ORE DEPOSITS AT BUTTE, MONT. 
By Walter Harvey Weed. 
INTRODUCTION. 
The geology of the Butte district and its ore deposits formed the 
subject of a report published by the Geological Survey as a geologic 
folio in 1897. a Subsequent development in the copper mines of the 
district, partly as a result of the greatly increased output of the prop- 
erties, but mainly because of the very large amount of work done to 
prove structural conditions, ore connections, and other evidence for 
use in the many lawsuits begun since 1896, has afforded opportunity 
to greatly extend the earlier work and to modify conclusions based 
upon the earlier incomplete data. A reexamination of the district, 
with a special study of the copper deposits, was therefore begun in 1 901. 
Owing to the necessity of completing other work for publication, and 
to the intricate nature of the study, involving a close 1 and detailed 
examination of over a hundred miles of underground workings, the 
field work was not completed until the autumn of 1902. The later 
workings show that the structural conditions are far more complex 
than was formerly supposed. The original veins are displaced by 
great faults, and these later fractures are themselves mineralized and 
again displaced. The working out of this structure has been diffi- 
cult because the deposits occur in a body of very homogeneous gran- 
ite, the rock alone affording no clue to the amount or direction of 
displacement. Nevertheless, the correlation of displaced areas is 
fairly satisfactory, based as it is upon a study of the quartz-porphyry 
and aplite intrusions in the granite and of the structural and miner- 
alogic variations of individual veins. 
SITUATION OF THE DISTRICT. 
The Butte district is situated in southwestern Montana, in the cen-| 
trai part of the Rocky Mountain region. The city which is built 
about and over the mines is the largest settlement of the State, while 
the neighboring city of Anaconda, 20 miles distant, is a dependent, 
a Geologic Atlas U. S., folio lis, Butte Special, Mont., 1897. 
170 
