e time, and
during the war were Germany's main source of copper. On Keweenaw Point,
Michigan, deposits of native copper formed in this manner in the
"Nonesuch" beds have been worked on a commercial scale. Other copper
ores on Keweenaw Point are replacements of conglomerate beds between
igneous flows, and are of a different origin already described (p. 200).

While much of the copper of sedimentary beds gives evidence that it was
deposited from solution in cracks and as replacements of the wall rocks,
often through the agency of abundant organic material in the beds, and
while also comparatively little of this copper can be identified as
having been deposited in detrital flakes or fragments along with the
other mineral fragments, there is, nevertheless, considerable evidence
that some of these deposits were formed essentially during the
sedimentation of the enclosing beds and as incidents to this process.
Such evidence consists of a close limitation of the copper to certain
beds, its wide and uniform distribution within these beds, its absence
in similar beds near at hand, the absence of evidence of feeding and
escape channels of the kind which would be necessary in case the
solutions were introduced long afterward, and often a minute
participation of the copper minerals in the minor structures of bedding,
false-bedding, and ripple-marks, which would be difficult to explain as
due to secondary concentration.

The Corocoro copper deposits of Bolivia occur in beds of sandstone with
no igneous rocks in the vicinity. However, they are all closely
associated with a fault plane, igneous rocks occur at distances of a few
miles, and the general mineralization is coextensive with the belt of
igneous rocks; the deposits are therefore ascribed to a magmatic source
rather than to sedimentary processes. Toward the surface the copper is
in part in the form of sulphides, somewhat altered to oxide minerals,
and farther down it is entirely native copper, associated with gypsum.
This is the only district outside of Lake Superior where native copper
has been mined on an important scale.

=General comments.= In general, the commercially prominent copper
deposits show a close relationship to igneous rocks in place, time, and
origin. Seldom do the ores extend more than 1,000 feet away from the
igneous rock.

The common downward order in sulphide deposits is: first, a weathered
zone, originally formed mainly above the water table, consisti