﻿AND 
  BANDED 
  CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  LIZARD. 
  533 
  

  

  diate 
  variety 
  whether 
  it 
  is 
  a 
  mica 
  or 
  a 
  chlorite. 
  This 
  intermediate 
  

   variety 
  is 
  highly 
  schillerized 
  along 
  the 
  cleavage-planes, 
  but 
  the 
  

   spaces 
  between 
  these 
  planes 
  are 
  not 
  green 
  but 
  of 
  an 
  extremely 
  pale 
  

   buff-colour. 
  This 
  variety 
  either 
  does 
  not 
  polarize 
  at 
  all 
  or, 
  more 
  

   frequently, 
  exhibits 
  a 
  peculiar 
  bluish 
  neutral 
  tint 
  between 
  crossed 
  

   nicols. 
  The 
  Lizard 
  granites 
  contain 
  a 
  mineral 
  closely 
  resembling 
  

   the 
  species 
  above 
  described, 
  and 
  I 
  regard 
  it 
  in 
  both 
  cases 
  as 
  a 
  

   degraded 
  biotite. 
  Some 
  slices 
  of 
  the 
  " 
  granulite 
  " 
  (as 
  also 
  of 
  the 
  

   granites 
  and 
  porphyritic 
  diorites) 
  contain, 
  in 
  addition 
  to 
  this, 
  a 
  rich 
  

   red-brown 
  mica 
  that 
  polarizes 
  brilliantly. 
  

  

  Apatite 
  is 
  present 
  in 
  all 
  the 
  slices 
  save 
  five, 
  and 
  it 
  is 
  often 
  rather 
  

   abundant. 
  Sphene 
  is 
  found 
  in 
  12 
  and 
  quartz 
  in 
  16 
  out 
  of 
  the 
  

   23 
  slices 
  entered 
  in 
  the 
  list. 
  The 
  quartz 
  abounds 
  in 
  liquid-cavities 
  

   with 
  moving 
  bubbles 
  ; 
  and 
  sphene, 
  when 
  present, 
  is 
  usually 
  abundant. 
  

   Hornblende 
  is 
  sparse 
  in 
  one 
  and 
  is 
  wholly 
  absent 
  from 
  11 
  slides. 
  

  

  The 
  microscopic 
  examination 
  (summarized 
  in 
  the 
  preceding 
  table) 
  

   shows 
  that 
  the 
  " 
  gran 
  uli 
  tic 
  " 
  group 
  consists 
  of 
  an 
  intermixture 
  of 
  

   granite 
  and 
  diorite, 
  and 
  it 
  is 
  favourable 
  to 
  the 
  view, 
  detailed 
  below, 
  

   that 
  the 
  white 
  bands 
  in 
  this 
  group 
  are 
  veins 
  of 
  granite 
  injected 
  into 
  

   dioritic 
  rocks 
  from 
  neighbouring 
  eruptive 
  masses. 
  The 
  presence 
  of 
  

   hornblende 
  in 
  some 
  of 
  the 
  granitic 
  portions 
  and 
  of 
  mica 
  in 
  the 
  dio- 
  

   rite 
  is 
  probably 
  due 
  to 
  contact 
  action. 
  

  

  Compared 
  with 
  the 
  hornblende-schists 
  the 
  " 
  granulites 
  " 
  differ 
  in 
  

   their 
  mineralogical 
  contents 
  by 
  the 
  absence 
  of 
  hornblende 
  in 
  11 
  out 
  

   of 
  23 
  samples 
  ; 
  in 
  the 
  presence 
  of 
  quartz 
  in 
  all 
  but 
  7 
  of 
  the 
  speci- 
  

   mens 
  ; 
  and 
  in 
  the 
  occurrence 
  of 
  mica 
  in 
  all 
  save 
  one, 
  whereas 
  only 
  one 
  

   of 
  the 
  hornblende-schists 
  contains 
  this 
  mineral. 
  Apatite, 
  sphene, 
  

   and 
  iron-oxides 
  are 
  common 
  to 
  both 
  series. 
  The 
  quartz 
  in 
  the 
  

   "granulites" 
  is 
  of 
  an 
  essentially 
  granitic 
  type. 
  

  

  Prof. 
  Bonney 
  regarded 
  the 
  " 
  granulitic 
  " 
  group 
  as 
  mainly 
  composed 
  

   of 
  bedded 
  rocks 
  ; 
  but 
  Mr. 
  Teall 
  held 
  that 
  true 
  igneous 
  rocks 
  form 
  

   no 
  inconsiderable 
  portion 
  of 
  this 
  group. 
  Both 
  views 
  are, 
  I 
  think, 
  

   correct. 
  Part 
  of 
  the 
  basic 
  portion 
  of 
  the 
  " 
  granulitic 
  " 
  group 
  is, 
  

   it 
  seems 
  to 
  me, 
  composed, 
  like 
  the 
  hornblende-schists, 
  of 
  volcanic 
  

   ashes 
  and 
  lavas, 
  which 
  thermo-aqueous 
  agencies, 
  succeeded 
  by 
  the 
  

   contact-action 
  of 
  igneous 
  masses, 
  have 
  reduced 
  to 
  a 
  condition 
  which 
  

   renders 
  it 
  impossible, 
  in 
  some 
  cases, 
  to 
  distinguish 
  between 
  them 
  

   and 
  the 
  diorites 
  that 
  have 
  invaded 
  the 
  rocks 
  of 
  this 
  group. 
  

  

  One 
  of 
  the 
  intrusive 
  diorites 
  is 
  well 
  seen 
  at 
  Polbarrow, 
  where 
  a 
  

   broad 
  dyke 
  of 
  it 
  cuts 
  right 
  across 
  the 
  " 
  granulitic 
  " 
  group 
  between 
  

   the 
  ruined 
  boat-house 
  and 
  the 
  cliff. 
  Down 
  below 
  veins 
  of 
  it 
  may 
  

   be 
  seen 
  intruding 
  into 
  the 
  "granulite." 
  This 
  is 
  'No. 
  1 
  of 
  the 
  

   microscopic 
  summary 
  given 
  further 
  on. 
  It 
  is 
  an 
  excessively 
  tough, 
  

   hard 
  rock. 
  

  

  Another 
  very 
  interesting 
  and 
  important 
  intruder 
  is 
  the 
  porphyritic 
  

   diorite, 
  to 
  which 
  attention 
  was 
  called 
  by 
  Mr. 
  Teall 
  in 
  a 
  paper 
  read 
  

   before 
  the 
  Geologists' 
  Association 
  in 
  April 
  1887 
  *, 
  and 
  by 
  Messrs. 
  

   Howard 
  Fox, 
  F.Gr.S., 
  and 
  Alex. 
  Somervail, 
  in 
  a 
  paper 
  read 
  at 
  the 
  

   meeting 
  of 
  the 
  British 
  Association 
  in 
  September 
  of 
  that 
  year 
  f 
  . 
  

  

  * 
  ' 
  Proceedings,' 
  vol. 
  x. 
  p. 
  75. 
  t 
  Geol. 
  Magazine, 
  February, 
  1888, 
  p. 
  74. 
  

  

  