﻿186 
  ON 
  THE 
  GROWTH 
  OP 
  CRYSTALS 
  IX 
  IGNEOUS 
  ROCKS. 
  

  

  Darwin, 
  "which 
  have 
  in 
  recent 
  years 
  received 
  such 
  valuable 
  illus- 
  

   tration 
  from 
  the 
  admirable 
  researches 
  of 
  Lossen, 
  Lehmann, 
  and 
  many 
  

   other 
  petrographers. 
  

  

  EXPLANATION 
  OF 
  PLATE 
  VII. 
  

  

  Outgrowths 
  to 
  felspar-crystals 
  in 
  labradorite-andesite, 
  Dun 
  da 
  Ghaoithe, 
  

   Isle 
  of 
  Mull. 
  

  

  Fig. 
  1. 
  Crystal 
  of 
  felspar 
  as 
  seen, 
  magnified 
  25 
  diameters, 
  by 
  ordinary 
  light. 
  

   The 
  contrast 
  between 
  the 
  original 
  crystal 
  with 
  its 
  numerous 
  cracks 
  

   (some 
  of 
  which 
  show 
  incipient 
  kaolinization 
  aloug 
  their 
  sides) 
  and 
  the 
  

   comparatively 
  fresh 
  and 
  irregular 
  outgrowths 
  of 
  secondary 
  origin 
  is 
  

   very 
  clearly 
  seen. 
  

  

  2. 
  The 
  same 
  crystal 
  as 
  seen 
  with 
  crossed 
  nicols. 
  The 
  difference 
  of 
  the 
  

  

  extinction-angle 
  between 
  the 
  central 
  core 
  and 
  the 
  surrounding 
  secondary 
  

   fringe 
  causes 
  the 
  latter 
  to 
  remain 
  light, 
  while 
  the 
  former, 
  except 
  where 
  

   traversed 
  bj* 
  twin-lamellse, 
  is 
  dark. 
  It 
  is 
  very 
  noticeable 
  that 
  newly 
  

   deposited 
  felspar-substance 
  filling 
  the 
  cracks 
  of 
  the 
  original 
  crystal 
  

   extinguishes 
  with 
  the 
  outer 
  fringe. 
  

  

  3. 
  Another 
  crystal 
  of 
  felspar 
  as 
  seen 
  magnified 
  75 
  diameters. 
  The 
  great 
  

  

  extent 
  of 
  the 
  outgrowths, 
  as 
  compared 
  with 
  the 
  original 
  rounded 
  grain 
  

   of 
  felspar, 
  is 
  very 
  striking 
  ; 
  as 
  is 
  also 
  the 
  very 
  irregular 
  development 
  of 
  

   the 
  secondary 
  fringe. 
  The 
  crystal 
  is 
  represented 
  as 
  seen 
  with 
  crossed 
  

   nicols, 
  the 
  stage 
  being 
  rotated 
  into 
  such 
  a 
  position 
  that 
  the 
  zone 
  of 
  

   extinction, 
  traversing 
  the 
  portions 
  of 
  the 
  crystal 
  which 
  have 
  the 
  same 
  

   chemical 
  composition, 
  lies 
  in 
  the 
  midst 
  of 
  the 
  secondary 
  outgrowth. 
  

   As 
  the 
  stage 
  is 
  gradually 
  rotated 
  this 
  zone 
  of 
  extinction 
  is 
  seen 
  to 
  pass 
  

   slowly 
  and 
  progressively 
  outwards, 
  without 
  any 
  kind 
  of 
  break, 
  till 
  it 
  

   reaches 
  the 
  most 
  distant 
  apophyses. 
  Some 
  of 
  these 
  last 
  must 
  have 
  a 
  

   composition 
  very 
  closely 
  approximating 
  to 
  that 
  of 
  albite. 
  

  

  