﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  495 
  

  

  (3) 
  Uniform 
  distribution 
  of 
  black 
  mica, 
  which 
  he 
  regards 
  as 
  of 
  

   secondary 
  origin. 
  

  

  Dr. 
  Barrois, 
  in 
  answer 
  to 
  my 
  inquiry, 
  informs 
  me 
  that 
  the 
  quartz- 
  

   granules 
  occur 
  also 
  in 
  the 
  hornblende. 
  These 
  changes 
  are 
  strikingly 
  

   similar 
  to 
  those 
  produced 
  under 
  like 
  conditions 
  at 
  Malvern. 
  

  

  I 
  have 
  investigated 
  contact-effects 
  on 
  material 
  from 
  my 
  Galway 
  

   collection. 
  Six 
  slides 
  were 
  cut 
  ; 
  they 
  are 
  from 
  the 
  fringe 
  of 
  the 
  

   great 
  mass 
  of 
  granite 
  north 
  of 
  Galway 
  Bay, 
  where 
  the 
  granite 
  sends 
  

   veins 
  into 
  the 
  diorite. 
  In 
  a 
  previous 
  paper 
  * 
  I 
  have 
  shown 
  that 
  the 
  

   granite 
  was 
  injected 
  into 
  the 
  diorite 
  along 
  the 
  joints. 
  All 
  the 
  

   specimens 
  are 
  from 
  the 
  actual 
  contact, 
  and 
  contain 
  both 
  granite 
  and 
  

   diorite. 
  

  

  Pour 
  slides 
  are 
  from 
  the 
  summit 
  of 
  Lettershinna, 
  where 
  there 
  are 
  

   no 
  marked 
  signs 
  of 
  pressure. 
  The 
  following 
  are 
  the 
  characters 
  

   noticed 
  in 
  all 
  : 
  — 
  

  

  (1) 
  The 
  hornblende 
  of 
  the 
  diorite 
  nearest 
  the 
  granite 
  is 
  more 
  or 
  

   less 
  perforated, 
  and 
  the 
  margins 
  are 
  cut 
  into 
  sinuate 
  or 
  dentate 
  

   outlines. 
  The 
  mineral 
  in 
  the 
  holes 
  and 
  in 
  contact 
  with 
  the 
  irregular 
  

   margin 
  is 
  often 
  water-clear. 
  In 
  a 
  few 
  cases 
  it 
  shows 
  the 
  twinning 
  

   of 
  plagioclase. 
  

  

  (2) 
  Near 
  the 
  granite, 
  the 
  place 
  of 
  the 
  hornblende 
  is 
  often 
  taken 
  

   by 
  chlorite. 
  It 
  is 
  pale 
  green, 
  slightly 
  pleochroic, 
  and 
  of 
  a 
  deep 
  

   indigo-colour 
  under 
  crossed 
  nicols. 
  

  

  (3) 
  The 
  chlorite 
  contains 
  some 
  granules 
  of 
  a 
  colourless, 
  highly 
  

   refracting, 
  brightly 
  polarizing 
  mineral, 
  probably 
  epidote. 
  

  

  (4) 
  Yery 
  rarely, 
  the 
  chlorite 
  is 
  interbanded 
  with 
  a 
  brown 
  mineral, 
  

   which 
  has 
  the 
  appearance 
  and 
  dichroism 
  of 
  biotite. 
  

  

  A 
  specimen 
  from 
  the 
  summit 
  of 
  Knockseefin 
  displays 
  signs 
  of 
  

   pressure. 
  The 
  hornblende 
  is 
  aggregated 
  into 
  irregular 
  undulating 
  

   folia 
  lying 
  roughly 
  parallel 
  to 
  the 
  Line 
  of 
  junction. 
  Some 
  of 
  the 
  

   crystals 
  are 
  dragged 
  out 
  in 
  the 
  same 
  direction. 
  Perforation 
  is 
  

   common. 
  At 
  the 
  junction, 
  the 
  hornblende 
  passes 
  into 
  chlorite, 
  

   occasionally 
  banded 
  with 
  well-marked 
  biotite 
  and 
  containing 
  some 
  

   epidote. 
  

  

  The 
  sixth 
  slide 
  is 
  from 
  the 
  south 
  of 
  Glendalough. 
  The 
  rock, 
  in 
  

   hand-specimens, 
  shows 
  a 
  rough 
  foliation. 
  The 
  hornblende 
  of 
  the 
  

   diorite 
  is 
  not 
  much 
  modified, 
  basal 
  and 
  prismatic 
  sections 
  being 
  well 
  

   shown, 
  but 
  it 
  displays 
  a 
  tendency 
  towards 
  linear 
  aggregation. 
  

   Along 
  the 
  line 
  of 
  junction 
  with 
  the 
  granite 
  lies 
  a 
  band 
  of 
  biotite 
  

   continuous 
  across 
  the 
  slide. 
  It 
  is 
  in 
  part 
  in 
  contact 
  with 
  the 
  vein, 
  

   but 
  in 
  places 
  it 
  winds 
  in 
  and 
  out 
  amongst 
  the 
  hornblendes 
  and 
  

   felspars 
  of 
  the 
  diorite. 
  It 
  is 
  a 
  sort 
  of 
  felt- 
  work 
  of 
  small 
  crystals, 
  

   lath-shaped 
  in 
  section, 
  averaging 
  0-25 
  millim. 
  in 
  breadth, 
  lying 
  with 
  

   their 
  longer 
  axes 
  in 
  all 
  directions. 
  The 
  dichroism 
  is 
  moderately 
  strong. 
  

   Similar 
  mica 
  and 
  some 
  epidote 
  also 
  appear 
  in 
  the 
  adjacent 
  granite, 
  

   the 
  former 
  arranged 
  with 
  an 
  imperfect 
  approach 
  to 
  foliation. 
  The 
  

   granite 
  consists 
  of 
  quartz 
  and 
  plagioclase, 
  the 
  latter 
  very 
  clear 
  and 
  

   well- 
  twinned, 
  both 
  quartz 
  and 
  felspar 
  being 
  moulded 
  upon 
  or 
  in- 
  

   cluding 
  the 
  mica 
  and 
  epidote. 
  There 
  would 
  appear 
  to 
  have 
  been 
  

   * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliii. 
  p, 
  519. 
  

  

  