emmons.]  NEGLECTED    MINE,   COLORADO.  125 
made  up  of  sandstones  and  calcareous  shales.  These  sediments  dip 
to  the  east  at  a  low  angle  and  are  cut  by  a  dike  of  monzonite-porphyry 
which  trends  nearly  east  and  west.  An  intrusive  sheet  of  monzonite- 
porphyry — a  grayish-green  rock  which  contains  light-gray  feldspar 
phenocrysts  and  which  lithologically  resembles  the  dike — appears  to 
cross  the  gulch  near  the  lower  workings  of  the  mine.  Owing  to  lim- 
ited exposures  above  ground,  relations  are  not  quite  clear,  but  it  is 
very  probable  that  this  sheet  is  in  part  the  porphyry  which  is  encoun- 
tered near  the  lowest  level.  There  is  a  shattered  or  fractured  zone 
which  trends  a  few  degrees  south  of  east,  crossing  both  the  porphyry 
and  the  sediments.  Near  this  zone  the  rocks  are  much  altered.  The 
feldspars  of  the  porphyry  have  completely  decayed  and  their  spaces 
are  filled  with  white  mud,  chiefly  kaolinite,  while  the  groundmass 
of  the  porphyry  has  been  silicified.  The  porphyry  and  also  the 
sedimentary  rocks  contain  a  considerable  quantity  of  pyrite  which 
appears  to  be  secondary. 
Ores. — The  principal  ores  of  the  mine  are  tellurides  of  gold  and 
silver,  and  of  these  sylvanite  is  much  the  most  important.  Native 
gold,  pyrite,  and  amalgam  are  present.  At  the  close  of  a  mill  run 
the  retorts  yielded  a  considerable  increase  of  quicksilver  over  that 
introduced  on  the  amalgamation  plates. 
The  ore  deposits. — The  ore  deposits  are  quite  irregular  in  form, 
but  in  a  broad  way  they  follow  the  vertical  fracture  zone  which 
crosses  the  property  east  and  west.  The  ore  occurs  as  small  rich 
veins  and  as  stringers  of  white  and  gray  quartz.  The  veins  and 
stringers  occur  in  the  porphyry  and  at  its  contact  with  the  sediments, 
and  to  a  certain  extent  in  the  sediments  also. 
The  plane  of  the  shaft  and  the  levels  is  at  or  near  the  contact  of  the 
porphyry  dike  and  the  sediments.  The  sediments  here  are  sandstones 
ind  silicified  shales.  Experience  in  the  development  of  the  mine  has 
shown  that  the  contact  between  the  sediments  and  the  porphyry  is 
the  most  favorable  place  to  look  for  the  ore,  and  so  far  the  shale 
pas  proved  the  more  favorable  horizon.  Apparently  on  account  of 
their  induration  the  shales  and  sandstones  were  at  the  time  of  their 
deformation  equally  easily  fractured,  and  the  water  which  deposited 
he  metals  penetrated  both  with  equal  facility.  The  most  profitable 
lorizon  has  been  the  zone  between  the  125-foot  and  the  LT5-foot  levels. 
There  has  also  been  replacement  and  impregnation  of  the  country 
I'ock  away  from  the  stringers  or  pay  streaks,  and  here  the  values,  con- 
iderably  lower,  are  said  to  run  largely  in  the  pyrite.     This  is  the 
illing  ore. 
Treatment. — The  rich  pay  streaks  are  separated  from  the  sur- 
ounding  rock  and  the  ore  is  sacked  and  sorted  in  the  mine.  This 
icher  ore  is  taken  to  the  smelter  by  pack  train.     The  remaining  low- 
