120  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bill.  260. 
Azurite  and  malachite  are  ofteD  abundant  as  coatings  on  minerals 
near  the  surface.  Specks  of  these  copper  carbonates  have  also  been 
found  in  the  sulphide  zone  at  considerable  depths."  Occasional  coat- 
ings and  stalactites  of  copper  sulphate  and  iron  sulphate  on  chalco- 
pyrite and  pyrit  !  ores  -how  the  method  of  alteration  of  these  ores. 
Below  the  oxidized  surface  zone  come  the  sulphides,  galena,  zinc 
blende,  pyrite,  chalcopyrite,  etc.  Sometimes,  as  in  parts  of  the 
Mendota  vein,  there  is  a  sharp  dividing  line  between  the  oxide  ores 
and  the  sulphide  ore-:   again  the  transition  is  very  gradual. 
At  the  gold  mines  of  Empire  the  surface  material  usually  consists 
of  free-milling  gold  ore-  with  very  little,  if  any.  sulphide-.  The 
surface  vein  filling  i-  chiefly  gold-bearing,  "honeycomb,"  cellulai 
quartz,  containing  cavities  left  by  dissolution  of  the  pyrite  crystals 
The  zone  often  extend-  60  or  70  feet  below  the  surface,  but  onh 
average-  between  $2.50  and  sd  per  ton.  Below  the  free-milling  surf 
face  ore-  of  the  Empire  vein-  are  pyrite  and  chalcopyrite  ore-,  often 
of  much  higher  grade. 
While  in  the  Empire  gold  district  the  oxidized  ores  are  poorer  thai 
the  sulphides  below,  the  reverse  i-  true  of  the  silver  districts  arouni 
Silver  Plume  and  Georgetown,  where  the  oxidized  ores  are  usually 
much  richer  than  the  sulphides  of  lower  depths. 
FAINT   ZONAL  DISTRIBUTION   OF  THE  ORES. 
There  are  evidences  in  some  of  the  mines  of  a  zonal  distribution! 
in  the  mineral-  of  the  sulphide  zone,  hut  nothing  regular  and  pet  I 
sistent  has  been  established. 
In  the  Mendota  mine  low-grade  zinc-galena  ores  of  practically 
uniform  nature  extend  from  the  bottom  of  the  narrow  oxide  bel 
15  or  20  feet  below  the  surface,  to  depths  of  over  1.000  feet.  In  tl  I 
Colorado  Central  mine  -hoot-  or  bunches  of  very  high-grade  leatl 
and  zinc  sulphide  ores  bearing  polybasite,  tetrahedrite,  ami  rut  I 
silver  may  be  found  occurring  at  any  depths  in  the  low-grade  galer: 
and  sphalerite  ore-  down  to  1,050  feet  below  the  surface. 
o  Coljrudu  Central,  l,uu'j-i'uut  Level. 
