180  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
is  no  evidence  of  any  such  enrichment.  Neither  is  there  any  evi- 
dence of  a  decided  change  of  volume  of  the  ore  bodies  such  as  this 
hypothesis  would  entail. 
It  has  also  been  suggested  that  these  ores  may  represent  very  basic 
intrusions  rich  in  iron.  Such  a  hypothesis,  however,  is  impossible, 
for  the  ores  show  stratification.  This  stratification  is  recognized  by 
distinct  differences  in  texture  and  is  not  at  all  to  be  confused  with 
cleavage.  Therefore,  as  the  rocks  show  stratification  it  is  evident 
that  they  can  not  be  igneous. 
From  a  study,  in  both  the  field  and  laboratory,  of  the  conditions 
under  which  the  ores  occur  it  seems  clear  that  the  gray  ores,  in  some 
stage  of  their  development,  have  been  hydra  ted  iron  ores,  and  that 
their  present  form  is  due  to  the  dehydration  of  the  original  limonite 
by  the  heat  and  pressure  produced  by  regional  deformation.  Where 
such  metamorphism  affects  a  deposit  of  limonite  the  first  result  is  to 
drive  off  the  water,  thus  forming  hematite.  If,  however,  under  the 
conditions  of  metamorphism  the  ore  body  was  shut  off  from  a  suffi- 
cient oxygen  supply,  so  that  the  iron  could  not  be  completely  oxi- 
dized, or  if  there  were  CO  or  some  other  deoxidizing  agent  present, 
the  result,  as  shown  by  Van  Hise,  would  be  a  change  of  the  hematite 
into  magnetite  and  carbon  dioxide.  The  formula  for  this  change 
would  be  3Fe203+CO  =  2Fe304  +  C02.  If,  then,  there  were  present 
at  the  time  of  metamorphism  enough  oxygen  to  completely  oxidize 
a  large  part  but  not  all  of  the  iron,  the  result  would  be  the  formation 
of  a  deposit  of  hematite  with  smaller  amounts  of  magnetite.  The 
abundance  of  the  magnetite  therefore4  varies  inversely  with  the 
amount  of  oxygen. 
The  belief  that  the  gray  ores  were  at  one  time  in  the  form  of  limon- 
ite  is  based  mainly  upon  their  field  occurrence.  In  many  parts  of  the 
field  brown  ore  deposits  occur  at  essentially  the  same  geologic  horizon 
as  the  gray  ores,  and  in  a  few  places  this  connection  is  so  close  that 
the  gray  ore  can  be  traced  into  brown  ore.  The  composition  of  the 
ore-,  with  their  high  phosphorus  and  silica  content;  also  strongly 
suggests  their  common  origin. 
While  the  gray  ores  have  probably  been  through  the  limonite  stage1 
it  is  not  believed  that  the  limonite  was  necessarily  the  original  form 
in  which  the  iron  ore  was  deposited.  The  ore  occurrence  on  the 
northwest  flanks  of  the  . Kahatchee  Hills,  already  described,  illus- 
trates this  point.  At  this  place  a  bed  of  quartzite  was  found  which 
had  been  opened  for  brown  iron  ore.  After  a  few  tons  of  this  had 
been  mined  and  the  pit  excavated  to  a  depth  of  about  5  feet  the  ore 
gave  place  to  a  disseminated  pyrite  body.  Here  the  limonite  was 
undoubtedly  the  residual  concentration  remaining  after  the  decom- 
position and  oxidation  of  the  pyrite.  The  most  interesting  point, 
however,  in  connection  with  this  occurrence  is  the  fact  that  on  the 
