500        ,         PKE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
canic  flows,  but  contains  thick  beds  of  inter  stratified  sandstones  and 
conglomerates',  especially  in  its  upper  part.  The  upper  division,  15,000 
feet  tliick,  is  wholly  of  detrital  material  which  is  largely  derived  from 
the  voleanics  of  the  same  series. 
The  unconformity  which  separates  the  Lower  Huronian  from  the 
Upper  Huronian  and  that  which  separates  the  latter  from  the  Kewee- 
nawan  each  represents  an  interval  of  time  sufficiently  long  to  raise  the 
land  above  the  sea,  to  fold  the  rocks,  to  carry  away  thousands  of  feet 
of  sediments,  and  to  depress  the  land  again  below  the  sea.  That  is, 
each  represents  an  amount  of  time  which  perhaps  is  as  long  as  any  of 
the  periods  of  deposition  themselves.  In  parts  of  the  region  the  lowest 
clastic  series  rest  unconformably  upon  the  fundamental  complex,  but 
in  certain  areas  the  relations  have  not  been  ascertained.  The  tipper  of 
the  three  clastic  series,  the  Keweenawan,  rests  unconformably  below 
the  Cambrian. 
In  the  lake  Superior  region  it  has  been  possible  with  a  considerable 
degree  of  certainty  to  refer  the  detached  areas  of  pre-Cambrian  clastic 
rocks  to  one  or  another  of  the  three  series  mentioned,  although  there 
have  been  aharp  differences  of  opinion  with  reference  to  certain  of  the 
areas.  It  has  been  possible  further  to  subdivide  the  series  into  forma- 
tions, some  of  which  have  a  widespread  extent  within  the  region.  The 
best  results  in  correlating  the  subdivisions  within  the  series  have  been 
reached  in  the  Penokee  and  Animikie  districts. 
Correlations  of  series  in  this  region  have  been  based  upon  uncon- 
formity, upon  lithological  similarity,  upon  the  belief  that  the  greater 
dynamic  movements  which  have  affected  the  region  have  been  wide- 
spread, and  upon  degree  of  crystallization  of  the  rocks.  The  correla- 
tion of  the  formations  within  a  given  series  has  been  based  upon  litho- 
logical characters  and  upon  a  similar  succession  of  like  beds. 
THE   BEGION   ABOUT  HUDSON   BAY. 
Within  the  main  Canadian  area  of  pre-C5ambrian  in  the  region  about 
Hudson  bay  exist  several  troughs  in  which  there  certainly  occur  frag- 
mental  rocks  such  as  slate-conglomerate,  limestone,  and  dolomite. 
These  are  associated  with  " imperfect"  gneisses,  a  great  variety  of 
schists,  and  schistose  and  jaspery  iron  ores.  The  Marble  island  series, 
with  that  of  the  adjacent  shore,  is  closely  analogous  in  lithological 
character  to  the  Upper  Huronian,  while  the  more  crystalline  phases 
resemble  the  Lower  Huronian.  The  chief  indication  available  as  to 
the  relations  of  these  rocks  to  the  basal  complex  is  the  presence  in  the 
slate  conglomerates  of  syenite  pebbles,  like  the  rocks  of  the  under- 
lying crystallines.  Resting  unconformably  upon  the  foregoing  elas- 
tics and  upon  the  basal  complex  is  the  Manitounuck  series,  which 
consists  of  siliceous  and  argillaceous  limestones,  sandstones,  quartz- 
ites,  shales,  ironstones,  writh  interbedded  amygdaloids  and  basalts, 
all  the  members  of  the  series  being  in  a  practically  unmetamorphosew 
