﻿46 
  ME. 
  T. 
  L. 
  WALKER 
  ON 
  THE 
  [Feb. 
  1897,- 
  

  

  (vi) 
  Grauwacke. 
  

  

  The 
  central 
  portion 
  of 
  the 
  trough 
  of 
  younger 
  Huronian 
  rocks 
  is 
  

   occupied 
  by 
  an 
  even 
  -grained, 
  ash-coloured, 
  quartzose 
  rock, 
  in 
  which 
  

   conglomeratic 
  fragments 
  may 
  be 
  detected 
  with 
  the 
  naked 
  eye. 
  

   This 
  is 
  the 
  youngest 
  member 
  of 
  the 
  Huronian 
  complex, 
  and, 
  as 
  it 
  is 
  

   never 
  cut 
  by 
  the 
  nickel-bearing 
  rocks, 
  I 
  am 
  inclined 
  to 
  regard 
  it 
  as- 
  

   later 
  than 
  these. 
  There 
  is 
  good 
  ground 
  for 
  looking 
  upon 
  the 
  nickel- 
  

   bearing 
  rocks 
  as 
  younger 
  than 
  the 
  volcanic 
  breccia, 
  and 
  hence 
  the 
  

   conclusion 
  that 
  the 
  nickel-bearing 
  rocks 
  are 
  of 
  Huronian 
  age, 
  and 
  that 
  

   they 
  occupy 
  a 
  position 
  lying 
  chronologically 
  between 
  the 
  volcanic 
  

   breccia 
  and 
  this 
  youngest 
  member. 
  It 
  may 
  be 
  mentioned, 
  however, 
  

   that 
  Bell 
  and 
  Selwyn 
  are 
  inclined 
  to 
  regard 
  the 
  grauwacke 
  as 
  Lower 
  

   Cambrian, 
  although 
  no 
  fossils 
  have 
  been 
  found 
  in 
  it. 
  

  

  Good 
  exposures 
  occur 
  at 
  several 
  points 
  along 
  the 
  railway,, 
  

   especially 
  at 
  Chelmsford 
  and 
  Larchwood 
  Stations. 
  

  

  IV. 
  The 
  Nickel-bearing 
  Eocks. 
  

   (i) 
  General 
  Observations. 
  

  

  These 
  rocks 
  have 
  attracted 
  much 
  more 
  attention 
  than 
  the 
  other 
  

   rocks 
  of 
  the 
  district. 
  The 
  discovery 
  of 
  areas 
  characterized 
  by 
  

   extensive 
  deposits 
  of 
  nickel 
  ores 
  gave 
  employment 
  to 
  large 
  numbers' 
  

   of 
  explorers 
  from 
  1887 
  to 
  1891. 
  It 
  was 
  soon 
  remarked 
  that 
  

   valuable 
  deposits 
  of 
  copper 
  and 
  nickel 
  ores 
  occurred 
  only 
  in 
  con- 
  

   nexion 
  with 
  greenish-coloured 
  rocks 
  of 
  medium 
  texture, 
  which 
  

   were 
  conveniently 
  referred 
  to 
  as 
  trap, 
  diorite, 
  or 
  greenstone. 
  

   Early 
  microscopic 
  examination 
  of 
  the 
  rocks 
  in 
  the 
  immediate 
  

   vicinity 
  of 
  the 
  nickel-deposits 
  showed 
  that 
  they 
  were 
  composed 
  in 
  

   general 
  of 
  hornblende 
  and 
  plagioclase, 
  and 
  in 
  smaller 
  proportions 
  

   of 
  quartz 
  and 
  biotite, 
  with 
  magnetite 
  and 
  apatite 
  as 
  accessories. 
  

   Thus 
  the 
  microscope 
  confirmed 
  in 
  a 
  measure 
  the 
  prospectors' 
  name 
  

   ' 
  diorite/ 
  but 
  it 
  also 
  showed 
  the 
  probability 
  of 
  the 
  secondary 
  nature 
  

   of 
  the 
  chief 
  constituents, 
  and 
  so 
  the 
  original 
  nature 
  of 
  the 
  : 
  

   country-rock 
  of 
  the 
  nickel-deposits 
  continued 
  to 
  be 
  a 
  matter 
  of 
  

   doubt. 
  As 
  the 
  hornblende 
  was 
  probably 
  derived 
  from 
  one 
  of 
  the 
  

   members 
  of 
  the 
  pyroxene 
  group, 
  the 
  original 
  rock 
  was 
  doubtless 
  one 
  

   of 
  the 
  gabbro 
  family. 
  

  

  In 
  the 
  published 
  descriptions 
  these 
  rocks 
  are 
  referred 
  to 
  as 
  diorite, 
  

   uralitic 
  diorite, 
  gabbro-diorite, 
  and 
  even 
  occasionally 
  as 
  diabase. 
  

   This 
  uncertainty 
  is 
  in 
  a 
  measure 
  accounted 
  for 
  by 
  the 
  fact 
  that 
  

   most 
  of 
  the 
  specimens 
  studied 
  microscopically 
  were 
  collected 
  quite 
  

   close 
  to 
  the 
  nickel-deposits, 
  where 
  the 
  rock 
  is 
  as 
  a 
  rule 
  completely 
  

   altered, 
  though 
  in 
  other 
  parts 
  of 
  the 
  ' 
  greenstone 
  ' 
  area 
  the 
  meta- 
  

   morphism 
  has 
  seldom 
  advanced 
  so 
  far, 
  and 
  in 
  some 
  places 
  the 
  rock 
  

   is 
  practically 
  unaltered. 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  nickel-bearing 
  rocks, 
  taken 
  as 
  a 
  whole 
  

   and 
  considered 
  geologically, 
  include 
  not 
  only 
  these 
  greenstone 
  areas 
  

   but 
  also 
  considerable 
  areas 
  of 
  gneissoid 
  and 
  microp 
  egmatiti 
  

  

  