112 
CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1905. 
per  cent  of  the  silver  production.     The  gold-copper  ores  of  Beaverhead,  Meagher,  and 
Jefferson  counties  furnish  a  total  of  17,300  ounces  of  silver  out  of  a  total  of  10,250,000  ounces 
The  statistics  of  the  1905  production  of  copper  for  the  individual  counties  are  not  yet 
available;  those  for  1904  are  as  follows: 
Production  of  copper  in  Montana,  1904,  Ml  counties  A 
Beaverhead  County 
Meagher  County 
Jefferson  County 
Lewis  and  Clarke  County. 
Madison  County 
Powell  County 
Silverbow  County 
Miscellaneous 
Total. 
Copper. 
Mint  report. 
Pounds. 
68,815 
101,100 
17,007 
15,815 
30,000 
283,070,422 
587, 111 
283, 945, 330 
U.  S.  G.  S. 
report. 
Pounds. 
543, 120 
75, 000 
11,100 
7,007 
11,300 
1,000 
290,032,979 
290,081,572 
Gold  in  cop- 
per ores. 
Silver  in 
copper  ores 
Ounces. 
31.15 
50. 00 
4.00 
465.  25 
43,889.90 
44,  440.  30 
Ounces. 
12,57 
2 
: 
10,218,79 
10, 230, 11 
a  The  figures  in  the  last  three  columns  are  from  the  chapter  on  Gold  and  Silver  Production  of  th 
United  States  in  Mineral  Resources  U.  S.  for  1904,  U.  S.  Geol.  Survey,  1905.  They  differ  materially  froi 
those  of  the  Director  of  the  Mint  in  his  report  on  Precious  Metals  of  the  United  States  for  1904,"])-  13! 
as  shown  in  first  column. 
It  will  be  seen  that  Silverbow  County,  according  to  these  figures,  yielded  99$  per  cent  c 
the  copper  product  of  the  State. 
GEOLOGY. 
The  geology  of  the  Butte  copper  district  has  been  somewhat  fully  presented  by  the  wrira 
in  a  previous  bulletin  (No.  213),  and  in  Folio  No.  39  of  the  Geologic  Atlas  of  the  Unite 
States.     The  main  facts  arc  us  follows: 
The  Butte  district  is  in  southwestern  Montana,  in  the  central  part  of  the  Rocky  Moui 
tain  region,  within  2  miles  of  the  continental  watershed.  The  rocks  of  the  region  are  a 
igneous,  the  prevailing  form  being  a  dark  basic  granite,  or  quartz-monzonite,«  which  is  pa  i 
of  a  great  mass  of  granite  forming  the  core  of  the  mountainous  area  extending  from  But 
to  Helena.  This  rock  is  cut  by  irregular  and  dike-like  bodies  of  a  lighter  colored  grani 
rock — an  aplite  called  the  "Bluebird"  granite,  largely  used  for  building  stone  at  Butt  | 
This  rock  covers  relatively  small  areas  on  the  surface,  but  is  more  abundant  in  deep  worl 
ings.  Both  the  granites  mentioned  are  cut  by  the  "Modoc"  porphyry,  a  light-colon 
quartz-porphyry,  carrying  large  and  well-formed  crystals  of  quartz.  The  latest  rocks  a 
rhyolitic  in  nature  and  later  than  the  veins  in  age. 
The  veins  occur  in  the  basic  or  Butte  granite,  but  cut  through  the  other  rocks.     Th< 
can  be  grouped  into  silver  veins  and  copper  veins,  and  the  two  classes  occupy  distin 
areas,  with  but  little  overlapping.     The  copper  area  is  about  one-half  mile  by  \\  miles  I 
extent. 
The  veins  belong  to  four  distinct  systems.  The  largest  veins,  embracing  the  Anacom 
and  its  dislocated  segment  the  Parrot,  the  Syndicate  ledge,  and,  in  fact,  all  the  greate 
producers  of  the  district,  run  east  and  west,  and  dip  steeply  to  the  south.  These  veins  a 
faulted  and  displaced  by  a  northwest-southeast  series,  including  the  Clear  Grit,  Bli 
Skyrme,  Gray  Rock,  and  other  important  veins.  A  still  later  set  has  a  direction  east 
northeast,  and  faults  and  displaces  both  the  earlier  vein  systems;  the  veins  of  this  set  a 
in  places  heavily  mineralized,  as  in  the  Stewart  mine.  The  latest  fracturing  is  by  northea* 
southwest  faults  which  contain  drag  ore,  but  little  if  any  newly  formed  ore. 
a  Granite  Rocks  of  Butte,  Montana,  and  vicinity:  Jour.  Geol.,  vol.  7,  1899,  pp.  737-750. 
