72  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  200. 
It  is  about  5  to  7  feet  in  total  thickness.  The  shoots  of  ore  here  are 
flat  masses  and  generally  fill  the  entire  space  between  roof  and  floor. 
Laterally  the  valuable  portions  of  the  ore  material  extend  from  5  to 
30  feet  from  the  fissure,  although  the  silicification  goes  much  farther. 
The  shoots  run  nearly  east  and  west,  parallel  to  the  direction  of  the 
fissure,  for  distances  often  of  300  feet  or  more.  Most  of  them  lie 
north  of  the  vein.  The  ore-bearing  limestone  bed  is  at  the  base  of  the 
McElmo  formation  of  the  Jurassic,  immediately,  above  the  La  Plata 
sandstone.  Its  position  may  be  seen  in  the  Gold  Hill  section  shown 
in  fig.  1  (p.  56). 
The  same  limestone  bed  extends  along  the  canyon  to  the  Pon}^  Ex- 
press mine,  where  it  is- about  30  feet  in  thickness,  and  is  overlain  by 
shales.     The  shoots  in  this  mine  are  much  larger  and  thicker.     They 
extend  into  the  hill  for  1,000  feet  along  the  course  of  the  vein,  and  are  i 
frequently   100   feet   wide.     The   slopes   now   form   great   chambers, . 
separated  by  irregular  pillars  of  limestone  or  ore,  and  are  very  sug- 
gestive of  a  coal  mine.     The  ore  sometimes  occupies  the  entire  30  feetr 
of  rock,  but  is  more  usually  8  to  10  feet  thick.     It  is  richer  imme- 
diately beneath  the  shale  roof,  but  is  not  always  in  contact  with, 
it.     The  limestone  where  unmineralized  is  much  brecciated  and  full! 
of  cavities,  which  have  probably  been  more  important  in  determining 
the  mineralization  of  the  rock  than  the  overlying  shales. 
In  the  Mineral  Farm  mine  the  limestone  belongs  to  the  Ouray 
epoch  of  the  Devonian.  The  ore  shoots  are  here  from  8  to  10  feet  in 
thickness  and  6  to  50  feet  in  width.  The  workings  follow  one  shoot 
for  a  distance  of  perhaps  600  feet.  This  shoot  has  a  strike  of  N.  63° 
E.  The  narrow  fissures  with  which  the  ores  are  associated  may  be 
often  seen  in  the  roof  of  the  ore,  but  are  not  noticeable  without  care- 
ful examination. 
The  ores. — The  ores  of  the  limestone  replacement  bodies  are  bluish- 
gray  or  buff  colored,  according  to  the  color  of  the  original  limestone, 
They  consist  of  fine-grained  silica  heavily  charged  with  crystalline 
barite  and  containing  argentiferous  gray  copper  (freibergite),  galena, 
and  sometimes  chalcopyrite.  The  metallic  minerals  are  either  dis- 
seminated in  small  grains  through  the  silica  and  barite — especially 
the  gray  copper — or  are  concentrated  in  bunches  of  irregular  fonr 
and  character  that  are  distributed  at  uncertain  intervals  througl 
the  siliceous  gangue  rock. 
Irregular  cavities  lined  with  druses  of  quartz-crystals  form  s 
prominent  feature  of  the  ore.  In  the  nonoxidized  portions  silvei 
chloride  and  secondary  black  copper  sulphide  may  frequently  b< 
observed.  The  silica  extends  to  great  distances  beyond  the  metallic 
replacements  and  continues  at  times  for  the  entire  visible  outcrop  o 
the   limestone   bed.      In   one  mine,  however,  the  Pony   Express,   th 
