﻿Laurentian 
  Highlands 
  of 
  Canada. 
  11 
  8 
  

  

  type 
  is 
  also 
  common 
  in 
  which 
  sericitized 
  feldspar 
  occurs 
  en- 
  

   closed 
  in 
  a 
  matrix 
  of 
  fresh, 
  granular 
  quartz 
  and 
  microcline. 
  

   In 
  some 
  thin 
  sections 
  the 
  minerals 
  show 
  by 
  their 
  undulatory 
  

   extinction 
  and 
  granulated 
  character 
  that 
  they 
  have 
  been 
  sub- 
  

   jected 
  to 
  intense 
  mechanical 
  deformation. 
  In 
  others, 
  however, 
  

   all 
  these 
  evidences 
  of 
  deformation 
  are 
  entirely 
  wanting. 
  

  

  Syenite 
  and 
  Syenite-gneiss. 
  — 
  The 
  syenite 
  and 
  syenite 
  gneiss 
  

   are 
  commonly 
  a 
  grey 
  to 
  rusty 
  red 
  rock 
  which, 
  in 
  most 
  localities, 
  

   shows 
  a 
  remarkable 
  tendency 
  to 
  disaggregate 
  into 
  its 
  constitu- 
  

   ent 
  mineral 
  grains 
  on 
  the 
  weathered 
  surface. 
  They 
  consist 
  

   essentially 
  of 
  orthoclase, 
  albite, 
  microperthite, 
  segirine, 
  and 
  

   dark 
  brown 
  biotite. 
  The 
  accessory 
  constituents 
  observed 
  are 
  

   titanite, 
  apatite, 
  zircon, 
  epidote, 
  and 
  magnetite. 
  Under 
  the 
  

   microscope 
  it 
  can 
  be 
  seen 
  that 
  the 
  disaggregation 
  on 
  the 
  weath- 
  

   ered 
  surface 
  arises 
  from 
  irregular 
  fractures 
  which 
  traverse 
  the 
  

   rock 
  along 
  the 
  contacts 
  of 
  the 
  mineral 
  grains. 
  The 
  cause 
  of 
  

   the 
  fractures 
  is 
  not 
  apparent, 
  but 
  they 
  are 
  most 
  probably 
  rela- 
  

   ted 
  in 
  their 
  origin 
  to 
  the 
  expansive 
  pressure 
  which 
  no 
  doubt 
  

   accompanied 
  the 
  slight 
  decomposition 
  which 
  has 
  occurred 
  in 
  

   the 
  segirine. 
  

  

  G-ranodiorite 
  and 
  granodiorite-gneiss. 
  — 
  The 
  granodiorite 
  

   and 
  granodiorite-gneiss 
  are 
  rocks 
  of 
  similar 
  appearance 
  to 
  the 
  

   granite 
  and 
  granite-gneiss, 
  but 
  their 
  mineralogical 
  composition 
  

   shows 
  them 
  to 
  occupy 
  an 
  intermediate 
  position 
  between 
  diorite 
  

   and 
  granite. 
  They 
  contain 
  much 
  less 
  quartz 
  and 
  orthoclase 
  

   than 
  the 
  granite 
  and 
  correspondingly 
  more 
  plagioclase, 
  and 
  

   biotite 
  is 
  replaced 
  by 
  hornblende 
  as 
  the 
  dominant 
  ferromagne- 
  

   sian 
  constituent. 
  The 
  accessory 
  mineral 
  constituents, 
  mineral 
  

   alterations 
  and 
  evidence 
  of 
  mineral 
  deformation 
  are 
  the 
  same 
  

   in 
  the 
  granodiorite 
  as 
  in 
  the 
  granite 
  and 
  granite 
  gneiss. 
  

  

  Diorite 
  and 
  diorite-gneiss. 
  — 
  The 
  diorite 
  and 
  diorite-gneiss 
  

   are 
  dark 
  rocks 
  containing 
  an 
  abundance 
  of 
  glistening 
  crystals 
  

   of 
  hornblende. 
  Examination 
  under 
  the 
  microscope 
  shows 
  most 
  

   of 
  the 
  rocks 
  of 
  this 
  class 
  to 
  consist 
  essentially 
  of 
  blue-green 
  

   hornblende 
  and 
  plagioclase, 
  either 
  albite, 
  oligoclase 
  or 
  andesine, 
  

   but 
  in 
  some 
  thin 
  sections 
  the 
  proportion 
  of 
  plagioclase 
  becomes 
  

   so 
  small 
  that 
  the 
  rock 
  might 
  be 
  more 
  appropriately 
  called 
  a 
  

   hornblendite. 
  The 
  common 
  accessory 
  minerals 
  observed 
  are 
  

   garnet, 
  magnetite, 
  biotite, 
  titanite, 
  epidote, 
  and 
  zircon. 
  The 
  

   hornblende 
  and 
  biotite 
  are 
  generally 
  more 
  or 
  less 
  altered 
  to 
  

   chlorite, 
  and 
  the 
  plagioclase 
  in 
  some 
  thin 
  sections 
  is 
  entirely 
  

   replaced 
  by 
  sericite 
  and 
  epidote. 
  

  

  Pegmatite 
  and 
  aplite. 
  — 
  These 
  rocks 
  are 
  among 
  the 
  most 
  

   common 
  in 
  the 
  Laurentian 
  gneissic 
  complex 
  occurring, 
  in 
  part, 
  

   as 
  thin 
  lenses 
  in 
  the 
  banded 
  gneiss 
  and, 
  in 
  part, 
  as 
  dikes 
  trans- 
  

   verse 
  to 
  the 
  foliation 
  and 
  banding. 
  They 
  consist 
  largely 
  of 
  

   quartz 
  and 
  alkalic 
  feldspar 
  (orthoclase, 
  microcline 
  and 
  albite), 
  

  

  