﻿AND 
  BANDED 
  CRYSTALLINE 
  ROCKS 
  OP 
  THE 
  LIZARD. 
  531 
  

  

  rupture 
  took 
  place, 
  and 
  the 
  fact 
  that 
  the 
  new 
  hornblende 
  -which 
  

   joins 
  the 
  severed 
  portions 
  of 
  divided 
  crystals 
  differs 
  radically 
  in 
  

   colour 
  from 
  the 
  old 
  hornblende 
  is 
  unfavourable 
  to 
  the 
  supposition 
  

   that 
  it 
  was 
  supplied 
  directly 
  from 
  the 
  uncrystallized 
  magma 
  prior 
  

   to 
  the 
  consolidation 
  of 
  the 
  rock. 
  The 
  correctness 
  of 
  this 
  inference 
  

   is 
  further 
  shown 
  by 
  the 
  fact 
  that 
  the 
  felspar 
  that 
  stops 
  the 
  major 
  

   portions 
  of 
  the 
  cracks 
  in 
  these 
  specimens 
  is 
  water-clear 
  and 
  presents 
  

   a 
  striking 
  contrast 
  in 
  this 
  respect 
  to 
  the 
  clouded, 
  semi-opaque, 
  and 
  

   highly 
  decomposed 
  character 
  of 
  the 
  felspar 
  in 
  the 
  body 
  of 
  the 
  thin 
  

   slices. 
  Undoubtedly 
  these 
  cracks 
  were 
  stopped 
  by 
  a 
  secondary 
  

   process 
  after 
  the 
  consolidation 
  of 
  the 
  rock 
  ; 
  and 
  that 
  this 
  was 
  not 
  a 
  

   dynamic 
  process 
  connected 
  with 
  shearing 
  or 
  friction 
  is 
  shown 
  by 
  

   the 
  fact 
  that 
  the 
  cracks 
  do 
  not 
  exhibit 
  faulting 
  even 
  on 
  a 
  microscopic 
  

   scale. 
  The 
  lesson 
  to 
  be 
  derived 
  from 
  the 
  above 
  examples 
  seems 
  to 
  

   me 
  to 
  be 
  an 
  important 
  one. 
  If 
  we 
  hold 
  a 
  sheet 
  of 
  paper 
  at 
  a 
  slope 
  

   with 
  a 
  magnet 
  under 
  it 
  and 
  allow 
  fine 
  fragments 
  of 
  pounded 
  minerals 
  

   to 
  slide 
  over 
  the 
  surface 
  of 
  the 
  paper, 
  minute 
  fragments 
  of 
  magnetite 
  

   will, 
  if 
  the 
  pounded 
  rock 
  contains 
  magnetite, 
  be 
  arrested 
  by 
  the 
  

   magnet 
  whilst 
  the 
  fragments 
  of 
  other 
  minerals 
  will 
  slide 
  past. 
  A 
  

   similar 
  polar 
  influence 
  appears 
  to 
  have 
  been 
  exercised 
  by 
  the 
  mole- 
  

   cules 
  of 
  the 
  hornblende-crystals 
  at 
  the 
  edges 
  of 
  the 
  cracks 
  on 
  the 
  

   material 
  in 
  aqueous 
  solution 
  that 
  dribbled 
  past 
  them, 
  and 
  the 
  chasm 
  

   that 
  divided 
  the 
  ruptured 
  fragments 
  of 
  hornblende 
  was 
  thus 
  bridged 
  

   over. 
  

  

  May 
  we 
  not 
  apply 
  the 
  lesson 
  taught 
  us 
  by 
  these 
  cracks 
  to 
  explain 
  

   the 
  banding 
  of 
  the 
  hornblende-schists 
  of 
  the 
  Lizard, 
  and 
  suppose 
  

   that 
  a 
  like 
  polar 
  influence 
  was 
  exercised 
  by 
  hornblende-crystals 
  

   already 
  formed 
  along 
  the 
  lines 
  of 
  sedimentation, 
  or 
  of 
  lamellar 
  

   cleavage, 
  on 
  the 
  molecules 
  carried 
  past 
  them 
  by 
  the 
  water 
  perco- 
  

   lating 
  through 
  the 
  rock, 
  and 
  that 
  as 
  the 
  water 
  charged 
  with 
  horn- 
  

   blendic 
  material 
  flowed 
  slowly 
  along 
  the 
  planes 
  of 
  deposition, 
  in 
  the 
  

   way 
  suggested, 
  a 
  segregation 
  of 
  hornblende 
  along 
  these 
  lines 
  took 
  

   place 
  and 
  the 
  banding 
  was 
  thus 
  produced 
  ? 
  

  

  The 
  explanation 
  above 
  suggested 
  is 
  only 
  intended 
  to 
  apply 
  to 
  the 
  

   banding 
  ; 
  for 
  ordinary 
  unhanded 
  hornblende-rocks 
  and 
  hornblende- 
  

   schists 
  it 
  is 
  not 
  necessary 
  to 
  suppose 
  that 
  water 
  flowed 
  through 
  

   the 
  rock 
  in 
  planes 
  parallel 
  to 
  the 
  bedding. 
  Heated 
  water 
  was, 
  I 
  

   see 
  good 
  reason 
  to 
  believe, 
  the 
  principal 
  agent 
  in 
  bringing 
  about 
  the 
  

   conversion 
  of 
  the 
  pyroxene 
  in 
  these 
  old 
  products 
  of 
  volcanic 
  action 
  

   into 
  hornblende 
  ; 
  but 
  if, 
  subsequent 
  to 
  this 
  conversion, 
  the 
  heat 
  

   derived 
  from 
  either 
  plutonic 
  or 
  dynamic 
  causes 
  was 
  sufficient 
  to 
  

   reduce 
  these 
  rocks 
  to 
  a 
  plastic, 
  or 
  semi 
  -plastic, 
  condition 
  without 
  

   actually 
  fusing 
  them, 
  the 
  materials 
  would 
  arrange 
  themselves 
  under 
  

   the 
  pressure 
  of 
  superincumbent 
  strata 
  in 
  a 
  direction 
  parallel 
  to 
  the 
  

   strata. 
  

  

  The 
  " 
  Granulitic 
  " 
  Group. 
  

  

  I 
  now 
  pass 
  on 
  to 
  consider 
  the 
  " 
  granulitic 
  " 
  group, 
  and 
  it 
  will 
  be 
  

   convenient, 
  I 
  think, 
  to 
  preface 
  my 
  remarks 
  by 
  giving 
  a 
  summary 
  in 
  

  

  2p2 
  

  

  