﻿Yol. 
  53.] 
  OEIGIN 
  OF 
  SOME 
  OF 
  THE 
  GNEISSES 
  OE 
  ANGLESEY. 
  349 
  

  

  26. 
  On 
  the 
  Okigin 
  of 
  some 
  of 
  the 
  Gneisses 
  of 
  Anglesey. 
  By 
  

  

  Chaeles 
  Callaway, 
  H.A., 
  D.Sc, 
  F.G.S. 
  (Eead 
  April 
  28th, 
  

  

  1897.) 
  

  

  Contents. 
  

  

  Page 
  

   Introduction 
  349 
  

  

  I. 
  The 
  Materials 
  out 
  of 
  which 
  the 
  Gneisses 
  are 
  formed 
  350 
  

  

  The 
  Granite. 
  The 
  Felsite. 
  The 
  Diorite. 
  

  

  II. 
  The 
  Products 
  of 
  the 
  Metamorphism 
  351 
  

  

  A. 
  Simple 
  Schists. 
  Mica-gneiss 
  from 
  Granite. 
  Horn- 
  

   blendic 
  and 
  Chloritic 
  Gneisses 
  from 
  Diorite. 
  Mica- 
  

   gneiss 
  from 
  Felsite 
  351 
  

  

  P. 
  Injection-Schists 
  353 
  

  

  (1) 
  Gneiss 
  of 
  Primary 
  Injection. 
  Various 
  Sections. 
  

  

  Diorite-blocks 
  in 
  the 
  Grey 
  Gneiss. 
  

  

  (2) 
  Gneiss 
  of 
  Secondary 
  Injection. 
  

  

  Introduction. 
  

  

  The 
  comparative 
  ages 
  of 
  the 
  subdivisions 
  of 
  the 
  schistose 
  and 
  

   gneissic 
  rocks 
  of 
  Anglesey 
  have 
  been 
  discussed 
  with 
  very 
  varied 
  

   conclusions, 
  but 
  these 
  discrepancies 
  have 
  been 
  in 
  great 
  part 
  the 
  

   result 
  of 
  the 
  unsettled 
  state 
  of 
  opinion 
  on 
  the 
  subject 
  of 
  meta- 
  

   morphism. 
  The 
  former 
  belief 
  that 
  the 
  granite 
  was 
  of 
  metamorphic 
  

   origin, 
  and 
  that 
  a 
  parallel 
  structure 
  necessarily 
  indicated 
  an 
  original 
  

   sedimentation, 
  could 
  not 
  fail 
  to 
  mislead 
  the 
  stratigraphical 
  geologist. 
  

   It 
  is 
  therefore 
  requisite 
  that 
  the 
  genesis 
  of 
  the 
  respective 
  rocks 
  

   should 
  be 
  determined 
  before 
  attempting 
  to 
  revise 
  the 
  old 
  classifications. 
  

   Before 
  entering 
  upon 
  my 
  task, 
  I 
  wish 
  to 
  indicate 
  briefly 
  the 
  

   present 
  state 
  of 
  our 
  knowledge 
  of 
  the 
  Archaean 
  (pre-Cambrian) 
  

   rocks 
  of 
  Anglesey. 
  Dr. 
  H. 
  Hicks, 
  F.R.S., 
  divided 
  them 
  into 
  three 
  

   groups 
  : 
  — 
  Dimetian 
  (granite), 
  Arvonian 
  (halleflinta), 
  and 
  Pebidian 
  

   (schistose), 
  these 
  divisions 
  being 
  named 
  in 
  ascending 
  order. 
  1 
  I 
  have 
  

   never 
  been 
  able 
  to 
  define 
  more 
  than 
  two 
  systems, 
  an 
  older 
  one, 
  

   which, 
  to 
  avoid 
  theory, 
  I 
  simply 
  designated 
  as 
  ' 
  Gneissic,' 
  and 
  a 
  

   newer 
  one, 
  which 
  I 
  described 
  as 
  ' 
  Slaty,' 
  and 
  provisionally 
  identified 
  

   with 
  the 
  Pebidian. 
  2 
  The 
  Rev. 
  J. 
  F. 
  Blake, 
  F.G.S., 
  admits 
  the 
  

   pre-Cambrian 
  age 
  of 
  all 
  these 
  rocks, 
  but 
  he 
  describes 
  them 
  as 
  one 
  

   continuous 
  series, 
  which 
  he 
  calls 
  ' 
  Monian.' 
  3 
  Prof. 
  T. 
  G. 
  Bonney, 
  

   F.R.S., 
  also 
  regards 
  the 
  crystalline 
  schists 
  as 
  Archaean, 
  4 
  but 
  Sir 
  A. 
  

   Geikie, 
  5 
  E.B.S., 
  and 
  Prof. 
  T. 
  M 
  C 
  K. 
  Hughes, 
  6 
  F.R.S., 
  place 
  some 
  

   of 
  the 
  less 
  altered 
  schists 
  at 
  a 
  higher 
  horizon. 
  The 
  pre-Cambrian 
  

   age 
  of 
  the 
  bulk 
  of 
  the 
  granitic, 
  gneissic, 
  and 
  schistose 
  rocks 
  of 
  

   Anglesey 
  may 
  therefore 
  be 
  regarded 
  as 
  a 
  settled 
  question; 
  but 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxv. 
  (1879) 
  p. 
  302. 
  

  

  2 
  Ibid. 
  vol. 
  xxxvii. 
  (1881) 
  p. 
  211. 
  

  

  3 
  Ibid. 
  vol. 
  xliv. 
  (1888) 
  p. 
  463. 
  

  

  4 
  Ibid. 
  vol. 
  xxxv. 
  (1879) 
  pp. 
  303, 
  304. 
  

   6 
  Ibid. 
  vol. 
  xlvii. 
  (1891) 
  Proc. 
  p. 
  130. 
  

  

  6 
  Proc. 
  Phil. 
  Soc. 
  Cambridge, 
  vol. 
  iii. 
  (1880) 
  p. 
  347. 
  

  

  