﻿184 
  PEOF. 
  J. 
  W. 
  JUDD 
  OjS" 
  THE 
  GEOWTH 
  OP 
  CEYSTALS 
  IN 
  

  

  crystal 
  and 
  the 
  outgrowths 
  extinguishing 
  together 
  (fig. 
  d). 
  Dr. 
  

   Haworth's 
  illustrations 
  of 
  these 
  phenomena 
  are 
  so 
  excellent 
  that 
  I 
  

   prefer 
  to 
  reproduce 
  them 
  rather 
  than 
  to 
  select 
  from 
  numerous 
  simi- 
  

   lar 
  cases 
  that 
  have 
  come 
  under 
  my 
  own 
  observation. 
  It 
  is 
  true 
  

   that 
  Dr. 
  Haworth 
  does 
  not 
  take 
  the 
  view 
  that 
  these 
  secondary 
  

   outgrowths 
  were 
  formed 
  after 
  the 
  solidification 
  of 
  the 
  mass, 
  but 
  

   rather 
  that 
  " 
  in 
  the 
  process 
  of 
  cooling 
  the 
  felspar-crystals 
  were 
  formed 
  

   and 
  floated 
  about 
  in 
  the 
  magma 
  for 
  some 
  time, 
  as 
  the 
  porphyritic 
  

   felspars 
  do 
  in 
  the 
  magma 
  of 
  a 
  porphyry 
  " 
  *. 
  After 
  the 
  proofs 
  I 
  have 
  

   now 
  given 
  that 
  such 
  secondary 
  growths 
  can 
  and 
  do 
  take 
  place 
  in 
  a 
  

   rock-mass, 
  long 
  after 
  it 
  has 
  become 
  solid, 
  the 
  objections 
  raised 
  by 
  

   Dr. 
  Haworth 
  to 
  this 
  explanation 
  disaj>pear. 
  

  

  When 
  it 
  is 
  remembered 
  how 
  crystals 
  are 
  developed 
  in 
  the 
  midst 
  

   of 
  solid 
  rock-masses, 
  during 
  contact-metamorphism, 
  this 
  growth 
  of 
  

   crystals 
  in 
  solid 
  igneous 
  rocks 
  ought 
  to 
  occasion 
  no 
  surprise. 
  

  

  My 
  friend 
  Prof. 
  G. 
  H. 
  AYilliams 
  — 
  in 
  whose 
  laboratory 
  have 
  been 
  

   carried 
  on 
  so 
  many 
  researches 
  having 
  an 
  important 
  bearing 
  on 
  the 
  

   questions 
  discussed 
  in 
  this 
  paper 
  — 
  has 
  in 
  his 
  various 
  memoirs 
  and 
  

   also 
  in 
  private 
  correspondence 
  called 
  my 
  attention 
  to 
  phenomena 
  

   which 
  seem 
  at 
  first 
  sight 
  to 
  be 
  inconsistent 
  with 
  the 
  theory 
  of 
  

   schillerization 
  which 
  I 
  have 
  propounded. 
  He 
  refers 
  to 
  the 
  case 
  of 
  

   felspar-crystals 
  which 
  contain 
  a 
  central 
  mass 
  filled 
  with 
  inclusions, 
  

   but 
  in 
  which 
  an 
  outer 
  zone 
  is 
  seen 
  quite 
  free 
  from 
  such 
  inclusions. 
  

  

  I 
  have 
  studied 
  many 
  such 
  cases, 
  and 
  in 
  some 
  of 
  them 
  have 
  been 
  

   able 
  to 
  prove 
  that 
  the 
  clear 
  outer 
  zone 
  is 
  really 
  a 
  secondary 
  outgrowth 
  

   to 
  the 
  crystal. 
  I 
  have 
  found 
  what 
  I 
  think 
  is 
  indisputable 
  evidence 
  

   that 
  the 
  outer 
  portion 
  of 
  a 
  crystal 
  may 
  be 
  removed 
  under 
  one 
  set 
  

   of 
  conditions, 
  and 
  that 
  under 
  other 
  conditions 
  its 
  growth 
  has 
  recom- 
  

   menced 
  again 
  ; 
  just 
  as 
  a 
  crystal 
  of 
  alum 
  would 
  behave 
  if 
  first 
  taken 
  

   from 
  a 
  saturated 
  solution 
  and 
  put 
  into 
  warm 
  water, 
  and 
  then 
  after 
  

   a 
  certain 
  interval 
  transferred 
  again 
  to 
  the 
  original 
  or 
  some 
  other 
  

   saturated 
  solution 
  of 
  an 
  alum. 
  

  

  I 
  must 
  postpone 
  to 
  a 
  future 
  occasion 
  the 
  discussion 
  of 
  the 
  exact 
  

   nature 
  of 
  the 
  operations 
  which 
  result 
  in 
  the 
  formation 
  of 
  the 
  different 
  

   varieties 
  of 
  micropegmatitic, 
  centric, 
  pseudospherulitic, 
  and 
  miaro- 
  

   litic 
  structures, 
  respectively. 
  In 
  all 
  these 
  cases, 
  I 
  believe, 
  I 
  shall 
  

   be 
  able 
  to 
  show 
  that, 
  in 
  vitreous 
  or 
  imperfectly 
  crystallized 
  material, 
  

   the 
  instability 
  which 
  exists 
  has 
  permitted 
  the 
  formation 
  in 
  the 
  

   solid 
  rock 
  of 
  outgrowths 
  to 
  preexisting 
  stable 
  crystals. 
  

  

  I 
  shall 
  show 
  that 
  there 
  is 
  every 
  gradation 
  from 
  a 
  glassy 
  ground- 
  

   mass 
  to 
  one 
  characteristic 
  of 
  the 
  so-called 
  granophyric 
  rocks, 
  and 
  

   prove 
  that 
  the 
  characteristic 
  structures 
  of 
  those 
  rocks 
  must 
  be 
  

   attributed 
  to 
  secondary 
  rather 
  than 
  to 
  primary 
  devitrification. 
  The 
  

   cavities 
  found 
  in 
  the 
  " 
  miarolitic 
  " 
  or 
  drusy 
  granites 
  — 
  which 
  are 
  so 
  

   difficult 
  to 
  account 
  for 
  on 
  any 
  other 
  hypothesis 
  — 
  will 
  be 
  shown 
  to 
  be 
  

   fully 
  explained 
  by 
  the 
  contraction 
  which 
  a 
  more 
  or 
  less 
  glassy 
  

   ground-mass 
  has 
  undergone 
  during 
  devitrification. 
  

  

  * 
  Loc. 
  cit. 
  pp. 
  15-16. 
  

  

  