﻿120 
  M. 
  E. 
  Wilson 
  — 
  Banded 
  Gneisses 
  of 
  the 
  

  

  great 
  importance 
  or 
  not, 
  is 
  unknown. 
  It 
  is 
  also 
  probable 
  that 
  

   basic 
  and 
  acidic 
  aggregations 
  and 
  other 
  variations 
  were 
  present 
  

   in 
  the 
  Laurentian 
  as 
  in 
  other 
  plutonic 
  masses, 
  but 
  it 
  is 
  doubtful 
  

   whether 
  all 
  of 
  these 
  differences, 
  as 
  developed 
  in 
  normal 
  plutonic 
  

   rocks, 
  would 
  account 
  for 
  the 
  excessive 
  -heterogeneity 
  which 
  

   would 
  have 
  to 
  be 
  present 
  to 
  result 
  in 
  such 
  variability 
  in 
  com- 
  

   position 
  as 
  occurs 
  in 
  the 
  Laurentian 
  complex. 
  

  

  Throughout 
  the 
  northern 
  geosynclinal 
  belt 
  formed 
  by 
  the 
  

   rocks 
  of 
  the 
  Abitibi 
  group, 
  small 
  batholiths 
  of 
  granite 
  occur 
  

   which 
  are 
  presumably 
  offsets 
  from 
  the 
  main 
  Laurentian 
  mag- 
  

   matic 
  mass. 
  These, 
  because 
  of 
  their 
  small 
  size, 
  would 
  no 
  

   doubt 
  consolidate 
  much 
  more 
  quickly 
  than 
  the 
  larger 
  central 
  

   complex, 
  so 
  that, 
  in 
  them, 
  we 
  should 
  expect 
  to 
  find 
  the 
  record 
  

   of 
  the 
  early 
  stages 
  in 
  the 
  series 
  of 
  events 
  which 
  resulted 
  in 
  

   the 
  development 
  of 
  the 
  Laurentian 
  banded 
  gneisses. 
  On 
  mak- 
  

   ing 
  an 
  examination 
  of 
  these 
  northern 
  batholiths, 
  it 
  is 
  found 
  

   that 
  although 
  they 
  consist 
  largely 
  of 
  granite 
  instead 
  of 
  gneiss 
  

   they 
  also 
  are 
  exceedingly 
  heterogeneous 
  and 
  the 
  heterogeneous 
  

   portions 
  are 
  similar 
  in 
  composition 
  to 
  the 
  most 
  common 
  bands 
  

   of 
  the 
  banded 
  gneisses. 
  Thus, 
  in 
  some 
  places 
  in 
  the 
  small 
  

   batholiths, 
  a 
  granite 
  containing 
  very 
  little 
  biotite 
  may 
  be 
  seen 
  

   to 
  cut 
  across 
  another 
  granite 
  in 
  which 
  this 
  mineral 
  is 
  more 
  

   abundant, 
  or 
  a 
  biotite 
  granite 
  may 
  cut 
  a 
  hornblende 
  granite 
  in 
  

   a 
  similar 
  manner. 
  Long 
  schlieren 
  of 
  granite 
  very 
  rich 
  in 
  

   biotite 
  are 
  also 
  common. 
  These 
  variations 
  are 
  generally 
  poorly 
  

   defined 
  and 
  are 
  gradational 
  from 
  basic 
  to 
  acidic 
  in 
  composition 
  

   from 
  which 
  it 
  is 
  inferred 
  that 
  they 
  are 
  differentiated 
  parts 
  of 
  

   the 
  same 
  magmatic 
  mass. 
  This 
  differentiation 
  was 
  evidently 
  

   assisted 
  by 
  deformation 
  which, 
  in 
  some 
  cases, 
  caused 
  movements 
  

   in 
  the 
  viscous 
  magma, 
  thus 
  dragging 
  it 
  out 
  into 
  long 
  schlieren 
  

   and, 
  in 
  other 
  cases, 
  broke 
  up 
  the 
  magma 
  after 
  it 
  had 
  become 
  solid 
  

   so 
  that 
  the 
  central 
  magma 
  of 
  more 
  acid 
  composition 
  flowed 
  in 
  

   to 
  fill 
  up 
  the 
  fractures. 
  By 
  this 
  process 
  of 
  deformation 
  during 
  

   consolidation, 
  a 
  magmatic 
  mass 
  originally 
  homogeneous 
  might 
  

   continue 
  to 
  become 
  more 
  and 
  more 
  heterogeneous 
  as 
  the 
  knead- 
  

   ing 
  process 
  continued, 
  material 
  of 
  progressively 
  more 
  salic 
  

   composition 
  being 
  squeezed 
  out 
  through 
  fractures 
  from 
  the 
  in- 
  

   terior.* 
  Not 
  only 
  would 
  heterogeneity 
  be 
  developed 
  by 
  this 
  

   process, 
  but 
  the 
  variations 
  in 
  the 
  magma 
  would, 
  as 
  consolidation 
  

   continued, 
  be 
  flattened 
  out 
  into 
  thin 
  lenses 
  which, 
  because 
  of 
  

   their 
  different 
  competency, 
  would 
  behave 
  like 
  sedimentary 
  

   beds 
  and 
  assume 
  a 
  folded 
  structure. 
  

  

  In 
  discussing 
  the 
  geological 
  relationships 
  of 
  the 
  banded 
  

   gneisses 
  it 
  was 
  pointed 
  out 
  that 
  they 
  apparently 
  formed 
  the 
  

   truncated 
  base 
  of 
  a 
  Pre-Cambrian 
  mountain 
  chain, 
  and 
  since 
  

   mountain 
  building 
  is 
  generally 
  accompanied 
  by 
  deformation 
  it 
  

  

  *Harker, 
  A., 
  The 
  Natural 
  History 
  of 
  Igneous 
  Eocks, 
  1909. 
  

  

  