, therefore, that copper and zinc deposits are
found through the downward exploitation of oxidized gold, silver, and
lead ores. The veins at Butte were first worked for silver, and the ore
bodies at Bingham, Utah, and Jerome, Arizona, were first mined for gold.
Exceptionally, copper ore in enriched, oxidized form outcrops, as at
Bisbee, Arizona.

It is not always true that valuable sulphide deposits have an
iron-stained outcrop, for in some of them iron sulphide or pyrite is so
scarce that the surface outcrops may be light-colored clayey and
siliceous rocks.

Silver is often represented in the outcrop by silver chloride or
cerargyrite, which may be easily identified. The prospecting for such
surface ores is sometimes called "chloriding."

The presence in the outcrop of dark manganese oxides associated with
vein quartz sometimes indicates the presence below of copper and zinc
and other minerals, as at Butte.

Extensive alterations of the country rock in the way of silicification
and sericitization, and the presence of minerals like garnet,
tourmaline, diopside, and others, known to be commonly deposited by the
same hot solutions which make many ore deposits, may furnish a clue for
exploration below. These characteristics of the country rock, however,
are likely to be masked at the outcrop by later weathering, which
superposes a kaolinic or clayey alteration.

TOPOGRAPHY AND CLIMATE AS AIDS IN SEARCHING FOR MINERAL OUTCROPS

The topographic expression of a mineral deposit depends upon its
hardness and resistance to erosion as compared with the adjacent rocks.
If more resistant it will stand out at the surface; if less resistant,
it will form a depression. The conditions determining resistance are
exceedingly variable, and no broad generalization can be made; but
within a local province a given group of mineral deposits may
characteristically form depressions or ridges, and thus topographic
criteria may be very useful in exploration. Even with such limitations,
the variations of the topographic factor may be so great as to require
much care in its use. Sulphide ores in quartzites are likely to develop
depressions under erosion. In limestones they are more likely to stand
out in relief, because of the softer character of the limestone, though
this does not always work out. Crystalline magnetite and hematite are
more resistant to erosion than almost any other type of rock, and stand
out at the surface with proportio