﻿Vol. 
  53.] 
  IGNEOUS 
  ROCKS 
  IN 
  NORTH 
  PEMBROKESHIRE. 
  4?5 
  

  

  In 
  conclusion, 
  a 
  few 
  words 
  must 
  be 
  said 
  concerning 
  the 
  diabases 
  

   or 
  dolerites, 
  which 
  are 
  by 
  far 
  the 
  most 
  conspicuous 
  igneous 
  rock 
  of 
  

   the 
  district. 
  

  

  They 
  are 
  found 
  throughout 
  the 
  greater 
  part 
  of 
  the 
  length 
  of 
  

   the 
  hills, 
  from 
  Rosebush 
  to 
  near 
  Crymmych. 
  On 
  some 
  of 
  the 
  

   southern 
  slopes 
  of 
  the 
  hills 
  near 
  the 
  former 
  village 
  the 
  outcrops 
  of 
  

   diabase 
  may 
  be 
  seen 
  to 
  make 
  a 
  high 
  angle 
  with 
  the 
  prevalent 
  

   strike 
  of 
  the 
  slates, 
  while 
  farther 
  north 
  and 
  east 
  diabase 
  is 
  found 
  

   both 
  above 
  and 
  below 
  the 
  acid 
  lava-flow 
  with 
  its 
  associated 
  frag- 
  

   mental 
  rocks. 
  Hence 
  the 
  diabase 
  must 
  be 
  considered 
  as 
  intrusive 
  

   and 
  the 
  youngest 
  igneous 
  rock 
  of 
  the 
  district. 
  There 
  is 
  also 
  a 
  

   general 
  linear 
  tendency 
  in 
  the 
  outcrops 
  which 
  suggests 
  that 
  part 
  of 
  

   the 
  intrusion 
  at 
  all 
  events 
  was 
  of 
  the 
  nature 
  of 
  a 
  laccolite. 
  

  

  As 
  a 
  typical 
  example 
  a 
  specimen 
  may 
  be 
  taken 
  from 
  Foel 
  Ervr, 
  at 
  

   the 
  western 
  end 
  of 
  the 
  range. 
  It 
  is 
  of 
  a 
  medium 
  grain, 
  of 
  a 
  general 
  

   greenish 
  tinge, 
  and 
  shows 
  distinct 
  black 
  augite-plates. 
  In 
  thin 
  

   section 
  the 
  predominating 
  mineral 
  is 
  seen 
  to 
  be 
  felspar, 
  in 
  crystals 
  

   generally 
  long 
  in 
  proportion 
  to 
  their 
  breadth 
  and 
  often 
  somewhat 
  

   decomposed. 
  These 
  felspars 
  are 
  embedded 
  in 
  the 
  plates 
  of 
  pale- 
  

   brown 
  augite. 
  The 
  more 
  fine-grained 
  varieties 
  — 
  for 
  example, 
  that 
  

   from 
  Cam 
  Ddu-fach 
  — 
  resemble 
  the 
  above 
  in 
  the 
  relations 
  of 
  their 
  

   augites 
  and 
  felspars; 
  the 
  latter 
  are 
  largely 
  converted 
  into 
  grey 
  

   saussuritic 
  products. 
  

  

  The 
  coarser 
  varieties 
  are 
  also 
  essentially 
  similar. 
  The 
  augites 
  are 
  

   rather 
  dark 
  brown 
  in 
  colour, 
  and 
  much 
  secondary 
  replacement 
  has 
  

   gone 
  on 
  in 
  the 
  felspars. 
  There 
  is, 
  however, 
  a 
  considerable 
  quantity 
  

   of 
  leucoxene 
  (ilmenite), 
  and 
  the 
  augites, 
  though 
  fairly 
  well 
  preserved 
  

   as 
  a 
  rule, 
  show 
  local 
  alteration, 
  either 
  into 
  a 
  brown 
  hornblende 
  

   giving 
  place 
  to 
  a 
  green 
  actinolitic 
  mineral, 
  which 
  itself 
  becomes 
  

   of 
  a 
  lighter 
  tint; 
  or 
  into 
  a 
  chlorite 
  — 
  apparently 
  the 
  latter 
  may 
  

   represent 
  a 
  further 
  stage 
  of 
  the 
  former 
  change. 
  As 
  in 
  the 
  other 
  

   cases, 
  the 
  felspars 
  are 
  considerably 
  altered. 
  

  

  In 
  places, 
  as, 
  for 
  , 
  example, 
  at 
  Cam 
  GyfTwy, 
  the 
  macroscopic 
  

   appearance 
  of 
  the 
  rock 
  varies 
  greatly 
  in 
  a 
  small 
  compass, 
  being 
  

   either 
  comparatively 
  light 
  in 
  colour 
  when 
  much 
  felspar 
  is 
  present, 
  

   or 
  almost 
  black 
  when 
  the 
  predominating 
  mineral 
  is 
  augite. 
  The 
  

   development 
  of 
  large, 
  irregular, 
  white 
  or 
  pink 
  felspars 
  with 
  no 
  

   definite 
  crystalline 
  shape, 
  showing 
  on 
  a 
  weathered 
  surface 
  simply 
  

   as 
  white 
  circular 
  lumps, 
  gives 
  rise 
  to 
  a 
  rather 
  handsome 
  variety. 
  

   This 
  is 
  found 
  principally 
  at 
  Cam 
  Meini. 
  As 
  in 
  former 
  cases, 
  

   the 
  felspars 
  are 
  replaced 
  by 
  saussuritic 
  products, 
  rendering 
  them 
  in 
  

   places 
  almost 
  opaque. 
  

  

  In 
  concluding 
  these 
  notes 
  I 
  wish 
  to 
  express 
  my 
  indebtedness 
  to 
  

   Prof. 
  Bonney 
  for 
  advice 
  and 
  invaluable 
  help 
  throughout 
  ; 
  to 
  Miss 
  

   Raisin 
  for 
  the 
  loan 
  of 
  slides; 
  and 
  to 
  Miss 
  E. 
  M. 
  R. 
  Wood 
  and 
  

   Dr. 
  Woodward 
  for 
  their 
  kindness 
  in 
  examining 
  the 
  fossils. 
  

  

  Postscriptum. 
  

  

  [xifter 
  the 
  meeting 
  Dr. 
  Gregory 
  suggested 
  to 
  me, 
  as 
  a 
  possible 
  

   explanation 
  of 
  the 
  incomplete 
  spherulites 
  of 
  PL 
  XXXVI, 
  fig. 
  4, 
  

  

  