﻿PELSITES 
  IIS 
  THE 
  BALA 
  GE0T7P 
  OF 
  XORTH 
  WALES. 
  293 
  

  

  I 
  pass 
  next 
  to 
  the 
  mass 
  above 
  the 
  Conway 
  Falls 
  Inn, 
  which 
  contains 
  

   the 
  large 
  pebbly 
  ;; 
  concretions." 
  The 
  " 
  matrix 
  " 
  gives, 
  as 
  before, 
  in- 
  

   dications 
  of 
  cleavage 
  ; 
  and 
  the 
  flattened 
  aspect 
  of 
  the 
  nodules 
  is 
  pro- 
  

   bably 
  due 
  to 
  pressure. 
  The 
  chief 
  difference 
  between 
  this 
  rock 
  and 
  the 
  

   former 
  (apart 
  from 
  the 
  greater 
  size 
  of 
  the 
  nodules) 
  is 
  that 
  the 
  sphe- 
  

   roidal 
  boundary 
  is 
  more 
  definite, 
  giving 
  the 
  rock 
  the 
  appearance 
  of 
  

   a 
  conglomerate, 
  and 
  the 
  " 
  pebbles 
  " 
  are 
  frequently 
  solid 
  to 
  the 
  core. 
  

  

  From 
  one 
  of 
  these, 
  nearly 
  three 
  inches 
  in 
  diameter, 
  I 
  have 
  had 
  a 
  

   slice 
  cut 
  ; 
  I 
  expected 
  to 
  find 
  some 
  traces 
  of 
  a 
  radial 
  structure, 
  deeming 
  

   it 
  a 
  kind 
  of 
  spherulite 
  ; 
  of 
  this 
  there 
  is 
  not 
  the 
  slightest 
  indication. 
  

   The 
  slide 
  (fig. 
  4) 
  exhibits 
  a 
  wavy 
  fluidal 
  structure, 
  and 
  does 
  not 
  

   materially 
  differ 
  from 
  that 
  of 
  the 
  felsite 
  obtained 
  near 
  the 
  inn, 
  show- 
  

   ing, 
  like 
  it, 
  one 
  or 
  two 
  crystals 
  of 
  felspar 
  and 
  aggregations 
  of 
  the 
  

   green 
  mineral. 
  The 
  fractured 
  face 
  of 
  the 
  nodule 
  gives 
  indications 
  

   of 
  cracks 
  concentric 
  with 
  the 
  exterior. 
  

  

  I 
  turn 
  now 
  to 
  a 
  rather 
  similar 
  rock, 
  obtained 
  several 
  years 
  since 
  

   from 
  the 
  neighbourhood 
  of 
  Beddgelert. 
  This 
  also 
  has 
  a 
  most 
  singu- 
  

   lar 
  resemblance 
  to 
  a 
  conglomerate 
  with 
  a 
  compact 
  matrix. 
  The 
  

   latter 
  is 
  rather 
  darker 
  in 
  colour 
  than 
  the 
  rock 
  we 
  have 
  been 
  descri- 
  

   bing; 
  the 
  " 
  pebbles" 
  show 
  a 
  similar 
  compact 
  homogeneous 
  aspect. 
  One 
  

   of 
  these 
  under 
  the 
  microscope 
  exhibits 
  the 
  usual 
  structure 
  of 
  a 
  devi- 
  

   trified 
  rhyolitic 
  lava, 
  except 
  that 
  it 
  is 
  rather 
  more 
  stained 
  with 
  ferrite, 
  

   opacite 
  &c, 
  with 
  some 
  scattered 
  grains 
  of 
  iron-oxide. 
  The 
  matrix 
  

   has 
  its 
  devitrification-structure 
  more 
  minute 
  ; 
  and 
  its 
  fluidal 
  struc- 
  

   ture 
  is 
  mainly 
  indicated 
  by 
  viridite*. 
  Several 
  parts 
  of 
  the 
  slide 
  

   present 
  % 
  distinct 
  though 
  rude 
  indications 
  of 
  a 
  perlitic 
  structure 
  f 
  . 
  

  

  In 
  the 
  lower 
  part 
  of 
  the 
  yellowish 
  felsite, 
  which 
  forms 
  the 
  mass 
  

   of 
  Conway 
  Mountain 
  J, 
  and 
  a 
  short 
  distance 
  to 
  the 
  west 
  of 
  that 
  town, 
  

   is 
  a 
  thick 
  band 
  of 
  spheroids 
  which 
  seems 
  to 
  run 
  rather 
  evenly 
  for 
  

   a 
  considerable 
  distance. 
  Another 
  band, 
  not 
  less 
  than 
  twenty 
  feet 
  

   thick 
  (the 
  rock 
  here, 
  I 
  was 
  informed, 
  had 
  been 
  quarried 
  for 
  china- 
  

   clay), 
  occurs 
  near 
  the 
  crest 
  of 
  the 
  mountain. 
  The 
  spheroids 
  are 
  some- 
  

   times 
  a 
  couple 
  of 
  inches 
  in 
  diameter 
  ; 
  they 
  are 
  often 
  hollow 
  at 
  the 
  

   core 
  ; 
  and 
  the 
  more 
  compact 
  rock 
  below 
  exhibits 
  occasional 
  small 
  

   cavities 
  with 
  irregular 
  flinty 
  -looking 
  patches, 
  sometimes 
  dark, 
  some- 
  

   times 
  pinkish 
  in 
  colour: 
  like 
  the 
  above- 
  described 
  green 
  patches, 
  they 
  

   are 
  rather 
  unctuous 
  and 
  easily 
  scratched 
  with 
  a 
  knife. 
  

  

  This 
  great 
  mass 
  of 
  felsite 
  is 
  overlain 
  by 
  a 
  grit 
  evidently 
  composed 
  

   of 
  materials 
  derived 
  from 
  it, 
  and 
  sometimes 
  by 
  no 
  means 
  easy 
  to 
  

   distinguish 
  with 
  the 
  unaided 
  eye. 
  

  

  A 
  similar 
  felsite 
  (part 
  of 
  the 
  same 
  mass 
  displaced 
  by 
  a 
  fault) 
  forms 
  

   the 
  Diganwy 
  Hills. 
  The 
  upper 
  surface 
  of 
  the 
  flow 
  was 
  well 
  exposed 
  

  

  * 
  The 
  term 
  is 
  used 
  advisedly 
  ; 
  most 
  of 
  the 
  mineral 
  is 
  not 
  doubly 
  refracting, 
  

   and 
  so 
  differs 
  from 
  that 
  described 
  above. 
  From 
  its 
  relation 
  to 
  cracks 
  &c. 
  it 
  is 
  

   clearly, 
  in 
  its 
  present 
  form, 
  a 
  mineral 
  of 
  secondary 
  origin. 
  

  

  t 
  See 
  fig. 
  4, 
  pi. 
  xxi. 
  vol. 
  xxxvii. 
  (illustrations 
  to 
  Mr. 
  F. 
  Kutley's 
  paper). 
  I 
  

   have 
  no 
  doubt 
  this 
  is 
  the 
  second 
  rock 
  mentioned 
  in 
  Mr. 
  G. 
  G. 
  Butler's 
  note. 
  

   p. 
  403. 
  

  

  J 
  See 
  Memoir, 
  p. 
  107, 
  considered, 
  like 
  those 
  last 
  described, 
  to 
  form 
  a 
  part 
  of 
  

   the 
  great 
  group 
  of 
  lava-flows 
  so 
  finely 
  exhibited 
  in 
  Snowdon. 
  

  

  Q.J.G.S. 
  No. 
  151. 
  x 
  

  

  