creasing distance from the igneous
source. The further investigation of this primary zonal arrangement
promises interesting results with a practical bearing on exploration and
development.

One of the newer features of the investigation of copper deposits has
been the recognition of the cyclic nature of the secondary
concentration. This process has been related not only to the present
erosion surface, but to older surfaces now partly buried under later
rocks. Ransome's[33] summary of conditions at the Ray-Miami camp has a
somewhat general application.

Supergene enrichment has generally been treated as a
continuously progressive process. There is considerable
probability, however, that it is essentially cyclic, although
the cyclic character may not be patent in all deposits. A full
development of the cycle can take place only under a certain
equilibrium of a number of factors, including climate,
erosion, topography, and character of rock. The essential fact
appears to be that as enrichment progresses and chalcocite
increases the process of enrichment becomes slower in action,
and erosion may, in some circumstances, overtake it. With the
removal of some of the protecting zone of chalcocite the
protore is again exposed to oxidation and a second cycle of
enrichment begins.

Although much of the enriched ore is now below ground-water
level, it probably was once above that level, and enrichment
is believed to have taken place mainly in the zone of rock
above any general water table.

Where the old erosion surface roughly coincides with the present erosion
surface, the deposits follow more or less the topography. Where the old
erosion surface pitches below later sediments, the ores pitch with it,
and therefore do not follow the present topography. The recognition of
the cyclic nature of secondary concentration is obviously of great
significance in exploration and development.

Although a vast amount of study has been devoted to the origin and
enrichment of copper deposits, and although the general conditions and
processes are pretty well understood, the results thus far have been
largely qualitative rather than quantitative.

LEAD ORES

ECONOMIC FEATURES

The most prominent uses of lead are in the manufacture of alloys, such
as type-metal, bearing metal, shot, solder, and casting metal; as the
oxide, red lead, and the basic ca