ikving.]  OURAY    DISTRICT,    COLORADO.  71 
Except  in  point  of  production  the  other  deposits  in  qnartzite 
exactly  resemble  those  of  the  American  Nettie  mine.  From  a  lower 
quartzite  in  the  Bright  Diamond  mine  high-grade  ore  to  the  value  of 
$32,000  was  obtained  from  small  shoots.  It  is  probable  that  further 
exploration  may  reveal  other  important  bodies  of  ore  similar  to  those 
of  the  American  Nettie. 
REPLACEMENT  DEPOSITS  IN  LIMESTONE. 
The  replacement  deposits  in  limestone  are  a  series  of  broad,  flat  ore- 
bodies  which  are  found  in  beds  of  limestone  either  adjoining  fissure 
veins  or  associated  with  numerous  small  vertical  (or  nearly  vertical) 
fissures  which  intersect  the  stratified  rocks.  They  have  been  formed 
by  the  removal  of  limestone  and  the  simultaneous  substitution  of  ore 
minerals  for  it  by  mineralizing  waters  which  here  gained  access  to  the 
more  soluble  rock  by  means  of  the  fissures.  They  are  more  regular 
than  the  replacements  of  quartzite  just  described,  and  much  more 
extensive,  a  few  of  them  being  fully  300  feet  wide,  ({old  predom- 
inates in  some  of  them,  silver  in  others,  but  the  ores  which  they  fur- 
nish are  uniformly  of  lower  grade  than  their  prototypes  in  quartz- 
ite. The  silver-bearing  deposits  are  associated  with  the  silver-bearing 
fissure  veins  alread}^  described,  and  are  found  only  where  these  veins 
penetrate  strata  of  soluble  limestone.  They  may  be  most  conveniently 
divided  and  subdivided  according  to  the  prevailing  gangue  minerals 
which  are  present,  as : 
1.  Baritic  siliceous  ores  (silver  bearing). 
2.  Magnetite-pyrite  ores  (gold  bearing). 
All  of  the  ores  of  both  groups  differ  a  good  deal  from  one  another, 
so  that  collective  description  will  give  only  a  most  general  idea  of 
their  character,  but  since  space  is  not  available  for  individual  discus- 
sion, the  reader  is  referred  for  a  full  treatment  to  the  paper  which 
will  appear  a  little  later. 
I5ARITIC    SILICEOUS    ORES. 
The  baritic  siliceous  ores  are  lateral  enrichments  of  silver  veins  or 
flat  masses  associated  with  small  vertical  fissures.  The  most  impor- 
tant are  in  the  Newsboy  mine,  the  Pony  Express  mine,  and  the  Min- 
eral Farm  mine,  In  the  Newsboy  and  Pony  Express  a  fissure  else- 
where productive  as  a  vein  is  connected  with  the  deposits;  in  the 
Mineral  Farm  no  important  vertical  fissure  veins  are  found,  and  it  is 
only  by  close  observation  that  the  knife-edge  fissures  may  be  noticed. 
At  the  Newsboy  mine  the  limestone  which  carries  the  ore  is  bluish- 
black  in  color,  and  is  overlain  by  a  clay  shale  and  sandstone  and 
underlain  by  thin-bedded  black  shales  and  a  heavy  gray  sandstone. 
