weed.] OEE DEPOSITS AT BUTTE, MONT. 179 
copper deposits with the Modoc porphyry occurrence, since the most 
productive lodes occur in the area penetrated by this rock. The 
veins cross the porphyry, however, even the earliest ones, and hence 
the vein fractures are of later occurrence. 
There is also a distinct genetic relation between ore and country 
rock, as a result of the deposition of the ore by metasoinatic replace- 
ment. Thus the Anaconda ledge is low grade where it crosses either 
the Bluebird granite or the Modoc porphyry, a feature explainable by 
the lack of easily replaceable, dark-colored, ferromagnesian minerals 
in those rocks. 
THE VEIN SYSTEMS. 
As a result of extensive legal development work the evidence is now 
conclusive that the east-west veins have been faulted. The identity 
of the Original-Parrot and Anaconda lodes is conclusively established, 
the displacement being due to the Blue vein. Farther east the Ana- 
conda ledge is again thrown to the north by the Mountain View fault, 
the displaced segment forming the South ledge of the Mountain View 
mine, terminated eastward by the Rarus fault, throwing the lode 
southward, so that its eastward extension appears in the Rarus mine. 
The same faulting has displaced the other veins of this part of the 
district. 
Earlier veins, east-west system. — The great veins of the district, the 
Anaconda, Parrot, Mountain View, West Colusa, Syndicate — in fact, 
all the great producers — belong to this east-west system, in which 
the trend is remarkably uniform, considering the length of the veins. 
The Silver Bow vein is a marked exception. There is some evidence 
to show that certain southeast fractures were mineralized in the ear- 
liest vein-forming period, and some of them reopened when the later 
faulting occurred. 
These earlier east-west veins are distinguished as lodes or com- 
i pound veins. They differ in structural and mineral character from the 
later lodes, and, except where faulted and enriched, lack the high sil- 
ver contents of the veins formed later. Fortunately they have been 
extensively fractured by strike faults, as well as the two other vein 
systems noted. 
Northwest fault veins. — The northwest system of fractures faulted 
and displaced the east- west veins. The three largest veins of this 
system, the Blue vein, Mountain View vein, and Grey Rock vein, are 
mineralized, but not so generally as the older veins; the ore occurs in 
chutes and is quite high grade and shows enrichment. The Blue vein 
has been developed for over a mile, and to a depth of 1,000 feet, prov- 
ing a heavy producer in several mines. It is cut and displaced by a 
northeast fracture in the Parrot workings. 
Northeast fault veins. — The veins of both the east- west and the 
northwest systems are cut and displaced by those of the northeast 
