irving.]  OURAY   DISTRICT,   COLORADO.  55 
free  from  intrusions  that  it  is  with  no  little  surprise  that  one  encoun- 
ters indications  of  mineralization. 
The  ore  deposits  are  difficult  to  classify  on  account  of  the  insensible 
gradations  between  even  the  most  diverse  types.  A  single  deposit 
frequently  displays  very  different  characters  in  different  rocks,  so 
that  it  will  conform  to  one  type  of  deposit  in  one  part  of  its  course 
and  another  type  at  another. 
For  convenience  of  description  the  classification  below  is  given.  It 
will  serve  to  bring  into  the  same  group  the  types  of  ore  bodies  most 
readily  discussed  together. 
1.  Fissure  veins. 
2.  Replacement  ores  in  quartzite. 
3.  Replacement  ores  in  limestone. 
These  three  main  classes  of  ore  bodies  owe  their  existence  to  the 
presence  of  fissures,  generally  nearly  vertical,  in  the  country  rock 
through  which  mineralizing  waters  have  circulated.  The  form  of  the 
nre  body  deposited  is  dependent  on  two  factors: 
1.  The  amount  of  open  space  in  the  fissures. 
2.  The  kind  of  rock  through  which  the  fissures  pass. 
Where  the  fissures  have  been  open  the  resulting  vein  frequently 
exhibits  a  roughly  parallel  alignment  of  minerals,  and  little  replace- 
ment of  the  wall  rock  is  noticeable.  Where,  on  the  other  hand, 
they  have  been  narrow  and  more  or  less  discontinuous  the  maximum 
replacement  is  to  be  observed  along  soluble  beds.  In  the  latter  case 
narrow  fissures  are  apt  to  be  developed  in  considerable  number,  and 
large  flat  masses  or  shoots  conformable  to  the  bedding  are  formed. 
Of  far  more  wide-reaching  importance  is  the  nature  of  the  rocks 
in  which  the  fissures  occur.  Most  of  the  veins  are  found  passing 
through  an  immensely  varied  series  of  rocks  and  may  frequently 
be  traced  from  the  red  beds  of  the  Hermosa  formation  into  the 
andesite  breccias  of  the  San  Juan,  which  form  the  massive  gray 
cap  rock  of  all  the  higher  hills  and  mountains. 
The  ore  bodies  are,  therefore,  as  widely  different  as  the  rocks  in 
which  they  occur.  Where  a  fissure  penetrates  only  impervious  rocks 
but  little  replacement  occurs  and  no  lateral  shoots  are  developed. 
Where  there  are  replaceable  limestones,  or  in  some  places  even 
quartzites  capped  by  impervious  shales,  lateral  enrichments  have 
:  been  developed,  often  of  great  extent. 
The  Bachelor  mine  is  an  example  of  the  simple  fissure  where 
replacement  has  operated  only  in  a  small  degree.  No  flat  ore  bodies 
of  any  size  are  observable  in  the  mine  workings,  because  the  latter 
have  not  yet  penetrated  deep  enough  to  reach  beds  of  limestone. 
The  Newsboy  mine  is  an  instance  of  a  vein  of  twofold  character 
