COPPER    DEPOSITS    OF    HARTVILLE    UPLIFT,    WYOMING.  103 
The  base  of  the  Guernsey  formation  is  here  a  heavily  iron-stained 
and  slightly  conglomeratic  sandstone  that  is  locally  cemented  with 
lime  carbonate.  The  underlying  pre-Cambrian  limestone  prior  to  the 
deposition  of  the  Guernsey  formation  had  a  very  uneven  surface, 
roughened  through  the  development  by  solution  of  enlarged  joints, 
small  sink  holes,  and  irregular  cave  galleries.  The  ore-bearing  sand- 
stone, which  is  as  a  rule  but  3  feet  thick,  extends  down  into  these  cavi- 
ties, some  of  which  are  20  feet  below  the  normal  contact  of  the  Guern- 
sey and  pre-Cambrian  formations.  The  complexity  of  the  ramifica- 
tions of  such  masses  of  copper-bearing  sandstone  is  indicated  by  the 
fact  that  they  are  in  places  separated  from  one  another  by  masses  of 
pre-Cambrian  limestone  on  the*  sides  of  the  workings.  Ore  does  not 
extend  from  the  sandstone  into  the  limestone  member  of  the  Guernsey 
formation.  The  mineralized  zone  is  exposed  for  150  feet  along  the 
valley  side. 
Malachite,  chrysocolla,  azurite,  and  chalcocite  form  the  cement  of 
the  sandstone.  The  vividly  colored  copper-stained  sandstone  alter- 
nates sharply  with  patches  of  unstained  pink  or  gray  srndstone.  The 
same  minerals,  together  with  the  yellowish-green  copper  arsenate 
already  mentioned,  fill  fractures  in  the  copper-stained  and  unstained 
sandstone,  forming  veinlets  and  nodules  of  ore.  Chalcocite  is  par- 
ticularly common  in  this  form.  Some  of  these  nodules  fill  cavities 
once  occupied  by  pebbles  of  pre-Cambrian  limestone  which  have  been 
removed  by  solution.  Where  the  cavities  have  not  been  completely 
filled  by  the  ore,  malachite  forms  tufted  crystal  aggregates,  whereas 
chrysocolla  occurs  as  botryoidal  masses.  White  or  yellowish  calcite, 
of  later  origin  than  the  copper  minerals,  incrusts  them,  and  in  many 
places  films  of  bluish-w\nte  chalcedony  cover  the  calcite.  Although 
jthese  gangue  minerals  are  clearly  of  later  origin  than  most  of  the  ore, 
'here  and  there  a  little  malachite  coats  the  chalcedony,  indicating  that 
'the  ore  was  undergoing  recrystallization  during  a  considerable  period 
I  of  time. 
Malachite,  chrysocolla,  and  azurite  have  been  leached  from  tins 
blanket  deposit  down  into  the  pre-Cambrian  limestones  as  irregular 
jstains  and  as  stringers  descending  from  the  blanket  deposit. 
Two  thin  sections  of  ore  from  the  Green  Hope  mine  were  examined 
microscopically.  One  is  a  well-assorted  quartzose  sandstone  in  which 
^the  half  stained  by  copper  is  separated  with  line-like  sharpness  from 
that  stained  by  limonite.  The  portion  of  the  rock  without  copper 
shas  as  a  cement  finely  divided  kaolinitic  matter,  with  probably  a 
[little  sericite.  In  the  other  portion  this  kaolinitic  substance  is 
(replaced  by  malachite  either  in  a  very  finely  divided  and  almost 
flocculent  condition  or  in  fibrous  aggregates,  the  fibers  of  some  of 
which  are  radially  arranged.  In  one  or  two  places  malachite  enters 
the  quartz  grains  in  blunted  embayments  and  probably  replaces  it. 
