﻿^ 
  r 
  °^ 
  53*] 
  ANNIVERSARY 
  ADDRESS 
  OF 
  THE 
  PRESIDENT. 
  lxXlii 
  

  

  be 
  doubtless 
  fairly 
  representative 
  of 
  the 
  majority. 
  Moreover, 
  an 
  

   examination 
  of 
  the 
  pre-Cambrian 
  rocks 
  which 
  are 
  now 
  found 
  

   exposed 
  tends 
  strongly 
  to 
  the 
  same 
  conclusion. 
  There 
  was 
  in 
  

   Pembrokeshire 
  a 
  typical 
  volcanic 
  group 
  which 
  had 
  undergone 
  a 
  

   considerable 
  amount 
  of 
  structural 
  change 
  in 
  pre-Cambrian 
  times, 
  

   and 
  before 
  any 
  of 
  the 
  fragments 
  in 
  the 
  Cambrian 
  conglomerates 
  had 
  

   been 
  broken 
  off. 
  Crushings 
  and 
  cleavage 
  had 
  produced 
  brecciated 
  

   and 
  schistose 
  structures, 
  and 
  along 
  many 
  fractured 
  lines 
  intrusions 
  

   of 
  basic 
  and 
  acid 
  rocks 
  had 
  taken 
  place. 
  There 
  seems 
  to 
  be 
  evidence 
  

   also 
  that 
  the 
  volcanic 
  outbursts 
  broke 
  through 
  a 
  floor 
  mainly 
  com- 
  

   posed 
  of 
  granitic 
  rocks 
  associated 
  probably 
  with 
  crystalline 
  schists, 
  

   for 
  in 
  the 
  lowest 
  of 
  the 
  agglomerates 
  fragments 
  of 
  such 
  rocks 
  occur 
  

   in 
  association 
  with 
  pieces 
  of 
  lava 
  showing 
  definite 
  flow-structure, 
  

   which 
  appear 
  as 
  if 
  they 
  had 
  been 
  torn 
  off 
  during 
  the 
  explosions. 
  

   Though 
  basic 
  and 
  acid 
  fragments 
  occur 
  in 
  the 
  lowest 
  agglomerates, 
  

   the 
  latter 
  certainly 
  are 
  greatly 
  in 
  excess, 
  and 
  the 
  immediately 
  

   succeeding 
  flows 
  and 
  breccias 
  are 
  mainly 
  of 
  an 
  acid 
  character. 
  

   Higher 
  in 
  the 
  series 
  the 
  basic 
  rocks 
  predominate, 
  but 
  there 
  again 
  

   some 
  acid 
  flows 
  are 
  intercalated 
  with 
  them. 
  The 
  more 
  acid 
  rocks 
  

   therefore 
  are, 
  as 
  I 
  formerly 
  suggested, 
  if 
  we 
  take 
  them 
  as 
  a 
  whole, 
  

   the 
  oldest 
  igneous 
  rocks 
  in 
  Pembrokeshire. 
  The 
  oldest 
  sediments 
  

   also, 
  the 
  porcellanites, 
  occur 
  in 
  association 
  with 
  these 
  lavas 
  and 
  

   breccias, 
  and 
  they 
  are 
  clearly 
  made 
  up 
  of 
  stratified 
  volcanic 
  dust 
  of 
  

   an 
  acid 
  character. 
  

  

  Higher 
  up 
  there 
  are 
  slaty 
  and 
  schistose 
  beds 
  which 
  are 
  largely 
  

   composed 
  of 
  basic 
  dust. 
  The 
  slates 
  are 
  purplish 
  and 
  greenish, 
  and 
  

   it 
  is 
  in 
  these 
  that 
  I 
  found 
  traces 
  of 
  worm-tubes 
  and 
  tracks. 
  They 
  

   occur 
  in 
  narrow 
  bands 
  interstratified 
  with 
  ashes 
  and 
  breccias. 
  I 
  

   have 
  hitherto 
  searched 
  them 
  in 
  vain 
  for 
  traces 
  of 
  other 
  fossils, 
  and 
  

   it 
  seems 
  to 
  me 
  that 
  the 
  conditions 
  under 
  which 
  they 
  must 
  have 
  been 
  

   deposited 
  were 
  very 
  unfavourable 
  to 
  the 
  existence 
  of 
  organisms 
  

   other 
  than 
  burrowing 
  forms. 
  The 
  break 
  between 
  the 
  Cambrian 
  

   and 
  the 
  pre-Cambrian 
  in 
  Pembrokeshire 
  is 
  a 
  most 
  important 
  one, 
  

   and, 
  in 
  my 
  opinion, 
  there 
  can 
  be 
  no 
  doubt 
  that 
  a 
  great 
  lapse 
  of 
  

   time 
  took 
  place 
  between 
  the 
  deposition 
  of 
  the 
  highest 
  of 
  the 
  pre- 
  

   Cambrian 
  beds 
  and 
  the 
  lowest 
  of 
  the 
  Cambrian. 
  

  

  The 
  pre-Cambrian 
  rocks 
  had 
  been 
  raised 
  to 
  form 
  dry 
  land, 
  had 
  

   suffered 
  from 
  the 
  effects 
  of 
  great 
  earth-movements 
  with 
  accom- 
  

   panying 
  structural 
  changes 
  in 
  the 
  rocks, 
  had 
  been 
  exposed 
  to 
  sub- 
  

   aerial 
  influences, 
  including 
  probably 
  that 
  of 
  ice-action, 
  which 
  assisted 
  

   the 
  process 
  of 
  disintegration, 
  and 
  had 
  been 
  scored 
  by 
  valleys 
  and 
  

  

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