LAKE    SUPERIOR   REGION.  267 
The  Keewatin  schists  and  Laurentian  granitoid  gneisses  are  in 
many  places  apparently  interbedded ;  in  other  places  the  junction  is 
of  the  most  irregular  sort,  tongues  of  schists  running  into  the  granite, 
or  masses  of  it  being  contained  in  the  granite  and  gneiss.  The  gneiss 
acts  in  many  places  as  though  it  had  in  a  fluid  state  intruded  the 
schists,  and  the  conclusion  is  reached  that  the  junction,  instead  of 
being  that  of  interlaminatecl  sediments,  is  that  of  a  set  of  schistose 
rocks  which  have  been  intruded  by  fluid  ones,  the  fluid  material  often 
placing  itself  along  the  parting  of  the  schist,  at  other  times  cutting 
across  it  or  including  fragments  of  it.  If  this  conclusion  is  true,  the 
supposed  conformable  junction  of  the  two  series  at  certain  localities 
is  no  proof  of  true  conformity,  because  the  foliation  of  the  granitoid 
gneisses,  if  these  rocks  were  once  viscid  or  plastic,  is  quite  independ- 
ent of  any  arrangement  due  to  sedimentation  that  they  may  have 
possessed.  This  conclusion  does  not  imply  that  the  gneisses  and 
schists  may  not  have  been  originally  sedimentary  and  conformable. 
However,  the  author  inclines  to  the  belief  that  the  granitoid  gneisses 
of  the  Laurentian  were  never  aqueous  sediments. 
The  granitic  intrusions  of  the  Lake  of  the  Woods  are  grouped  into 
ten  main  centers.  The  granite  cuts  both  the  granitoid  gneiss  (Lau- 
rentian) and  the  various  rocks  of  the  Keewatin  series,  and  is  there- 
fore of  later  age  than  either.  A  granite,  the  intrusive  character  of 
which  is  undoubted,  sometimes  in  the  same  rock  mass  merges  into  a 
granitoid  gneiss.  There  is  a  marked  association  of  felsites  or  micro- 
granites  with  the  main  granite  mass,  there  being  an  apparent  tend- 
ency on  the  part  of  the  former  to  an  arrangement  concentric  with 
the  periphery  of  the  granite.  In  various  sections  a  certain  periodic 
arrangement  of  the  Keewatin  is  made  out,  upon  which  as  a  basis  it  is 
found  that  the  maximum  thickness  of  the  series  is  in  the  neighbor- 
hood of  20,000  feet.  As  to  the  general  stratigraphical  relations  of 
the  Keewatin,  the  conclusion  is  reached  that  the  beds  have  been  laid 
down  and  folded  within  a  trough  in  the  Laurentian  formation. 
Herrick,  Tight,  and  Jones,248  in  1886,  find  on  the  north  shore  of 
Lake  Superior  three  distinct  groups  of  rocks  with  their  respective  in- 
trusives,  granitic,  schistose,  and  conglomeratic.  The  granites  are 
found  underlying  the  schists  in  such  a  way  as  to  suggest  that  they 
have  been  intruded  beneath  them,  although  similar  granites  consti- 
tute the  pebbles  of  the  basement  conglomerates  in  the  schists.  The 
schists  are  metamorphosed  at  contacts  with  the  granites;  the  schists 
and  schist  conglomerates  especially,  in  several  places,  have  been 
altered  to  porphyry  and  felsite  porphyry  by  contact  with  the  erup- 
tives.  The  third  group  consists  of  basement  conglomerates,  contain- 
ing fragments  of  all  the  verieties  of  rock  included  in  the  other  two 
series.     Periodic  overflows  of  igneous  matter  have  left  vast  sheets  of 
