﻿496 
  Geological 
  Society. 
  

  

  April 
  15th.— 
  Dr. 
  J. 
  J. 
  Harris 
  Teall, 
  M.A., 
  F.E.S., 
  

  

  Vice-President, 
  in 
  the 
  Chair. 
  

  

  The 
  following 
  communications 
  were 
  read 
  : 
  — 
  

  

  ■ 
  1. 
  ' 
  The 
  Geological 
  Structure 
  of 
  the 
  St. 
  David's 
  Area 
  (Pembroke- 
  

   shire).' 
  By 
  J". 
  Frederick 
  N. 
  Green, 
  B.A., 
  F.G.S. 
  

  

  With 
  a 
  view 
  to 
  the 
  elucidation 
  of 
  controverted 
  points 
  in 
  the 
  

   geology 
  of 
  the 
  St. 
  David's 
  area, 
  the 
  author 
  has 
  mapped 
  the 
  district 
  

   in 
  considerable 
  detail 
  on 
  the 
  6-inch 
  scale. 
  The 
  Cambrian 
  rocks, 
  

   the 
  succession 
  of 
  which 
  is 
  well 
  known, 
  were 
  first 
  traced 
  and 
  were 
  

   found 
  to 
  be 
  greatly 
  faulted. 
  The 
  faults 
  have 
  been 
  followed 
  into 
  

   the 
  underlying 
  volcanic 
  tuffs 
  (Pebidian), 
  and 
  the 
  succession 
  within 
  

   the 
  intervening 
  blocks 
  of 
  country 
  determined 
  and 
  pieced 
  together. 
  

   In 
  this 
  way 
  the 
  Pebidian 
  has 
  been 
  subdivided 
  into 
  fourteen 
  

   horizons, 
  with 
  a 
  total 
  visible 
  thickness 
  of 
  over 
  3000 
  feet. 
  The 
  

   tuffs 
  are 
  described 
  in 
  some 
  detail, 
  and 
  the 
  subdivisions 
  classified 
  

   into 
  four 
  series 
  — 
  the 
  lower 
  two 
  of 
  which 
  are 
  composed 
  chiefly 
  of 
  

   trachytic 
  pebbles 
  in 
  a 
  cbloritic 
  matrix, 
  and 
  are 
  usually 
  separated 
  by 
  

   a 
  peculiar 
  schistose 
  quartz-felspar-porphyry 
  sill. 
  The 
  third 
  series 
  

   is 
  composed 
  of 
  fragments 
  of 
  rhyolite 
  and 
  halleninta 
  in 
  a 
  silicified 
  

   matrix, 
  and 
  the 
  topmost 
  now 
  principally 
  consists 
  of 
  highly- 
  

   sheared 
  schistose 
  beds. 
  The 
  volcanic 
  fragments 
  throughout 
  the 
  

   Pebidian 
  are 
  rolled, 
  and 
  have 
  often 
  undergone 
  changes 
  before 
  deposi- 
  

   tion 
  ; 
  thus 
  the 
  tuffs 
  appear 
  to 
  be 
  mainly, 
  if 
  not 
  wholly, 
  detrital. 
  

  

  By 
  the 
  aid 
  of 
  this 
  succession, 
  an 
  unconformity 
  between 
  the 
  

   Pebidian 
  and 
  the 
  Cambrian 
  is 
  demonstrated, 
  the 
  position 
  of 
  the 
  

   basal 
  Cambrian 
  conglomerate 
  on 
  the 
  volcanic 
  series 
  varying 
  by 
  

   at 
  least 
  1000 
  feet. 
  The 
  red 
  coloration 
  of 
  the 
  beds 
  immediately 
  

   underlying 
  the 
  conglomerate 
  is 
  due 
  to 
  staining. 
  

  

  The 
  schistose 
  sill 
  has 
  been 
  traced 
  into 
  the 
  porphyritic 
  margin 
  of 
  

   the 
  St. 
  David's 
  granophyre 
  (Dimetian), 
  to 
  which 
  it 
  is 
  allied 
  petro- 
  

   graphically 
  ; 
  and 
  it 
  is 
  inferred 
  that 
  the 
  granophyre 
  is 
  a 
  laccolitic 
  

   intrusion 
  in 
  the 
  Pebidian. 
  The 
  boundaries 
  between 
  the 
  granophyre 
  

   and 
  the 
  Cambrian 
  are 
  prolongations 
  of 
  faults 
  proved 
  in 
  the 
  latter 
  ; 
  

   except 
  at 
  one 
  point 
  in 
  the 
  well-known 
  Porthclais 
  district, 
  which, 
  

   on 
  account 
  of 
  its 
  importance, 
  has 
  been 
  mapped 
  on 
  the 
  scale 
  of 
  

   25 
  inches 
  to 
  the 
  mile. 
  A 
  trench 
  specially 
  opened 
  at 
  this 
  point 
  

   exposed 
  basal 
  Cambrian 
  rocks 
  resting 
  upon 
  a 
  denuded 
  surface 
  of 
  

   the 
  granophyre, 
  which 
  is 
  therefore 
  of 
  pre-Cambrian 
  but 
  of 
  post- 
  

   Pebidian 
  age. 
  

  

  The 
  relationships 
  of 
  the 
  basic 
  igneous 
  rocks 
  west 
  of 
  St. 
  David's, 
  

   which 
  have 
  hitherto 
  been 
  held 
  to 
  be 
  Pebidian 
  lavas, 
  are 
  discussed, 
  

   and 
  the 
  conclusion 
  is 
  reached 
  that 
  they 
  are 
  all 
  post-Cambrian 
  

   intrusions. 
  Finally, 
  it 
  is 
  suggested 
  that 
  the 
  word 
  Pebidian 
  

   should 
  be 
  revived 
  as 
  a 
  general 
  term. 
  

  

  2.' 
  Notes 
  on 
  the 
  Geology 
  of 
  Burma.' 
  By 
  Leonard 
  Y. 
  Dalton, 
  

   B.Sc, 
  F.R.G.S. 
  

  

  