﻿Vol. 
  53-] 
  SOME 
  OF 
  THE 
  GNEISSES 
  OF 
  ANGLESEY. 
  357 
  

  

  That 
  the 
  foliation 
  of 
  the 
  grey 
  gneiss 
  should 
  follow 
  the 
  margins 
  

   of 
  the 
  diorite 
  is 
  not 
  difficult 
  to 
  explain. 
  When 
  the 
  rocks 
  were 
  

   sheared 
  the 
  diorite 
  was 
  a 
  dyke-like 
  mass 
  with 
  nearly 
  vertical 
  sides, 
  

   and 
  the 
  foliation 
  in 
  both 
  felsite 
  and 
  diorite 
  was 
  parallel 
  to 
  them. 
  

   A 
  vertical 
  pressure 
  subsequently 
  distorted 
  the 
  western 
  margin, 
  and 
  

   at 
  the 
  same 
  time 
  contorted 
  the 
  gneiss 
  in 
  contact, 
  compelling 
  its 
  

   foliation 
  to 
  follow 
  the 
  outlines 
  of 
  the 
  diorite. 
  

  

  The 
  distance 
  from 
  the 
  diorite 
  to 
  which 
  the 
  chlorite, 
  etc., 
  are 
  

   injected 
  varies 
  considerably. 
  Where 
  the 
  shear-planes 
  are 
  numerous 
  

   and 
  close 
  the 
  rock 
  is 
  very 
  permeable 
  to 
  liquids, 
  and 
  a 
  considerable 
  

   fringe 
  of 
  dark-lined 
  gneiss 
  is 
  the 
  result. 
  This 
  variety 
  of 
  injection- 
  

   gneiss 
  is 
  sometimes 
  seen 
  where 
  no 
  basic 
  rock 
  appears 
  in 
  proximity, 
  

   though 
  of 
  course 
  hidden 
  masses 
  of 
  it 
  may 
  be 
  near. 
  

  

  I 
  need 
  hardly 
  point 
  out 
  that 
  the 
  genesis 
  of 
  this 
  kind 
  of 
  gneissic 
  

   structure 
  in 
  the 
  felsite 
  was 
  posterior 
  to 
  its 
  consolidation, 
  since 
  

   planes 
  of 
  discontinuity 
  could 
  not 
  have 
  been 
  produced 
  in 
  a 
  fluid 
  or 
  

   plastic 
  magma. 
  It 
  is 
  further 
  to 
  be 
  noted 
  that 
  the 
  gneiss 
  is 
  some- 
  

   times 
  traversed 
  by 
  irregular 
  cracks, 
  and 
  that 
  the 
  infiltrated 
  liquids 
  

   have 
  passed 
  into 
  these 
  cracks 
  as 
  well 
  as 
  into 
  the 
  shear-planes. 
  

  

  The 
  diorite 
  also 
  must 
  have 
  been 
  in 
  a 
  solid 
  state 
  at 
  the 
  time 
  of 
  

   injection. 
  It 
  is 
  not 
  molten 
  rock 
  that 
  has 
  been 
  injected 
  into 
  the 
  

   grey 
  gneiss, 
  but 
  the 
  products 
  of 
  decomposition, 
  such 
  as 
  are 
  caused 
  

   by 
  the 
  passage 
  of 
  liquids 
  through 
  a 
  crushed 
  solid. 
  

  

  Summary. 
  

  

  1. 
  The 
  gneisses 
  described 
  in 
  this 
  paper 
  have 
  been 
  formed 
  out 
  of 
  

  

  plutonic 
  igneous 
  rocks 
  by 
  crushing 
  and 
  shearing 
  subsequently 
  

   to 
  their 
  consolidation. 
  

  

  2. 
  Occasionally 
  the 
  process 
  has 
  been 
  accompanied 
  by 
  a 
  partial 
  re- 
  

  

  melting 
  of 
  the 
  rock. 
  

  

  3. 
  The 
  most 
  interesting 
  varieties 
  of 
  gneiss 
  have 
  been 
  produced 
  

  

  where 
  there 
  has 
  been 
  a 
  complex 
  of 
  diorite 
  and 
  felsite, 
  primary 
  

   injection 
  giving 
  rise 
  to 
  banded 
  gneiss, 
  and 
  secondary 
  injection 
  

   accentuating 
  gneissic 
  structure 
  in 
  sheared 
  felsite 
  by 
  dark 
  lines. 
  

  

  4. 
  Simple 
  gneisses 
  have 
  been 
  formed 
  out 
  of 
  granite, 
  diorite, 
  and 
  

  

  felsite 
  respectively, 
  the 
  last-named 
  originating 
  the 
  well-known 
  

   grey 
  gneiss 
  of 
  Southern 
  and 
  Central 
  Anglesey. 
  

  

  5. 
  The 
  so-called 
  halleflinta 
  is 
  merely 
  the 
  felsite 
  in 
  the 
  first 
  stage 
  of 
  

  

  conversion 
  into 
  gneiss. 
  

  

  Discussion. 
  

  

  The 
  President 
  said 
  that 
  the 
  Author 
  was 
  doubtless 
  right 
  in 
  

   regarding 
  the 
  older 
  pre-Cambrian 
  rocks 
  in 
  Anglesey 
  as 
  formed 
  from 
  

   a 
  great 
  igneous 
  complex 
  ; 
  but 
  it 
  was 
  difficult 
  to 
  say 
  in 
  all 
  cases 
  how 
  

   and 
  when 
  the 
  gneissic 
  structure 
  had 
  been 
  produced. 
  As 
  to 
  the 
  

   blocks 
  of 
  diorite 
  in 
  gneiss, 
  shown 
  in 
  fig. 
  2 
  (p. 
  355), 
  he 
  thought 
  it 
  

   possible 
  that 
  they 
  might 
  be 
  broken 
  portions 
  of 
  a 
  thin 
  dyke, 
  the 
  

   result 
  of 
  pressure 
  and 
  crushing, 
  rather 
  than 
  masses 
  of 
  rock 
  caught 
  up 
  

  

  