﻿V°l« 
  53-] 
  IGNEOUS 
  BOCKS 
  IN 
  NORTH 
  PEMBROKESHIRE. 
  465 
  

  

  33. 
  Some 
  Igneous 
  Rocks 
  in 
  North 
  Pembrokeshire. 
  

   By 
  John 
  Parkinson, 
  Esq., 
  F.G.S. 
  (Read 
  June 
  9th, 
  1897.) 
  

  

  [Plate 
  XXXYI.] 
  

  

  The 
  acid 
  rocks 
  which 
  form 
  the 
  subject 
  of 
  the 
  bulk 
  of 
  the 
  

   following 
  notes 
  are 
  situated 
  at 
  the 
  eastern 
  end 
  of 
  the 
  Prescelly 
  

   Hills 
  in 
  North 
  Pembrokeshire. 
  Although 
  igneous 
  rocks 
  differing 
  

   from 
  these 
  do 
  occur 
  to 
  a 
  large 
  extent, 
  their 
  petrological 
  interest 
  is 
  

   not 
  great, 
  and 
  the 
  mention 
  made 
  of 
  them 
  will 
  be 
  brief. 
  The 
  

   Prescelly 
  range 
  has 
  a 
  general 
  trend 
  in 
  an 
  easterly 
  and 
  westerly 
  

   direction, 
  and 
  may 
  be 
  considered 
  as 
  extending 
  roughly 
  from 
  near 
  

   the 
  village 
  of 
  Rosebush 
  — 
  noteworthy 
  for 
  its 
  slate-quarries 
  — 
  to 
  the 
  

   village 
  of 
  Crymmych, 
  some 
  7 
  miles 
  to 
  the 
  west. 
  The 
  slates 
  of 
  the 
  

   country 
  are 
  coloured 
  on 
  the 
  Geological 
  Survey 
  maps 
  as 
  Llandeilo 
  ; 
  

   and 
  it 
  is 
  unfortunate 
  that 
  the 
  difficulty 
  of 
  finding 
  fossils 
  prevents 
  

   much 
  further 
  and 
  more 
  definite 
  information 
  being 
  added. 
  

  

  Although 
  apparently 
  organisms 
  are 
  sometimes 
  found 
  in 
  the 
  Rose- 
  

   bush 
  slate-quarries, 
  neither 
  my 
  endeavours 
  nor 
  those 
  of 
  the 
  men 
  

   employed 
  in 
  the 
  place 
  were 
  successful 
  during 
  my 
  visit. 
  About 
  

   J 
  mile 
  west 
  of 
  Crymmych, 
  however, 
  a 
  few 
  fragmentary 
  fossils 
  were 
  

   obtained. 
  

  

  One 
  species 
  of 
  graptolite 
  was 
  found, 
  which 
  has 
  been 
  kindly 
  

   identified 
  by 
  Miss 
  E. 
  M. 
  R. 
  Wood 
  as 
  Didymograptus 
  comp. 
  indentus, 
  

   var. 
  nanus, 
  Lapw., 
  and 
  some 
  fragments 
  of 
  at 
  least 
  one 
  species 
  of 
  

   Trinucleus, 
  which 
  Dr. 
  Woodward 
  has 
  kindly 
  allowed 
  me 
  to 
  submit 
  

   to 
  him, 
  but 
  which 
  were 
  unfortunately 
  not 
  sufficiently 
  well 
  preserved 
  

   for 
  a 
  specific 
  identification. 
  Miss 
  Wood 
  is 
  inclined 
  to 
  think 
  that 
  

   the 
  rocks 
  are 
  of 
  Lower 
  Llandeilo 
  age, 
  though 
  there 
  is 
  a 
  possibility 
  

   that 
  they 
  may 
  be 
  Lower 
  Arenig. 
  

  

  This 
  conclusion 
  is 
  interesting, 
  since 
  the 
  masses 
  of 
  acid 
  rock 
  which 
  

   occur 
  at 
  the 
  places 
  marked 
  Eoel 
  Trigarn 
  and 
  Cam 
  Alw 
  show 
  all 
  the 
  

   characteristics 
  of 
  a 
  true 
  lava-flow. 
  Those 
  two 
  localities 
  are 
  about 
  

   1} 
  mile 
  apart, 
  and 
  although 
  the 
  development 
  of 
  the 
  rock 
  at 
  either 
  

   is 
  but 
  small, 
  yet 
  there 
  are 
  a 
  few 
  points 
  which 
  may 
  be 
  of 
  interest. 
  

  

  As 
  regards 
  the 
  general 
  characteristics 
  of 
  the 
  rocks, 
  slides 
  cut 
  

   from 
  two 
  specimens 
  obtained 
  at 
  Foel 
  Trigarn 
  may 
  be 
  taken 
  as 
  

   examples. 
  

  

  The 
  first 
  of 
  these, 
  which 
  is 
  quarried 
  for 
  road-metal 
  on 
  the 
  northern 
  

   side 
  of 
  the 
  hill, 
  is 
  a 
  compact, 
  somewhat 
  light 
  blue-grey 
  rock, 
  often 
  

   conspicuously 
  banded, 
  and 
  weathering 
  to 
  a 
  deep 
  brown. 
  In 
  thin 
  

   section 
  it 
  is 
  seen 
  to 
  be 
  cryptocrystalline, 
  with 
  occasional 
  larger 
  

   grains 
  of 
  irregular 
  shape 
  and 
  brighter 
  polarization-tints. 
  The 
  slide 
  

   contains 
  pyrite 
  in 
  some 
  abundance, 
  with 
  a 
  little 
  haematite 
  and 
  also 
  

   flakes 
  of 
  opacite 
  arranged 
  in 
  a 
  manner 
  suggestive 
  of 
  flow. 
  

  

  A 
  variety 
  of 
  this 
  rock 
  from 
  the 
  southern 
  side 
  of 
  the 
  hill 
  is 
  porphy- 
  

   ritic, 
  of 
  a 
  uniform 
  slate-grey 
  colour, 
  weathers 
  in 
  a 
  very 
  slaggy 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  212. 
  2 
  i 
  

  

  