﻿182 
  peoe. 
  j. 
  w. 
  jctdd 
  on 
  THE 
  GEOWTH 
  OE 
  CETSTALS 
  IN" 
  

  

  nature 
  of 
  the 
  glassy 
  base 
  which 
  is 
  found 
  in 
  igneous 
  rocks 
  *. 
  An 
  

   admirable 
  discussion 
  of 
  Lagorio's 
  results 
  has 
  been 
  given 
  by 
  Mr. 
  

   Teallt, 
  who 
  has 
  supplied 
  further 
  illustrations 
  of 
  the 
  subject. 
  The 
  

   interesting 
  analyses 
  of 
  Lagorio 
  show 
  very 
  clearly 
  that 
  the 
  glassy 
  

   magmas 
  of 
  igneous 
  rocks 
  are 
  just 
  of 
  such 
  a 
  composition 
  as 
  to 
  permit 
  

   anges 
  of 
  the 
  kind 
  I 
  have 
  been 
  describing 
  to 
  take 
  place 
  in 
  them. 
  

  

  6. 
  The 
  pectjliae 
  Conditions 
  which 
  have 
  given 
  eise 
  to 
  the 
  pheno- 
  

   mena 
  OP.SEEYED 
  IN 
  THE 
  PAETICULAE 
  CASE 
  NOW 
  DESCEIBED. 
  

  

  It 
  may 
  not 
  unreasonably 
  be 
  asked 
  why 
  the 
  felspars 
  in 
  this 
  par- 
  

   ticular 
  rock 
  should 
  exhibit 
  this 
  phenomenon 
  in 
  such 
  a 
  very 
  marked 
  

   and 
  unmistakable 
  manner; 
  and 
  it 
  may 
  fairly 
  be 
  objected 
  to 
  the 
  

   views 
  I 
  have 
  expressed 
  that, 
  if 
  the 
  phenomenon 
  of 
  crystal-growth 
  

   be 
  a 
  common 
  one, 
  it 
  ought 
  to 
  be 
  more 
  frequently 
  and 
  easily 
  traced. 
  

   To 
  this 
  objection 
  I 
  think 
  that 
  I 
  am 
  able 
  to 
  give 
  a 
  complete 
  reply. 
  

  

  Careful 
  study 
  of 
  a 
  large 
  number 
  of 
  cases 
  convinces 
  me 
  that 
  this 
  

   growth 
  of 
  felspar-crystals 
  at 
  the 
  expense 
  of 
  the 
  surrounding 
  unstable 
  

   magma 
  is 
  of 
  very 
  frequent 
  occurrence 
  in 
  rocks 
  which 
  have 
  been 
  

   deep-seated 
  at 
  any 
  part 
  of 
  their 
  history. 
  A 
  very 
  interesting 
  series 
  

   of 
  rocks 
  which 
  I 
  have 
  received 
  from 
  New 
  South 
  Wales 
  by 
  the 
  

   kindness 
  of 
  my 
  friend 
  T. 
  Edgeworth 
  David, 
  P.Gr.S., 
  shows 
  that 
  the 
  

   secondary 
  growth 
  of 
  a 
  felspar 
  may 
  even 
  take 
  place 
  after 
  the 
  ad- 
  

   vanced 
  kaolinization 
  of 
  the 
  original 
  crystal. 
  

  

  But 
  in 
  the 
  great 
  majority 
  of 
  cases, 
  these 
  secondary 
  outgrowths 
  

   are 
  undoubtedly 
  formed 
  while 
  the 
  original 
  crystal 
  is 
  still 
  fresh 
  and 
  

   unaltered, 
  and 
  thus 
  it 
  is 
  not 
  easy 
  to 
  demonstrate 
  that 
  the 
  growth 
  

   has 
  gone 
  on 
  after 
  the 
  surrounding 
  magma 
  has 
  become 
  solid. 
  

  

  The 
  clearness 
  of 
  the 
  evidence 
  in 
  the 
  particular 
  case 
  described 
  is 
  

   the 
  result 
  of 
  the 
  circumstance 
  of 
  the 
  alteration 
  of 
  the 
  felspar 
  before 
  

   its 
  growth 
  recommenced 
  ; 
  and 
  of 
  this 
  peculiarity 
  I 
  am 
  able 
  to 
  give 
  

   a 
  very 
  simple 
  explanation. 
  The 
  labradorite-andesite 
  in 
  which 
  these 
  

   crystals 
  occur 
  is 
  one 
  of 
  the 
  old 
  series 
  of 
  " 
  felstone 
  "-lavas 
  which 
  I 
  

   described 
  in 
  1874 
  as 
  belonging 
  to 
  the 
  earliest 
  period 
  of 
  eruption 
  in 
  

   the 
  Mull 
  volcano. 
  The 
  lava-stream 
  had 
  been 
  exposed 
  to 
  denudation 
  

   and 
  weathering 
  action 
  for 
  long 
  periods 
  of 
  time, 
  sufficient 
  to 
  allow 
  

   of 
  the 
  mechanical 
  injury 
  and 
  partial 
  kaolinization 
  of 
  the 
  felspar. 
  

  

  Subsequently, 
  this 
  old 
  lava 
  was 
  buried 
  to 
  the 
  depth 
  of 
  several 
  

   thousands 
  of 
  feet 
  by 
  the 
  later 
  out-welling 
  of 
  basaltic 
  and 
  other 
  lavas 
  — 
  

   the 
  consequence 
  being 
  that 
  the 
  mass 
  was 
  placed 
  under 
  just 
  the 
  

   conditions 
  which 
  are 
  favourable 
  to 
  the 
  renewed 
  growth 
  of 
  the 
  felspar- 
  

   crystals. 
  In 
  the 
  presence 
  of 
  these 
  conditions 
  — 
  pressure, 
  heat, 
  and 
  

   the 
  free 
  passage 
  of 
  liquids 
  and 
  gases 
  through 
  the 
  solid 
  rock-mass 
  — 
  

   the 
  crystals 
  already 
  corroded 
  by 
  their 
  surrounding 
  magma, 
  and 
  

   altered 
  by 
  surface-agencies, 
  renewed 
  their 
  youth 
  and 
  recommenced 
  

   their 
  growth. 
  But, 
  depending, 
  as 
  they 
  did, 
  for 
  the 
  supply 
  of 
  fresh 
  

   material 
  on 
  the 
  limited 
  volumes 
  of 
  glass 
  surrounding 
  them, 
  the 
  

   remarkable 
  change 
  in 
  composition, 
  as 
  the 
  lime 
  was 
  extracted 
  from 
  

   the 
  glass 
  faster 
  than 
  the 
  soda, 
  was 
  brought 
  about. 
  

  

  * 
  Min. 
  u. 
  petr. 
  Mitth. 
  viii. 
  (1887), 
  p. 
  421. 
  

   t 
  'British 
  Petrography' 
  (1888), 
  pp. 
  391-401. 
  

  

  