126  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY;  1904.        [bull.  260. 
grade  ore  goes  to  the  mill,  where  it  is  amalgamated  and  concentrated. 
About  28  tons  are  concentrated  into  one,  and  the  concentrates  are- 
carried  to  the  smelter. 
THE  DURANGO   GIRL  MINE. 
Location  and  production. — The  Durango  Girl  mine  is  situated  on 
the  east  slope  of  Lewis  Mountain,  in  Walls  Gulch,  which  is  tributary 
to  Junction  Creek.  Its  altitude  is  9,900  feet  above  sea  level.  The 
mine  was  located  in  1893  and  has  been  worked  in  a  small  way  ever 
since.  The  ores  are  chiefly  tellurides  and  the  sorted  shipping  prod- 
uct is  very  rich.  In  1903  and  in  the  first  half  of  1904  its  productior 
was  36.27  tons,  which  netted  something  over  $400  per  ton.  The  ore 
is  sacked  at  the  mine  and  carried  by  pack  mules  and  by  wagon  t( 
Durango. 
Geology. — The  geologic  relations  are  almost  the  same  as  at  thii 
Neglected  mine.  The  lower  "  Red  Beds,"  the  Cutler  formation,  dip 
sharply  eastward  and  are  intruded  by  sheets  of  monzonite-porphyn 
and  cut  by  a  dike  of  the  same  rock.  A  fracture  zone  or  fissure  cut 
the  sediments  and  the  porphyry,  having  nearly  the  same  general  clirec 
tion  as  that  in  the  Neglected  mine.  The  mine  is  worked  by  a  tunne 
which  follows  this  fissure,  and  much  of  the  ground  above  the  tunne 
has  been  stoped  out.  Two  dikes  of  a  dark,  fine-grained  basic  rock 
trending  nearly  east  and  west,  cut  the  porphyry  and  the  sediments 
These  dikes  have  nearly  the  same  direction  as  the  vein. 
The  ore  deposits. — The  ore  deposits  occur  along  the  vertical  frac 
ture  zone  which  passes  through  the  sediments  and  the  green  porphyrj  I 
The  ores  appear  to  be  in  part  a  replacement  or  alteration  product  oil 
the  porphyry,  which,  since  its  intrusion,  has  been  crushed  and  impregi 
nated.     The  sediments  near  the  porphyry  have  likewise  been  mineraal 
ized.     Small  pay  streaks  or  stringers  of  very  rich  ore  occur  in  th: 
altered  zone.     Some  of  these  appear  to  be  composed  mainly  of  whit 
hydrous  silica  which  contains  flakes  and  specks  of  sylvanite.     The* 
are  light  gray  in  color.     Other  and  richer  streaks  are  composed  ( 
clear,  colorless  quartz,  talc,  kaolinite,  tellurides,  and  sulphides.    The* 
metallic  constituents  sometimes  make  up  as  much  as  one-fourth  of  tl 
volume  of  the  pay  streaks.  A  mechanical  separation  of  a  sample  < 
this  rock  was  made  in  the  Survey  laboratory  and  the  metallic  coi 
stituents  were  examined  by  Doctor  Hillebrand.  Sylvanite,  free  gol 
tetrahedrite,  and  a  mercury  sulphide  were  found  to  be  present.  Son 
of  the  richer  ores  occur  also  in  the  sedimentary  rocks;  a  quartziij 
member  which  occurs  below  a  bed  of  shale  has,  so  far,  proved  tl 
more  productive  horizon. 
