ORES OF COPPER 91 
1872 to 1874. In 1881 Marcus Daly of the Anaconda Silver 
Mining Co. found rich copper ores below those of silver ; the 
Anaconda Smelter, which began operations in 1884, led to 
Butte becoming the most prolific copper-field in the world. 
The mines occur in an Eocene granite rock, a quartz-mon- 
zonite, which in the Miocene was invaded by masses of aplite 
and dykes of quartz-porphyry, and covered by rhyolitic 
lavas. © The monzonite is traversed by three series of lodes. 
The. oldest are quartz-veins which trend E. to W., and contain 
silver in the northern and copper beneath silver in the southern 
parts of the field. These lodes 
were torn by N.W. to S.E. clefts 
and faults, which are charged 
with copper ores. The third series 
trends from N.E. to S.W., cuts 
across the two earlier series, and 
Contains ore broken from them and 
Some primary ore deposited by 
solution. The famous Anaconda 
Lode (Fig. 31) is one of the oldest 
Series, and has been worked for a 
mile and a half long, and to 2400 
feet deep; its width is in places 
100 feet, and large sections aver- 
aged 40 feet. The upper part of 
the lode consisted of iron-stained 
Quartz with silver ores; between 
200 and 400 feet deep it held 
oxidized copper ores; and below } 
400 feet occur large secondary bodies of chalcocite (Cu,S), 
Which are especially rich where the lode is crossed by faults. 
The Anaconda Lode is traversed by compound faults, such 
as the Rarus Fault, which has shattered a band of monzonite 
'n places 130 feet wide; the broken rock is seamed with 
quartz-veins and impregnated with sulphides. 
. The Butte ore was at one time attributed to lateral secre- 
ton; but there seems no reason why some of the lodes should 
contain silver and others copper if both sets were filled from 
ey Same rock. The ores have probably been deposited by 
Solutions which came from the ore-zone beneath the mon- 
Zonite, and reached the surface through fractures consequent 
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