﻿ON 
  THE 
  NORTHERN 
  8LOPES 
  OP 
  CADER 
  IDRIS. 
  429 
  

  

  Microscopic 
  examination 
  shows 
  that 
  the 
  tuffs 
  with 
  felspar 
  crystals 
  

   and 
  slaty 
  fragments 
  preserve, 
  throughout 
  some 
  2500 
  feet 
  of 
  sedi- 
  

   ments 
  and 
  intercalated 
  sheets, 
  characters 
  closely 
  similar 
  to 
  those 
  of 
  

   the 
  bed 
  at 
  Penrhyn-gwyn. 
  In 
  specimens 
  collected 
  at 
  the 
  higher 
  

   levels, 
  the 
  proportion 
  of 
  palagonitic 
  scoriae 
  is 
  diminished 
  ; 
  and 
  on 
  

   the 
  west 
  flank 
  of 
  Tyrau 
  Mawr 
  grains 
  of 
  quartz 
  containing 
  liquid- 
  

   inclusions 
  appear 
  among 
  the 
  ejected 
  materials. 
  Here 
  also, 
  shortly 
  

   above 
  the 
  massive 
  eurite, 
  is 
  a 
  bed 
  containing 
  white 
  and 
  very 
  ir- 
  

   regular 
  fragments, 
  some 
  of 
  them 
  an 
  inch 
  or 
  two 
  across. 
  The 
  forms 
  

   of 
  these, 
  under 
  the 
  microscope, 
  strongly 
  suggest 
  those 
  of 
  shattered 
  

   scoriaceous 
  glass 
  ; 
  and 
  the 
  resistance 
  they 
  have 
  offered 
  to 
  decom- 
  

   position, 
  and 
  the 
  extreme 
  sharpness 
  of 
  their 
  outlines, 
  would 
  ally 
  

   them 
  to 
  the 
  trachytes 
  or 
  even 
  to 
  the 
  rhyolites. 
  They 
  exhibit 
  

   traces 
  of 
  brown 
  spherulitic 
  and 
  " 
  microfelsitic 
  " 
  structures, 
  which 
  

   are 
  now 
  mere 
  dendritic 
  patches 
  through 
  the 
  development 
  around 
  

   them 
  and 
  in 
  them 
  of 
  the 
  transparent 
  products 
  of 
  secondary 
  devitri- 
  

   fication. 
  A 
  few 
  porphyritic 
  felspars 
  occur 
  in 
  the 
  larger 
  fragments 
  

   of 
  the 
  glass, 
  and 
  minute 
  chalcedonic 
  spherules, 
  giving 
  a 
  black 
  cross 
  

   and 
  vivid 
  colours 
  between 
  crossed 
  nicols, 
  are 
  in 
  places 
  developed 
  

   very 
  freely. 
  We 
  find 
  here, 
  then, 
  a 
  deposit 
  that 
  links 
  the 
  normal 
  

   slaty 
  tuffs 
  with 
  the 
  undoubtedly 
  acid 
  products 
  of 
  the 
  latest 
  phases 
  of 
  

   eruption. 
  

  

  Some 
  1100 
  feet 
  lower 
  in 
  the 
  series, 
  omitting 
  the 
  thickness 
  of 
  the 
  

   intrusive 
  eurite, 
  the 
  metamorphosed 
  porcellaneous 
  tuffs 
  on 
  Mynydd- 
  

   y-Gader 
  contain 
  blocks 
  that 
  weather 
  white 
  on 
  the 
  outside, 
  and 
  

   exhibit 
  within 
  the 
  compact 
  blue-grey 
  appearance 
  of 
  the 
  altered 
  

   rhyolites 
  of 
  Wales. 
  While 
  many 
  of 
  these 
  were 
  undoubtedly 
  scoriae, 
  

   others 
  prove 
  to 
  be, 
  as 
  Prof. 
  Ramsay 
  stated, 
  concretions 
  developed 
  in 
  

   the 
  tuff, 
  or 
  at 
  any 
  rate, 
  as 
  the 
  micrw&eope 
  would 
  indicate, 
  ejected 
  

   lumps 
  from 
  previously 
  consolidated 
  tuffs. 
  One 
  of 
  these 
  " 
  concre- 
  

   tions," 
  five 
  inches 
  in 
  length, 
  has 
  a 
  specific 
  gravity 
  of 
  2-62. 
  Wisps 
  

   of 
  colourless 
  devitrified 
  glass 
  and 
  larger 
  fragments 
  of 
  pumice 
  form 
  

   the 
  ground-mass 
  of 
  this 
  specimen, 
  while 
  the 
  adjacent 
  tuff 
  contains 
  

   more 
  slaty 
  matter 
  and 
  palagonite. 
  

  

  A 
  true 
  ejected 
  scoria 
  from 
  the 
  south-west 
  corner 
  of 
  Mynydd-y- 
  

   Gader 
  contains 
  corroded 
  grains 
  of 
  quartz, 
  exhibits 
  traces 
  of 
  fluidal 
  

   structure, 
  and 
  would 
  rank, 
  could 
  it 
  be 
  restored 
  to 
  its 
  original 
  con- 
  

   dition, 
  with 
  the 
  quartz-andesites 
  or 
  the 
  rhyolites. 
  The 
  nature 
  of 
  

   the 
  porphyritic 
  felspars 
  has 
  been 
  masked, 
  in 
  this 
  case, 
  by 
  kaolini- 
  

   zation. 
  

  

  Another 
  and 
  greyer 
  lump 
  from 
  close 
  below 
  Llyn-y-Gafr 
  is, 
  allowing 
  

   for 
  devitrification, 
  a 
  good 
  porphyritic 
  trachyte, 
  all 
  the 
  more 
  typical 
  

   for 
  containing 
  some 
  plagioclastic 
  as 
  well 
  as 
  orthoclastic 
  felspar. 
  

   The 
  cracks 
  in 
  the 
  compact 
  matrix 
  of 
  this 
  rock 
  have 
  a 
  rude 
  perlitic 
  

   tendency. 
  

  

  In 
  one 
  at 
  least 
  of 
  the 
  ash-beds 
  on 
  Mynydd-y-Gader 
  numerous 
  

   quartz-grains 
  occur, 
  often 
  rounded 
  and 
  containing 
  intruded 
  material; 
  

   these 
  have 
  been 
  derived, 
  it 
  would 
  appear, 
  from 
  some 
  fairly 
  acid 
  and 
  

   vitreous 
  lava. 
  Felspar 
  crystals 
  are 
  comparatively 
  rare 
  in 
  this 
  deposit^ 
  

  

  