ikving.J  OURAY    DISTRICT,    COLORADO.  63 
fragments  in  the  clastic  dike  are  generally  arranged  with  their  flat 
surfaces  parallel  to  the  walls  of  the  vein.  They  were  probably  de- 
rived in  part  from  the  overlying  black  shale  and  partly  from  the 
numerous  black-shale  beds  which  occur  just  above  the  La  Plata 
sandstone  at  depths  not  yet  reached  by  the  mine  workings. 
The  high-grade  ore  is  shipped  to  the  smelters  and  gives  no  trouble 
other  than  that  involved  in  a  careful  sorting. 
For  the  treatment  of  the  lean  ore  a  small  mill  has  been  erected  at 
the  mine.  It  is  fitted  with  jigs,  Bartlett  tables,  slime-settling  tanks, 
and  canvas  slime  tables.  Great  difficulty  has  been  experienced  here, 
as  in  the  other  silver  veins,  in  saving  the  silver  values  contained  in 
the  gray  copper.  This  mineral  is  converted  into  slime,  even  with  the 
most  careful  crushing,  and  floats  on  the  top  of  the  water  so  that  a 
large  part  of  it  is  lost.  The  canvas  tables  save  a  little,  and  are  a 
very  commendable  feature  of  this  mill,  but  even  by  their  aid  a  con- 
siderable loss  of  silver  values  can  not  be  avoided. 
Veins  in  Portland  amphitheater. — A  group  of  silver-bearing  fissure 
veins  of  a  very  different  character  is  found  at  the  head  of  the  Port- 
land amphitheater,  in  the  almost  inaccessible  cliffs  which  surround 
the  glaciated  valley.  They  exhibit  many  different  strikes  and  dips 
and  pass  from  the  limestones  and  shales  into  the  andesite  breccias  of 
the  San  Juan  formation. 
They  contain  both  silver  and  gold,  in  about  equal  proportions. 
The  silver  usually  preponderates,  but  the  reverse  is  occasionally  true. 
The  gangue  is  usually  white  quartz  and  rhodochrosite,  or  some  car- 
bonate intermediate  between  that  and  calcite.  In  many  of  the  veins 
the  gangue  is  entirely  quartz.  They  vary  in  width  from  a  few  inches 
to  4  feet,  and  are  apt  to  pinch  out  within  short  distances.  In  the 
sediments  below  the  andesite  breccia  they  often  form  lateral  enrich- 
ments in  the  limestone,  although  none  of  those  noted  were  extensive. 
Ore  minerals  occur  in  very  small  quantities  in  the  gangue.  Those 
observed  are  galena  and  gray  copper.  Stephanite  and  other  rich 
silver  minerals  have  been  reported  from  these  veins.  The  ore  deposits 
of  this  district  are  as  yet  of  merely  prospective  value. 
t     Commercial  considerations. — The  ore  from  the  silver  mines  ranges 
n  value  from  $30  to  $800  per  ton.     Very  high   values  have   been 
►btained  from  the  oxidized  portions  where  the  native  silver  is  in  large 
amount,  and  the  occasional  bonanzas  of  ruby  silver  found  have  also 
ielded  exceptionally  high  returns.     In  the  majority  of  cases,  how- 
ver,  values  of  from  00  to  200  ounces  per  ton  may  be  regarded  as  an 
iverage  grade  of  shipping  ore.     The  rich  ore  is  usually  shipped  by 
*ail  to  smelting  centers  and  there  smelted,  and  where  lean  ore  is  con- 
centrated, as  in  the  case  of  the  Bachelor  mine,  the  concentrates  are 
hipped  in  like  manner.     A  local  smelter  at  Ouray  was  in  operation 
it  the  time  the  district  was  visited,  and  treated  the  lower-grade 
