﻿NODULAR 
  FELSTONES 
  OF 
  THE 
  LLEYN. 
  251 
  

  

  marked 
  at 
  parts 
  (to 
  the 
  N.W. 
  and 
  the 
  N.E.) 
  by 
  light- 
  and 
  dark-grey 
  

   lines 
  bending 
  around 
  pinkish 
  crystals, 
  and 
  the 
  rock, 
  under 
  the 
  

   microscope, 
  is 
  seen 
  to 
  be 
  much 
  broken 
  and 
  silicified. 
  Southwards 
  

   the 
  mass 
  is 
  compact 
  and 
  uniform 
  ; 
  it 
  shows 
  at 
  one 
  place 
  curvitabular 
  

   jointing, 
  similar 
  to 
  that 
  figured 
  by 
  Professor 
  Bonney 
  from 
  the 
  

   basalt 
  of 
  La 
  Prudelle 
  *. 
  

  

  II. 
  Some 
  Petrological 
  Characters 
  of 
  the 
  Rocks. 
  

  

  The 
  general 
  character 
  of 
  the 
  ground 
  -mass 
  has 
  been 
  indicated 
  as 
  

   fluidal 
  and 
  sometimes 
  slaggy, 
  with 
  a 
  finer 
  or 
  coarser 
  devitrification. 
  

   The 
  distinct 
  crystals 
  are, 
  many 
  of 
  them, 
  alteration-products, 
  such 
  as 
  

   forms 
  of 
  viridite 
  or 
  of 
  chlorite, 
  sometimes 
  vermicular 
  or 
  spherulitic. 
  

   Leucoxene 
  is 
  found, 
  often 
  with 
  cuneiform 
  magnetite 
  within, 
  and, 
  in 
  

   one 
  rock, 
  some 
  apatite. 
  Epidote 
  is 
  not 
  uncommon, 
  and, 
  in 
  several 
  

   slides, 
  is 
  connected 
  with 
  aggregations 
  of 
  dark 
  grains, 
  which, 
  although 
  

   obscure, 
  I 
  believe 
  to 
  be 
  themselves 
  mainly 
  epidote, 
  either 
  skeleton- 
  

   crystals, 
  or 
  formed 
  as 
  pseudomorphs, 
  possibly 
  after 
  augite. 
  The 
  

   spaces 
  between 
  the 
  grains 
  cross 
  rectangularly, 
  as 
  might 
  happen 
  

   from 
  a 
  corrosion 
  of 
  that 
  mineral. 
  There 
  is 
  some 
  free 
  quartz, 
  but 
  

   the 
  felspars 
  form 
  the 
  chief 
  of 
  the 
  porphyritic 
  crystals. 
  A 
  few 
  of 
  

   these 
  exhibit 
  under 
  polarization 
  deep 
  purple 
  and 
  red 
  colours 
  ; 
  some 
  

   are 
  kaolinized, 
  some 
  are 
  clear, 
  possibly 
  a 
  result 
  of 
  pseudomorphism 
  ; 
  

   and 
  others 
  have 
  undergone 
  replacements, 
  which 
  I 
  will 
  discuss 
  

   later. 
  

  

  The 
  normal 
  perlitic 
  structure 
  of 
  glassy 
  rocks 
  is 
  well 
  shown 
  f 
  . 
  

   Sometimes 
  it 
  is 
  marked 
  by 
  viridite 
  or 
  a 
  ferruginous 
  deposit 
  ; 
  and 
  in 
  

   slides 
  from 
  some 
  noduliferous 
  localities, 
  fibrous 
  chalcedony 
  stretches 
  

   across 
  the 
  cracks 
  and 
  fills 
  them 
  up, 
  resembling 
  in 
  miniature 
  the 
  

   chalcedonic 
  bands 
  in 
  the 
  crust 
  of 
  many 
  agate 
  nodules. 
  

  

  Spherulites 
  showing 
  the 
  black 
  cross 
  occur, 
  sometimes 
  isolated 
  and 
  

   rounded 
  J. 
  Some 
  have 
  viridite 
  or 
  a 
  ferruginous 
  deposit 
  aggregated 
  

   towards 
  the 
  centre. 
  Others 
  are 
  clear 
  and 
  homogeneous, 
  and 
  look 
  as 
  if 
  

   silicified 
  ; 
  these 
  are 
  most 
  marked 
  where 
  chalcedony 
  has 
  been 
  deposited 
  

   in 
  perlitic 
  cracks. 
  They 
  often 
  form 
  the 
  whole 
  ground-mass, 
  "inter- 
  

   locking 
  with 
  irregular 
  outlines," 
  as 
  described 
  by 
  Professor 
  Bonney 
  §, 
  

   and 
  doubtless 
  indicate, 
  as 
  he 
  suggests, 
  some 
  secondary 
  change. 
  They 
  

   are 
  similar 
  to 
  the 
  spherulites 
  (" 
  herissees 
  d'asperites 
  ") 
  developed 
  

   in 
  glass 
  by 
  Daubree's 
  experiments 
  || 
  . 
  In 
  some 
  slides 
  the 
  spherulites 
  

   are 
  not 
  without 
  relation 
  to 
  perlitic 
  cracks, 
  which 
  may 
  encircle 
  them, 
  

   or 
  may 
  be 
  directed 
  towards 
  the 
  interior 
  and 
  be 
  traceable 
  again 
  

   beyond 
  the 
  interruption. 
  In 
  one 
  example 
  the 
  ground 
  shows 
  perlitic 
  

  

  * 
  " 
  On 
  Columnar, 
  Fissile, 
  and 
  Spheroidal 
  Structure," 
  Quart. 
  Journ. 
  Geol. 
  

   Soc. 
  vol. 
  xxxii. 
  1876, 
  p. 
  146, 
  fig. 
  6. 
  

  

  t 
  Compare 
  Mr. 
  Rutley, 
  " 
  Lavas 
  of 
  Glyder 
  Fawr," 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  

   1879, 
  vol. 
  xxxv. 
  p. 
  508. 
  " 
  Rocks 
  from 
  Beddgelert 
  and 
  Snowdon," 
  Quart. 
  

   Journ. 
  Geol. 
  Soc. 
  1881, 
  vol. 
  xxxvii. 
  p. 
  403. 
  

  

  X 
  '"British 
  Petrography,' 
  J. 
  J. 
  H. 
  Teall, 
  pi. 
  38. 
  

  

  § 
  Pres. 
  Addr. 
  Geol. 
  Soc. 
  1885, 
  p. 
  68. 
  

  

  || 
  Geol. 
  Exper. 
  p. 
  170, 
  fig. 
  45 
  ; 
  see 
  also 
  a 
  group 
  in 
  the 
  Saulieu 
  rock, 
  Min. 
  

   Micr., 
  Fouque 
  and 
  Levy, 
  pi. 
  xii. 
  fig. 
  1. 
  

  

  