﻿IGNEOUS 
  KOCKS 
  AFTER 
  THEIE 
  CONSOLIDATION. 
  179 
  

  

  a 
  perfectly 
  fresh 
  condition, 
  while 
  in 
  Mull 
  and 
  other 
  parts 
  of 
  the 
  

   Western 
  Isles 
  they 
  may 
  be 
  traced 
  undergoing 
  certain 
  changes 
  due 
  

   to 
  both 
  deep-seated 
  and 
  surface-action, 
  and 
  also 
  exhibiting 
  in- 
  

   teresting 
  examples 
  of 
  the 
  so-called 
  propylitic 
  modification. 
  

  

  It 
  is 
  in 
  a 
  labradorite-andesite 
  from 
  Dun 
  da 
  Ghaoithe 
  (Dun-da-gu) 
  

   in 
  Mull, 
  a 
  locality 
  where 
  rocks 
  of 
  this 
  type 
  are 
  admirably 
  displayed, 
  

   that 
  the 
  enlarged 
  crystals 
  which 
  I 
  propose 
  to 
  describe 
  occur. 
  The 
  

   principal 
  minerals 
  in 
  the 
  rock 
  are 
  large, 
  idiomorphic 
  felspar- 
  crystals 
  

   (which 
  are 
  shown 
  by 
  their 
  extinctions, 
  their 
  specific 
  gravity, 
  and 
  

   their 
  flame-reactions 
  to 
  approximate 
  to 
  labradorite), 
  and 
  there 
  

   are 
  also 
  a 
  few 
  individuals 
  of 
  augite 
  and 
  magnetite. 
  Between 
  these 
  

   crystals, 
  glass 
  is 
  frequently 
  caught 
  up, 
  sometimes 
  in 
  angular 
  portions, 
  

   giving 
  rise 
  to 
  the 
  appearance 
  called 
  by 
  Professor 
  Rosenbusch 
  " 
  inter- 
  

   sertal 
  structure." 
  The 
  glassy 
  matrix, 
  in 
  many 
  places, 
  is 
  seen 
  to 
  

   have 
  undergone 
  much 
  change, 
  while 
  the 
  crystals 
  are 
  comparatively 
  

   fresh 
  and 
  unaltered. 
  

  

  On 
  close 
  examination, 
  the 
  felspar-crystals 
  in 
  the 
  rock-sections 
  are 
  

   found 
  to 
  exhibit 
  remarkably 
  irregular 
  and 
  ragged 
  outlines 
  ; 
  and 
  a 
  

   minute 
  scrutiny 
  reveals 
  the 
  fact 
  that 
  each 
  crystal 
  has 
  a 
  central 
  core, 
  

   which 
  shows 
  the 
  rounded 
  and 
  sometimes 
  corroded 
  forms 
  so 
  frequently 
  

   seen 
  in 
  the 
  porphyritic 
  crystals 
  of 
  rocks. 
  When 
  viewed 
  between 
  

   crossed 
  nicols, 
  the 
  distinction 
  between 
  the 
  central 
  core 
  and 
  the 
  

   irregular 
  fringe 
  surrounding 
  it 
  becomes 
  very 
  striking. 
  

  

  Looked 
  at 
  very 
  carefully, 
  the 
  portion 
  lying 
  in 
  the 
  centre 
  of 
  each 
  

   crystal 
  is 
  seen 
  not 
  only 
  to 
  exhibit 
  corroded 
  surfaces 
  and 
  glass- 
  

   enclosures, 
  but 
  to 
  be 
  traversed 
  by 
  cracks, 
  to 
  contain 
  bands 
  of 
  

   secondary 
  inclusions, 
  and 
  to 
  present 
  planes 
  of 
  decomposition 
  (kao- 
  

   linization) 
  which 
  do 
  not 
  exist 
  in 
  the 
  outer 
  clear 
  and 
  transparent 
  

   fringe 
  In 
  most 
  cases 
  the 
  outer 
  fringe 
  is 
  quite 
  subordinate 
  to 
  the 
  

   mass 
  of 
  the 
  crystals 
  (see 
  PI. 
  VII. 
  figs. 
  1, 
  2) 
  ; 
  but 
  in 
  a 
  few 
  instances 
  the 
  

   outer 
  fresh 
  portion 
  is 
  equal 
  in 
  area, 
  as 
  seen 
  in 
  thin 
  sections, 
  to 
  the 
  

   altered 
  and 
  more 
  or 
  less 
  rounded 
  core 
  which 
  it 
  encloses 
  (see 
  PI. 
  VII. 
  

   fig. 
  3). 
  Occasionally 
  the 
  enveloped 
  crystal 
  can 
  be 
  seen 
  to 
  have 
  

   undergone 
  actual 
  fracture; 
  and 
  in 
  these 
  cases, 
  the 
  cracks 
  in 
  the 
  

   fractured 
  crystals 
  are 
  found 
  to 
  be 
  filled 
  up, 
  and 
  the 
  portions 
  of 
  the 
  

   crystal 
  to 
  be 
  cemented 
  together 
  by 
  felspar-material, 
  which 
  extin- 
  

   guishes 
  with 
  the 
  surrounding 
  mass 
  and 
  not 
  with 
  the 
  central 
  core 
  

   itself 
  (see 
  PI. 
  VII. 
  fig. 
  2). 
  It 
  is 
  a 
  very 
  significant 
  circumstance 
  

   that 
  the 
  crystals 
  have 
  only 
  undergone 
  enlargement 
  where 
  they 
  are 
  

   in 
  contact 
  with 
  the 
  glassy 
  matrix, 
  and 
  that 
  where 
  other 
  crystals 
  

   lie 
  against 
  them, 
  all 
  growth 
  has 
  been 
  prevented. 
  

  

  5. 
  Changes 
  in 
  the 
  Composition 
  oe 
  the 
  Outer 
  Zones 
  oe 
  

   Felspar-crystals 
  during 
  their 
  Growth. 
  

  

  When 
  studied 
  by 
  polarized 
  light, 
  these 
  felspar-crystals 
  exhibit 
  

   one 
  very 
  important 
  difference 
  from 
  similarly 
  enlarged 
  quartz-crystals. 
  

   The 
  crystallographic 
  continuity 
  of 
  the 
  inner 
  and 
  outer 
  portions 
  of 
  

   the 
  crystals 
  is 
  show* 
  1 
  by 
  the 
  way 
  the 
  twin-planes 
  sometimes 
  pass 
  

   from 
  the 
  one 
  to 
  the 
  other 
  (see 
  PI. 
  VII. 
  fig. 
  2) 
  ; 
  but 
  the 
  inner 
  and 
  

  

  