﻿434 
  MESSRS. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  V. 
  JENNINGS 
  

  

  composed 
  of 
  black 
  slates 
  and 
  stratified 
  ash. 
  But 
  along 
  the 
  face 
  of 
  

   Cyfrwy, 
  around 
  the 
  gloomy 
  pool 
  of 
  Llyn-y-Gader, 
  and 
  on 
  the 
  higher 
  

   front 
  of 
  Mynydd 
  Moel. 
  this 
  rock 
  stretches 
  with 
  marked 
  uniformity, 
  

   dipping 
  southward 
  or 
  south-westward 
  with 
  the 
  strata, 
  and 
  strewing 
  

   the 
  taluses 
  with 
  angular 
  fragments 
  and 
  columnar 
  blocks 
  that 
  en- 
  

   croach 
  upon 
  the 
  grass 
  below. 
  The 
  weather 
  has 
  little 
  effect 
  on 
  this 
  

   detritus, 
  beyond 
  a 
  slight 
  browning 
  of 
  the 
  surfaces, 
  and 
  the 
  steeply 
  

   piled 
  grey 
  accumulations 
  are 
  well 
  contrasted 
  with 
  the 
  soft 
  terraces 
  

   of 
  the 
  underlying 
  tuffs. 
  

  

  The 
  thickness 
  of 
  this 
  principal 
  mass 
  is 
  about 
  1500 
  feet 
  : 
  another 
  

   sheet 
  comes 
  in 
  below 
  it 
  on 
  the 
  level 
  of 
  the 
  slates 
  of 
  Penrhyn-gwyn 
  ; 
  

   and 
  an 
  additional 
  mass, 
  perhaps 
  a 
  faulted 
  repetition 
  of 
  the 
  second, 
  

   is 
  recognizable 
  in 
  the 
  rounded 
  hill-outlines 
  and 
  white 
  exposures 
  that 
  

   extend 
  from 
  Llynau 
  Crogenen 
  to 
  Gelli-lwyd-fawr. 
  West 
  of 
  Tyrau 
  

   Mawr, 
  though 
  at 
  a 
  lower 
  level, 
  the 
  main 
  mass 
  forms 
  the 
  steep 
  slopes 
  

   near 
  the 
  road, 
  and 
  curves 
  south, 
  rising 
  at 
  the 
  same 
  time, 
  till 
  it 
  

   resumes 
  its 
  place 
  on 
  the 
  cliff 
  above 
  Llyn 
  Cyri. 
  The 
  whole 
  of 
  the 
  

   flat 
  upland 
  around 
  this 
  lake 
  reveals 
  the 
  presence 
  of 
  the 
  eurite, 
  

   and 
  between 
  the 
  white 
  and 
  angular 
  fragments 
  a 
  bog 
  has 
  formed 
  

   rather 
  than 
  a 
  soil. 
  

  

  Everywhere 
  its 
  fractured 
  surfaces 
  present 
  the 
  same 
  compact 
  blue- 
  

   grey 
  appearance, 
  faintly 
  tinged 
  with 
  brown, 
  the 
  splintery 
  fragments 
  

   broken 
  from 
  it 
  cutting 
  like 
  flakes 
  of 
  glass. 
  The 
  uniformity 
  of 
  its 
  

   characters 
  over 
  separate 
  exposures 
  three 
  and 
  four 
  miles 
  in 
  length 
  

   would 
  alone 
  be 
  strong 
  evidence 
  for 
  regarding 
  it 
  as 
  intrusive. 
  The 
  

   Survey 
  map 
  shows 
  how 
  it 
  cuts 
  on 
  a 
  large 
  scale 
  across 
  the 
  ashes 
  and 
  

   more 
  basic 
  sheets 
  ; 
  and 
  near 
  the 
  Roman 
  road 
  on 
  Tyrau 
  ITawr 
  a 
  short 
  

   offshoot 
  appears 
  to 
  us 
  to 
  run 
  down 
  into 
  the 
  ash 
  and 
  slates 
  below. 
  We 
  

   are 
  not 
  at 
  present 
  prepared 
  to 
  discuss 
  the 
  continuity 
  of 
  the 
  eurite 
  from 
  

