﻿350 
  DR. 
  C. 
  CALLAWAY 
  ON 
  THE 
  ORIGIN 
  OF 
  [Aug. 
  1 
  89 
  7, 
  

  

  whether 
  they 
  are 
  to 
  be 
  referred 
  to 
  one, 
  two, 
  or 
  three 
  epochs 
  remains 
  

   still 
  in 
  dispute. 
  

  

  My 
  own 
  views 
  as 
  to 
  the 
  broad 
  outlines 
  remain 
  unchanged 
  after 
  

   many 
  years' 
  intermittent 
  study 
  of 
  Anglesey 
  geology. 
  The 
  newer 
  

   series 
  I 
  still 
  regard 
  as 
  Pebidian, 
  and 
  I 
  have 
  nothing 
  material 
  to 
  alter 
  

   in 
  the 
  details 
  of 
  the 
  distribution 
  of 
  this 
  group 
  and 
  the 
  succession 
  of 
  

   its 
  members. 
  The 
  gneissic 
  rocks 
  belong, 
  I 
  have 
  no 
  doubt, 
  to 
  an 
  older 
  

   epoch 
  ; 
  but 
  I 
  now 
  believe 
  them 
  to 
  be 
  an 
  igneous 
  complex, 
  which 
  

   has 
  undergone 
  great 
  chemical 
  and 
  structural 
  changes 
  under 
  the 
  

   influence 
  of 
  pressure 
  and 
  heat. 
  In 
  these 
  rocks 
  I 
  once 
  described 
  a 
  

   succession-in-time, 
  which 
  must 
  of 
  course 
  be 
  abandoned. 
  

  

  In 
  this 
  paper 
  I 
  shall 
  not 
  touch 
  upon 
  the 
  Pebidian 
  rocks, 
  or 
  

   attempt 
  an 
  exhaustive 
  study 
  of 
  the 
  older 
  masses 
  ; 
  but 
  will 
  confine 
  

   myself 
  to 
  a 
  description 
  of 
  the 
  production 
  of 
  the 
  gneissic 
  structure 
  

   in 
  some 
  very 
  interesting 
  sections 
  in 
  the 
  south 
  of 
  the 
  island. 
  This 
  

   will, 
  I 
  hope, 
  be 
  of 
  service 
  in 
  clearing 
  the 
  ground 
  for 
  further 
  

   research. 
  

  

  I. 
  The 
  Materials 
  out 
  of 
  which 
  the 
  Gneisses 
  are 
  formed. 
  

  

  These 
  gneisses 
  have 
  been 
  produced 
  out 
  of 
  a 
  granite, 
  a 
  felsite, 
  and 
  

   a 
  diorite, 
  either 
  separately 
  or 
  in 
  combination. 
  

  

  The 
  Granite. 
  — 
  This 
  rock 
  has 
  been 
  described 
  by 
  Prof. 
  Bonney 
  l 
  as 
  

   consisting 
  mainly 
  of 
  quartz 
  and 
  felspar. 
  It 
  is 
  clearly 
  of 
  igneous 
  

   origin, 
  for, 
  on 
  the 
  shore 
  near 
  Porth 
  JNobla, 
  masses 
  and 
  veins 
  of 
  it 
  

   are 
  seen 
  to 
  penetrate 
  the 
  adjacent 
  rocks. 
  It 
  will 
  be 
  but 
  slightly 
  

   noticed 
  in 
  this 
  paper. 
  

  

  The 
  Felsite. 
  — 
  This 
  is 
  in 
  part 
  the 
  Arvonian 
  of 
  Dr. 
  Hicks. 
  Great 
  

   difficulty 
  has 
  attended 
  the 
  study 
  of 
  this 
  rock, 
  owing 
  to 
  the 
  enormous 
  

   mechanical 
  pressures 
  to 
  which 
  the 
  igneous 
  complex 
  has 
  been 
  

   subjected. 
  Ordinarily 
  it 
  presents 
  the 
  appearance 
  of 
  the 
  rock 
  which 
  

   has 
  been 
  vaguely 
  described 
  as 
  ' 
  halleflinta.' 
  In 
  hand-specimens 
  it 
  

   is 
  seen 
  to 
  be 
  fine 
  in 
  grain 
  and 
  of 
  uniform 
  texture, 
  more 
  like 
  a 
  fel- 
  

   spathic 
  mud 
  than 
  an 
  eruptive 
  rock. 
  A 
  specimen 
  of 
  it 
  was 
  described 
  

   by 
  Prof. 
  Bonney 
  2 
  in 
  1881 
  as 
  similar 
  to 
  the 
  Treftgarn 
  (St. 
  David's) 
  

   halleflinta, 
  but 
  as 
  ' 
  more 
  gneissic 
  ' 
  under 
  the 
  microscope. 
  At 
  that 
  

   date 
  he 
  leaned 
  to 
  the 
  opinion 
  that 
  the 
  Anglesey 
  halleflinta 
  3 
  is 
  of 
  

   ' 
  fragmental 
  origin,' 
  but 
  admitted 
  that 
  the 
  structure 
  sometimes 
  comes 
  

   ' 
  nearer 
  to 
  that 
  of 
  some 
  microcrystalline 
  felsites.' 
  Mr. 
  Blake 
  

   considers 
  4 
  the 
  halleflinta 
  of 
  Roche 
  Castle 
  (St. 
  David's) 
  to 
  be 
  an 
  

   altered 
  andesite, 
  and 
  probably 
  few 
  will 
  now 
  dispute 
  that 
  halleflinta 
  

   is 
  often 
  an 
  eruptive. 
  The 
  subsequent 
  study 
  of 
  a 
  large 
  number 
  of 
  

   the 
  Anglesey 
  slides 
  has 
  convinced 
  me 
  that 
  the 
  fragmental 
  structure 
  

   is 
  merely 
  hypoclastic, 
  and 
  is 
  the 
  result 
  of 
  crushing, 
  being 
  in 
  fact 
  an 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxv. 
  (1879) 
  pp. 
  307, 
  308, 
  nos. 
  vii, 
  ix, 
  & 
  xi. 
  

  

  2 
  Ibid. 
  vol. 
  xxxvii. 
  (1881) 
  p. 
  233. 
  

  

  3 
  This 
  rock 
  must 
  not 
  be 
  confounded 
  with 
  the 
  quartz-felsites 
  associated 
  with 
  

   the 
  granite, 
  which 
  are 
  truly 
  a 
  part 
  of 
  the 
  granitic 
  intrusion, 
  and 
  often 
  imper- 
  

   ceptibly 
  grade 
  ho 
  to 
  the 
  granite. 
  

  

  4 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884) 
  p. 
  308. 
  

  

  