LAKE    SUPERIOR   REGION.  269 
The  thickness  of  the  Keewatin  rocks  is  calculated  to  be  about  5 
miles.  The  rocks  are  found  in  the  main  to  be  of  clastic  origin,  but 
probably  in  the  nature  of  volcanic  debris  rather  than  water-deposited 
sediments,  although  a  small  quantity  of  the  materials  is  of  the  latter 
character.  The  volcanic  debris  of  the  Keewatin  has  a  basic  and  an 
acidic  division,  the  latter  being  higher  in  the  series  than  the  former. 
Microscopic  study  leads  to  the  conclusion  that  the  series  has  been 
subjected  to  a  great  pressure,  as  is  evidenced  by  the  fracture  of  the 
grains  as  well  as  by  the  schistose  structure.  The  granite  bowlders 
found  in  the  conglomerates  may  either  be  ordinary  detritus  or  have 
been  brought  up  from  beneath  by  volcanic  forces,  although  the  for- 
mer is  the  more  probable.  All  of  the  granites  of  the  region  appear  to 
be  in  some  degree  later  than  the  Keewatin  rocks,  but  this  does  not 
imply  that  there  was  not  a  granite  shore  for  the  basin  in  which  the 
Keewatin  rocks  were  deposited.  The  author  has  no  doubt  that  the 
original  floor  upon  which  the  Keewatin  and  Coutchiching  rocks  were 
deposited  was  fused  at  the  time  of  disturbance  and  appears  now  to  us 
as  the  foliated  granite  of  the  Laurentian.  The  granite  bowlders  of 
the  Keewatin  agglomerates  may  have  been  derived  from  a  granite 
basement  obliterated  by  subsequent  plutonic  fusion. 
The  Coutchiching  series  consists  of  mica  schists,  or  mica  schists  in 
which  feldspar  is  present.  Hornblende  has  been  observed  only  in 
one  instance.  No  limestones  or  conglomerates  have  been  discovered. 
The  schistose  structure  is  believed  to  represent  original  sedimenta- 
tion. Upon  this  basis  the  formation  has  an  average  thickness  of  4  or 
5  miles.  The  relations  of  the  Coutchiching  series  to  the  Laurentian 
are  found  to  be  precisely  the  same  as  between  the  Keewatin  and  the 
Laurentian.  In  the  Coutchiching  series  there  are  no  intercalations 
of  recognizable  volcanic  rocks  such  as  are  found  in  the  Keewatin, 
the  rocks  being  acidic  crystalline  schists  which  are  regarded  as  ordi- 
nary metamorphosed  quartzose  sediments,  with  perhaps  also  volcanic 
material,  although  in  no  place  has  it  been  possible  conclusively  to 
prove  this. 
The  Laurentian  gneiss  is  intermixed  with  granite  in  such  a  way  as 
to  make  it  impossible  at  times  to  separate  them.  Its  structure,  if  it  has 
any,  is  so  complex  that  no  attempt  was  made  to  work  it  out,  and, 
while  parts  of  the  granite  seem  to  belong  to  the  Laurentian  proper, 
it  is  certain  that  granitic  eruptions  have  occurred  in  this  series  after 
the  main  mass  of  the  rock  had  solidified.  The  belts  of  Keewatin 
rocks  which  encircle  the  Laurentian  areas  are  anastomosing  or  con- 
fluent, forming  a  continuous  retiform  area,  the  meshes  being  occupied 
by  the  Laurentian  gneiss.  At  their  nodes  or  points  of  confluence 
these  belts  have  their  greatest  width.  The  Coutchiching  schists  dip 
away  from  the  Laurentian  bosses  in  all  directions,  so  that  the  gen- 
eral anticlinal  structure  of  the  belt  is  made  up  of  three  anticlinal 
