"kansomb.5"*  ]         CRIPPLE    CREEK    DISTRICT,    COLORADO.  93 
the  small  volume  of  available  vein  matter  is  particularly  noticeable. 
The  walls  of  such  fractures  and  the  fragments  of  the  shattered  rock 
are  usually  merely  coated  with  a  thin  deposit  of  quartz,  fluorite, 
and  other  minerals.  As  the  rich  tellurides  were  usually  among  the 
minerals  last  to  form,  and  are  particularly  abundant  on  the  walls  of 
the  vugs,  it  is  probable  that  had  quartz,  fluorite,  or  other  gangne 
minerals  been  more  abundantly  deposited  the  ores  would  have  been 
of  much  lower  grade. 
Sheeted  reins. — The  mineralized  sheeted  zones  constitute  the  most 
characteristic  deposits  of  the  district  and  occur  in  practically  all  the 
rocks,  although  particularly  common  in  breccia.  They  consist  of  a 
varying  number  of  narrow,  approximately  parallel  fissures,  together 
composing  a  sheeted  zone  that  may  range  from  a  fraction  of  a  foot 
to  50  or  60  feet  in  width. 
As  a  rule,  the  fissures  are  mere  cracks,  showing  no  brecciation, 
slickensiding,  or  other  evidence  of  tangential  movement  of  the  walls. 
Usually  the  tellurides  are  exclusively  confined  to  the  narrow  fissures 
and  cracks,  and  rarely,  in  this  type  of  deposit,  constitute  a  replace- 
ment of  the  country  rock.  The  rocks  in  the  vicinity  of  the  fissures 
are  partly  replaced  by  dolomite,  pyrite,  and  a  little  fluorite.  The 
fissures  are  not,  in  general,  planes  of  faulting.  Appreciable  move- 
ment has  undoubtedly  occurred  in  some  instances,  but  the  displace- 
ment probably  rarely  exceeded  1  or  '2  feet. 
Although  found  most  abundantly  in  the  breccia  or  trachytie  phono- 
lite,  sheeted  zones  and  single  fissures  are  often  well  developed  in  the 
granite,  as  in  the  El  Paso,  C.  K.  &  N.,  and  Gold  Coin  mines.  While 
l in  some  of  these  lodes  the  ore  minerals  are  as  plainly  confined  to  the 
fissures  as  in  the  breccia,  in  other  cases  the  ore  to  some  extent  per- 
meates the  granite  alongside  the  fissure,  this  constituting  a  deposit 
intermediate  in  nature  between  types  1  and  ±     They  also  frequently 
! follow  phonolite  dikes,  the  general  tendency  of  these  dikes  to  develop 
a  platv  parting  parallel  to  their  walls  being  particularly  favorable 
to  the  production  of  a  well-defined  sheeted  zone  when  the  direction 
iof  Assuring  happens  to  coincide  with  that  of  the  dike. 
The  metasomatic  alteration  accompanying  these  sheeted  zones  is 
surprisingly  slight,  and  consists  of  a  partial  replacement  of  the  brec- 
cia, phonolite,  trachytie  phonolite,  or  "  basalt  "  by  dolomite  and  pyrite 
accompanied  by  a  small  amount  of  sericite  and  a  little  secondary 
potash  feldspar.  The  alteration  in  granite  exhibits  a  somewhat 
different  phase,  described  in  a  subsequent  paragraph. 
Not  all  the  sheeted  zones  carry  ore,  nor  is  the  ore  of  a  productive 
sheeted  zone  necessarily  coextensive  with  the  Assuring.  The  ore 
►ccurs  in  pay  shoots  up  to  2,000  feet  in  length  and  1,000  feet  in  depth, 
but  usually  very  much  smaller  than  is  indicated  by  these  limits. 
