﻿Laurentian 
  Highlands 
  of 
  Canada. 
  Ill 
  

  

  which 
  occur 
  so 
  extensively 
  throughout 
  the 
  northern 
  part 
  of 
  

   the 
  St. 
  Lawrence 
  basin. 
  As 
  a 
  result 
  of 
  the 
  investigations 
  

   carried 
  on 
  by 
  Logan 
  and 
  his 
  associates 
  during 
  the 
  early 
  years 
  

   of 
  the 
  Canadian 
  Geological 
  Survey, 
  it 
  was 
  concluded 
  that 
  the 
  

   Pre-Cambrian 
  rocks 
  of 
  eastern 
  Canada 
  fell 
  naturally 
  into 
  two' 
  

   main 
  stratigraphical 
  divisions 
  ; 
  an 
  older 
  complex, 
  the 
  Lauren- 
  

   tian, 
  and 
  a 
  younger 
  series, 
  the 
  Huronian. 
  Logan 
  further 
  

   attempted 
  to 
  subdivide 
  the 
  basement 
  complex 
  into 
  an 
  Upper 
  

   Laurentian, 
  consisting 
  of 
  anorthosite 
  and 
  anorthosite 
  gneiss, 
  and 
  

   a 
  Lower 
  Laurentian 
  composed 
  of 
  two 
  groups, 
  the 
  younger 
  of 
  

   which 
  consisted 
  largely 
  of 
  limestone, 
  and 
  the 
  lower 
  of 
  gneiss. 
  

   Logan's 
  conception 
  of 
  the 
  Pre-Cambrian 
  succession 
  may 
  thus 
  

   be 
  tabulated 
  as 
  follows 
  : 
  

  

  f 
  Huronian 
  

  

  Azoic 
  

  

  f 
  Upper 
  Laurentian, 
  Labrador, 
  anorthosite 
  

   (Pre-Cambrian) 
  \ 
  Laurentian 
  or 
  Norian 
  series. 
  

  

  \ 
  ( 
  Grenville 
  series. 
  

  

  Lower 
  Laurentian 
  j 
  

  

  [_ 
  (_ 
  Ottawa 
  gneiss. 
  

  

  Although 
  Logan 
  included 
  the 
  rocks 
  of 
  the 
  Abitibi 
  group 
  in 
  

   his 
  Huronian 
  instead 
  of 
  placing 
  them 
  in 
  the 
  Laurentian 
  where 
  

   they 
  actually 
  belonged, 
  and 
  thus 
  went 
  astray 
  in 
  applying 
  his 
  

   Pre-Cambrian 
  classification, 
  yet 
  the 
  work 
  of 
  later 
  years 
  has 
  

   shown 
  that, 
  theoretically, 
  his 
  classification 
  of 
  the 
  Pre-Cambrian 
  

   into 
  Laurentian 
  and 
  Huronian 
  was 
  wholly 
  in 
  accord 
  with 
  the 
  

   facts. 
  Unfortunately, 
  the 
  importance 
  of 
  the 
  great 
  strati- 
  

   graphic 
  break 
  which 
  separates 
  the 
  Huronian 
  from 
  the 
  basal 
  

   complex 
  and 
  the 
  consequent 
  necessity 
  for 
  a 
  group 
  name 
  to 
  

   include 
  all 
  the 
  rocks 
  of 
  the 
  older 
  complex 
  was 
  not 
  generally 
  

   recognized, 
  and 
  in 
  subsequent 
  years 
  it 
  became 
  customary 
  to 
  

   limit 
  the 
  term 
  Laurentian 
  to 
  merely 
  the 
  granite 
  and 
  gneiss.* 
  

   It 
  is 
  with 
  approximately 
  the 
  latter 
  significance 
  that 
  the 
  name 
  

   is 
  here 
  used 
  by 
  the 
  writer 
  to 
  include 
  all 
  the 
  acid 
  plutonic 
  rocks 
  

   of 
  the 
  basement 
  complex 
  regardless 
  of 
  possible 
  difference 
  in 
  

   age. 
  Defined 
  in 
  terms 
  of 
  geological 
  relationships, 
  the 
  Lau- 
  

   rentian 
  includes 
  the 
  granite 
  and 
  gneiss 
  which 
  intrude 
  the 
  

   Grenville 
  series 
  and 
  the 
  volcanic 
  complex 
  (Abitibi 
  group) 
  

   of 
  the 
  Lake 
  Mistassini-Lake 
  Timiskaming-Lake 
  Huron 
  re- 
  

   gion. 
  It 
  would 
  also 
  include 
  any 
  granite 
  or 
  gneiss 
  which 
  

   might 
  lie 
  unconformable 
  beneath 
  either 
  the 
  Grenville 
  series 
  

  

  * 
  Report 
  of 
  the 
  Special 
  International 
  Committee 
  on 
  the 
  Correlation 
  of 
  the 
  

   Pre-Cambrian 
  Rocks 
  of 
  the 
  Adirondack 
  Mountains 
  and 
  the 
  "Original 
  

   Laurentian 
  Area" 
  of 
  Eastern 
  Ontario, 
  Jour, 
  of 
  Geology, 
  vol. 
  xv, 
  pp. 
  191— 
  

   217, 
  1907. 
  

  

  