ral minerals. Copper is known to be taken
into solution as copper sulphate at the surface, and to be redeposited
as chalcocite where these sulphate solutions come in contact with
chalcopyrite or pyrite below. Not only has the process been duplicated
in the laboratory, but the common coating of chalcocite around grains of
pyrite and chalcopyrite below the water level indicates that this
process has been really effective. Sulphides of zinc, lead, silver, and
other metals are similarly concentrated, in varying degrees. The zone of
deposition of secondary sulphides thus formed is called the zone of
_secondary sulphide enrichment_. Ores consisting mainly of secondary
sulphides are also called _supergene_ ores (p. 33). In some deposits, as
in the copper deposits of Ray and Miami, there is found, below the
secondary sulphide zone, a lean sulphide zone which is evidently of
primary nature. The mineralized material of this zone, where too lean to
mine, has been called a _protore_.

With the discovery of undoubted evidence of secondary sulphide
enrichment, there was a natural tendency to magnify its importance as a
cause of values. Continued study of sulphide deposits, while not
disproving its existence and local importance, has in some districts
shown clearly that the process has its limitations as a factor in ore
concentration, and that it is not safe to assume its effectiveness in
all camps or under all conditions. At Butte for instance, secondary
chalcocite is clearly to be recognized. The natural inference was that
as the veins were followed deeper the proportion of chalcocite would
rapidly diminish, and that a leaner primary zone of chalcopyrite,
enargite and other primary minerals would be met. However, the great
abundance of chalcocite in solid masses which have now been proved to a
depth of 3500 feet, far below the probable range of waters from the
surface in any geologic period, seems to indicate that much of the
chalcocite is primary. The present tendency at Butte is to consider as
secondary chalcocite only certain sooty phases to be found in upper
levels. The solid masses of chalcocite in the Kennecott copper mines
seem hardly explainable as the result of secondary sulphide enrichment.
No traces of other primary minerals are present and the chalcocite here
is regarded as probably primary.

The possible magnification of the process of secondary enrichment above
referred to has had for its logical consequence a tendenc