﻿NODULAR 
  EELSTONES 
  OF 
  THE 
  LLEYN. 
  267 
  

  

  lithologically 
  a 
  marked 
  resemblance, 
  and 
  contains 
  similar 
  nodular 
  

   structures. 
  

  

  II. 
  By 
  their 
  texture 
  the 
  rocks 
  must 
  he 
  classed 
  as 
  petrosiliceous, 
  

   and 
  they 
  illustrate 
  in 
  a 
  marked 
  manner 
  many 
  structures 
  which 
  are 
  

   probably 
  due 
  to 
  secondary 
  devitrification 
  *. 
  They 
  break 
  with 
  sub- 
  

   conch 
  oidal 
  fracture 
  ; 
  slides 
  from 
  them 
  exhibit 
  various 
  micrographic 
  

   and 
  dendritic 
  growths, 
  and 
  the 
  ground-mass 
  may 
  be 
  formed 
  of 
  

   spherulites 
  "interlocking 
  with 
  irregular 
  outlines." 
  Other 
  sphe- 
  

   rulitic 
  formation 
  occurs 
  on 
  a 
  large 
  scale 
  in 
  the 
  pyromerides 
  and 
  in 
  the 
  

   radial 
  crust 
  of 
  agate-nodules. 
  Felspar 
  crystals 
  may 
  be 
  modified 
  by 
  

   micrographic 
  structure, 
  and 
  are 
  sometimes 
  silicified, 
  and 
  chalcedony 
  

   or 
  quartz 
  is 
  found 
  filling 
  veins, 
  the 
  interior 
  of 
  nodules, 
  and 
  the 
  hollows 
  

   of 
  vesicular 
  lavas. 
  Thus 
  the 
  rocks 
  exhibit 
  much 
  evidence 
  of 
  a 
  sili- 
  

   cification 
  (and 
  often 
  of 
  a 
  radialization 
  possibly 
  connected 
  with 
  it 
  f 
  ), 
  

   and 
  we 
  may 
  imagine 
  that 
  the 
  lessening 
  activity 
  of 
  the 
  volcanoes 
  

   manifested 
  itself 
  in 
  the 
  percolation 
  of 
  heated 
  waters 
  carrying 
  silica 
  

   in 
  solution. 
  Geysers 
  may 
  have 
  been 
  in 
  eruption 
  near 
  this 
  locality 
  ; 
  

   and 
  some 
  of 
  the 
  Ordovician 
  conditions 
  of 
  the 
  Lleyn 
  may 
  have 
  been 
  

   similar 
  to 
  the 
  Miocene 
  activity 
  of 
  Schemnitz 
  or 
  to 
  modern 
  pheno- 
  

   mena 
  in 
  Iceland. 
  Possibly 
  some 
  of 
  the 
  marked 
  perlitic 
  and 
  similar 
  

   structures 
  were 
  originated, 
  or 
  intensified, 
  during 
  the 
  secondary 
  alter- 
  

   nations 
  of 
  temperature 
  in 
  this 
  Solfatara-stage 
  of 
  the 
  district. 
  

  

  III. 
  The 
  nodular 
  structures 
  seem 
  capable 
  of 
  being 
  classed 
  in 
  the 
  

   following 
  groups 
  : 
  — 
  

  

  1. 
  Contraction-spheroids 
  or 
  magnified 
  perlitic 
  structures. 
  

  

  2. 
  Masses 
  resulting 
  from 
  flow-brecciation. 
  

  

  3. 
  Solid 
  spherulites 
  or 
  pyromerides. 
  

  

  4. 
  Agate-nodules, 
  with 
  an 
  outer 
  spherulitic 
  crust. 
  

  

  5. 
  Quartzose 
  amygdaloids. 
  

  

  6. 
  Spheroidal 
  formations 
  developed 
  around 
  a 
  nucleus, 
  such 
  as 
  

   an 
  agate-nodule, 
  a 
  group 
  of 
  crystals, 
  or 
  an 
  original 
  vesicle 
  of 
  

   the 
  lava. 
  

  

  The 
  large 
  spherulites, 
  where 
  their 
  relations 
  are 
  evident, 
  seem 
  

   developed 
  either 
  along 
  certain 
  strata 
  or 
  within 
  masses 
  of 
  flow- 
  

   brecciation. 
  Generally, 
  neighbouring 
  examples 
  show 
  some 
  approxi- 
  

   mation 
  to 
  a 
  uniformity 
  in 
  their 
  size. 
  They 
  stand 
  out 
  by 
  weathering 
  

   of 
  the 
  mass, 
  as 
  is 
  very 
  noticeable 
  in 
  the 
  Jersey 
  rock, 
  where 
  Prof. 
  

   Bonney 
  stated 
  that, 
  so 
  far 
  as 
  he 
  could 
  see, 
  they 
  were 
  well 
  defined 
  

   only 
  on 
  an 
  exposed 
  surface, 
  and 
  were 
  scarcely 
  traceable 
  on 
  a 
  fresh 
  

   fracture. 
  They 
  appear 
  also 
  to 
  be 
  generally 
  uninfluenced 
  by 
  the 
  

   pressure 
  which 
  has 
  modified 
  the 
  matrix 
  in 
  certain 
  Welsh 
  localities. 
  

   Thus 
  the 
  spherulite 
  seems 
  to 
  be 
  the 
  most 
  durable 
  part 
  of 
  the 
  rock. 
  

   The 
  mass 
  of 
  the 
  rock 
  very 
  commonly 
  exhibits 
  an 
  originally 
  vesicular 
  

   character, 
  resembling 
  that 
  of 
  a 
  modern 
  trachyte. 
  The 
  matrix 
  

   surrounding 
  the 
  nodular 
  spherulites 
  consists, 
  for 
  the 
  most 
  part, 
  of 
  what 
  

   must 
  have 
  been 
  a 
  compact, 
  laminated, 
  glassy 
  lava, 
  now 
  de 
  vitrified, 
  

  

  * 
  Pres. 
  Addr. 
  Geol. 
  Soc, 
  Prof. 
  Bonney, 
  1885, 
  pp. 
  68, 
  69. 
  

  

  t 
  Compare 
  Delesse, 
  Bull. 
  Soc. 
  GeoJ. 
  de 
  Fr. 
  1852, 
  2 
  e 
  ser. 
  t. 
  ix. 
  p. 
  175. 
  

  

  