﻿lxXViii 
  PROCEEDINGS 
  OE 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [May 
  1897, 
  

  

  * 
  According 
  to 
  this 
  view 
  beds 
  belonging 
  to 
  many 
  different 
  horizons 
  

   in 
  geological 
  succession 
  would 
  now 
  appear 
  on 
  the 
  surface 
  faulted 
  

   against 
  the 
  pre-Cambrian 
  rocks 
  ; 
  and 
  this 
  it 
  is 
  that 
  occurs 
  wherever 
  

   they 
  can 
  be 
  examined. 
  I 
  have 
  found 
  beds 
  occupying 
  every 
  position 
  

   from 
  the 
  lowest 
  Cambrian 
  to 
  the 
  Bala 
  Beds 
  in 
  direct 
  contact 
  with 
  the 
  

   pre-Cambrian 
  rocks 
  as 
  the 
  result 
  of 
  faults. 
  The 
  constant 
  recognition 
  

   of 
  these 
  facts 
  has 
  been 
  of 
  great 
  value 
  in 
  attempting 
  to 
  unravel 
  so 
  

   difficult 
  a 
  region 
  as 
  the 
  Lleyn 
  Promontory 
  has 
  proved 
  to 
  be/ 
  

  

  That 
  the 
  pre-Cambrian 
  rocks 
  had 
  been 
  greatly 
  affected 
  by 
  lateral 
  

   or 
  thrust-movements 
  before 
  the 
  Cambrian 
  rocks 
  were 
  deposited 
  is 
  

   certain, 
  and 
  most 
  of 
  the 
  schistosity 
  and 
  cleavage 
  in 
  them 
  had 
  been 
  

   produced 
  at 
  that 
  early 
  time. 
  Before 
  I 
  pass 
  from 
  the 
  consideration 
  

   of 
  the 
  basal 
  Cambrian 
  rocks 
  in 
  Caernarvonshire, 
  where 
  they 
  are 
  

   found 
  in 
  contact 
  with 
  or 
  overlying 
  the 
  pre-Cambrian, 
  especially 
  where 
  

   this 
  is 
  of 
  igneous 
  origin, 
  I 
  may 
  once 
  more 
  call 
  attention 
  to 
  some 
  of 
  

   the 
  difficulties 
  with 
  which 
  earlier 
  observers 
  had 
  to 
  contend 
  when 
  

   attempting 
  to 
  define 
  the 
  exact 
  boundary-lines 
  between 
  the 
  formations 
  

   in 
  greatly-disturbed 
  areas. 
  The 
  crushings 
  and 
  crumplings 
  due 
  to 
  

   earth-movements, 
  sometimes 
  aided 
  by 
  faults, 
  produced 
  an 
  appear- 
  

   ance 
  of 
  conformity, 
  whilst 
  in 
  other 
  cases 
  the 
  results 
  were 
  such 
  that 
  

   experienced 
  observers 
  had 
  been 
  led 
  to 
  look 
  upon 
  the 
  broken 
  and 
  

   irregular 
  junctions 
  as 
  evidences 
  of 
  igneous 
  intrusions. 
  Here, 
  

   usually, 
  the 
  illusion 
  had 
  been 
  strengthened 
  by 
  the 
  fact 
  that 
  in 
  the 
  

   case 
  of 
  many 
  of 
  the 
  pre-Cambrian 
  rocks, 
  especially 
  the 
  granites, 
  

   much 
  decomposition 
  had 
  taken 
  place 
  in 
  them, 
  mainly 
  by 
  atmospheric 
  

   agencies, 
  before 
  the 
  overlying 
  rocks 
  had 
  been 
  deposited, 
  and 
  the 
  

   arkose 
  derived 
  from 
  the 
  granite 
  had 
  been 
  at 
  first 
  re-arranged 
  by 
  

   water 
  with 
  but 
  little 
  additional 
  matter. 
  

  

  I 
  particularly 
  called 
  attention 
  to 
  some 
  of 
  these 
  causes 
  in 
  my 
  first 
  

   paper 
  on 
  the 
  Caernarvonshire 
  rocks 
  in 
  1877, 
  as 
  may 
  be 
  seen 
  from 
  

   the 
  following 
  passage 
  : 
  — 
  ' 
  The 
  supposed 
  intrusive 
  nature 
  of 
  these 
  

   ribs, 
  and 
  the 
  apparent 
  passage 
  by 
  gradual 
  alteration 
  mentioned 
  by 
  

   various 
  observers, 
  are 
  mainly 
  due 
  to 
  the 
  fact 
  that 
  the 
  matrix 
  in 
  the 
  

   conglomerates 
  has 
  been 
  derived 
  from 
  rocks 
  immediately 
  below 
  or 
  

   from 
  similar 
  ones, 
  and 
  from 
  a 
  slight 
  subsequent 
  change 
  in 
  the 
  

   matrix, 
  due, 
  probably, 
  to 
  proximity 
  to 
  the 
  intrusive 
  dykes, 
  aided 
  by 
  

   a 
  readiness 
  perhaps 
  in 
  the 
  material 
  to 
  assume 
  this 
  change. 
  This 
  is 
  

   clearly 
  observed 
  by 
  watching 
  the 
  weathering 
  of 
  these 
  conglomerates 
  

   even 
  when 
  in 
  direct 
  contact 
  with 
  the 
  porphyritic 
  series 
  ; 
  for 
  any 
  

   apparent 
  melting-away 
  of 
  the 
  hard 
  pebble 
  is 
  shown 
  not 
  to 
  be 
  a 
  

   fact, 
  since 
  on 
  very 
  slight 
  weathering 
  the 
  pebble 
  becomes 
  easily 
  

   separable 
  from 
  the 
  matrix, 
  and 
  its 
  outline 
  is 
  as 
  perfect 
  as 
  on 
  the 
  day 
  

  

  