﻿474 
  me. 
  j. 
  pakkinson 
  on 
  some 
  [Dec. 
  1897, 
  

  

  "been 
  less 
  of 
  the 
  finer 
  dust 
  and 
  debris, 
  and 
  the 
  rock 
  is 
  then 
  of 
  a 
  lighter 
  

   colour. 
  

  

  Fine 
  ashes 
  are 
  also 
  found 
  in 
  the 
  neighbourhood 
  of 
  Foel 
  Trigarn. 
  

   These 
  in 
  thin 
  sections 
  are 
  seen 
  to 
  be 
  finely 
  granular 
  or 
  crystalline. 
  

  

  In 
  some 
  eases 
  the 
  true 
  fragmental 
  nature 
  of 
  the 
  rock 
  is 
  not 
  easy 
  

   to 
  make 
  out, 
  more 
  especially 
  as 
  in 
  places 
  it 
  has 
  been 
  considerably 
  

   crushed. 
  

  

  The 
  occurrence 
  of 
  these 
  pyroclastic 
  rocks, 
  together 
  with 
  the 
  petro- 
  

   graphical 
  evidence, 
  leaves 
  no 
  doubt 
  that 
  the 
  rocks 
  which 
  have 
  been 
  

   described 
  constitute 
  part 
  of 
  a 
  true 
  lava-flow. 
  

  

  Associated 
  principally 
  with 
  the 
  southern 
  slopes 
  of 
  the 
  hills 
  are 
  

   a 
  series 
  of 
  rather 
  more 
  basic 
  rocks, 
  of 
  somewhat 
  uniform 
  and 
  

   monotonous 
  appearauce, 
  which 
  will 
  be 
  briefly 
  noticed 
  here. 
  They 
  

   occur 
  intermittently 
  over 
  the 
  entire 
  length 
  of 
  the 
  ground 
  under 
  

   notice, 
  but 
  seldom 
  attain 
  to 
  any 
  great 
  thickness. 
  The 
  best 
  exposure 
  

   is 
  found 
  to 
  the 
  right 
  of 
  the 
  path 
  leading 
  from 
  Rosebush 
  village 
  to 
  

   the 
  disused 
  slate-quarry 
  at 
  Craig-y-cwm. 
  Here 
  they 
  form 
  a 
  rather 
  

   conspicuous 
  cliff. 
  

  

  The 
  common 
  type 
  is 
  greenish 
  grey 
  in 
  colour, 
  breaking 
  with 
  an 
  

   uneven 
  or 
  subconchoidal 
  fracture, 
  and 
  in 
  general 
  of 
  a 
  homogeneous 
  

   appearance, 
  except 
  for 
  the 
  presence 
  — 
  occasionally 
  in 
  some 
  quantity 
  — 
  

   of 
  chloritic 
  or 
  quartzose 
  amygdaloids. 
  This 
  even 
  appearance 
  may, 
  

   however, 
  be 
  somewhat 
  detracted 
  from 
  by 
  dark 
  patches 
  resulting 
  from 
  

   flow-brecciation. 
  These 
  are 
  occasionally 
  present 
  in 
  quantity, 
  and, 
  

   in 
  addition, 
  in 
  those 
  cases 
  where 
  a 
  crush 
  has 
  taken 
  place 
  the 
  rocks 
  

   are 
  difficult 
  to 
  distinguish 
  in 
  the 
  field 
  from 
  true 
  ashes. 
  

  

  In 
  thin 
  sections 
  the 
  rocks 
  are 
  seen 
  to 
  contain 
  a 
  large 
  number 
  of 
  

   microporphyritic 
  felspars, 
  both 
  monoclinic 
  and 
  triclinic. 
  The 
  base 
  

   in 
  which 
  these 
  are 
  set 
  contains 
  much 
  microfelsitic 
  matter, 
  with 
  

   but 
  little 
  evidence 
  of 
  fluxional 
  movement 
  in 
  those 
  cases 
  in 
  which 
  

   flow-brecciation 
  is 
  absent. 
  Opacite 
  is 
  often 
  abundantly 
  present, 
  

   and 
  somewhat 
  frequently 
  the 
  structure 
  is 
  obscured 
  by 
  secondary 
  

   products. 
  

  

  West 
  of 
  Rosebush 
  a 
  slightly 
  different 
  type 
  is 
  found. 
  It 
  is 
  

   often 
  bluer 
  in 
  colour 
  than 
  the 
  rocks 
  just 
  noticed, 
  and 
  contains 
  

   porphyritic 
  felspars 
  about 
  '05 
  inch 
  in 
  length 
  which 
  are 
  referable 
  to 
  

   orthoclase. 
  The 
  cryptocrystalline 
  base 
  contains 
  large 
  quantities 
  of 
  

   oligoclase-microlites. 
  The 
  arrangement 
  of 
  these, 
  together 
  with 
  the 
  

   manner 
  of 
  formation 
  of 
  a 
  considerable 
  quantity 
  of 
  green, 
  filmy, 
  

   chloritic 
  matter, 
  affords 
  evidence 
  of 
  flow-movement. 
  

  

  East 
  of 
  Rosebush 
  an 
  agglomerate 
  occurs, 
  consisting 
  both 
  of 
  these 
  

   rocks 
  and 
  of 
  pieces 
  of 
  slate. 
  

  

  This 
  fact, 
  taken 
  in 
  conjunction 
  with 
  the 
  general 
  parallelism 
  of 
  the 
  

   outcrop 
  of 
  these 
  rocks 
  to 
  the 
  strike 
  of 
  the 
  slates, 
  and 
  added 
  to 
  such 
  

   microscopical 
  evidence 
  as 
  is 
  afforded 
  by 
  their 
  fine 
  texture 
  and 
  

   amygdaloidal 
  habit, 
  with 
  occasional 
  evidences 
  of 
  flow, 
  leads 
  to 
  the 
  

   conclusion 
  that 
  they 
  constitute 
  a 
  lava 
  -flow 
  antecedent 
  to, 
  and 
  

   slightly 
  more 
  basic 
  than, 
  that 
  which 
  occurs 
  at 
  the 
  north-eastern 
  

   extremity 
  of 
  the 
  range. 
  

  

  