﻿118 
  M. 
  E. 
  Wilson 
  — 
  Banded 
  Gneisses 
  of 
  the 
  

  

  Banding, 
  folding 
  and 
  foliation. 
  — 
  If 
  the 
  banded 
  gneisses 
  of 
  

   the 
  Lanrentian 
  complex 
  are 
  igneous 
  in 
  origin, 
  then 
  it 
  becomes 
  

   necessary 
  to 
  frame 
  an 
  hypothesis 
  which 
  will 
  account 
  for 
  the 
  

   development 
  of 
  a 
  banded, 
  folded 
  and 
  foliated 
  structure 
  in 
  rocks 
  

   which 
  originally 
  constituted 
  a 
  batholithic 
  magmatic 
  mass. 
  The 
  

   various 
  ways 
  by 
  which 
  a 
  banded 
  structure 
  might 
  develop 
  in 
  an 
  

   igneous 
  rock 
  are 
  the 
  following 
  : 
  — 
  (1) 
  by 
  the 
  flattening 
  out 
  of 
  

   fragments 
  of 
  the 
  invaded 
  rock 
  forming 
  the 
  batholithic 
  roof 
  ; 
  

  

  (2) 
  by 
  lit 
  jpar 
  lit 
  injection, 
  — 
  that 
  is, 
  by 
  the 
  intrusion 
  of 
  dikes 
  

   parallel 
  the 
  foliation 
  of 
  a 
  gneiss 
  ; 
  (3) 
  by 
  the 
  deformation 
  of 
  

   (a) 
  a 
  heterogeneous 
  complex 
  of 
  igneous 
  rocks 
  in 
  the 
  zone 
  of 
  

   flowage 
  long 
  after 
  consolidation 
  ; 
  (b) 
  a 
  heterogeneous 
  magma 
  

   during 
  or 
  immediately 
  after 
  consolidation. 
  

  

  (1) 
  The 
  development 
  of 
  a 
  banded 
  structure 
  along 
  the 
  con- 
  

   tact 
  of 
  the 
  Lanrentian 
  batholiths 
  by 
  the 
  flattening 
  out 
  of 
  xeno- 
  

   liths 
  has 
  been 
  advocated 
  by 
  a 
  number 
  of 
  Canadian 
  geologists,* 
  

   and 
  is 
  undoubtedly 
  an 
  important 
  mode 
  of 
  origin 
  for 
  the 
  struc- 
  

   ture 
  in 
  some 
  places, 
  but 
  this 
  method 
  alone 
  cannot 
  account 
  for 
  

   the 
  banded 
  structure 
  of 
  the 
  Laurentian 
  axial 
  complex 
  because 
  

   the 
  composition 
  of 
  the 
  bands 
  is, 
  for 
  the 
  most 
  part, 
  wholly 
  dif- 
  

   ferent 
  to 
  that 
  of 
  the 
  rocks 
  constituting 
  the 
  batholithic 
  roof. 
  

  

  (2) 
  A 
  banded 
  structure 
  may 
  originate 
  by 
  lit 
  par 
  lit 
  injection, 
  

   wherever 
  a 
  magma 
  intrudes 
  a 
  rock 
  which, 
  because 
  of 
  its 
  bedded 
  

   or 
  foliated 
  structure, 
  possesses 
  a 
  prominent 
  cleavage. 
  Thus, 
  

   where 
  the 
  Laurentian 
  rocks 
  intrude 
  some 
  of 
  the 
  sedimentary 
  

   schists 
  of 
  the 
  Abitibi 
  group, 
  the 
  granite, 
  aplite 
  and 
  pegmatite 
  

   commonly 
  occur, 
  as 
  sheets 
  or 
  dikes 
  paralleling 
  the 
  foliation. 
  

   There 
  are 
  also 
  some 
  sharply 
  defined 
  dikes 
  of 
  pegmatite 
  and 
  

   aplite 
  in 
  the 
  gneissic 
  complex 
  which 
  parallel 
  the 
  foliation 
  of 
  

   the 
  gneiss 
  and 
  were 
  probably 
  intruded 
  in 
  this 
  way, 
  but 
  it 
  is 
  

   scarcely 
  possible 
  that 
  the 
  banded 
  structure 
  of 
  the 
  Laurentian 
  

   gneisses 
  has 
  originated 
  to 
  any 
  great 
  extent 
  by 
  lit 
  par 
  -lit 
  injec- 
  

   tion 
  for 
  if 
  such 
  were 
  the 
  case, 
  the 
  bands 
  formed 
  by 
  intrusion 
  

   (1) 
  should 
  be 
  connected 
  in 
  places 
  by 
  dikes 
  transverse 
  to 
  the 
  

   foliation, 
  (2) 
  should 
  be 
  sharply 
  defined 
  on 
  their 
  contacts, 
  and 
  

  

  (3) 
  should 
  be 
  continuous 
  for 
  considerable 
  distances 
  when 
  fol- 
  

   lowed 
  along 
  the 
  strike. 
  Instead 
  of 
  these 
  features 
  being 
  pres- 
  

   ent, 
  the 
  bands 
  were 
  never 
  seen 
  to 
  connect 
  transversely, 
  the 
  con- 
  

   tacts 
  of 
  the 
  bands 
  are 
  generally 
  poorly 
  defined^ 
  the 
  mineral 
  

   grains 
  interlocking 
  across 
  the 
  line 
  of 
  junction, 
  and 
  the 
  bands 
  

   commonly 
  pinch 
  out 
  in 
  short 
  distances 
  when 
  followed 
  in 
  the 
  

   direction 
  of 
  their 
  trend. 
  

  

  (3) 
  The 
  third 
  hypothesis 
  to 
  explain 
  the 
  origin 
  of 
  the 
  banded 
  

   gneisses 
  has 
  been 
  divided 
  into 
  two 
  subdivisions 
  according 
  to 
  

  

  *Lawson. 
  A. 
  G., 
  Ann. 
  Rep. 
  G. 
  S. 
  C, 
  vol. 
  iii. 
  Part 
  I, 
  p. 
  138F, 
  1887. 
  

   Adams, 
  F. 
  D., 
  and 
  Barlow, 
  A. 
  E., 
  Memoir 
  No. 
  6, 
  Geol. 
  Snrv. 
  Dept. 
  of 
  Mines, 
  

   Can., 
  1910. 
  Miller, 
  W. 
  G., 
  and 
  Knight 
  C, 
  Ann. 
  Rep., 
  Bur. 
  of 
  Mines. 
  Ont., 
  

   vol. 
  xx, 
  pp. 
  280-284, 
  1911. 
  

  

  