THE   CORDILLERAS.  883 
sion  are  the  met  amorphic  rocks,  which  may,  but  probably  do  not, 
include  any  pre-Cambrian  horizons.  The  metamorphic  rocks  of 
undetermined  age,  believed  to  be  in  part  Paleozoic,  though  the  definite 
proof  is  still  wanting,  are  thrown  in  a  third  group,  and  it  is  in  this 
that  pre-Cambrian  sediments  should  be  sought,  though  their  presence 
has  not  been  established. 
ARCHEAN. 
Dawson  has  mapped  a  number  of  elliptical  areas  of  gneissoid0 
rocks  which  have  a  general  linear  arrangement,  stretching  from  south- 
ern British  Columbia  to  Alaska,  and  included  in  these  his  Shuswap 
"  series,"  which  is  made  up  not  only  of  gneisses  and  crystalline  schists, 
but  also  of  crystalline  limestones  and  quartzites.  He  points  out  their 
lithological  and  structural  similarity  to  the  Grenviile h  series  of  east- 
ern Canada,  and  provisionally  assigns  them  to  the  Archean.6"  It 
appears  to  the  writer  that  such  criteria  of  metamorphic  rocks  can 
have  little  weight,  though  the  conclusion  that  these  gneisses  belong 
to  the  basal  complex  of  western  Canada  appears  to  be  borne  out  by 
the  general  structural  relations.  Whether  or  not  this  complex  will 
eventually  be  found  to  include  sedimentary  beds  is  a  question  which 
must  be  left  in  abeyance  for  the  present.  The  fact  that  the  older 
sedimentary  terranes  of  this  province  sometimes  include  crystalline 
schists  and  sheared  eruptives  makes  a  lithologic  determination  of 
Archean  age  of  doubtful  value. 
The  northernmost  of  these  gneissoid  areas  stretches  into  Alaska 
between  the  Tanana  and  the  Yukon  and  is  well  exposed  along  lower 
White  River.d  Here  is  a  complex  of  gneisses  and  mica  schists,  with 
massive  and  schistose  and  basic  and  acidic  intrusives,  characterized 
by  being  highly  crystalline.  In  general,  it  may  be  said  to  resemble 
the  rocks  which  in  other  regions  are  usually  assigned  to  the  Archean. 
McConnell  c  has  recently  suggested  terming  the  gneissic  rocks  of  the 
Yukon  basin  the  Pelly  gneisses.  It  is  significant  that,  as  studies  and 
mapping  progress,  the  areas  of  the  basal  gneisses  have  been  much 
reduced,  for  the  rocks  first  assigned  to  the  Archean  have  proved  to 
be  in  part  metamorphosed  Paleozoic  sediments  and  in  part  sheared 
intrusive  eruptions  in  the  Paleozoic  terranes.  McConnell  t  has  shown 
that  some  of  the  granites  which  have  been  classed  as  basal  are  intru- 
°  Dawson,  G.  M.,  Geological  record  of  the  Rocky  Mountain  region  in  Canada:  Bull. 
Geol.  Soc.  America,  vol.  12,  L901,  pp.  62-64.  Geological  map  of  Dominion  of  Canada 
(Western  sheel   No.   783),  Geol.   Survey   Canada. 
^Op.  cit,  p.  (•»•".. 
c  In  the  sense  of   pre-( 'ambriaii.    following   the   Canadian    usage. 
d  Brooks,  Alfred  EL,  Reconnaissance  of  the  Tanana  and  White  river  basins:  Twentieth 
Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  7,  1900,  pp.    160    166. 
r  Personal  letter  to  the  writer. 
fMcConnell,  R.  G.,  The  so-called  basal  granite  of  the  Yukon:  Am.  Geologist,  vol.  30, 
1902,  pp.  55-62. 
