﻿Vol. 
  53.] 
  SOME 
  OF 
  THE 
  GNEISSES 
  OF 
  ANGLESEY. 
  351 
  

  

  early 
  stage 
  in 
  the 
  conversion 
  of 
  felsite 
  into 
  schist. 
  Evidence 
  of 
  this 
  

   conclusion 
  will 
  be 
  offered 
  further 
  on. 
  

  

  The 
  Diorite. 
  — 
  This 
  rock 
  is 
  now 
  well 
  known. 
  Near 
  Gaerwen 
  it 
  

   is 
  modified 
  into 
  hornblendic 
  and 
  chloritic 
  gneiss, 
  as 
  I 
  showed 
  x 
  in 
  

   1887. 
  Mr. 
  Blake 
  2 
  has 
  confirmed 
  and 
  extended 
  this 
  result. 
  

  

  II. 
  The 
  Pkoducts 
  of 
  the 
  Metamorphism. 
  

  

  In 
  a 
  series 
  of 
  papers 
  3 
  on 
  the 
  Malvern 
  crystallines, 
  I 
  have 
  

   expounded 
  the 
  changes 
  which, 
  in 
  that 
  area, 
  take 
  place 
  in 
  plutonic 
  

   rocks 
  during 
  dynamo-metamorphism. 
  It 
  was 
  seen 
  that 
  schists 
  

   and 
  gneisses 
  are 
  formed 
  sometimes 
  by 
  pressure 
  acting 
  upon 
  a 
  

   single 
  rock, 
  but 
  the 
  most 
  important 
  results 
  were 
  shown 
  to 
  be 
  pro- 
  

   duced 
  in 
  diorite 
  interlaced 
  by 
  granite-veins. 
  Similar 
  results 
  have 
  

   been 
  observed 
  by 
  me 
  in 
  Anglesey. 
  Diorite 
  alone 
  is 
  converted 
  into 
  

   various 
  gneisses, 
  felsite 
  alone 
  is 
  changed 
  to 
  mica-gneiss 
  ; 
  while 
  

   extremely 
  interesting 
  gneisses 
  of 
  another 
  type 
  are 
  formed 
  where 
  

   diorite 
  is 
  interveined 
  with 
  felsite. 
  

  

  The 
  processes 
  of 
  metamorphism 
  in 
  Anglesey 
  bear 
  so 
  close 
  a 
  

   resemblance 
  to 
  those 
  which 
  have 
  operated 
  at 
  Malvern 
  that 
  the 
  

   classification 
  of 
  gneisses 
  and 
  schists 
  adopted 
  in 
  my 
  researches 
  in 
  the 
  

   last-named 
  area 
  will 
  be 
  usually 
  applicable 
  in 
  this 
  paper. 
  

  

  A. 
  Simple 
  Schists, 
  or 
  those 
  formed 
  from 
  one 
  kind 
  of 
  rock. 
  

  

  Mica- 
  gneiss 
  from 
  Granite. 
  — 
  This 
  change 
  is 
  sometimes 
  seen 
  near 
  

   the 
  margin 
  of 
  the 
  central 
  granite-band, 
  but 
  it 
  is 
  of 
  the 
  ordinary 
  

   kind, 
  and 
  need 
  not 
  be 
  described 
  here. 
  

  

  Hornblendic 
  and 
  Chloritic 
  Gneisses 
  from 
  Diorite. 
  — 
  These 
  types 
  

   have 
  been 
  already 
  noticed, 
  and 
  references 
  to 
  descriptions 
  of 
  their 
  

   origin 
  have 
  been 
  given. 
  

  

  Mica-gneiss 
  from 
  Felsite. 
  — 
  A 
  fine 
  section 
  showing 
  this 
  change 
  is 
  

   exposed 
  along 
  the 
  western 
  end 
  of 
  a 
  crag 
  in 
  a 
  field 
  at 
  Y 
  Graig, 
  near 
  

   Gaerwen, 
  at 
  a 
  little 
  distance 
  from 
  the 
  gate 
  leading 
  into 
  it 
  from 
  the 
  

   road 
  from 
  Gaerwen. 
  The 
  rock 
  is 
  very 
  sound 
  and 
  clean, 
  and 
  the 
  trans- 
  

   ition 
  can 
  be 
  traced 
  inch 
  by 
  inch 
  without 
  any 
  break 
  caused 
  by 
  frac- 
  

   ture 
  or 
  turf- 
  covering. 
  At 
  the 
  northern 
  end 
  is 
  felsite, 
  which 
  passes 
  

   gradatim 
  through 
  the 
  intermediate 
  form 
  of 
  halleflinta 
  into 
  a 
  typical 
  

   mica-gneiss. 
  At 
  the 
  British 
  Association 
  Meeting 
  at 
  Manchester 
  in 
  

   1887, 
  I 
  exhibited 
  a 
  series 
  of 
  slides 
  from 
  this 
  section. 
  The 
  first 
  

   (No. 
  379 
  4 
  ) 
  of 
  the 
  set 
  was 
  admitted 
  by 
  Profs. 
  Bonney 
  and 
  Eenard, 
  

   and 
  by 
  Mr. 
  J. 
  J. 
  H. 
  Teall, 
  to 
  be 
  a 
  true 
  felsite. 
  Under 
  the 
  microscope, 
  

   it 
  polarizes 
  as 
  an 
  aggregate 
  of 
  granules 
  of 
  various 
  sizes, 
  with 
  foliate 
  

   interlocking 
  margins. 
  Several 
  crystals 
  of 
  felspar, 
  some 
  of 
  them 
  

   plagioclase, 
  are 
  scattered 
  through 
  the 
  groundmass. 
  A 
  few 
  of 
  

   them 
  have 
  been 
  broken, 
  and 
  the 
  parts 
  shifted, 
  and 
  the 
  granular 
  

  

  1 
  Brit. 
  Assoc. 
  Kep. 
  (Manchester) 
  1887, 
  p. 
  706. 
  

  

  2 
  Ibid. 
  (Bath) 
  1888, 
  p. 
  405. 
  

  

  3 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliii. 
  (1 
  887) 
  p. 
  525 
  ; 
  ibid. 
  vol. 
  xlv. 
  (1 
  889) 
  p. 
  475 
  ; 
  

   ibid. 
  vol. 
  xlix. 
  (1893) 
  p. 
  398 
  ; 
  Geol. 
  Mag. 
  1892, 
  p. 
  545 
  ; 
  ibid. 
  1893, 
  p. 
  535; 
  ibid. 
  

   1894, 
  p. 
  217 
  ; 
  & 
  ibid. 
  1895, 
  p. 
  220. 
  

  

  4 
  The 
  numbers 
  in 
  parentheses 
  are 
  those 
  of 
  the 
  slides 
  in 
  my 
  cabinet. 
  

  

  