Irving.]  OURAY    DISTRICT,    COLORADO.  65 
bottom  of  the  canyon,  in  the  steep  faces  of  the  cliffs,  and  the  ore  is 
brought  clown  by  means  of  wire-rope  tramways,  all  of  which  are  of 
antiquated  model  and  now  in  disuse.  Owing  to  their  prominent  posi- 
tion these  mines  were  among  the  first  to  be  operated  in  the  district. 
They  have  never  been  very  productive. 
The  values  are  largely  in  gold,  although  a  little  silver  is  generally 
present.  Exact  and  reliable  data  on  the  values  are  difficult  to  obtain, 
but  the  gold  values,  when  present,  are  undoubtedly  high.  The  aver- 
age yield  is  probably  between  2  and  3  ounces  gold  per  ton.  making  a 
high-grade  ore  of  $50.  In  some  instances,  as  in  the  Grand  View, 
the  ore  occasionally  runs  up  to  8  ounces,  and  in  a  few  cases  as  much 
as  30  ounces  is  reported. 
The  mines  are  now  either  idle  or  operated  under  lease.  They 
were  never  very  profitable,  presumably  because  the  irregularity  of 
the  veins  and  their  small  size  rendered  their  working  uncertain,  and 
the  methods  of  treatment  were  not  adapted  to  the  ore. 
REPLACEMENT    DEPOSITS    IN    QUARTZITE. 
Character  and  genesis. — The  ore  bodies  that  have  been  grouped 
under  the  class  "  replacement  deposits  in  quartzite  "  form  a  series 
of  gold  ores,  with  very  subordinate  silver,  which  occur  in  irregular 
masses  in  strata  of  massive  quartzite.  They  are  termed  iw  replace- 
ment ores"  because  they  have  been  formed,  not  by  the  filling  of 
iopen  spaces  existing  previous  to  their  deposition,  but  by  a  chemical 
\ interchange  of  ore  material  for  original  country  rock.  The  miner- 
alizing waters  by  which  they  were  produced  have  been  both  the 
(solvents  of  the  country  rock  and  the  agents  which  have  effected  the 
ore  deposition.  In  some  cases  the  solvent  action  of  these  waters  has 
been  greater  than  their  depositing  action,  so  that  cavities  exactly 
similar  to  solution  caves  in  limestone  have  been  produced,  and  yet 
no  ore  has  been  deposited. 
Inclosing  rocks. — These  ores  are  found  in  a  number  of  the  quartz- 
ites  which  outcrop  on  either  side  of  the  Uncompahgre  Canyon.     The 
uppermost — the  one  in  which  the  majority  of  occurrences  have  been 
round — is  the  Dakota   quartzite  of  the  Cretaceous.     The  others  lie 
it  varying  intervals  below  this  formation  and  constitute  a  portion 
)f  the  McElmo  formation  of  the  Jurassic,  but  there  are  many  similar 
>eds,  and  they  vary  so  much  from  place  to  place  that  they  can  not  be 
orreiated  with  one  another.     It   is  a   prevailing  impression    in   the 
listrict  that  there  are  two  ore-bearing  quartzites,  an  "  upper  "  and  a 
lower,"  which  are  readily  traceable  for  great  distances.     This  ini- 
ression  is  founded  on  error.     The  Dakota  quartzite  alone  is  a  promi- 
nent and   constant   stratum.     Below   this  are  many  other  quartzite 
Bull.  260—05  m 5 
