tion of substances
already present; and the varying emphasis placed on these two processes
by different observers has led to some controversy.

SECONDARY CONCENTRATION IN PLACE OF THE FOREGOING CLASSES OF MINERAL
DEPOSITS THROUGH THE AGENCY OF SURFACE SOLUTIONS

Mineral deposits of direct magmatic segregation are seldom much affected
by surficial alteration, perhaps because of their coarse crystallization
and their intermingling with resistant crystalline rocks. Mineral
deposits of the "igneous after-effect" type may be profoundly altered
through surficial agencies. The more soluble constituents are taken
away, leaving the less soluble. The parts that remain are likely to be
converted into oxides, carbonates, and hydrates, through reaction with
oxygen, carbon dioxide, and water, which are always present at the
surface and at shallow depths. These processes are most effective at the
surface and down to the level of permanent ground-water, though locally
they may extend deeper. This altered upper part of the ore bodies is
usually called the _oxide zone_. It may represent either an enrichment
or a depletion of ore values, depending on whether the ore minerals are
taken into solution less rapidly or more rapidly than the associated
minerals and rocks; all are removed to some extent. In certain deposits,
there is evidence that both zinc and copper have been taken out of the
upper zone in great quantity; but they happen to be associated with
limestone, which has dissolved still more rapidly, with the result that
there is a residual accumulation of copper and zinc values. Manganese,
iron, and quartz are usually more resistant than the other minerals and
tend to remain concentrated above. The same is true to some extent of
gold and silver. The abundance of iron oxide thus left explains the name
"iron cap" or "gossan" so often applied to the upper part of the oxide
zone. Not infrequently, and especially in copper ores, the upper part of
the oxide zone is nearly or entirely barren of values and is called the
_capping_.

The depth or thickness of the oxide zone depends on topography, depth of
water table, climatic conditions, and speed of erosion. A fortunate
combination of conditions may result in a deep oxide zone with important
accumulations of values. In other cases erosion may follow oxidation so
rapidly as to prevent the growth of a thick oxide zone.

It is clear from the study of many ore deposits that the process of