214  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
approximately  500  tons  a  day  in  a  total  output  of  2,000  tons  per 
day.  At  Globe,  whose  output  is  very  much  smaller  than  that  of 
the  two  other  camps,  being  about  one-ninth  that  of  Bisbee  and  one- 
fourth  that  of  Clifton,  the  oxidized  ore  is  as  yet  predominant,  and 
the  sulphide  ores  supply  barely  enough  sulphur  for  matting  in  the 
furnaces. 
NATIVE  COPPER  ORES. 
The  Lake  Superior  district. — The  native  copper  ore  produced  in 
the  United  States  is  all  derived  from  the  Lake  Superior  district. 
This  district  is  unique  among  the  world's  producers  in  the  character 
and  occurrence  of  the  ore,  the  cheapness  with  which  it  is  mined  and 
converted  into  bar  copper,  and  the  enormous  sum  paid  out  in  divi- 
dends. The  ore  occurs  in  a  series  of  upturned  conglomerates  and 
lava  flows,  which  aggregate  many  thousands  of  feet  in  thickness, 
and  which  dip  at  steep  angles  to  the  west.  These  rocks  extend 
along  the  northern  border  of  the  Keweenaw  Peninsula  of  Michigan, 
the  outcrops  forming  the  Copper  Range,  which  is  from  4  to  G  miles 
wide,  curving  with  the  peninsula,  and  passing  through  the  center  off 
the  peninsula  at  Hancock  and  Houghton. 
At  the  present  time  production  is  limited  to  the  mines  in  the 
vicinity  of  the  two  towns  just  named,  coming  either  from  the 
group  of  mines  north  of  Portage  Lake,  including  the  Calumet  andi 
Hecla  and  other  world-famous  producers,  or  from  the  southern 
part  of  the  range,  southwest  of  the  towns  noted.  The  geologic 
conditions  are  simple,  the  ore  occurring  either  as  the  cementing 
material  of  conglomerates,  formed  of  reddish  and  chocolate-colored 
felsites  and  porphyries,  or  in  irregular  pellets  filling  cavities 
(amygdaloids)  in  the  porous  trap  sheets.  The  ore  is,  however,  con- 
fined to  well-defined  shoots  of  limited  horizontal  extent,  and  does 
not  occur  in  paying  quantity  throughout  the  entire  extent  of  a  bed. 
The  Calumet  and  Hecla  ore  shoot  is,  however,  very  large,  being  3 
miles  long  and  productive  to  a  depth  of  5,000  feet,  the  greatest 
depth  yet  reached.  The  ore  body  is  12  to  15  feet  thick,  and  three- 
quarters  of  the  entire  production  of  the  region  comes  from  the 
mines  on  this  ore  shoot. 
The  discovery  of  the  rich  ore  shoot  of  the  Champion  and  Trimoun- 
tain  mines  and  the  reopening  of  the  Baltic  mine  emphasize  the  fact 
that  but  little  is  known  of  the  character  of  this  southern  part  of  the 
range,  where  about  250  feet  of  glacial  drift  hides  the  rocks  in  all  buti 
a  few  ravines.  There  is  no  probability  of  a  lack  of  ore  from  this 
region  for  a  very  long  period,  though  the  existing  mines  are  not 
likely  to  increase  their  production  rapidly. 
The  native  copper  ores  are  treated  by  mechanical  concentration, 
resulting  in  the  production  of  a  shotlike  material,  composed  almost 
