leith.]  THE    LAKE    SUPERIOR    IRON    REGION    DURING    1903.  219 
and  banded  siliceous  phases.  A  very  small  amount  of  hydrated  hema- 
tite or  limonite  is  also  present.  The  rocks  immediately  associated  with 
the  ore  and  into  which  the  ore  grades  are  dolomite,  cherty  ferruginous 
dolomite,  ferruginous  chert,  ferruginous  slate,  and  ferruginous  dolo- 
mitie  slate — in  fact,  all  possible  gradations  and  mixtures  of  the  minerals 
dolomite,  hematite,  quartz,  and  such  argillaceous  minerals  as  kaolin  and 
chlorite.  In  the  ferruginous  rocks  associated  with  the  iron  ore  the 
iron  occurs  as  hematite  and  also  in  the  form  of  carbonate,  isomorphous 
with  carbonate  of  calcium,  magnesium,  and  manganese,  in  the  form  of 
ferro-dolomite  and  manganic-ferro-dolomite  and  as  silicates  combined 
with  various  proportions  of  alumina,  lime,  magnesia,  and  manganese,  as 
chlorite  and  mica,  and  also  very  probably  to  a  small  extent  as  iron 
phosphate. 
The  ore  deposits  thus  far  found  are  all  in  the  valley  between  the 
quartzite  ranges,  and  because  of  the  structure  of  the  pre-Cambrian 
series  it  is  hardly  possible  that  ore  deposits  will  be  found  elsewhere 
than  in  this  valley. 
The  iron  ore  is  a  stratified  formation  and  is  conformable  with  the 
associated  stratified  rocks,  both  below  and  above.  The  ore  bodies 
therefore  have  the  dip  and  strike  of  the  associated  rocks,  and  are  found 
dipping  at  various  angles  from  nearly  horizontal  to  nearly  vertical. 
At  the  Illinois  mine.  3  miles  southwest  of  North  Freedom,  and  on 
the  south  limb  of  the  syncline,  the  ore  deposit  has  a  thickness  of  30  to 
35  feet,  and  bears  an  average  of  51  to  58  per  cent  metallic  iron.  The 
dip  of  the  ore  body  and  adjacent  formation  is  about  50  X.  Between 
the  ore  and  underlying  Seeley  slate  are  100  feet  of  alternating  beds 
and  thin  seams  of  iron  ore  with  a  considerable  thickness  of  dolomite 
and  ferruginous  dolomite,  ferruginous  chert,  and  ferruginous  slate. 
Between  the  ore  deposit  and  the  overlying  dolomite  are  alternating 
strata  of  similar  character  grading  up  into  the  nearly  pure  dolomite. 
North  of  the  Illinois  mine  a  thickness  of  600  feet  of  dolomite  is  known 
to  occur. 
Underground  exploration  seems  to  show  that  the  conditions  and 
character  of  the  rock  existing  at  the  Illinois  mine  prevail  generally, 
as  should  be  expected,  throughout  the  valley  wherever  the  iron  for- 
mation and  overlying  rocks  have  not  been  eroded  by  the  subsequent 
pre-Potsdam  erosion.  In  general,  there  appears  to  be  a  highly  fer- 
ruginous horizon  near  the1  base  of  the  dolomitic  formation,  having  a 
variable  thickness,  probably  ranging  from  400  to  500  feet.  This  fer- 
ruginous horizon  bears  one  or  more  deposits  of  iron  ore  separated  by 
intervening  strata  of  associated  ferruginous  rock.  In  one  of  the  drill 
holes  in  the  west  end  of  the  district  nearly  200  feet  of  iron  ore  and 
paint  rock  were  penetrated  immediately  beneath  the  unconformable 
Potsdam  sandstone,  the  average  contentof  iron  for  this  distance  being 
about  45  per  cent. 
