﻿ON 
  THE 
  NOKTHERN 
  SLOPES 
  OF 
  CADER 
  IDRIS. 
  433 
  

  

  structure 
  and 
  the 
  imperfect 
  development 
  of 
  the 
  felspars 
  indicate 
  

   that 
  it 
  was 
  more 
  glass}' 
  than 
  the 
  other 
  compact 
  sheets 
  of 
  Cader 
  

   Idris. 
  The 
  coarsely 
  granular 
  quartz, 
  with 
  inclusions 
  of 
  vermicular 
  

   chlorite, 
  fills 
  cavities 
  that 
  may 
  have 
  been 
  originally 
  gas-vesicles. 
  

   It 
  is 
  noteworthy 
  that 
  this 
  development 
  of 
  quartz 
  occurs, 
  just 
  as 
  in 
  

   the 
  slates 
  of 
  the 
  district, 
  close 
  to 
  the 
  contact 
  with 
  the 
  massive 
  and 
  

   intrusive 
  eurite. 
  

  

  These 
  " 
  greenstones 
  " 
  or 
  " 
  diabases 
  " 
  form, 
  then, 
  a 
  series 
  related 
  

   rather 
  to 
  the 
  augite-diorites 
  and 
  augite-andesites 
  than 
  to 
  the 
  normal 
  

   gabbros 
  and 
  olivine-bearing 
  basalts. 
  They 
  range 
  from 
  dolerites 
  

   without 
  olivine 
  and 
  aphanites 
  to 
  andesitic 
  rocks 
  with 
  an 
  originally 
  

   glassy 
  matrix. 
  

  

  On 
  Rhobell-fawr, 
  on 
  the 
  other 
  hand, 
  the 
  imposing 
  mass 
  to 
  the 
  

   north 
  of 
  our 
  area, 
  we 
  find 
  examples 
  of 
  far 
  more 
  basic 
  rocks, 
  con- 
  

   taining 
  pophyritic 
  altered 
  olivine. 
  We 
  believe, 
  moreover, 
  that 
  the 
  

   beautiful 
  " 
  hornblende-diabase 
  " 
  of 
  this 
  mountain 
  has 
  no 
  repre- 
  

   sentative 
  on 
  Cader 
  Idris. 
  

  

  In 
  concluding 
  this 
  portion 
  of 
  our 
  subject, 
  we 
  are 
  not 
  unmindful 
  

   of 
  the 
  observations 
  made 
  by 
  the 
  late 
  Mr. 
  John 
  Arthur 
  Phillips 
  on 
  

   the 
  alteration 
  of 
  uralite 
  in 
  a 
  diabase 
  from 
  the 
  Dolgelley 
  area 
  *. 
  

  

  IV. 
  The 
  Massive 
  Eurite 
  f. 
  

  

  Undoubtedly 
  the 
  most 
  striking 
  feature 
  of 
  Cader 
  Idris 
  is 
  the 
  

   precipitous 
  escarpment 
  of 
  grey 
  columnar 
  rock 
  that 
  forms 
  so 
  large 
  a 
  

   portion 
  of 
  the 
  mountain-wall. 
  The 
  main 
  mass, 
  running 
  east 
  and 
  

   west, 
  is 
  faulted 
  down 
  at 
  Tyrau 
  Mawr, 
  the 
  bold 
  cliff 
  of 
  which 
  is 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxiii. 
  (1877), 
  p. 
  427. 
  

  

  t 
  We 
  have 
  adopted 
  this 
  name, 
  after 
  some 
  deliberation, 
  for 
  a 
  rock 
  that 
  might 
  

   be 
  classed 
  in 
  this 
  country 
  as 
  a 
  compact 
  quartz-felsite. 
  The 
  term 
  " 
  felsite 
  " 
  is 
  

   now 
  so 
  generally 
  employed 
  on 
  the 
  continent 
  for 
  that 
  portion 
  of 
  a 
  rock 
  in 
  which 
  

   "felsitic" 
  structure 
  may 
  be 
  recognized, 
  that 
  its 
  retention 
  as 
  a 
  generic 
  name 
  

   has 
  seemed 
  to 
  most 
  geologists 
  unadvisable. 
  Klaproth's 
  " 
  felsit 
  " 
  appears 
  to 
  

   have 
  been 
  based 
  on 
  chemical 
  characters, 
  and 
  Gerhard 
  (Abhandl. 
  der 
  Berlin. 
  

