deposits exhibits widely different phenomena
as to continuity and no generalization is of any value. In gold
deposits of this type in West Australia, Colorado, and Nevada,
continuity far beyond a sampled face must be received with the
greatest skepticism. Much the same may be said of most copper
replacements in limestone. On the other hand the most phenomenal
regularity of values have been shown in certain Utah and Arizona
copper mines, the result of secondary infiltration in porphyritic
gangues. The Mississippi Valley lead and zinc deposits, while irregular
in detail, show remarkable continuity by way of reoccurrence over
wide areas. The estimation of the prospective value of mines where
continuity of production is dependent on reoccurrence of ore-bodies
somewhat proportional to the area, such as these Mississippi deposits
or to some extent as in Cobalt silver veins, is an interesting
study, but one that offers little field for generalization.

THE POSITION OF THE OPENINGS IN RELATION TO SECONDARY ALTERATION.--The
profound alteration of the upper section of ore-deposits by oxidation
due to the action of descending surface waters, and their associated
chemical agencies, has been generally recognized for a great many
years. Only recently, however, has it been appreciated that this
secondary alteration extends into the sulphide zone as well. The
bearing of the secondary alteration, both in the oxidized and upper
sulphide zones, is of the most sweeping economic character. In
considering extension of values in depth, it demands the most rigorous
investigation. Not only does the metallurgical character of the ores
change with oxidation, but the complex reactions due to descending
surface waters cause leaching and a migration of metals from one
horizon to another lower down, and also in many cases a redistribution
of their sequence in the upper zones of the deposit.

The effect of these agencies has been so great in many cases as
to entirely alter the character of the mine and extension in depth
has necessitated a complete reequipment. For instance, the Mt.
Morgan gold mine, Queensland, has now become a copper mine; the
copper mines at Butte were formerly silver mines; Leadville has
become largely a zinc producer instead of lead.

From this alteration aspect ore-deposits may be considered to have
four horizons:--

1. The zone near the outcrop, where the dominating feature
is oxidation and leaching of the soluble miner