﻿ON 
  THE 
  GEOWTH 
  OF 
  CKYSTALS 
  IN 
  IGNEOUS 
  EOCKS. 
  175 
  

  

  11. 
  On 
  the 
  Geowth 
  of 
  Ceystals 
  in 
  Igneous 
  Eocks 
  after 
  their 
  

   Consolidation. 
  By 
  Professor 
  John 
  "W. 
  Judd, 
  F.R.S., 
  F.G.S. 
  

   (Eead 
  January 
  9, 
  1889.) 
  

  

  [Plate 
  VII.] 
  

  

  Contents. 
  

  

  Page 
  

  

  1. 
  Introduction. 
  Secondary 
  Origin 
  of 
  the 
  " 
  Granophyric" 
  structures 
  175 
  

  

  2. 
  Enlargements 
  of 
  detrital 
  fragments 
  of 
  Crystals 
  : 
  — 
  Quartz, 
  Felspar, 
  Horn- 
  

  

  blende, 
  and 
  Mica 
  176 
  

  

  3. 
  Enlargement 
  of 
  Crystals 
  in 
  Igneous 
  Eocks 
  177 
  

  

  4. 
  Proofs 
  that 
  Growth 
  of 
  Porphyritic 
  Felspars 
  may 
  take 
  place 
  after 
  the 
  

  

  consolidation 
  and 
  the 
  alteration 
  by 
  weathering 
  of 
  an 
  Igneous 
  Rock 
  178 
  

  

  5. 
  Changes 
  in 
  the 
  Composition 
  of 
  the 
  Outer 
  Zones 
  of 
  Felspar-crystals 
  

  

  during 
  their 
  Growth 
  179 
  

  

  6. 
  Peculiar 
  Conditions 
  which 
  have 
  given 
  rise 
  to 
  the 
  phenomena 
  observed 
  in 
  

  

  the 
  particular 
  case 
  now 
  described 
  182 
  

  

  7. 
  Relation 
  of 
  these 
  facts 
  to 
  Dr. 
  Lehmann's 
  theory 
  of 
  the 
  Origin 
  of 
  the 
  

  

  Perthite-structure, 
  and 
  other 
  recent 
  researches 
  183 
  

  

  8. 
  Bearing 
  of 
  these 
  considerations 
  on 
  the 
  problem 
  of 
  the 
  Origin 
  of 
  Folia- 
  

  

  tion 
  in 
  Metamorphic 
  Rocks 
  185 
  

  

  1. 
  Inteoeuction. 
  Second 
  aey 
  Oeigin 
  oe 
  the 
  " 
  Geanoehyeic 
  " 
  

   steuctuees. 
  

  

  Theee 
  years 
  ago, 
  in 
  describing 
  some 
  basic 
  igneous 
  rocks 
  from 
  Scot- 
  

   land 
  and 
  Ireland, 
  I 
  expressed 
  my 
  conviction 
  that 
  a 
  part, 
  at 
  least, 
  of 
  

   the 
  structures 
  known 
  as 
  micropegmatitie 
  and 
  granophyric 
  were 
  

   really 
  of 
  secondary 
  origin*. 
  At 
  the 
  same 
  time, 
  I 
  stated 
  that 
  these 
  

   structures 
  were 
  so 
  much 
  more 
  clearly 
  illustrated 
  by 
  the 
  intermediate 
  

   and 
  acid 
  rocks 
  of 
  the 
  same 
  district 
  that 
  it 
  would 
  be 
  well 
  to 
  defer 
  

   the 
  discussion 
  of 
  the 
  subject 
  till 
  my 
  examination 
  and 
  description 
  of 
  

   the 
  rocks 
  in 
  question 
  should 
  be 
  completed. 
  

  

  The 
  study 
  of 
  the 
  intermediate 
  and 
  acid 
  lavas 
  (which, 
  in 
  1874, 
  I 
  

   grouped 
  under 
  the 
  old 
  English 
  field-name 
  of 
  " 
  felstones 
  ") 
  and 
  of 
  

   their 
  plutonic 
  representatives 
  has 
  occupied 
  much 
  of 
  my 
  time 
  and 
  

   thought 
  during 
  the 
  last 
  fifteen 
  years. 
  But 
  so 
  numerous 
  and 
  

   interesting 
  are 
  the 
  problems 
  connected 
  with 
  the 
  wonderful 
  meta- 
  

   morphoses 
  which 
  the 
  minerals 
  of 
  these 
  rocks 
  have 
  undergone, 
  and 
  

   so 
  singular 
  are 
  the 
  changes 
  which 
  have 
  taken 
  place 
  in 
  the 
  structures 
  

   of 
  these 
  rocks, 
  that, 
  in 
  spite 
  of 
  a 
  number 
  of 
  visits 
  to 
  the 
  district 
  

   and 
  much 
  labour 
  in 
  the 
  laboratory, 
  some 
  time 
  must 
  yet 
  elapse 
  before 
  

   the 
  examination 
  of 
  the 
  chief 
  types 
  and 
  the 
  elucidation 
  of 
  their 
  mode 
  

   of 
  origin 
  is 
  completed. 
  

  

  In 
  the 
  meanwhile, 
  however, 
  certain 
  discussions 
  which 
  have 
  taken 
  

   place 
  concerning 
  rock-structures 
  and 
  their 
  significance 
  — 
  especially 
  

   as 
  a 
  basis 
  of 
  rock-nomenclature 
  and 
  classification 
  — 
  render 
  it 
  desirable 
  

   that 
  the 
  very 
  definite 
  and 
  unmistakable 
  evidence 
  which 
  I 
  have 
  ob- 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol 
  xlii. 
  (1886), 
  pp. 
  72, 
  73, 
  pi. 
  vii. 
  fig. 
  8. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  178. 
  p 
  

  

  