HARTVILLE    IRON-ORE    RANGE,  WYOMING.  203 
body  of  detrital  ore  at  the  base  of  the  Guernsey  formation,  1,850  feet 
south  and  525  feet  west  of  the  northeast  corner  of  sec.  26,  may  have 
been  derived  from  some  pre-Cambrian  ore  lens  along  this  contact. 
DETRITAL  ORES  AT  THE  BASE  OF  THE  GUERNSEY  FORMATION. 
The  basal  bed  of  the  Guernsey  formation,  which  is  at  many  places 
iron  stained,  is  a  quartzite  or  a  limestone  containing  clastic  grains. 
When  conglomeratic  the  pebbles  ordinarily  consist  of  quartz,  but 
locally  of  iron  ore.  The  iron-ore  pebbles  are  usually  confined  to 
the  immedate  vicinity  of  known  deposits  of  pre-Cambrian  iron  ore. 
The  pebbles  are  well  rounded,  many  of  them  being  beautifully  pol- 
ished, and  reach  a  maximum  diameter  of  1  foot.  The  ore  is  typic- 
ally the  hard  gray  ore,  which  in  some  places  has  been  altered  to 
soft  ore.  Where  such  iron-ore  pebbles  with  the  associated  iron- 
stained  quartzite  lie  in  the  irregular  sink  holes  and  cave  galleries 
that  extend  into  the  pre-Cambrian  limestone  (see  p.  195),  the  ore 
has  in  many  places  been  considerably  recrystallized.  Each  sink 
hole,  when  partially  cut  away  by  erosion,  has  acted  as  an  imper- 
vious trough,  in  which  iron  has  been  concentrated  by  downward 
circulating  waters.  In  consequence  the  quartzite  is  wholly  or  par- 
tially replaced  by  red  hematite,  which  is  often  of  excellent  grade. 
In  places  the  detrital  iron-ore  pebbles  are  intact ;  in  others  they  have 
been  completely  destroyed  by  recrystallization.  Calcite,  quartz, 
chalcedony,  siderite,  and  copper  ore  are  at  many  places  closely  asso- 
ciated with  this  hematite  and  ordinarily  fill  fractures  in  it.  So  far 
as  known,  none  of  these  deposits  are  of  large  size,  and  as  a  rule  the 
ore  is  too  siliceous  to  be  of  economic  value. 
DEPOSITS    AT    THE    BASE    OF    THE    HARTVILLE. 
The  base  of  the  Hartville  formation  is  an  iron-stained  sandstone 
which  at  a  few  places  contains  small  pebbles  of  iron  ore.  The  mate- 
rial of  these  pebbles  was  directly  or  indirectly  derived  from  pre- 
Cambrian  ore  bodies  through  the  breaking  up  of  the  detrital  pebbles 
of  the  Guernsey  formation.  North  and  west  of  Hartville  the  base  of 
the  Hartville  formation  at  a  number  of  places  is  an  iron-stained  shale, 
upon  which  lies  the  typical  Hartville  sandstone.  Where  this  shale 
has  been  folded  into  gentle  synclines  the  iron  of  the  sandstone  has 
been  redeposited  upon  the  shale,  and  in  consequence  blanket  beds  of 
low-grade  hematite  mark  this  horizon.  Ore  bodies  of  this  class  will 
probably  never  be  of  economic  importance. 
RESIDUAL    AND    PLACER    ORE. 
In  the  early  days  of  mining  at  Sunrise  the  surface  was  covered  by 
bowlders  of  iron  ore  which  were  derived  from  the  breaking  down 
