﻿Vol. 
  53.] 
  SOME 
  OF 
  THE 
  GNEISSES 
  OF 
  ANGLESEY. 
  353 
  

  

  B. 
  Injection-Schists. 
  

  

  In 
  two 
  of 
  my 
  Malvern 
  papers 
  I 
  have 
  described 
  two 
  kinds 
  of 
  

   injection-schist, 
  those 
  produced 
  by 
  the 
  injection 
  of 
  rock 
  (primary 
  

   injection) 
  and 
  those 
  formed 
  by 
  the 
  infiltration 
  of 
  mineral 
  matter 
  

   along 
  shear-planes 
  subsequent 
  to 
  consolidation 
  (secondary 
  injection). 
  

   Both 
  of 
  these 
  varieties 
  of 
  schist 
  l 
  are 
  to 
  be 
  seen 
  in 
  Anglesey. 
  

  

  In 
  Anglesey 
  the 
  felsite 
  plays 
  the 
  part 
  which 
  at 
  Malvern 
  is 
  taken 
  

   by 
  the 
  granite, 
  but 
  certain 
  important 
  differences 
  arise. 
  The 
  granite 
  

   of 
  Malvern 
  is 
  very 
  coarse 
  in 
  grain, 
  and 
  the 
  shear-lenticles 
  are 
  

   normally 
  short, 
  thick 
  in 
  the 
  middle, 
  and 
  irregular, 
  so 
  that 
  the 
  

   gneissic 
  banding 
  is 
  also 
  more 
  or 
  less 
  irregular. 
  The 
  Anglesey 
  

   felsite, 
  however, 
  being 
  fine-grained 
  and 
  compact, 
  shears 
  under 
  

   pressure 
  into 
  very 
  regular 
  lenticles. 
  These 
  are 
  often 
  very 
  thin, 
  

   almost 
  like 
  sheets 
  of 
  paper. 
  When 
  the 
  intervening 
  shear-planes 
  

   are 
  covered 
  by 
  infiltration-products 
  and 
  the 
  laminae 
  are 
  puckered, 
  a 
  

   very 
  striking 
  and 
  beautiful 
  effect 
  is 
  produced. 
  

  

  (1) 
  Gneiss 
  of 
  Primary 
  Injection. 
  

  

  This 
  variety 
  is 
  formed 
  by 
  the 
  parallel 
  interveining 
  of 
  felsite 
  and 
  

   diorite. 
  According 
  to 
  the 
  Eev. 
  J. 
  F. 
  Blake, 
  2 
  the 
  diorite 
  is 
  intrusive 
  

   in 
  the 
  gneissic 
  rock, 
  which, 
  in 
  my 
  view, 
  has 
  been 
  formed 
  from 
  the 
  

   felsite. 
  My 
  own 
  impression 
  is 
  that 
  the 
  diorite 
  is 
  the 
  older 
  rock, 
  

   and 
  that 
  portions 
  of 
  it 
  which 
  look 
  like 
  dykes 
  are 
  really 
  included 
  

   masses. 
  The 
  sections 
  at 
  Llangaffo 
  and 
  Y 
  Graig, 
  which 
  I 
  am 
  about 
  

   to 
  describe, 
  seem 
  to 
  support 
  this 
  opinion, 
  but 
  the 
  point 
  is 
  not 
  very 
  

   material. 
  

  

  Section 
  in 
  the 
  Llangaffo 
  Railway-cutting. 
  

  

  In 
  fig. 
  1 
  (p. 
  354) 
  we 
  see 
  numerous 
  3 
  felsite-veins 
  included 
  in 
  a 
  

   dark-green 
  rock 
  of 
  somewhat 
  gneissose 
  structure, 
  passing 
  within 
  a 
  

   few 
  feet 
  into 
  foliated 
  diorite, 
  and 
  even 
  into 
  a 
  diorite 
  which 
  is 
  

   scarcely 
  foliated 
  at 
  all. 
  Mr. 
  Blake 
  truly 
  affirms 
  (loc. 
  cit.) 
  that 
  this 
  

   dark 
  rock 
  agrees 
  in 
  all 
  respects 
  with 
  the 
  well-known 
  diorite 
  at 
  

   Gaerwen. 
  The 
  felsite-veins 
  vary 
  in 
  thickness 
  from 
  several 
  inches 
  

   or 
  even 
  feet 
  to 
  the 
  tenuity 
  of 
  a 
  wafer. 
  Many 
  of 
  them 
  are 
  entirely 
  

   surrounded 
  (in 
  section) 
  by 
  the 
  diorite. 
  They 
  lie 
  in 
  the 
  direction 
  of 
  

   the 
  prevailing 
  schistosity, 
  and 
  frequently 
  run 
  out 
  into 
  sharp 
  points, 
  

   as 
  if 
  compressed. 
  

  

  Section 
  at 
  Y 
  Graig, 
  Gaerwen. 
  

  

  Further 
  evidence 
  of 
  the 
  eruptive 
  origin 
  of 
  the 
  grey 
  gneiss 
  is 
  

   seen 
  in 
  a 
  small 
  quarry 
  on 
  the 
  east 
  side 
  of 
  the 
  road 
  at 
  Y 
  Graig. 
  

   The 
  vertical 
  face 
  exposed 
  in 
  the 
  bank 
  mainly 
  consists 
  of 
  horn- 
  

   blendic 
  gneiss 
  (modified 
  diorite) 
  dipping 
  at 
  nearly 
  90°. 
  Running 
  

  

  1 
  I 
  use 
  the 
  term 
  'schist' 
  in 
  preference 
  to 
  'gneiss,' 
  since 
  felspar 
  may 
  be 
  

   sometimes 
  absent 
  in 
  this 
  type 
  of 
  rock. 
  

  

  2 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliv. 
  (1888) 
  p. 
  504. 
  

  

  3 
  Only 
  a 
  small 
  proportion 
  of 
  them 
  are 
  shown. 
  

  

  Q. 
  J.G. 
  S. 
  No. 
  211. 
  2 
  b 
  

  

  