LAKE    SUPERIOR   REGION.  2 
lava.  The  horizontal  parting  is  caused  by  a  later  shrinkage  along 
the  least  diameters  of  the  columns  formed  by  the  prismatic  jointing. 
The  banding  is  due  to  the  removal  of  silica  and  the  entrance  of  iron 
along  the  parting. 
The  ore  deposits  rest  upon  the  Pewabic  quartzite,  or  upon  the  hard 
and  little  altered  iron-bearing  rock,  in  areas  of  especial  weakness  or 
disturbance,  as  (1)  actual  fault  lines,  (2)  incipient  fault  lines,  (3) 
apices  of  anticlinal  folds,  and  the  troughs  of  synclines.  These  are 
places  of  fracture  and  where  abundant  waters  were  converged,  often 
wide  areas,  and  therefore  place's  where  large  quantities  of  iron  were 
supplied.  The  downward-percolating  water,  taking  iron  carbonate  in 
solution,  precipitated  the  iron  as  oxide  in  those  places  where  there  was 
an  abundance  of  oxygen,  and  at  the  same  time  took  the  silica  in  solu- 
tion, thus  forming  the  ore  bodies.  Between  those  of  the  largest  size 
and  the  small  local  concentrations  there  are  all  gradations.  The 
larger  deposits  of  ore  occur  where  they  are  protected  from  glacial 
erosion  on  the  north  by  a  hard  ridge  of  the  Keewatin  rocks,  and  espe- 
cially when  the  hard  rocks  give  slight  elevations  on  either  side,  so  as 
to  present  a  basin-like  depression. 
The  glauconite  in  origin  is  believed  to  be  the  same  as  modern  glau- 
conites — that  is,  it  has  developed  within  Foraminifera  and  other 
minute  shells,  as  a  result  of  a  reaction  between  the  organic  matter 
within  the  shells  and  fine  ferriferous  clay.  As  the  formation  con- 
tains only  a  small  quantity  of  ordinary  fragmental  quartz  grains,  it 
formed  in  water  at  a  depth  beyond  which  much  of  these  materials 
was  deposited.  As  its  upper  horizon  grades  into  limestone  this  indi- 
cates a  further  subsidence  of  the  area,  so  that  the  distance  from  the 
shore  line  became  so  great  that  very  little  mechanical  detritus  was 
furnished,  and  the  deposit  was  made  up  of  calcareous  matter. 
In  the  eastern  Mesabi  district  the  Animikie  strata  are  pierced  and 
intermingled  with  the  northern  border  of  the  Keweenawan  rocks,  so 
that  their  normal  attitude  is  often  much  disturbed.  With  this  change 
the  iron  of  the  iron-bearing  member  becomes  lare;elv  magnetic  and 
the  silica  hard  and  crystalline.  It  is  concluded  that  the  iron  before 
Keweenawan  time  was  here  in  the  state  of  sesquioxide,  and  that  the 
heat  of  the  igneous  Keweenawan  rocks  and  the  disturbances  of  the 
Animikie  series  produced  by  them  are  the  causes  of  the  change  of  the 
sesquioxide  of  iron  to  its  magnetic  form.  Thus  the  normal  process  of 
decomposition  and  concentration  was  brought  to  a  close,  and  this 
probably  explains  the  poverty  of  this  part  of  the  district  in  large 
ore   deposits. 
At  the  base  of  (lie  Cretaceous  are  ferriferous  detrital  deposits  de- 
rived from  the  Animikie.  A  study  of  these  indicates  that  the  nieta- 
somatic  processes  had  gone  far  before  Cretaceous  time,  although  they 
have  since  continued  to  the  present  time. 
