HARTVILLE    IRON-ORE    RANGE,  WYOMING.  197 
ORE  AND  GANGUE  MINERALS. 
The  chief  ore  of  these  deposits  is  hydrated  hematite.  It  is  either 
(1)  a  hard  gray  ore  filled  with  numerous  cavities,  which  are  lined  with 
finely  crystalline  specular  hematite,  or  (2)  a  soft  greasy  ore  of  brown- 
ish-red color.  Fibrous  varieties  of  hematite,  including  mammillary 
ore,  grape  ore,  and  stalactitic  ore,  occur  less  frequently.  Minor  quan- 
tities of  siderite  and  limonite  are  associated  with  the  hematite.  The 
limonite  is  in  some  places  compact  and  finely  granular;  at  others  it 
is  mammillary,  and  at  still  others  it  is  a  soft  yellow  ocher.  Mag- 
netite is  not  present  in  masses  large  enough  to  attract  the  eye, 
although  slight  local  magnetic  variations  noted  at  some  places  in 
the  vicinity  of  the  ore  deposits  may  indicate  its  presence.  Pyrite 
and  marcasite  were  not  observed  and  probably  do  not  occur  in  the 
ore  bodies. 
The  gangue  minerals  are  calcite,  quartz,  gypsum,  chalcedony, 
barite,  and  a  kaolinlike  mineral,  and  the  copper  minerals  are  chrys- 
ocolla,  malachite,  chalcocite,  azurite,  and  native  copper.  Calcite 
occurs  in  colorless  or  slightly  yellow  crystals.  The  most  important 
development  of  quartz,  like  that  of  calcite,  is  clearly  later  than  the 
ore.  It  also  occurs  as  brecciated  fragments  in  the  ore  and  is  then 
older  than  the  hematite,  while  to  a  less  extent  it  is  contemporaneous 
with  the  ore.  At  the  Chicago  mine  the  quartz  sometimes  is  of  a 
beautiful  amethystine  tint.  The  copper  minerals  a  occur  in  fractures 
in  the  hematite  and  associated  rocks  and  are  the  younger. 
Irregular  masses  of  iron-stained  schist,  "soapstone,"  and  to  a  less 
extent  iron-stained  limestone  and  iron-stained  siliceous  rocks  occur 
as  vein  material  in  the  ore  bodies.  Schist  and  soapstone  occur  as 
irregular  horses  throughout  the  ore  body.  The  " soapstone"  is  an 
unctuous  substance  of  pale  green  color. 
PARAGENESIS. 
The  hard  and  soft  ores  grade  into  schist  and  it  is  evident  that  each 
was  formed  through  the  replacement  of  the  schist  by  hematite. 
The  soft  ore  is  thus  in  part  derived  directly  from  the  schist,  but  a 
considerable  portion  of  it  is  derived  secondarily  from  the  hard  ore. 
Pseudomorphous  replacements  of  hard  ore  by  soft  ore  are  common. 
Perhaps  the  best  evidence  of  this  change  is  seen  in  the  pebbles  of  soft 
ore  of  beautifully  rounded  form  that  occur  in  the  detrital  deposits 
at  the  base  of  the  Guernsey  formation.  Pebbles  of  this  formation 
are  usually  of  the  dense  hard  gray  ore,  and  it  is  absolutely  impossible 
that  a  substance  offering  as  little  resistance  to  attrition  as  the  soft 
ore  could  form  such  well-rounded  pebbles.  The  secondary  char- 
acter of  much  of  the  soft  ore  is  further  indicated  by  the  fact  that  on 
a  See  this  volume,  pp.  93-107. 
