﻿424 
  MESSES. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  V. 
  JENNINGS 
  

  

  The 
  calcite, 
  however, 
  that 
  fills 
  up 
  the 
  vesicles 
  of 
  this 
  rock 
  has 
  

   become 
  bent 
  and 
  even 
  granulated, 
  as 
  may 
  be 
  seen 
  in 
  microscopic 
  

   sections 
  ; 
  the 
  porphyritic 
  felspars, 
  which 
  are 
  plagioclastic, 
  are 
  also 
  

   largely 
  replaced 
  by 
  calcite, 
  and 
  lie 
  in 
  a 
  fine-grained 
  matrix 
  of 
  pris- 
  

   matic 
  plagioclase 
  -felspar 
  and 
  granular 
  altered 
  pyroxene. 
  The 
  beds 
  

   immediately 
  below 
  have 
  been 
  converted 
  into 
  spotted 
  slate, 
  and 
  the 
  

   igneous 
  mass 
  appears 
  to 
  be 
  an 
  intrusive 
  sheet 
  rather 
  than 
  an 
  actual 
  

   lava-flow. 
  

  

  However 
  this 
  may 
  be, 
  it 
  is 
  followed 
  by 
  a 
  considerable 
  thickness 
  of 
  

   grey 
  volcanic 
  tuff, 
  in 
  marked 
  contrast 
  to 
  the 
  compact 
  dark 
  slates 
  

   below 
  ; 
  felspar 
  crystals, 
  about 
  one 
  eighth 
  of 
  a 
  inch 
  in 
  length, 
  are 
  

   freely 
  scattered 
  through 
  this 
  deposit, 
  accompanied 
  by 
  slaty 
  frag- 
  

   ments, 
  the 
  abundance 
  of 
  which 
  points 
  to 
  considerable 
  volcanic 
  

   activity. 
  The 
  flakes 
  and 
  lenticular 
  fragments 
  of 
  clay 
  and 
  shale 
  

   thus 
  torn 
  from 
  the 
  old 
  sea-floor 
  appear 
  to 
  have 
  shrunk 
  and 
  cracked 
  

   internally 
  during 
  the 
  consolidation 
  of 
  the 
  tuff. 
  On 
  exposure, 
  a 
  

   septarian 
  structure 
  is 
  often 
  revealed 
  towards 
  the 
  centre, 
  which 
  

   becomes 
  brown 
  on 
  weathering, 
  while 
  the 
  outer 
  portion, 
  and 
  the 
  entire 
  

   flake 
  in 
  smaller 
  instances, 
  has 
  become 
  dense 
  and 
  irregularly 
  fissile, 
  

   exhibiting 
  a 
  lustrous 
  surface 
  like 
  that 
  of 
  slickensided 
  slate. 
  These 
  

   argillaceous 
  fragments, 
  sometimes 
  six 
  or 
  seven 
  inches 
  long, 
  flung 
  

   out 
  imperfectly 
  consolidated 
  among 
  the 
  first 
  products 
  of 
  eruption, 
  

   may 
  have 
  been 
  baked 
  and 
  metamorphosed, 
  externally 
  at 
  any 
  rate, 
  

   by 
  the 
  invading 
  igneous 
  sheets, 
  and 
  have 
  subsequently 
  acquired 
  

   some 
  of 
  the 
  characters 
  of 
  slate. 
  The 
  larger 
  fragments 
  appear 
  as 
  

   elongated 
  and 
  flattened 
  " 
  eyes 
  " 
  in 
  the 
  midst 
  of 
  the 
  more 
  yielding 
  

   ash 
  ; 
  while 
  the 
  smaller 
  pellets, 
  on 
  the 
  other 
  hand, 
  spread 
  out 
  by 
  

   dynamic 
  action, 
  have 
  given 
  rise 
  to 
  the 
  thin 
  black 
  lustrous 
  films 
  

   that 
  occur 
  so 
  frequently 
  in 
  this 
  rock. 
  

  

  Under 
  the 
  microscope 
  the 
  tuff 
  is 
  seen 
  to 
  abound 
  in 
  particles 
  of 
  

   scoriaceous 
  andesite-glass, 
  now 
  converted 
  into 
  a 
  green 
  palagonite; 
  and 
  

   round 
  these, 
  as 
  well 
  as 
  round 
  the 
  scattered 
  plagioclastic 
  felspars, 
  

   the 
  fine 
  interstitial 
  ash 
  has 
  assumed 
  a 
  kind 
  of 
  flow. 
  Flecks 
  of 
  iron 
  

   pyrites 
  are 
  very 
  common. 
  

  

  Similar 
  tuffs 
  with 
  black 
  clayey 
  inclusions 
  are 
  well 
  seen 
  at 
  the 
  base 
  

   of 
  the 
  Snowdonian 
  eruptive 
  series, 
  as 
  at 
  Llyn 
  Llydau 
  and 
  between 
  

   Rhyddu 
  and 
  Beddgelert. 
  Comparison 
  is 
  naturally 
  suggested 
  with 
  

   the 
  well-known 
  beds 
  in 
  the 
  Yorder 
  Eifel, 
  where, 
  however, 
  a 
  still 
  

   greater 
  contrast 
  is 
  afforded 
  by 
  the 
  hard 
  cleaved 
  particles 
  of 
  Devonian 
  

   slate 
  and 
  the 
  loose 
  ash 
  in 
  which 
  they 
  lie. 
  

  

  The 
  upper 
  portion 
  of 
  this 
  slate-tuff, 
  or 
  shale-tuff 
  at 
  the 
  time 
  of 
  

   its 
  formation, 
  passes 
  into 
  a 
  compacter 
  ash, 
  above 
  which 
  dolerite 
  

   occurs. 
  The 
  higher 
  slopes, 
  rising 
  another 
  thousand 
  feet 
  as 
  far 
  as 
  

   the 
  floor 
  of 
  Llyn-y-Gader, 
  consist 
  of 
  repetitions 
  of 
  slate-tuff 
  and 
  

   abundant 
  finely 
  bedded 
  ash, 
  in 
  which 
  intrusive 
  sheets, 
  of 
  fairly 
  basic 
  

   character 
  and 
  of 
  very 
  various 
  grain, 
  have 
  produced 
  extensive 
  altera- 
  

   tion. 
  

  

  Below 
  the 
  north-east 
  face 
  of 
  Tyrau 
  Mawr, 
  and 
  among 
  the 
  earlier 
  

   layers, 
  one 
  of 
  the 
  coarser 
  tuffs 
  contains 
  bomb-like 
  masses 
  six 
  inches 
  

  

  