LAKE    SUPERIOR   REGION.  221 
has  been  deposited  in  cracks  and  fissures  in  the  slate,  which  lie  at  a  lower 
elevation,  but  stratigraphically  above  the  ore.  The  source  of  the  iron 
is  believed  to  have  been  chemical  and  mechanical  oceanic  deposits, 
which  have  simply  concentrated  in  the  present  situation,  perhaps  from 
rocks  now  completely  removed  by  erosion.  The  water  which  brought 
in  the  iron  ore  to  supply  the  place  of  the  silica  taken  away  in 
solution  followed  the  natural  drainage  courses — either  the  drainage 
slopes  or  the  points.  The  Giants  range  is  regarded  as  having  been 
uplifted  at  the  time  of  the  gabbro  outflows,  and  to  have  been  caused 
by  them. 
Grant,185  in  1894,  gives  a  general  account  of  the  geology  of  the 
Gunflint  Lake  district.  In  Tps.  65  and  66  N.,  Rs.  4,  5,  and  6  W.,  are 
Keewatin  rocks,  including  the  usual  types — volcanic  tuff,  greenstone 
schists,  greenstone,  and  the  Ogishke  conglomerate.  The  Saganaga 
granite  is  intrusive  in  the  Keewatin.  The  more  crystalline  schists  of 
the  district  have  been  called  Coutchiching  and  Vermilion.  It  ap- 
pears, however,  that  these  rocks  in  this  area  are  a  more  crystalline 
phase  of  the  Keewatin,  and  that  they  owe  their  crystalline  nature  to 
the  proximity  of  intrusive  granite. 
The  iron-bearing  rocks  of  Akeley  Lake  lie  upon  the  Keewatin 
greenstone  to  the  north,  and  on  the  south  are  overlain  by  the  great 
gabbro  mass.  The  belt  has  a  width  of  300  to  1,300  feet,  and  a  dip 
varying  from  20°  to  almost  vertical,  but  averaging  45°  to  50°.  Where 
widest  it  has  an  a  Average  dip  of  30°,  which  would  make  a  maximum 
thickness  of  650  feet.     The  iron  ore  is  a  nontitaniferous  magnetite. 
The  Animikie  rocks  are  little  disturbed,  except  locally,  having  an 
average  dip  of  8°  or  10°  a  little  east  of  south.  The  Animikie  beds 
are  interleaved  with  diabase  sills.  These  give  parallel  east-west 
ridges,  the  south  sides  of  which  are  gentle  slopes  and  the  north  steep 
mural  descents.  This  topography  has  led  Lawson  to  the  conclusion 
that  the  apparently  large  number  of  sills  are  due  to  monoclinal  fault- 
ing of  fewer  layers,  but  of  this  there  is  no  evidence.  The  Animikie 
strata  are  divided  as  follows:  An  upper  or  graywacke  slate  member, 
1,900  feet  thick,  composed  of  slates  and  graywackes  with  fine-grained 
quartzites  and  quartz  slates;  a  middle  or  black  slate  member,  1,050  feet 
thick,  composed  mainly  of  black  slates,  apparently  carbonaceous,  with 
a  line-grained  siliceous  and  flinty  layer  at  the  base  60  feet  thick;  and 
a  lower  or  iron-bearing  member,  composed  largely  of  jaspery,  actino- 
litic,  siliceous,  and  magnetitic  slates,  usually  thinly  laminated,  and 
some  beds  of  cherty  iron  carbonate.  The  Akeley  Lake  rocks,  first 
called  Pewabic  quartzite,  are  similar  to  the  Gunflint  .iron-bearing 
rocks  and  different  from  the  Pewabic  quartzite  of  the  western  Mesabi 
range,  and  if  these  iron-bearing  rocks  are  put  at  the  base  of  the  Anim- 
ikie there  seems  to  be  serious  objection  to  regarding  them  as  the 
basal  quartzite  and  the  equivalent  of  the  quartzite  of  the  western 
