﻿Laurentian 
  Highlands 
  of 
  Canada. 
  117 
  

  

  tigation 
  to 
  the 
  problem, 
  the 
  sedimentary 
  hypothesis 
  was 
  grad- 
  

   ually 
  abandoned 
  — 
  'as 
  regards 
  the 
  major 
  part 
  of 
  the 
  Laurentian 
  

   complex 
  — 
  in 
  favor 
  of 
  the 
  igneous 
  hypothesis, 
  which 
  is 
  now 
  

   generally 
  accepted.* 
  

  

  In 
  describing 
  the 
  lithological 
  character 
  of 
  the 
  gneissic 
  com- 
  

   plex 
  it 
  was 
  noted 
  that 
  within 
  the 
  axial 
  belt 
  particularly 
  near 
  

   its 
  northern 
  border, 
  garnetiferous 
  mica 
  schists 
  occur 
  which 
  are 
  

   probably 
  of 
  sedimentary 
  origin. 
  Likewise, 
  along 
  the 
  southern 
  

   border 
  of 
  the 
  central 
  Laurentian 
  gneissic 
  complex, 
  fine-grained 
  

   rusty 
  gneisses 
  and 
  amphibolites 
  occur 
  which 
  are 
  believed 
  from 
  

   their 
  lithological 
  character 
  and 
  chemical 
  composition 
  to 
  be 
  

   altered 
  sediments, 
  the 
  former 
  being 
  a 
  mashed 
  quartzite 
  or 
  

   arkose 
  and 
  the 
  latter 
  a 
  metamorphosed 
  limestone.f 
  Thus 
  it 
  is 
  

   probable 
  that 
  the 
  proportion 
  of 
  sediments 
  associated 
  with 
  the 
  

   Laurentian 
  is 
  somewhat 
  larger 
  than 
  is 
  generally 
  supposed, 
  yet 
  

   the 
  characteristics 
  of 
  the 
  major 
  part 
  of 
  the 
  complex 
  is 
  such 
  as 
  

   to 
  point 
  conclusively 
  to 
  an 
  igneous 
  origin. 
  The 
  evidence 
  upon 
  

   which 
  this 
  conclusion 
  is 
  based 
  may 
  be 
  summarized 
  briefly 
  as 
  

   follows 
  : 
  — 
  

  

  (1) 
  The 
  complex 
  is 
  largely 
  composed 
  of 
  granite, 
  diorite, 
  

   granodiorite 
  and 
  pegmatite, 
  and 
  hence 
  is 
  composed 
  of 
  rocks 
  

   having 
  the 
  mineral 
  ogical 
  and 
  chemical 
  composition 
  J 
  and 
  tex- 
  

   ture 
  which 
  belong 
  to 
  rocks 
  of 
  igneous 
  origin. 
  

  

  (2) 
  Pegmatite 
  constitutes 
  a 
  large 
  and 
  essential 
  portion 
  of 
  the 
  

   Laurentian 
  complex 
  and 
  occurs 
  not 
  only 
  in 
  parallel 
  bands 
  but 
  

   as 
  dikes 
  transverse 
  to 
  the 
  banding. 
  

  

  (3) 
  The 
  bands 
  in 
  the 
  gneiss 
  pinch 
  out 
  when 
  followed 
  along 
  

   the 
  strike, 
  whereas 
  sedimentary 
  beds 
  composing 
  uniformly 
  

   stratified 
  series 
  are 
  generally 
  continuous 
  for 
  long 
  distances. 
  

  

  (4) 
  The 
  extreme 
  local 
  heterogeneity 
  of 
  the 
  Laurentian 
  com- 
  

   plex 
  and 
  the 
  uniformity 
  of 
  this 
  heterogeneity 
  over 
  many 
  thous- 
  

   and 
  square 
  miles 
  is 
  not 
  characteristic 
  of 
  sedimentary 
  rocks. 
  

  

  (5) 
  The 
  dominant 
  sediments 
  which 
  result 
  from 
  the 
  decom- 
  

   position 
  of 
  igneous 
  rocks 
  are 
  argillaceous, 
  and 
  since 
  the 
  Lau- 
  

   rentian 
  banded 
  gneisses 
  have 
  an 
  areal 
  extent 
  in 
  Canada 
  of 
  not 
  

   less 
  than 
  2 
  million 
  square 
  miles, 
  it 
  might 
  be 
  expected 
  that 
  a 
  

   considerable 
  proportion 
  of 
  the 
  complex 
  would 
  consist 
  of 
  slates, 
  

   but 
  on 
  the 
  contrary 
  it 
  is 
  almost 
  entirely 
  composed 
  of 
  rocks 
  

   approaching 
  the 
  composition 
  of 
  arkose 
  or 
  quartzite. 
  

  

  It 
  may 
  therefore 
  be 
  assumed 
  as 
  unquestioned 
  that 
  the 
  banded 
  

   gneisses 
  of 
  the 
  Laurentian 
  plateau, 
  in 
  their 
  most 
  typical 
  develop- 
  

   ment, 
  are 
  of 
  igneous 
  origin. 
  

  

  * 
  Adams, 
  F; 
  D., 
  Jour, 
  of 
  GeoL, 
  vol. 
  i, 
  pp. 
  325-340, 
  1893. 
  Barlow, 
  A. 
  E., 
  

   Ann. 
  Rep., 
  C. 
  G. 
  S., 
  p. 
  51, 
  i, 
  1897. 
  

  

  f 
  Adams, 
  F. 
  D., 
  this 
  Journal, 
  vol. 
  1, 
  pp. 
  58-69, 
  1894. 
  Adams, 
  F. 
  D., 
  

   Ann. 
  Rep., 
  G. 
  S. 
  C, 
  vol. 
  viii, 
  1895. 
  Adams 
  and 
  Barlow, 
  Memoir 
  No. 
  6, 
  

   Geol. 
  Surv., 
  Dept. 
  of 
  Mines, 
  Can., 
  1910. 
  

  

  X 
  Barlow, 
  A. 
  E., 
  Ann. 
  Rep. 
  G. 
  S. 
  C, 
  p. 
  55, 
  I, 
  1897. 
  

  

  