GOLD    AND    SILVER    ORES    OF    REAR    CREEK,    COLORADO.  27 
dollars'  worth  of  ore.  They  are  all  in  hard,  rather  siliceous  phases  of  the  slate,  and  it  is 
reported  that  in  the  development  of  the  Gold  Bug  mine  the  profitable  ore  pockets  were 
found  in  siliceous  walls. 
The  veins  vary  in  width  from  the  thickness  of  a  knife  blade  to  2  or  3  feet,  and  the  Gold 
Bug  vein  is  said  to  reach  a  maximum  of  6  feet.  While  they  usually  have  pretty  well- 
defined  limits,  there  is  often  a  considerable  amount  of  the  country  rock  contained  in  the 
vein  matter,  so  that  in  certain  cases  where  the  country  rock  is  cut  by  quartz  stringers  it 
is  impossible  to  determine  the  wall  definitely.  For  considerable  distances  the  veins  may  be 
entirely  barren  of  the  precious  metals,  while  at  the  end  of  this  barren  streak  a  rich  pocket 
sometimes  occurs  in  which  are  large  quantities  of  high-grade  ore.  When  this  pocket  lias 
been  removed  another  barren  streak  is  apt  to  be  encountered.  This  erratic  distribution 
of  the  ore  has  had  a  tendency  to  discourage  continued  effort  at  exploitation,  and  while  a 
considerable  amount  of  development  work  has  been  done  the  principal  mines  are  at  pres- 
ent idle. 
Summary. — At  Bear  Creek,  Needle  Mountains,  Colorado,  gold  and  silver  tellurlde,  gray 
copper,  and  other  minerals  occur  as  pockets  and  stringers  in  quartz  veins  which  cut  the 
Uncompahgre  quartzites  and  shales  and  older  pre-Cambrian  formations.  These  veins 
appear  to  have  no  intimate  relation  to  the  folding,  faulting,  or  schistosity  of  the  rocks. 
They  are  most  clearly  defined  in  the  quartzite  or  in  the  more  silicified  portions  of  the  shale. 