   Craig-y-Llyn 
  (Gallt-y-Llyn) 
  across 
  the 
  southern 
  slopes 
  of 
  the 
  moun- 
  

   tain 
  to 
  Llyn-y-Cau, 
  since 
  the 
  rock 
  on 
  Llyn-y-Cau 
  has 
  more 
  of 
  the 
  

   characters 
  of 
  a 
  lava-flow, 
  and 
  part 
  of 
  the 
  area 
  mapped 
  as 
  " 
  felspathic 
  

   trap, 
  intrusive 
  " 
  is 
  occupied 
  by 
  highly 
  silicated 
  ashes 
  and 
  spherulitic 
  

   rhyolites 
  like 
  those 
  described 
  on 
  Craig-y-Llam. 
  Curiously 
  enough, 
  

   further 
  to 
  the 
  north-east, 
  in 
  a 
  hollow 
  under 
  Gau 
  Graig, 
  the 
  rock 
  

   appears 
  to 
  have 
  recovered 
  its 
  intrusive 
  character, 
  and 
  abuts 
  against 
  

   altered 
  slates 
  in 
  which 
  quartz-veins 
  are 
  abundantly 
  developed. 
  It 
  

   is 
  at 
  this 
  point 
  that 
  a 
  reddish 
  rock 
  with 
  chloritic 
  bands 
  arises, 
  

   simulating 
  a 
  gneiss 
  when 
  lying 
  broken 
  on 
  the 
  roads. 
  Probably 
  on 
  

  

  rapprochera 
  beaucoup 
  du 
  feldspath 
  coinpacte 
  . 
  . 
  . 
  mais 
  si 
  le 
  quartz 
  est 
  abondant 
  

   dans 
  le 
  granite, 
  la 
  rocbe 
  . 
  . 
  . 
  se 
  rapprochera 
  du 
  quartz 
  coinpacte 
  (Jiornstein) 
  ou 
  

   keratite." 
  

  

  Tbe 
  " 
  haelleflinta 
  " 
  of 
  Sweden 
  is 
  appropriately 
  introduced 
  for 
  comparison, 
  

   and 
  tbe 
  connexion 
  of 
  tbe 
  darker 
  amphibolic 
  eurites 
  with 
  apbanite 
  is 
  also 
  

   pointed 
  out. 
  Tbe 
  specific 
  gravity 
  of 
  eurite 
  is 
  stated 
  to 
  be 
  26 
  to 
  27 
  

   (Gerbard 
  gives 
  tbat 
  of 
  felsite 
  as 
  2'980 
  to 
  3 
  - 
  020). 
  Tbe 
  name 
  is 
  derived 
  from 
  

   tbe 
  fusibility 
  of 
  tbe 
  rock 
  (? 
  evpoos). 
  

  

  Apart 
  from 
  the 
  Neptunian 
  views 
  of 
  d'Aubuissou, 
  tbe 
  lucidity 
  and 
  exactitude 
  

   of 
  these 
  statements 
  bave 
  impressed 
  us 
  in 
  no 
  slight 
  degree. 
  Unable 
  to 
  accept 
  

   " 
  quartz-porphyry," 
  a 
  term 
  covering 
  many 
  ancient 
  lavas 
  as 
  well 
  as 
  crystalline 
  

   elvans. 
  or 
  " 
  microgranulite," 
  as 
  used 
  by 
  several 
  prominent 
  French 
  authors, 
  it 
  

   is 
  with 
  satisfaction 
  that 
  we 
  fall 
  back 
  on 
  " 
  eurite 
  " 
  as 
  a 
  name 
  for 
  rocks 
  inter- 
  

   mediate 
  between 
  granite 
  and 
  tbe 
  fresh 
  or 
  altered 
  types 
  of 
  acid 
  lavas. 
  

  

  