hiving. j  OURAY   DISTRICT,   COLORADO.  75 
2.  That  the  metamorphic  minerals  were  formed  by  the  same  solu- 
tions as  the  ore  minerals,  and  these  solutions  reached  the  limestone 
by  means  of  the  fissures. 
3.  That  ore  minerals  were  deposited  by  circulating  waters  subse- 
quent to  the  porphyry  intrusion  and  were  not  derived  from  them. 
4.  That  the  circulating  waters  supplied  not  only  the  ore  minerals 
but  also  the  silica  and  other  ingredients  necessary  to  produce  meta- 
morphic minerals  in  a  limestone  otherwise  too  pure  to  be  affected  by 
the  contact  with  igneous  rock. 
5.  That  both  ore  and  metamorphic  minerals  were  produced  by 
mineralizing  Avaters  and  are  not  due  to  contact  metamorphism. 
Too  little  space  is  available  for  a  full  discussion  of  this  question, 
but  these  conclusions  are  here  briefly  stated  preparatory  to  a  fuller 
discussion  in  a  forthcoming  paper. 
GEOLOGIC  AGE  OF  THE  ORE  DEPOSITS. 
In  all  cases  where  the  geologic  age  of  the  ore  deposits  can  be 
ascertained  with  measurable  accuracy  the  evidence  points  to  a  single 
period  of  mineralization  for  all  of  the  deposits  discussed.  They  are 
of  very  recent  formation;  in  all  cases  later  than  the  latest  of  the 
igneous  rocks,  as  the  fractures  from  which  they  have  been  formed 
frequently  extend  into  the  eruptives  and  some  of  the  veins  lie  wholly 
within  the  dikes. 
These  eruptives  were  formed  later  than  the  andesites  of  the  San 
Juan  formation,  often  cutting  the  latter  and  spreading  out  beneath 
it  in  the  form  of  horizontal  sills.  The  eruptives  are  therefore  later 
than  the  andesite  breccia,  which  is  of  Eocene  age.  We  can  then 
conclude  that  the  ores  were  formed  during  or  later  than  the  Miocene 
epoch. 
SUPPOSED  GOLD-BEARING  CONGLOMERATES. 
Underlying  the  San  Juan  andesite  breccia,  which  forms  the  cap 
rock  of  the  hills  in  the  Ouray  quadrangle,  and  rests  unconformably 
on  the  stratified  rocks  below,  is  a  conglomerate  known  as  the  Tellu- 
ride  conglomerate.  This  is  not  always  present,  but  occurs  in  large 
development  in  the  vicinity  of  Cobbs  Gulch  of  Cow  (reek.  It  has 
been  prospected  recently  for  gold,  and  an  experimental  stamp  mill 
has  been  erected  near  the  mouth  of  Cobbs  Gulch  to  test  its  value. 
The  possible  occurrence  of  placer  gold  in  this  rock  forms  a  problem 
of  unusual  scientific  interest  on  account  of  the  bearing  it  has  on  the 
geological  period  during  which  the  ore  deposits  of  the  region  were 
formed. 
If  the  conglomerates  carry  placer  gold,  there  must  have  been  ore 
bodies  containing  free  gold  in  existence  before  their  deposition.     It 
