64  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
material,   but    it    has   not  yet  received   from   the  mine   owners   the 
encouragement  necessary  for  profitable  operation. 
GOLD-BEARING    VEINS. 
General  character. — The  gold-bearing  veins  are  very  subordinate 
to  the  silver  mines  in  their  economic  importance.  They  are  devel- 
oped along  the  course  of  the  Uncompahgre  River  between  the  mouth; 
of  Dexter  Creek  and  the  town  of  Ouray,  and  outcrop  in  the  steep 
clitls  on  either  side  of  the  canyon. 
The  country  rocks  which  form  these  cliffs  are,  from  below  upward, 
(1)  the  red  sandstones,  conglomerates,  and  sandy  shales  of  the  Her- 
mosa  and  Cutler  formations  of  the  Carboniferous;  (2)  the  red  shales 
and  sandstones  of  the  Dolores  formation  of  the  Triassic  lying  uncon- 
formably  above  at  a  slightly  different  angle;  (3)  the  white  and  gray 
sandstone  of  the  La  Plata  Jurassic;  (4)  the  alternating  shales,  sand- 
stones, and  limestones  of  the  McElmo  Cretaceous,  and  (5)  the  Dakota 
quartzite  and  black  Mancos  shales  of  the  Cretaceous. 
The  inner  and  steeper  walls  of  the  canyon  are  formed  of  rocks  o1 
the  lower  part  of  this  column ;    that  is,  the  red  beds  below  the  Jam 
Plata  sandstone. 
Irregular  intrusions  of  quartz-bearing  monzonite-porphyry  cut  di 
agonally  across  the  sedimentary  series,  some  of  them  vertical,  other 
with  marked  dip  toward  the  south. 
On  the  west  side  of  the  river  irregular  sheets  extend  outward  fron  j 
these  dikes  and  pass  between  the  sedimentary  beds.     When  smal 
the  dikes  weather  so  much  more  readily  than  the  sandstones  that  the; 
may  be  found  only  at  the  bottom  of  deep  clefts  in  the  cliffs,  althougl 
the  larger  ones  often  lie  nearly  flush  with  the  canyon  walls.     This 
veins  occur  either  wholly  within  the  dikes  or  are  closely  associate ! 
with  them.     They  comprise  a  group  of  highly  inclined  sheeted  zone) 
made  up  of  a  series  of  irregular  and  closely  spaced  fissures,  usuall 
of  small  throw,  and  often  aggregating  5  or  6  feet  in  width.     The  oi  I 
consists  of  gold-bearing  pyrite  and  chalcopyrite  in  a  gangue  of  com 
try  rock  and  clay — the  clay  having  been  derived  partly  from  intei 
calated  shale  beds  and  partly  from  porphyry.     The  filling  of  ope 
spaces  seems  to  have  been  subordinate  to  the  replacement  of  the  wa 
rock,  which  has  ocurred  to  a  considerable  extent.     The  ore  occurs  ii 
small  shoots  in  the  vein,  and  is  so  irregular  in  its  distribution  that  tl  < 
veins  are  mined  with  some  difficulty. 
The  veins  are  later  than  the  porphyry,  as  they  may  be  seen  distinct  j 
faulting  that  rock.     Both  the  dikes  and  the  associated  veins  stril 
approximately  east  and  west,  parallel  to  the  majority  of  silver-bearh 
fissures.     The  dip  is  sometimes  vertical,  but  in  other  cases  is  as  shallc  j 
as  50°.    Nearly  all  of  the  mine  workings  are  situated  high  above  tiJ 