   Akad. 
  der 
  Wissensch. 
  1814 
  & 
  1815, 
  p. 
  18), 
  to 
  whom 
  the 
  name 
  is 
  usually 
  as- 
  

   cribed, 
  while 
  considering 
  felsite 
  as 
  forming 
  the 
  ground-mass 
  of 
  various 
  rocks, 
  

   includes 
  under 
  it 
  Werner's 
  " 
  Thonstein" 
  and 
  the 
  Labradorite-rock 
  of 
  Labrador. 
  

   He 
  adduces, 
  moreover, 
  as 
  allies, 
  the 
  pitchstone 
  of 
  Meissen 
  and 
  other 
  substance?, 
  

   on 
  the 
  ground 
  that 
  they 
  all 
  consist 
  of 
  " 
  Kiesel, 
  Thonerde, 
  Kalk, 
  und 
  Natrum." 
  

   After 
  this, 
  it 
  is 
  idle 
  to 
  seek 
  authority, 
  other 
  than 
  Anglo-Saxon 
  usage, 
  for 
  re- 
  

   stricting 
  " 
  felsite 
  " 
  to 
  any 
  defined 
  class 
  of 
  rocks. 
  

  

  Compare 
  with 
  this 
  laxity 
  of 
  expression 
  the 
  definition 
  of 
  "eurite" 
  by 
  dAu- 
  

   buisson 
  de 
  Voisins 
  (Traite 
  de 
  Geognosie, 
  1819, 
  t. 
  ii. 
  p 
  117) 
  : 
  — 
  " 
  Dans 
  le 
  porphyre 
  

   ordinaire, 
  celui 
  qui 
  correspond 
  au 
  granite 
  proprement 
  dit, 
  cette 
  pate 
  aura 
  le 
  

   feldspath 
  pour 
  principe 
  principal 
  : 
  nous 
  lui 
  donnons 
  le 
  nom 
  d'eurite 
  et 
  nous 
  

   la 
  definissons 
  en 
  disant 
  : 
  L'eurite 
  est 
  un 
  granite 
  compacte, 
  .... 
  d 
  apparence 
  

   homogene, 
  dans 
  laquelle 
  le 
  feldspath 
  est 
  le 
  principe 
  dominant, 
  et 
  dont 
  les 
  divers 
  

   principes 
  sont 
  comme 
  fondus 
  les 
  uns 
  dans 
  les 
  autres. 
  S'il 
  etait 
  possible 
  de 
  la 
  

   redissoudre, 
  et 
  de 
  faire 
  cristalliser 
  tranquillement 
  la 
  solution, 
  de 
  maniere 
  a 
  ce 
  

   que 
  les 
  principes 
  integrants 
  pussent 
  se 
  former 
  en 
  cristaux 
  distiucts, 
  elle 
  pro- 
  

   duirait 
  un 
  granite." 
  

  

  It 
  is 
  clear 
  from 
  what 
  immediately 
  follows 
  in 
  d'Aubuisson's 
  treatise 
  that 
  the 
  

   predominance 
  of 
  felspar 
  is 
  not 
  sti'ictly 
  insisted 
  on. 
  On 
  page 
  118 
  we 
  read, 
  " 
  Si, 
  

   dans 
  le 
  granite 
  qui 
  a 
  produit 
  l'eurite, 
  ou 
  qui 
  est 
  cense 
  l'avoir 
  produit, 
  en 
  

   devenant 
  compacte, 
  le 
  feldspath 
  etait 
  en 
  tres 
  grande 
  quantite, 
  l'eurite 
  se 
  

  

  Q.J.G.S. 
  No. 
  179. 
  2 
  1 
  

  

  