious (see p. 325).

It should be clearly recognized that oxidizing processes are not limited
to the zone above the ground-water level. Locally oxidizing solutions
may penetrate and do effective work to much greater depths, especially
where the rocks traversed at higher elevations are of such composition
or in such a stage of alteration as not to extract most of the oxygen.
Consequently the presence of oxide ores below the water table is not
necessarily proof that the water table has risen since their formation.
On the other hand, the facts of observation do indicate generally a
marked difference, in circulation and chemical effect, between waters
above and below this horizon, and show that oxidation is dominantly
accomplished above rather than below this datum surface.

During the formation of the oxide zone, erosion removes some of the ore
materials entirely from the area, both mechanically and in solution.
Part of the material in solution, however, is known to penetrate
downward and to be redeposited in parts of the ore body below the oxide
zone,--that is, usually below the water table. Evidence of this process
is decisive in regard to seve