HARTVILLE    IRON-ORE    RANGE,  WYOMING.  201 
to  judge  from  their  even  distribution  throughout  large  masses  of  the 
schist,  were  original  to  the  recrystallization  of  the  schist.  Pyrite  also 
appears  to  be  present  as  an  original  constituent  in  some  of  the  quartz- 
ose  beds  interbedded  with  the  schist  member.  The  chemical  compo- 
sition of  the  schist  shows  that  it  had  a  low  iron  content  prior  to  its 
alteration. 
Locally  there  are  probably  small  amounts  of  iron  carbonate,  fer- 
ruginous dolomite,  or  ferruginous  calcite  in  the  limestone.  The 
unstained  character  of  much  of  its  weathered  surface,  however,  indi- 
cates that  the  quantities  of  such  minerals  in  the  limestone  are  so 
small  as  to  be  absolutely  inadequate  to  supply  material  for  the  con- 
centration of  large  iron-ore  bodies.  Further,  this  iron  carbonate  of 
the  limestone  lies  below  the  main  course  of  circulating  waters.  In 
the  schist  and  limestone  there  are  a  few  quartz  veins,  probably  of 
pegmatitic  origin,  and  a  small  amount  of  hematite  forms  a  constituent 
of  these  veins.  The  role  of  the  pegmatite  as  an  original  source  of  the 
ore  may,  however,  be  neglected. 
It  is  believed,  then,  that  the  iron  ores  were  concentrated  by  surface 
waters  from  magnetite  and  iron  pyrite  of  the  schLt  tying  above  the 
limestone  foot  wall.  During  pre-Cambrian  erosion  large  bodies  of  this 
schist  were  carried  away,  and  carbonated  surface  waters  probably 
broke  down  the  iron  minerals  into  iron  carbonate  or  other  soluble  iron 
salts.  This  material  was  carried  downward  in  solution  along  the 
impervious  limestone  foot  wall,  where  it  was  precipitated  by  oxygen- 
bearing  waters  descending  more  directly  from  the  surface  through 
cross  faults  or  other  passages  of  free  water  circulation 
AGE  OF  DEPOSITS. 
These  lenses  of  ore  were  evidently  formed  after  the  pre-Cambrian 
rocks  had  been  subjected  to  the  synclinal  folding,  which  has  already 
been  described.  This  is  shown  by  the  fact  that  the  position  of  the 
ore  deposits  depends  closely  upon  rock  structure.  As  to  the  relative 
age  of  the  granite  intrusion  and  the  ore  deposition  certain  evidence  is 
lacking,  although  it  is  believed  that  the  ore  was  deposited  after  the 
granite  was  intruded.  This  much,  at  least,  is  certain  that  in  the  ore 
deposits  there  are  zones  of  brecciated  quartz,  which  were  probably  once 
pegmatitic  veins  cutting  schist  that  is  now  replaced  by  ore.  It  is  also 
certain  that  in  the  main  the  ore  was  formed  prior  to  the  deposition  of 
the  Guernsey  formation,  since  this  Carboniferous  terrane  locally  con- 
tains at  its  base  pebbles  of  hard  ore  similar  to  that  of  the  Sunrise 
deposit.  To  provide  water  circulation  adequate  for  the  deposition  of 
bodies  of  iron  ore  that  lay  so  deep  the  topography  at  the  time  of  their 
formation  must  have  had  sufficient  relief  to  cause  deep  circulation  of 
surface  waters.     In   consequence  it  is  probable    that    the    ore  was 
