﻿Vol. 
  $$.'] 
  IGNEOUS 
  ROCKS 
  IN 
  NORTH 
  PEMBROKESHIRE. 
  471 
  

  

  those 
  just 
  described. 
  The 
  evidences 
  of 
  flow 
  seem 
  rather 
  stronger 
  

   on 
  the 
  whole, 
  and 
  apparently 
  the 
  disturbance 
  has 
  taken 
  place 
  

   entirely 
  after 
  the 
  formation 
  of 
  the 
  spherulites. 
  In 
  the 
  case 
  of 
  the 
  

   Prescelly 
  rock 
  the 
  structure 
  of 
  the 
  slides 
  indicates 
  that 
  movement 
  

   has 
  accompanied 
  spherulitic 
  growth, 
  although 
  in 
  places 
  here 
  and 
  

   there 
  one 
  or 
  the 
  other 
  may 
  appear 
  to 
  have 
  been 
  the 
  stronger. 
  

  

  A 
  second 
  division 
  of 
  incomplete 
  spherulites 
  includes 
  those 
  in 
  

   which 
  flow-brecciation, 
  accompanied 
  by 
  but 
  very 
  feeble 
  radial 
  

   growth, 
  has 
  resulted 
  in 
  the 
  formation 
  of 
  globular 
  bodies 
  sometimes 
  

   nearly 
  a 
  inch 
  in 
  diameter; 
  possessing 
  a 
  complete 
  discontinuity 
  

   from, 
  and 
  showing 
  a 
  striking 
  contrast 
  to, 
  the 
  matrix 
  in 
  which 
  they 
  

   lie. 
  As 
  is 
  common 
  in 
  the 
  neighbouring 
  rock, 
  the 
  specimen 
  is 
  

   marked 
  by 
  lighter 
  bands, 
  the 
  appearance 
  of 
  which 
  is 
  identical 
  with 
  

   that 
  of 
  the 
  nodules 
  (if 
  the 
  term 
  be 
  allowed), 
  so 
  that 
  it 
  scarcely 
  needs 
  

   a 
  microscopic 
  examination 
  to 
  see 
  that 
  the 
  causes 
  which 
  have 
  

   produced 
  the 
  former 
  must 
  have 
  been 
  nearly 
  concerned 
  in 
  the 
  

   formation 
  of 
  the 
  latter 
  also. 
  

  

  In 
  hand-specimens 
  the 
  nodules 
  much 
  resemble 
  true 
  spherulites, 
  

   standing 
  out 
  prominently 
  on 
  a 
  fractured 
  surface, 
  and 
  leaving 
  an 
  oval 
  

   cavity 
  on 
  removal. 
  In 
  thin 
  section 
  (PI. 
  XXXVI, 
  fig. 
  6) 
  they 
  are 
  

   distinguished 
  from 
  the 
  matrix 
  by 
  their 
  browner 
  colour 
  and 
  consequent 
  

   greater 
  opacity, 
  while 
  under 
  crossed 
  nicols 
  they 
  show 
  irregular 
  patches 
  

   of 
  light 
  with 
  indefinite 
  wavy 
  clouds 
  of 
  shadow 
  as 
  the 
  stage 
  is 
  rotated, 
  

   again 
  in 
  contrast 
  to 
  the 
  body 
  of 
  the 
  rock, 
  which 
  is 
  micro- 
  and 
  crypto- 
  

   crystalline 
  (often 
  quite 
  coarse), 
  and 
  which 
  contains 
  a 
  green 
  chloritic 
  

   mineral 
  in 
  some 
  quantity. 
  The 
  ' 
  nodules 
  ' 
  are 
  often 
  elongated, 
  and 
  

   are 
  connected 
  one 
  to 
  another 
  in 
  the 
  manner 
  which 
  is 
  usual 
  in 
  those 
  

   portions 
  of 
  a 
  rock 
  which 
  have 
  become 
  differentiated 
  from 
  their 
  

   surroundings 
  and 
  influenced 
  by 
  flow-movements, 
  since 
  occasionally 
  

   circular, 
  but 
  more 
  frequently 
  elliptical 
  or 
  irregularly 
  oval, 
  outlines 
  

   are 
  found, 
  which 
  often 
  taper 
  to 
  a 
  point. 
  

  

  There 
  is 
  in 
  places 
  a 
  somewhat 
  indefinite 
  suggestion 
  of 
  radial 
  growth, 
  

   which 
  in 
  a 
  few 
  cases 
  becomes 
  more 
  pronounced. 
  Rather 
  more 
  

   often 
  there 
  is 
  a 
  tendency 
  towards 
  a 
  concentric 
  banding 
  at 
  the 
  edge 
  

   of 
  the 
  patch, 
  giving 
  the 
  idea 
  that 
  after 
  it 
  had 
  assumed 
  its 
  

   present 
  shape 
  segregation 
  was 
  again 
  operative. 
  In 
  the 
  interior 
  

   of 
  these 
  ' 
  nodules 
  ' 
  there 
  are 
  irregular 
  but 
  more 
  or 
  less 
  central 
  

   quartzose 
  areas, 
  roughly 
  concentric 
  with 
  the 
  periphery. 
  As 
  regards 
  

   the 
  origin 
  of 
  these 
  masses 
  there 
  can 
  be 
  no 
  doubt 
  — 
  they 
  are 
  

   evidently 
  one 
  of 
  the 
  results 
  of 
  flow-brecciation, 
  as 
  indeed 
  has 
  been 
  

   suggested 
  in 
  connexion 
  with 
  some 
  of 
  the 
  Eoulay 
  Bay 
  pyromerides. 
  1 
  

  

  It 
  seems, 
  therefore, 
  that 
  a 
  mixing 
  of 
  two 
  materials, 
  or 
  the 
  dif- 
  

   ferentiation 
  of 
  one, 
  has 
  occurred, 
  resulting 
  after 
  brecciation 
  in 
  the 
  

   formation 
  of 
  clots 
  which 
  have 
  been 
  drawn 
  out 
  by 
  the 
  action 
  of 
  their 
  

   more 
  liquid 
  surroundings, 
  as 
  just 
  described, 
  subsequently 
  often 
  

   losing 
  their 
  connexion 
  with 
  the 
  original 
  mass. 
  The 
  quartzose 
  

   areas 
  are 
  sharply 
  defined, 
  and 
  roughly 
  follow, 
  as 
  above 
  mentioned, 
  

   the 
  outline 
  of 
  the 
  nodule, 
  with 
  occasional 
  irregularities 
  in 
  various 
  

  

  1 
  Miss 
  0. 
  A. 
  Eaisin, 
  ' 
  Nodular 
  Felsites 
  of 
  the 
  Lleyn,' 
  Quart. 
  Journ. 
  G-eol. 
  

   Soc. 
  vol. 
  xlv. 
  (1889) 
  pp. 
  264, 
  265. 
  

  

  