﻿112 
  M. 
  E. 
  Wilson 
  — 
  Banded 
  Gneisses 
  of 
  the 
  

  

  or 
  the 
  Abitibi 
  group. 
  The 
  upper 
  limits 
  of 
  the 
  Laurentian 
  

   are 
  defined 
  by 
  the 
  erosion 
  surface 
  that 
  separates 
  the 
  basement 
  

   complex 
  from 
  the 
  Huronian, 
  or 
  defined 
  more 
  locally, 
  the 
  

   plane 
  which 
  separates 
  the 
  Cobalt 
  series 
  from 
  the 
  older 
  complex, 
  

   in 
  the 
  Timiskaming 
  region, 
  and 
  the 
  original 
  Huronian 
  rocks 
  

   from 
  the 
  basement 
  complex, 
  on 
  the 
  north 
  shore 
  of 
  Lake 
  Huron. 
  

   It 
  is 
  probable, 
  according 
  to 
  this 
  definition, 
  that 
  the 
  anorthosites 
  

   (Logan's 
  Upper 
  Laurentian) 
  should 
  be 
  classed 
  as 
  Laurentian 
  

   for 
  these 
  rocks 
  are 
  in 
  part 
  transformed 
  into 
  gneiss 
  and 
  prob- 
  

   ably 
  belong 
  essentially 
  to 
  the 
  basement 
  complex.* 
  

  

  Lithological 
  Character. 
  

  

  ■ 
  The 
  rocks 
  of 
  the 
  Laurentian 
  complex 
  may 
  be 
  classified 
  (1) 
  

   according 
  as 
  to 
  whether 
  they 
  are 
  massive 
  or 
  foliated, 
  and 
  

   (2) 
  according 
  to 
  their 
  mineralogical 
  composition. 
  To 
  the 
  mas- 
  

   sive 
  rock 
  types 
  belong 
  nearly 
  all 
  of 
  the 
  small 
  batholitic 
  masses 
  

   which 
  intrude 
  the 
  northern 
  volcanic 
  complex 
  (Abitibi 
  group) 
  

   and, 
  to 
  a 
  much 
  more 
  limited 
  extent, 
  some 
  of 
  the 
  batholiths 
  

   which 
  intrude 
  the 
  southern 
  limestonef 
  complex 
  (G-renville 
  

   series). 
  The 
  foliated 
  rocks, 
  on 
  the 
  other 
  hand, 
  include 
  nearly 
  

   the 
  whole 
  of 
  the 
  central 
  gneissic 
  belt 
  which 
  separates 
  the 
  Abi- 
  

   tibi 
  group 
  from 
  the 
  Grenville 
  series. 
  Classified 
  according 
  to 
  

   mineralogical 
  composition, 
  the 
  rocks 
  of 
  the 
  Laurentian 
  com- 
  

   plex 
  include 
  the 
  following 
  types 
  : 
  granite, 
  syenite, 
  granodiorite, 
  

   diorite, 
  pegmatite, 
  aplite, 
  pyroxenite, 
  amphibolite, 
  and 
  garnet- 
  

   iferous 
  mica 
  schist. 
  

  

  Granite 
  and 
  Granite-gneiss. 
  — 
  The 
  granite 
  and 
  granite-gneiss 
  

   of 
  the 
  Laurentian 
  complex 
  are 
  granular, 
  fine- 
  to 
  coarse-grained 
  

   rocks 
  consisting 
  essentially 
  of 
  quartz 
  and 
  alkalic 
  feldspar 
  

   (orthoclase, 
  microcline, 
  albite, 
  and 
  oligoclase), 
  with 
  biotite 
  or 
  

   hornblende 
  or 
  biotite 
  and 
  hornblende 
  together 
  as 
  ferromagne- 
  

   sian 
  constituents. 
  The 
  biotite 
  granite 
  and 
  granite 
  gneiss 
  is, 
  

   however, 
  much 
  more 
  common 
  than 
  the 
  hornblende 
  variety. 
  

   The 
  common 
  accessory 
  minerals 
  present 
  are 
  titanite, 
  epidote, 
  

   muscovite, 
  garnet, 
  apatite, 
  zircon, 
  and 
  magnetite, 
  but 
  allanite, 
  

   tourmaline, 
  rutile, 
  graphite, 
  cyanite, 
  arfvedsonite, 
  and 
  segirine 
  

   have 
  also 
  been 
  found 
  in 
  some 
  thin 
  sections 
  of 
  the 
  granite 
  

   gneiss.;}: 
  From 
  the 
  microscopic 
  examination 
  of 
  the 
  granite 
  and 
  

   granite 
  gneiss 
  it 
  is 
  seen 
  that, 
  in 
  some 
  places, 
  the 
  constituent 
  

   minerals 
  are 
  remarkably 
  fresh 
  while, 
  in 
  others, 
  the 
  feldspars 
  

   are 
  largely 
  replaced 
  by 
  sericite, 
  and 
  the 
  hornblende 
  and 
  biotite 
  

   by 
  chlorite. 
  Between 
  these 
  two 
  extremes 
  an 
  intermediate 
  rock 
  

  

  * 
  Adams, 
  F. 
  D.: 
  "Uber 
  das 
  Norian 
  oder 
  Ober-Laurentian 
  von 
  Canada" 
  ; 
  

   Neues 
  Jahrbuch 
  fur 
  Mineralogie, 
  etc., 
  pp. 
  419-498, 
  1892. 
  

  

  f 
  According 
  to 
  Adams 
  and 
  Barlow, 
  Memoir 
  No. 
  6, 
  G. 
  S. 
  Branch, 
  Dept. 
  of 
  

   Mines, 
  Can., 
  1910. 
  

  

  % 
  Barlow, 
  A. 
  E., 
  Ann. 
  Eep. 
  G. 
  S. 
  C, 
  p. 
  871, 
  1897. 
  Wilson, 
  M. 
  E., 
  Sum. 
  

   Eep., 
  G. 
  S. 
  Dept. 
  of 
  Mines, 
  Can., 
  1913. 
  

  

  