topographic forms. In such a case it is important to realize
that the diversity is only superficial. On the other hand, a slight
local divergence from the usual topographic forms in a given region may
reflect a similar local divergence in the underground structure. Thus it
is that an appreciation of the physiographic details may suggest
important variations in the underground structure which would otherwise
pass undiscovered.

Many mineral deposits owe their origin or enrichment to weathering and
other related processes which are preliminary to erosion. These
processes vary in intensity, distribution, and depth, with the stage of
erosion, or in relation to the phase of the erosion cycle. They vary
with the climatic conditions which obtain on the erosion surface.
Mineral deposits are therefore often closely related to the topographic
features, present and past, in kind, shape, and distribution. A few
illustrative cases follow.

Many of the great copper deposits of the western United States owe their
values to a secondary enrichment through the agency of waters working
down from the surface. When this fact of secondary enrichment was
discovered, it was naturally assumed that the process was related to the
present erosion surface and to present climatic and hydrologic
conditions. Certain inferences were drawn, therefore, as to depth and
distribution of the enriched ores. This conception, however, proved to
be too narrow; for evidences were found in many cases that the copper
deposits had been concentrated in previous erosion cycles, and therefore
in relation to erosion surfaces, now partly buried, different from the
present surface. The importance of this knowledge from an exploring and
development standpoint is clear. It has made it possible to find and
follow rich ores, far from the present erosion surface, which would
otherwise have been disclosed solely by chance. Studies of this kind in
the copper camps are yet so recent that much remains to be learned. The
economic geologist advising exploration and development in copper ores
who does not in the future take physiographic factors into account is
likely to go wrong in essential ways, as he has done in some cases in
the past.

Not only is it necessary to relate the secondary enrichment of copper
deposits to the erosion surface, present or past, but by a study of the
conditions it must be ascertained how closely erosion has followed after
the processes of enrichment.