ibving.]  ORE    DEPOSITS    NEAR    LAKE    CITY,    COLO.  81 
uniform  in  width  and  more  nearly  approximate  the  ideal  type  of 
fissure  veins.  The  filling  of  gangue  materials  and  ore  extends  often 
uninterruptedly  from  one  wall  to  the  other. 
The  veins  usually  outcrop  in  the  high  hills  which  border  the  can- 
3^ons,  so  that  they  have  generally  been  worked  first  in  their  upper 
portions  high  above  the  stream  bed  and  later  at  a  very  much  lower 
level,  either  by  drifts  on  the  vein  or  long  tunnels  through  barren 
rock.  On  account  of  the  large  vertical  extent  of  vein  materials 
thus  developed  it  has  been  possible  to  gather  considerable  evidence 
as  to  the  behavior  of  these  fissures  in  depth.  While  there  are  many 
exceptions  and  it  is  a  little  difficult  to  form  an  accurate  conclusion,  the 
mass  of  evidence  seems  to  show  that  the  fissures  tend  to  die  out  in  their 
deeper  portions.  They  frequently  branch  as  they  are  followed  down- 
ward, inclusions  of  rock  material  become  more  frequent,  and  the 
open  spaces  of  the  fissures  become  more  constricted  and  narrower. 
It  is  also  observable  that  the  ore  minerals  become  leaner,  less  in 
amount,  and  of  a  different  character  from  those  exploited  in  the 
higher  levels  of  the  mine.  It  is  for  these  reasons  that  many  of  the 
long  tunnels  which  mark  the  latest  phase  of  mining  development 
on  these  fissures  have  been  costly  and  unprofitable.  It  is  probable 
that  a  clear  understanding  of  the  geologic  features  which  have  pro- 
duced these  characteristics  may  avert  some  loss  of  capital  in  ill- 
advised  exploration. 
Minerals. — The  minerals  which  fill  the  fissures  may  be  grouped  in 
two  classes — ore  minerals  and  gangue  minerals.     The  ore  minerals 
are  galena,  tetrahedrite,  chalcopyrite,  sphalerite,  and  pyrite,  which 
collectively  constitute  the  unaltered  sulphides  representing  original 
ore  minerals.     In  the  upper  portions  of  the  veins  are  found  ruby 
silver,  argentite,  anglesite,  limonite,  pyrolusite,  and  cerussite.     These 
have  been  produced  by  downward-moving  waters,  either  by  processes 
of  simple   oxidation   or   secondary   reactions  between   material   dis- 
solved from  the  upper  portions  of  the  deposits  and  the  unaltered  sul- 
phides below.     Anglesite  is  quite  a  common  oxidation  product,  and 
>ften  occurs  in  large  masses,  the  carbonate  of  lead  being  usually 
subordinate  owing  to  the  absence  of  calcium  carbonate  from  the  coun- 
try rock.     The  ruby  silver  is  usually  irregularly  distributed  and  often 
si  K'curs  as  bonanzas  in  the  deeper  portions  of  the  mine.     It  is  thought 
ttfjto  have  been  produced  as  a  secondary  mineral  by  reduction  of  anti- 
nonial  sulphates  from  downward-seeping  solutions  in  contact  with 
he  unaltered  sulphides  below.     Such  argentite  as  is  present,  and  also 
tiative  silver,  can  also  be  ascribed  to  the  same  oxidizing  agency.     It 
s  due  in  large  measure  to  the  absence  of  such  secondary  enrichment 
hat  the  poverty  of  the  deeper  portions  of  some  of  the  lodes  must  be 
ittributed.     Sphalerite  is  variable  in  amount.     In  the  Ute  and  Hid-' 
nll;itl!  Bull.  260—05  m 6 
