ransom*:.] SECONDARY ALTERATION OF THE LODE ORES. 101 
their possible changes in value with depth. The indications, however, 
are in favor of a varying, but on the whole more nearly constant, 
tenor as they are mined to greater depths, than in the case of silver- 
load ores. It is fair to assume that the considerable depth to which 
the lodes of California have been mined without revealing any regular 
increase or decrease in the tenor of the pay shoots below the zone of 
oxidation is something more than a mere local phenomenon, and 
holds for gold-quartz Lodes somewhat generally, provided that the 
fissures are continuous and the country rock is uniform. 
RELATION <>F ORES TO KIND OF COUNTRY ROOK. 
As far as known, 1 h<> influence of the country rock upon the deposi- 
tion of the lode ores in the Silverton quadrangle is small. Extensive 
productive ore bodies occur in monzonite and in the volcanic rocks of 
both the Silverton and San. I nan series; and, although none are worked 
in the Algonkian schists, there is no indication that, given a suitable 
fissure, ores would not be deposited as readily in these as in other 
rocks. The influence of the wall rock upon ore deposition appears to 
be limited to a tendency toward impregnation and replacement in the 
deposition of ores in some of the rhyolitic and andesitic flow breccias 
and tuffs. This, however, is a phenomenon relating to the form of the 
deposit, and to the subject of the alteration of the wall rocks rather 
than to the character of the ores themselves. 
SECONDARE ALTERATION OF THE LODE ORES. 
Owing to the fact that erosion in this high region is effected by 
mechanical disintegration rather than by secular decay, the oxidation 
of the ores is not, as a rule, vot extensive, and is sometimes negligi- 
ble. In the North Star mine, on Little Giant Peak, oxidized ore, con- 
sisting chiefly of anglesite, is reported to have extended to a depth of 
200 feet. Ordinarily, however, complete oxidation is very superficial, 
although traces may be found at much greater depths than 200 feet. 
Carbonate and sulphate of lead and carbonates of copper, often asso- 
ciated with the unaltered sulphides, occur in the croppings of most of 
the lodes carrying galena and tetrahedrite or chalcopyrite. In the 
Silver Lake mine sulphate and carbonate of lead occurred abundantly 
in the highest level (No. 4) with unoxidized ore. At the level below, 
however, the secondary ores were unimportant. As is commonly the 
case in this region, there is no sharp line separating an upper zone of 
oxidation from the sulphide ores. The decomposition of the original 
ore minerals has penetrated irregularly downward wherever oxidizing 
waters found opportunity to descend, and is seldom limited by a dis- 
tinct water level. Very commonly the level of permanent water lies 
far below the oxidized portion of the lode. In the Tomboy and Camp 
Bird mines black oxide of manganese occurs in the deepest workings 
and usually indicates good ore. In these cases the oxide appears to 
