﻿^°1' 
  53'] 
  IGNEOUS 
  ROCKS 
  IN 
  NORTH 
  PEMBROKESHIRE. 
  473 
  

  

  the 
  case 
  of 
  a 
  fragment 
  of 
  a 
  spheralite 
  embedded 
  in 
  the 
  matrix 
  and 
  

   having 
  an 
  appearance 
  by 
  no 
  means 
  suggestive 
  of 
  crush. 
  

  

  It 
  seems 
  then 
  that 
  brecciation 
  in 
  situ 
  will 
  scarcely 
  account 
  for 
  

   the 
  facts, 
  and 
  one 
  is 
  obliged 
  to 
  believe 
  that 
  this 
  is 
  either 
  a 
  case 
  of 
  

   a 
  true 
  agglomerate 
  or 
  a 
  rather 
  exceptional 
  one 
  of 
  now-brecciation. 
  

   The 
  appearance 
  in 
  thin 
  section 
  presents 
  some 
  difficulties 
  in 
  the 
  

   way 
  of 
  accepting 
  the 
  former 
  view, 
  for 
  although 
  in 
  some 
  instances 
  

   the 
  edge 
  of 
  the 
  felstone-fragments 
  is 
  well 
  defined, 
  more 
  often 
  the 
  

   boundary 
  is 
  almost 
  impossible 
  to 
  determine 
  between 
  crossed 
  nicols. 
  

   The 
  matrix, 
  if 
  such 
  it 
  may 
  be 
  called, 
  is 
  pale 
  green 
  with 
  ordinary 
  

   light, 
  having 
  low 
  polarization-tints 
  and 
  crowded 
  with 
  minute 
  clear 
  

   spaces 
  occupied 
  by 
  grains 
  of 
  quartz 
  or 
  felspar; 
  it 
  also 
  contains 
  patches 
  

   free 
  from 
  these 
  grains 
  and 
  a 
  lighter 
  green 
  in 
  colour, 
  as 
  if 
  some 
  dif- 
  

   ferentiation 
  had 
  gone 
  on 
  in 
  it, 
  but 
  the 
  outlines 
  are 
  not 
  well 
  denned, 
  

   and 
  the 
  clotting 
  appearance 
  which 
  it 
  suggests 
  is 
  not 
  specially 
  

   marked. 
  

  

  In 
  one 
  case 
  there 
  is 
  a 
  long 
  porphyritic 
  felspar 
  which 
  has 
  been 
  

   bent 
  and 
  cracked, 
  without 
  displacement 
  of 
  the 
  parts, 
  in 
  a 
  manner 
  

   suggestive 
  of 
  softening 
  and 
  heat. 
  

  

  These 
  facts, 
  together 
  with 
  the 
  absence 
  of 
  scoria, 
  seem 
  scarcely 
  

   to 
  favour 
  the 
  idea 
  of 
  an 
  agglomerate, 
  while 
  the 
  agency 
  of 
  flow- 
  

   brecciation, 
  by 
  which 
  a 
  second 
  onflow 
  of 
  lava 
  has 
  broken 
  up 
  and 
  

   separated 
  a 
  mass 
  probably 
  still 
  in 
  a 
  plastic 
  state, 
  seems 
  to 
  account 
  

   for 
  the 
  observed 
  facts, 
  tinder 
  these 
  circumstances 
  one 
  would 
  cer- 
  

   tainly 
  have 
  expected 
  to 
  find 
  more 
  evidence 
  of 
  flow-movement 
  ; 
  but 
  

   if, 
  as 
  seems 
  likely, 
  this 
  ' 
  matrix 
  ' 
  represent 
  in 
  some 
  measure 
  a 
  

   hydrated 
  glass, 
  the 
  changes 
  which 
  this 
  would 
  have 
  undergone 
  since 
  

   its 
  solidification 
  may 
  have 
  masked 
  any 
  traces 
  of 
  original 
  fluxion. 
  

  

  Specimens 
  of 
  brecciated 
  felstone 
  from 
  Pen-y-chain, 
  for 
  an 
  

   examination 
  of 
  which 
  I 
  am 
  indebted 
  to 
  Miss 
  Raisin, 
  show 
  macro- 
  

   scopically 
  considerable 
  resemblances 
  to 
  this 
  Pembrokeshire 
  rock 
  

   both 
  in 
  the 
  characters 
  of 
  ' 
  matrix' 
  and 
  fragments. 
  A 
  thin 
  section, 
  

   however, 
  shows 
  that 
  the 
  structure 
  is 
  not 
  due 
  to 
  nuxional 
  action. 
  

  

  The 
  Fragmental 
  Rocks 
  

  

  associated 
  with 
  those 
  which 
  have 
  been 
  just 
  described 
  are 
  not 
  

   sufficiently 
  remarkable 
  to 
  demand 
  anything 
  but 
  a 
  brief 
  notice. 
  One 
  

   of 
  the 
  common 
  varieties 
  consists 
  of 
  a 
  felstone-agglomerate, 
  the 
  

   chips 
  having 
  an 
  average 
  length 
  of 
  0*3 
  inch 
  and 
  a 
  breadth 
  equal 
  to 
  

   about 
  half 
  their 
  length. 
  There 
  are 
  also 
  occasional 
  fragments 
  of 
  

   more 
  basic 
  rock, 
  often 
  in 
  well-rounded 
  pebbles, 
  and 
  identical 
  with 
  

   those 
  which 
  have 
  in 
  places 
  been 
  caught 
  up 
  by 
  the 
  lavas 
  themselves. 
  

   They 
  are 
  never 
  of 
  large 
  size. 
  

  

  •Scattered 
  through 
  the 
  matrix 
  are 
  detached 
  felspars 
  and 
  quartz- 
  

   fragments, 
  with 
  a 
  great 
  quantity 
  of 
  a 
  greenish-grey, 
  filmy, 
  secondary 
  

   product, 
  together 
  with 
  which 
  occur 
  patches 
  of 
  a 
  green, 
  dichroic, 
  and 
  

   probably 
  chloritic 
  mineral, 
  showing 
  yellow 
  and 
  violet 
  polarization- 
  

   tints, 
  and 
  which 
  seems 
  minutely 
  fibrous 
  or 
  filmy. 
  It 
  is 
  soft, 
  and 
  

   can 
  be 
  easily 
  grooved 
  with 
  a 
  knife. 
  In 
  places 
  there 
  seems 
  to 
  have 
  

  

  