﻿Laurentian 
  Highlands 
  of 
  Canada. 
  115 
  

  

  rock 
  outcrop 
  are 
  scarcely 
  capable 
  of 
  description, 
  yet 
  when 
  

   examined 
  over 
  broad 
  areas 
  this 
  complexity 
  and 
  heterogeneity 
  is 
  

   so 
  uniform 
  that 
  it 
  becomes 
  monotonous. 
  The 
  banding 
  of 
  the 
  

   gneisses 
  may 
  arise 
  either 
  from 
  (1) 
  a 
  variation 
  in 
  the 
  proportion 
  

   of 
  minerals 
  present 
  in 
  the 
  same 
  rock 
  or 
  (2) 
  by 
  the 
  alteration 
  of 
  

   bands 
  of 
  different 
  rock. 
  Thus, 
  one 
  of 
  the 
  most 
  common 
  types 
  

   of 
  banding 
  is 
  brought 
  about 
  by 
  the 
  alternation 
  of 
  bands 
  of 
  bio- 
  

   tite 
  gneiss, 
  containing 
  varying 
  proportions 
  of 
  biotite 
  so 
  that 
  a 
  

   light 
  band, 
  in 
  which 
  little 
  biotite 
  is 
  present, 
  alternates 
  with 
  a 
  

   dark 
  band 
  containing 
  a 
  large 
  proportion 
  of 
  biotite. 
  In 
  a 
  sim- 
  

   ilar 
  manner, 
  variations 
  in 
  the 
  proportion 
  of 
  hornblende 
  in 
  the 
  

   hornblende 
  granite 
  gneiss, 
  the 
  granodiorite-gneiss 
  or 
  the 
  diorite- 
  

   gneiss" 
  result 
  in 
  a 
  banded 
  structure. 
  The 
  second 
  type 
  of 
  

   banded 
  structure, 
  in 
  which 
  the 
  alternate 
  bands 
  are 
  composed 
  

   of 
  different 
  rocks, 
  may 
  also 
  be 
  combined 
  with 
  bands 
  of 
  the 
  

   first 
  types, 
  and 
  in 
  this 
  way 
  an 
  almost 
  infinite 
  variation 
  in 
  the 
  

   composition 
  of 
  the 
  bands 
  may 
  occur. 
  The 
  commonest 
  rock 
  of 
  

   the 
  banded 
  gneiss 
  is 
  the 
  biotite, 
  or 
  biotite 
  hornblende 
  granite 
  

   gneiss 
  ; 
  but 
  pegmatite 
  and 
  aplite 
  are 
  also 
  important, 
  composing 
  

   not 
  less 
  than 
  15 
  per 
  cent 
  of 
  the 
  whole. 
  The 
  proportion 
  of 
  

   other 
  rocks 
  is 
  small, 
  so 
  that 
  the 
  central 
  belt 
  of 
  Laurentian 
  

   gneisses, 
  considered 
  as 
  a 
  whole, 
  is 
  granitic 
  rather 
  than 
  dioritic 
  in 
  

   composition. 
  The 
  width 
  of 
  the 
  bands 
  may 
  vary 
  from 
  a 
  frac- 
  

   tion 
  of 
  an 
  inch 
  to 
  hundreds 
  of 
  feet. 
  When 
  followed 
  along 
  the 
  

   strike 
  they 
  are 
  found 
  to 
  pinch 
  out 
  as 
  though 
  they 
  were, 
  in 
  

   reality 
  thin 
  lenses. 
  This 
  lenticular 
  character 
  is 
  particularly 
  

   evident 
  in 
  the 
  case 
  of 
  the 
  pegmatite, 
  which 
  commonly 
  occurs 
  

   in 
  a 
  succession 
  of 
  lenses 
  around 
  which 
  the 
  foliation 
  in 
  the 
  sur- 
  

   rounding 
  gneiss 
  bends 
  in 
  a 
  manner 
  very 
  similar 
  to 
  that 
  which 
  

   occurs 
  on 
  a 
  small* 
  scale 
  around 
  the 
  augen 
  of 
  feldspar 
  in 
  the 
  

   augen 
  gneiss. 
  

  

  Q-ranulation. 
  — 
  That 
  granulation 
  has 
  occurred 
  to 
  a 
  large 
  

   extent 
  in 
  the 
  banded 
  gneisses 
  is 
  apparent 
  from 
  the 
  abundance 
  

   of 
  augen 
  gneisses 
  and 
  from 
  the 
  evidences 
  of 
  strain 
  and 
  frag- 
  

   mentation 
  seen 
  in 
  some 
  thin 
  sections. 
  Recrystallization 
  has 
  

   followed 
  granulation 
  in 
  many 
  cases, 
  however, 
  for 
  in 
  many 
  

   rocks 
  which 
  have 
  very 
  evidently 
  suffered 
  granulation, 
  the 
  

   granular 
  quartz 
  and 
  feldspar 
  which 
  surrounded 
  the 
  lense 
  of 
  

   the 
  augen 
  contain 
  a 
  large 
  proportion 
  of 
  microcline 
  and 
  are 
  

   much 
  fresher 
  in 
  appearance 
  than 
  the 
  central 
  core. 
  

  

  Folding 
  and 
  Faulting. 
  — 
  The 
  stud}^ 
  of 
  the 
  structure 
  of 
  the 
  

   banded 
  gneisses 
  indicates 
  that 
  they 
  have 
  been 
  folded 
  in 
  a 
  

   manner 
  very 
  similar 
  to 
  that 
  exhibited 
  by 
  deformed 
  sedimentary 
  

   rocks. 
  While 
  the 
  bands 
  are 
  not 
  continuous 
  over 
  wide 
  areas 
  

   like 
  sedimentary 
  beds, 
  yet, 
  all 
  the 
  various 
  types 
  of 
  folds 
  are 
  

   present 
  on 
  a 
  small 
  scale 
  and, 
  in 
  places, 
  anticlines 
  and 
  synclines 
  

   nearlv 
  a 
  half-mile 
  in 
  cross 
  section, 
  can 
  be 
  recognized. 
  These 
  

  

  