64  PRE-CAMBRIAN   ROCKS    OF   NORTH   AMERICA.  [bull.  86. 
not  ordinary  elastics,  bnt  are  of  volcanic  origin.  Both  paste  and  in- 
cluded fragments  have  evidently  had  a  common  origin  and  been  laid 
down  together,  perhaps  not  even  always  under  water.  The  fragments 
are  usually  more  or  less  elongated  or  lens- shaped,  due  to  pressure. 
The  greatest  planes  in  the  fragments  are  parallel  with  the  planes  of 
schistosity,  which  are  usually  observably  identical  with  those  of  bed- 
ding; at  times  the  agglomerates  merge  into  mica-schists  on  the  one 
hand  and  into  hornblende-schists  on  the  other.  The  mica-schists, 
micaceous  slates,  clay-slates,  and  quartzites  constitute  a  natural  group 
of  rocks  intimately  associated,  both  as  regards  their  origin  and  their 
present  relations  in  the  field.  They  are  all  probably  ordinary  meta- 
morphic  elastics.  The  felsitic,  sericitic  and  hydromicaceous  schists 
are  probably  sediments,  the  material  of  which  was  probably  volcanic. 
The  mica- schists  quite  frequently  pass  into  finely  laminated  mica- 
gneisses.  The  limestones  are  found  only  in  small  masses  and  seem  to 
be  vein  stones  rather  than  bedded  strata. 
The  Keewatin  schists  an  d  Lauren tian  granitoid  gneisses  are  found 
oftentimes  to  be  apparently  interbedded.  At  other  times  the  junction 
is  of  the  most  irregular  sort,  tongues  of  schists? unninginto  the  granite, 
or  masses  of  it  being  contained  in  the  granite  and  gneiss.  The  gneiss 
acts  in  many  places  as  though  it  had  been  in  a  fluid  state  intruding 
the  schists,  and  the  conclusion  is  reached  that  the  junction,  instead  of 
being  that  of  interlaminated  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  independent 
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  Lauren- 
tian  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  (Lauren- 
tian)  and  the  various  rocks  of  the  Keewatin  series,  and  is  therefore  of 
later  age  than  either.  A  granite,  the  intrusive  character  of  which  is 
undoubted,  sometimes  merges  in  the  same  rock  mass  into  a  granitoid 
gneiss.  There  is  a  marked  association  of  felsites  or  microgranites  with 
the  main  granite  mass,  there  being  an  apparent  tendency  on  the  part 
of  the  former  to  an  arrangement  concentric  with  the  periphery  of  the 
granite.  Upon  various  sections  a  certain  periodic  arrangement  of  the 
Keewatin  is  made  out,  upon  which  as  a  basis  it  is  found  that  the  maxi- 
mum thickness  of  the  series  is  in  the  neighborhood  of  20,000  feet.  As 
to  the  general  stratigraphical  relations  of  the  Keewatin,  the  conclusion 
