200  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
is  evident  that  the  ore  is  but  a  replacement  of  the  schist,  since  no 
hard-and-fast  line  can  be  drawn  between  unaltered  schist,  iron- 
stained  schist,  siliceous  ore,  and  good  ore.  Where  the  contact 
between  the  country  rock  and  the  ore  is  sharp,  considerable  differential 
movement  has  taken  place,  and  most  of  such  contacts  are  lines  of 
recognized  faulting. 
ORIGIN  OF  THE  ORES. 
The  ore  lenses  in  the  pre-Cambrian  rocks  are  of  secondary  origin. 
This  is  proved  by  the  close  relationship  between  them  and  secondary 
structures,  such  as  folding,  jointing,  faulting,  and  breeciation.  Fur- 
ther, the  ore  was  deposited  by  descending  water.  This  is  indicated  by 
the  position  of  the  ore  along  a  contact  which  is  a  maximum  zone  of 
downward  water  circulation  and  by  the  presence  of  lenses  and  veins 
of  iron  ore  at  a  distance  from  the  main  ore  bodies  along  joints  and 
faults,  natural  sites  of  maximum  water  circulation.  Further,  the  ore- 
is  associated  with  calcite,  quartz,  and  limonite,  minerals  known  to  be 
deposited  by  water.  Circulating  waters  naturally  flow  in  some  per- 
vious stratum  above  an  impervious  bed,  or  follow  more  or  less  open 
channels  along  zones  of  maximum  rock  crushing.  That  the  limestone 
is  relatively  impervious  and  the  schist  relatively  pervious  is  indicated 
by  a  number  of  the  characteristics  of  these  rocks.  Thus  the  pre- 
Cambrian  diabase  dikes  in  limestone  are  comparatively  fresh,  while 
those  in  schist  are  greatly  altered.  Likewise  veins  of  pegmatitic 
quartz,  presumably  deposited  by  very  dilute  aqueous  solutions,  are 
much  more  abundant  in  schist  than  in  limestone.  Further,  the  lime- 
stone, when  folded,  appears  to  have  escaped  important  breeciation, 
while  the  more  siliceous  bands  of  the  schist  were  intensely  fractured. 
The  limestones  naturally  confined  the  circulation  of  the  water  to  the 
more  pervious  rock  that  overlay  it.  The  main  circulation,  then,  was 
in  schist  down  the  dip  slope  of  limestone.  The  faults  crossing  the 
pre-Cambrian  formations  would  furnish  outlets  by  which  the  descend- 
ing water  could  reach  the  surface.  From  the  depth  of  the  deposits  it 
is  inferred  that  the  topography,  when  the  ores  were  deposited,  was  of 
a  rugged  character. 
The  possible  sources  of  the  iron  ore  are  (1)  magnetite,  hematite,  and 
pyrite  in  schist;  (2)  pyrite  in  quartzose  beds;  (3)  iron  carbonate  in 
the  limestone;  and  (4)  hematite  in  the  pegmatite  veins. 
Pyrite  and  magnetite  are  very  common  minerals  in  much  of  the 
schist  of  the  Ilartville  iron  range.  Pyrite  in  diamond-drill  cores  of 
biotitic  schist  occurs  usually  along  the  planes  of  schistosity  in  very 
small  flakes.  In  the  muscovite  schists,  particularly  in  those  near  the 
ore  bodies,  are  many  tiny  cavities  which  are  heavily  iron-stained. 
Some  of  these  represent  hematite  and  magnetite  crystals ;  others  are, 
without  much  question,  the  casts  of  small  crystals  of  pyrite,  which, 
