﻿484 
  DK. 
  C. 
  CALLAWAY 
  ON 
  SECONDAKY 
  MINEEALS 
  IN 
  THE 
  

  

  must 
  be 
  regarded 
  as 
  the 
  effect 
  of 
  secondary 
  action. 
  This 
  hornblende, 
  

   compared 
  with 
  that 
  of 
  the 
  preceding 
  slides, 
  is 
  darker, 
  brighter, 
  

   clearer, 
  and 
  more 
  strongly 
  pleochroic. 
  Chlorite 
  sometimes 
  appears 
  

   in 
  the 
  cleavage-planes 
  of 
  the 
  hornblende, 
  and 
  increases 
  in 
  quantity 
  

   at 
  some 
  points 
  so 
  as 
  to 
  form 
  nests 
  of 
  rosettes, 
  irregular 
  blotches, 
  or 
  

   patches 
  running 
  with 
  the 
  foliation. 
  Under 
  crossed 
  nicols 
  it 
  gives 
  a 
  

   deep 
  indigo 
  -colour, 
  passing 
  into 
  grey. 
  In 
  one 
  of 
  the 
  slides 
  there 
  is 
  

   a 
  fair 
  quantity 
  of 
  epidote. 
  

  

  Much 
  of 
  the 
  felspar 
  is 
  clear 
  and 
  sound, 
  and 
  a 
  large 
  proportion 
  of 
  

   it 
  shows 
  the 
  twinning 
  of 
  plagioclase 
  ; 
  but 
  some 
  of 
  it 
  has 
  begun 
  to 
  

   break 
  up 
  into 
  an 
  indistinct 
  mosaic, 
  and 
  displays 
  numerous 
  microlitbs 
  

   of 
  a 
  clear 
  mineral 
  (white 
  mica 
  ?). 
  It 
  is 
  important 
  to 
  note 
  that 
  

   this 
  felspar 
  is 
  moulded 
  upon 
  all 
  the 
  preceding 
  minerals, 
  and 
  even 
  

   includes 
  shreds 
  of 
  the 
  chlorite 
  and 
  minute 
  fragments 
  of 
  broken 
  

   hornblende- 
  crystals. 
  The 
  chlorite 
  is 
  certainly 
  derived 
  from 
  the 
  

   hornblende, 
  and 
  the 
  distortion 
  and 
  fracture 
  of 
  the 
  hornblende 
  is 
  a 
  

   secondary 
  result 
  produced 
  by 
  pressure 
  subsequent 
  to 
  consolidation, 
  

   since, 
  as 
  I 
  have 
  stated, 
  it 
  is 
  frequently 
  connected 
  with 
  the 
  formation 
  

   of 
  planes 
  of 
  discontinuity. 
  That 
  this 
  felspar 
  is 
  secondary 
  receives 
  

   some 
  confirmation 
  from 
  its 
  clear 
  and 
  fresh 
  appearance. 
  

  

  The 
  slides 
  contain 
  a 
  small 
  proportion 
  of 
  granular 
  quartz, 
  some 
  of 
  

   which 
  contains 
  liquid-cavities 
  with 
  air-bubbles. 
  

  

  No. 
  314 
  is 
  taken 
  from 
  within 
  a 
  few 
  inches 
  of 
  the 
  veined 
  plexus. 
  

   Much 
  of 
  the 
  hornblende 
  is 
  dragged 
  out 
  in 
  the 
  direction 
  of 
  foliation, 
  

   and 
  the 
  whole 
  of 
  it 
  is 
  more 
  or 
  less 
  dirty 
  and 
  opaque. 
  The 
  opacity 
  is 
  

   apparently 
  due 
  to 
  liberated 
  ferric 
  oxide. 
  Most 
  of 
  the 
  felspar 
  is 
  

   either 
  cloudy 
  or 
  contains 
  countless 
  clear 
  microliths 
  (white 
  mica 
  ?), 
  

   and 
  in 
  some 
  there 
  are 
  patches 
  of 
  white 
  mica. 
  There 
  is 
  also 
  a 
  great 
  

   quantity 
  of 
  finely 
  granular, 
  almost 
  opaque, 
  matter 
  in 
  the 
  felspar. 
  

   It 
  may 
  be 
  epidote 
  or 
  zoisite. 
  Granules 
  of 
  quartz 
  are 
  scattered 
  through 
  

   the 
  slide. 
  

  

  No. 
  391. 
  About 
  the 
  same 
  distance 
  from 
  the 
  zone, 
  but 
  a 
  little 
  

   higher 
  up. 
  The 
  hornblende 
  is 
  like 
  the 
  last, 
  but 
  the 
  curvilinear 
  

   outlines 
  may 
  be 
  sometimes 
  made 
  out. 
  The 
  felspar 
  is 
  also 
  similar. 
  

   The 
  proportion 
  of 
  granular 
  quartz 
  is 
  greater. 
  The 
  noteworthy 
  

   feature 
  of 
  the 
  slide 
  is 
  the 
  coming-in 
  of 
  black 
  mica. 
  It 
  occurs 
  in 
  

   elongated 
  flakes 
  several 
  laminae 
  in 
  thickness 
  and 
  pointed 
  at 
  each 
  end. 
  

   They 
  run 
  parallel 
  to 
  the 
  general 
  foliation, 
  often 
  passing 
  through 
  the 
  

   middle 
  of 
  hornblende-crystals, 
  and 
  are 
  occasionally 
  associated 
  with 
  

   a 
  little 
  chlorite. 
  The 
  habit 
  of 
  these 
  flakes 
  is 
  precisely 
  that 
  of 
  the 
  

   chlorite, 
  which 
  forms 
  in 
  the 
  interior 
  of 
  the 
  hornblende, 
  and 
  frequently 
  

   projects 
  beyond 
  it. 
  The 
  felspar 
  is 
  clearly 
  moulded 
  upon 
  the 
  mica, 
  

   and 
  sometimes 
  completely 
  encloses 
  it. 
  

  

  No. 
  316 
  is 
  in 
  contact 
  with 
  the 
  shear-zone. 
  The 
  rock 
  is 
  rather 
  

   more 
  compressed 
  than 
  in 
  the 
  last 
  two 
  slides. 
  The 
  hornblende 
  is 
  

   similar. 
  It 
  sometimes 
  passes 
  into 
  chlorite, 
  which 
  becomes 
  banded 
  

   here 
  and 
  there 
  with 
  black 
  mica 
  or 
  is 
  entirely 
  replaced 
  by 
  it. 
  

   Sometimes 
  a 
  little 
  opaque 
  flake, 
  probably 
  ferruginized 
  hornblende, 
  

   projects 
  from 
  a 
  bit 
  of 
  hornblende 
  into 
  a 
  patch 
  of 
  chlorite, 
  and 
  just 
  

   round 
  the 
  margin 
  of 
  the 
  flake 
  black 
  mica 
  is 
  formed, 
  showing 
  marked 
  

  

  