﻿lxxii 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [May 
  1 
  89 
  7, 
  

  

  he 
  says 
  : 
  — 
  ' 
  The 
  view 
  that 
  the 
  Cambrian 
  conglomerate 
  at 
  St. 
  David's 
  

   encloses 
  much 
  waterworn 
  debris 
  of 
  the 
  Dimetian 
  [i. 
  e. 
  the 
  granite 
  

   of 
  St. 
  David's] 
  is, 
  I 
  think, 
  fully 
  justified 
  by 
  the 
  evidence 
  now 
  

   adduced 
  from 
  the 
  examination 
  of 
  many 
  slides 
  of 
  this 
  rock, 
  few 
  of 
  

   which 
  have 
  failed 
  to 
  afford 
  evidence 
  of 
  the 
  presence, 
  not 
  only 
  of 
  

   pebbles 
  of 
  a 
  rock 
  which 
  under 
  the 
  microscope 
  could 
  not 
  be 
  dis- 
  

   tinguished 
  from 
  it, 
  but 
  also 
  of 
  its 
  individual 
  mineral 
  constituents. 
  

   The 
  slides 
  examined 
  and 
  described 
  here 
  are 
  not 
  selected 
  ones, 
  but 
  

   have 
  been 
  taken 
  as 
  they 
  were 
  cut.' 
  l 
  

  

  In 
  1886 
  Prof. 
  Bonney 
  kindly 
  examined 
  many 
  slides 
  submitted 
  

   by 
  me 
  to 
  him, 
  prepared 
  from 
  Cambrian 
  conglomerates 
  obtained 
  

   from 
  different 
  areas 
  in 
  Pembrokeshire, 
  and 
  at 
  p. 
  362, 
  Quart. 
  Journ. 
  

   Geol. 
  Soc. 
  vol. 
  xlii. 
  he 
  says 
  : 
  — 
  

  

  1 
  A. 
  When 
  the 
  Chanter's 
  Seat 
  conglomerate 
  was 
  formed 
  the 
  

   following 
  rocks 
  were 
  undergoing 
  denudation 
  : 
  — 
  

   ' 
  (1) 
  Granitoid 
  rocks, 
  identical 
  with 
  the 
  existing 
  Dimetian. 
  

   ' 
  (2) 
  Trachytic 
  rocks, 
  among 
  which 
  were 
  probably 
  true 
  lava- 
  

   flows. 
  

   * 
  (3) 
  Quartzites 
  and 
  schists, 
  the 
  latter 
  resembling 
  those 
  which 
  in 
  

  

  many 
  districts 
  occur 
  rather 
  high 
  in 
  the 
  Archaean 
  series. 
  

   1 
  (4) 
  Ordinary 
  sedimentary 
  rocks. 
  

  

  1 
  Hence 
  there 
  was 
  in 
  this 
  district 
  a 
  series 
  of 
  rocks, 
  some 
  much 
  

   older 
  than 
  others, 
  which 
  contributed 
  to 
  the 
  formation 
  of 
  the 
  

   Cambrian 
  conglomerate. 
  

  

  1 
  B. 
  The 
  conglomerate 
  above 
  the 
  Trefgarn 
  series 
  is 
  formed 
  from 
  

   rocks 
  which 
  occur 
  in 
  the 
  latter. 
  

  

  ' 
  C. 
  The 
  peculiar 
  characteristics 
  distinctive 
  of 
  certain 
  members 
  

   of 
  the 
  Trefgarn 
  series 
  had 
  been 
  assumed 
  by 
  them 
  when 
  the 
  con- 
  

   glomerate 
  was 
  formed. 
  

  

  ' 
  D. 
  Either 
  the 
  Dimetian 
  is 
  a 
  member 
  of 
  an 
  old 
  gneissoid 
  series 
  

   or, 
  if 
  it 
  is 
  the 
  core 
  of 
  a 
  volcanic 
  group 
  from 
  which 
  the 
  trachytic 
  

   lavas 
  had 
  been 
  ejected, 
  this 
  had 
  been 
  laid 
  bare 
  by 
  denudation 
  before 
  

   the 
  Cambrian 
  conglomerate 
  was 
  formed. 
  Hence 
  in 
  either 
  case 
  both 
  

   the 
  Dimetian 
  and 
  the 
  felstones 
  are 
  pre-Cambrian.' 
  

  

  These 
  notes 
  by 
  Mr. 
  T. 
  Davies 
  and 
  Prof. 
  Bonney 
  enable 
  us 
  to 
  

   realize 
  with 
  fair 
  accuracy 
  what 
  rocks 
  composed 
  the 
  pre-Cambrian 
  

   land 
  in 
  Pembrokeshire 
  when 
  the 
  oldest 
  of 
  the 
  Cambrian 
  rocks 
  in 
  

   that 
  area 
  were 
  deposited. 
  Of 
  course 
  it 
  is 
  possible 
  that 
  there 
  were 
  

   other 
  rocks 
  in 
  the 
  area 
  than 
  those 
  hitherto 
  found 
  as 
  fragments 
  in 
  

   the 
  conglomerates, 
  but 
  we 
  have 
  in 
  these 
  lists 
  all 
  such 
  as 
  would 
  

   1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884) 
  p. 
  555. 
  

  

  