﻿NODULAR 
  FELSTONES 
  OF 
  THE 
  LLEYN. 
  

  

  265 
  

  

  gested 
  those 
  which 
  have 
  been 
  denned 
  by 
  spheroidal 
  cracks 
  forming 
  

   around 
  gas-vesicles, 
  as 
  explained 
  by 
  Professor 
  Bonoey 
  in 
  felsites 
  of 
  

   North 
  Wales 
  *. 
  I 
  have, 
  from 
  near 
  Pwllheli, 
  one 
  beach-specimen, 
  

   apparently 
  formed 
  by 
  a 
  similar 
  process. 
  It 
  is 
  a 
  devitrified 
  rhyolite, 
  

   with 
  fluidal 
  structure, 
  somewhat 
  chalcedonized, 
  and 
  has 
  patches 
  of 
  

   perlitic 
  formations 
  with 
  viridite 
  deposit. 
  The 
  nodules 
  in 
  this 
  specimen 
  

   are 
  half 
  aD 
  inch 
  or 
  smaller, 
  oval 
  in 
  shape, 
  and 
  contain 
  quartz-filled 
  

   vesicles 
  (fig. 
  10). 
  The 
  quartz 
  is 
  traversed 
  by 
  lines 
  of 
  enclosures 
  

  

  Pig. 
  10. 
  — 
  Nodules, 
  not 
  spherulitic, 
  in 
  a 
  ground-mass, 
  which 
  shows 
  at 
  

   places 
  Perlitic 
  Cracks 
  with 
  Viridite 
  deposit. 
  From 
  the 
  beach 
  

   towards 
  Pen-y-Chain. 
  (Enlarged 
  3 
  diameters.) 
  

  

  

  

  The 
  crust 
  of 
  the 
  nodule 
  has 
  a 
  fluidal 
  structure, 
  and 
  is 
  more 
  opaque 
  along 
  an 
  

   exterior 
  zone. 
  The 
  interior 
  is 
  filled 
  up 
  with 
  granular 
  chalcedony 
  ; 
  but 
  in 
  

   one 
  example 
  the 
  nodule 
  encloses 
  porphyritic 
  felspars. 
  

  

  with 
  moving 
  bubbles, 
  and 
  the 
  lines 
  pass 
  from 
  grain 
  to 
  grain, 
  as 
  in 
  

   the 
  example 
  figured 
  by 
  Mr. 
  Cole 
  f 
  . 
  The 
  lines 
  are 
  parallel 
  to 
  the 
  

   longer 
  diameter 
  of 
  the 
  nodules, 
  as 
  would 
  result 
  if 
  the 
  enclosures 
  

   had 
  originated 
  from 
  the 
  action 
  of 
  pressure, 
  in 
  the 
  way 
  described 
  by 
  

   Prof. 
  Judd 
  %. 
  The 
  edge 
  of 
  the 
  nodule 
  for 
  a 
  little 
  way 
  inwards 
  is 
  

   more 
  opaque, 
  but 
  otherwise 
  the 
  crust 
  is 
  similar 
  to 
  the 
  surrounding 
  

   mass, 
  and 
  exhibits 
  a 
  fluidal 
  lamination, 
  like 
  that 
  described 
  by 
  Prof. 
  

   Bonney 
  in 
  nodules 
  from 
  near 
  Conway 
  §. 
  One 
  of 
  the 
  spheroidal 
  

   cracks 
  seems 
  to 
  have 
  formed 
  partially 
  around 
  a 
  group 
  of 
  broken 
  

   porphyritic 
  felspar, 
  something 
  like 
  the 
  crack 
  around 
  olivine 
  described 
  

   by 
  Mr. 
  Rutley 
  ||. 
  A 
  somewhat 
  different 
  explanation 
  for 
  these 
  

   nodules 
  was 
  suggested 
  to 
  me 
  as 
  a 
  possibility 
  by 
  Prof. 
  Bonney 
  — 
  that 
  

   fragments 
  of 
  the 
  lava 
  crust, 
  enclosing 
  vesicles, 
  might 
  have 
  been 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  1882, 
  vol. 
  xxxviii. 
  p. 
  295. 
  

   t 
  Ibid. 
  1886, 
  vol. 
  xlii. 
  p. 
  188. 
  

   X 
  Ibid. 
  1885, 
  vol. 
  xli. 
  p. 
  376. 
  

   § 
  Ibid. 
  1882, 
  vol. 
  xxxviii. 
  p. 
  294. 
  

   || 
  Proc. 
  Roy. 
  Soc. 
  1886, 
  p. 
  437. 
  

  

  