LAKE    SUPERIOR   REGION.  223 
cate  that  the  granite  is  eruptive.  Thrust  planes  are  numerous,  and 
generally  have  either  vertical  or  very  steep  dips. 
On  Big  Fork  River,  a  few  miles  above  the  mouth  of  Rice  Jliver, 
diorite  was  found,  and  also  at  Koochiching  Falls  in  Rainy  River. 
Greenstones  constitute  the  chief  exposures  between  Rice  River  and 
Big  Falls.  .They  comprise  beds  which  are  purely  eruptive,  other  beds 
which  are  consolidated  tuffs,  and  other  phases  which  it  is  not  possible 
to  place  certainly  in  either  class.  The  mica  schists  constitute  an  im- 
mense series,  extending  on  Big  Fork  River  from  a  point  12  miles 
below  Little  Falls  to  within  15  miles  of  Rainy  River.  At  various 
places  the  mica  schist  is  cut  by  granite.  The  mica  schist  gradually 
becomes  veined  with  a  granite,  which  increases  in  abundance  until 
granite  becomes  the  predominating  rock.  The  schists  are  also  cut 
by  dikes  of  greenstone. 
Elftman,187  in  1894,  publishes  his  field  notes  on  northeastern  Min- 
nesota. In  the  region  north  of  Snowbank  Lake  are  found  conglom- 
erate, mica  schist,  sericite  schist,  argillite,  diabase,  conglomeratic 
greenstone,  porphyry,  augite  granite,  and  hornblende  granite.  The 
former  of  these  granites  has  heretofore  been  called  gray  syenite,  and 
the  latter  red  syenite. 
On  the  west  shore  of  Boot  Lake,  in  the  SW.  J  NW.  i  sec.  21,  T. 
64  N.,  R.  8  W.,  are  several  large  dikes  of  porphyry  cutting  the  gray- 
wacke  and  schist.  In  the  SW.  J  NE.  J  of  the  same  section,  on  the 
east  side  of  a  long  point,  dikes  of  granite  are  found  cutting  the  con- 
glomerate in  all  directions  and  distorting  the  strata  in  a  very  compli- 
cated manner.  In  the  conglomerate  are  bowlders  up  to  4  feet  in 
diameter  of  gneiss,  slate,  diabase,  and  granite,  the  last  being  scarcely 
distinguishable  from  the  granite  which  cuts  the  conglomerate.  In 
some  instances  a  granite  dike  was  found  to  cut  some  of  the  large 
bowlders  of  the  conglomerate,  when  the  contact  between  the  dike  and 
the  granite  bowlders  could  not  easily  be  determined.  The  exact  rela- 
tions of  the  hornblende  granite  and  the  augite  granite  to  each  other, 
and  the  relations  of  the  latter  to  the  sedimentaries,  are  still  doubtful. 
The  gray  granite  has  not  been  found  in  contact  with  the  schists, 
argillites,  and  conglomerates,  and  it  is  cut  by  the  red  granite,  which 
also  cuts  the  schist.  The  hornblende  schists  and  mica  schists  of 
Snowbank  and  White  Iron  lakes  grade  into  argillaceous  slates  and 
conglomerates,  the  schistose  character  being  most  fully  developed  at 
the  contact  with  the  granite. 
The  Animikie  nctinolite-magnetite  schists  are  derived  from  rock 
containing  an  original  iron  carbonate.  As  the  formation  thins  out 
toward  the  east  and  passes  under  the  gabbro  it  becomes  more  crys- 
talline. Near  the  contact  of  the  gabbro  augite  and  olivine  occur  inti- 
mately associated  with  the  aetinolite  and  magnetite  of  the  Animikie 
schists.     The  black  slates  have  been  changed  into  biotite  schist  in  the 
