﻿ISO 
  PROF. 
  J. 
  W. 
  JFDD 
  OX 
  THE 
  GBOW1H 
  OF 
  CRYSTALS 
  FX 
  

  

  outer 
  portions 
  of 
  the 
  crystal 
  do 
  not 
  extinguish 
  simultaneously 
  when 
  

   the 
  section 
  is 
  rotated 
  between 
  crossed 
  nicols. 
  On 
  the 
  contrary, 
  in 
  

   rotatiDg 
  the 
  crystal, 
  after 
  the 
  position 
  of 
  extinction 
  of 
  the 
  internal 
  

   core 
  has 
  been 
  passed, 
  a 
  dark 
  zone 
  of 
  extinction 
  makes 
  ius 
  appearance 
  

   around 
  the 
  central 
  mass, 
  and, 
  as 
  rotation 
  goes 
  on, 
  this 
  dark 
  zone 
  

   passes 
  slowly 
  and 
  gradually 
  outwards 
  through 
  the 
  surrounding 
  fringe 
  

   (see 
  PI. 
  YIL 
  fig. 
  3). 
  

  

  Xothing 
  can 
  be 
  more 
  striking 
  and 
  suggestive 
  than 
  this 
  beautiful 
  

   phenomenon, 
  which 
  it 
  is 
  possible 
  to 
  observe 
  and 
  verify 
  in 
  the 
  case 
  

   of 
  many 
  of 
  the 
  crystals 
  in 
  this 
  very 
  interesting 
  rock. 
  

  

  I 
  think 
  there 
  cannot 
  be 
  any 
  real 
  doubt 
  as 
  to 
  the 
  true 
  explanation 
  

   of 
  the 
  remarkable 
  appearances 
  which 
  I 
  have 
  been 
  describing. 
  

  

  The 
  more 
  or 
  less 
  decomposed 
  character 
  of 
  the 
  glassy 
  ground-mass 
  

   in 
  these 
  rocks, 
  as 
  compared 
  with 
  the 
  crystallized 
  minerals, 
  shows 
  

   that 
  the 
  vitreous 
  part 
  is 
  in 
  a 
  less 
  stable 
  condition 
  than 
  the 
  crystal- 
  

   line. 
  That 
  long 
  after 
  the 
  consolidation 
  of 
  the 
  rock 
  the 
  crystals 
  

   grew 
  outwards 
  irregularly, 
  at 
  the 
  expense 
  of 
  the 
  surrounding 
  glassy 
  

   magma, 
  is 
  clear: 
  this 
  being 
  shown 
  by 
  the 
  fact 
  that 
  where 
  two 
  

   crystal 
  faces 
  are 
  in 
  juxtaposition 
  without 
  the 
  intervention 
  of 
  

   glassy 
  material, 
  no 
  exterior 
  fringe 
  is 
  formed. 
  The 
  phenomenon 
  

   which 
  these 
  particular 
  crystals 
  exhibit 
  — 
  and 
  which 
  it 
  is 
  especially 
  

   important 
  to 
  bear 
  in 
  mind 
  — 
  is 
  that 
  they 
  have 
  not 
  only 
  been 
  de- 
  

   veloped 
  before 
  the 
  secondary 
  outgrowths 
  have 
  been 
  formed 
  around 
  

   them, 
  but 
  that 
  they 
  have 
  suffered 
  a 
  considerable 
  amount 
  of 
  injury 
  and 
  

   alteration 
  from 
  the 
  action 
  of 
  mechanical 
  and 
  chemical 
  forces. 
  This 
  

   is 
  especially 
  well 
  shown 
  when 
  the 
  junctions 
  of 
  the 
  old 
  and 
  the 
  new 
  

   portions 
  of 
  the 
  crystal 
  are 
  studied 
  with 
  high 
  microscopic 
  powers. 
  

  

  On 
  the 
  other 
  hand, 
  it 
  can 
  be 
  proved 
  in 
  the 
  same 
  way 
  that 
  the 
  

   period 
  since 
  the 
  development 
  of 
  the 
  outer 
  fringe 
  of 
  new 
  material 
  has 
  

   been 
  sufficient 
  for 
  the 
  formation 
  of 
  new 
  cracks 
  and 
  bands 
  of 
  

   enclosures, 
  which 
  traverse 
  both 
  the 
  old 
  and 
  new 
  portions 
  of 
  the 
  

   crystals 
  alike. 
  In 
  most 
  cases 
  the 
  twin-lamella? 
  pass 
  from 
  one 
  por- 
  

   tion 
  to 
  the 
  other 
  ; 
  but 
  whether 
  this 
  twinning 
  was 
  developed 
  before 
  

   or 
  after 
  the 
  outgrowths 
  were 
  formed 
  can 
  only 
  be 
  proyed 
  in 
  the 
  cases 
  

   (which 
  sometimes 
  occur) 
  when 
  incipient 
  kaolinization 
  has 
  taken 
  place 
  

   along 
  the 
  twin-planes, 
  which 
  are 
  thus 
  shown 
  to 
  have 
  been 
  'solution- 
  

   planes, 
  before 
  the 
  outward 
  growth 
  took 
  place 
  (see 
  PI. 
  Til. 
  fig. 
  1). 
  

  

  In 
  a 
  few 
  instances, 
  I 
  have 
  found 
  proof 
  that 
  secondary 
  twinning 
  

   has 
  been 
  developed 
  in 
  the 
  fringe 
  of 
  new 
  material, 
  but 
  does 
  not 
  

   extend 
  into 
  the 
  old 
  nucleus. 
  

  

  The 
  remarkable 
  behayiour 
  of 
  the 
  zone 
  of 
  extinction 
  is 
  well 
  worthy 
  

   of 
  study 
  and 
  consideration. 
  The 
  existence 
  of 
  zoned 
  plagioclastic 
  

   felspars 
  with 
  areas 
  giving 
  different 
  extinction 
  is 
  well 
  known 
  ; 
  and 
  in 
  

   most 
  of 
  these 
  the 
  more 
  basic 
  portions 
  form 
  the 
  centre 
  and 
  approximate 
  

   to 
  the 
  Anorthite-extreme, 
  while 
  the 
  outer 
  zones 
  are 
  successively 
  

   more 
  acid 
  in 
  character 
  and 
  approach 
  towards 
  the 
  Albite-limit. 
  But, 
  

   in 
  nearly 
  all 
  such 
  cases 
  the 
  successive 
  zones 
  are 
  clearly 
  and 
  sharply 
  

   marked 
  off 
  from 
  one 
  another, 
  and 
  they 
  not 
  unfrequently 
  exhibit 
  

   numerous 
  solid 
  and 
  liquid 
  inclusions, 
  or 
  other 
  indications 
  that 
  the 
  

   growth 
  of 
  the 
  crystal 
  had 
  been 
  arrested 
  at 
  successive 
  stages, 
  and 
  

  

  