﻿263 
  mss 
  c. 
  a. 
  rajhdt 
  ox 
  soivrx 
  

  

  generally 
  perlitic 
  and 
  often 
  spheralitie. 
  The 
  interior 
  of 
  the 
  nodule 
  

   is 
  in 
  many 
  cases 
  filled 
  with 
  chalcedony, 
  and 
  is 
  not 
  distinguishable 
  in 
  

   form 
  from 
  an 
  original 
  vesicle 
  of 
  the 
  lava 
  : 
  it 
  is 
  sometimes 
  rounded 
  

   and 
  amygdaloidal. 
  often 
  it 
  is 
  irregular 
  and 
  may 
  be 
  somewhat 
  

   stellate. 
  

  

  As 
  illustrating 
  their 
  mode 
  of 
  formation, 
  the 
  agate-nodules 
  of 
  the 
  

   Lleyn 
  are 
  wanting 
  in 
  the 
  freshness 
  of 
  those 
  recently 
  described 
  by 
  

   Mr. 
  Iddings 
  from 
  Obsidian 
  Cliff, 
  although 
  in 
  these 
  old 
  and 
  more 
  

   obscure 
  forms, 
  notwithstanding 
  their 
  siliceous 
  alteration, 
  a 
  connexion 
  

   with 
  vesicles 
  seems 
  to 
  be 
  indicated 
  *. 
  The 
  evidence 
  from 
  them 
  

   appears 
  to 
  support 
  these 
  considerations 
  : 
  — 
  

  

  (1 
  ) 
  That 
  spherulitic 
  growth 
  originated, 
  on 
  the 
  principle 
  described 
  

   by 
  Prof. 
  Bonney. 
  from 
  a 
  surface 
  of 
  discontinuity. 
  This 
  is 
  illustrated 
  

   by 
  the 
  radial 
  tufts 
  which 
  have 
  formed 
  in 
  one 
  rock 
  along 
  the 
  line 
  

   of 
  an 
  adjacent 
  stratum, 
  and 
  in 
  others 
  along 
  perlitic 
  cracks. 
  Gas- 
  

   filled 
  vesicles 
  might 
  be 
  specially 
  liable 
  to 
  cause 
  such 
  change 
  in 
  the 
  

   rock 
  ; 
  and 
  radial 
  growth 
  may 
  have 
  thus 
  begun 
  around 
  cavities, 
  such 
  

   as 
  those 
  forming 
  the 
  amygdaloids 
  of 
  the 
  south-east 
  of 
  the 
  Pen-y- 
  

   chain 
  promontory. 
  

  

  i 
  That 
  possibly 
  contraction 
  may 
  have 
  acted 
  (sometimes 
  around 
  

   vesicles) 
  to 
  produce 
  weakened 
  concentric 
  surfaces, 
  or 
  to 
  have 
  induced 
  

   incipient 
  spherulitic 
  growth 
  f. 
  

  

  1 
  3 
  ! 
  The 
  spherulitic 
  formation 
  sometimes 
  paused 
  and 
  recommenced, 
  

   giving 
  rise 
  to 
  a 
  concentric 
  banding. 
  Protuberant 
  ridges 
  may 
  thus 
  

   have 
  been 
  formed, 
  where 
  a 
  communication 
  was 
  kept 
  open 
  into 
  the 
  

   cavity 
  of 
  the 
  nodule. 
  The 
  renewed 
  growth 
  might 
  be 
  connected 
  with 
  

   recurrence 
  of 
  conditions 
  of 
  moderate 
  heating, 
  since 
  spherulitic 
  

   ■f.r^Lopuient 
  has 
  been 
  induced 
  artificially 
  in 
  glass, 
  under 
  temperatures 
  

   sufficient 
  to 
  soiten 
  without 
  fusing 
  its 
  substance^. 
  

  

  i 
  4: 
  | 
  Such 
  changes, 
  by 
  heating 
  or 
  an 
  alteration 
  of 
  pressure, 
  might 
  

   cause 
  irregularity 
  of 
  vesicles 
  already 
  existing, 
  or 
  brecciation 
  of 
  the 
  

   spherulitic 
  nodules, 
  possibly 
  even 
  giving 
  rise 
  to 
  fresh 
  evolution 
  of 
  

   gas, 
  if 
  the 
  volcanic 
  glass 
  were 
  of 
  the 
  hydrated 
  character, 
  which 
  has 
  

   been 
  proved 
  of 
  many 
  examples. 
  

  

  •:< 
  Cavities 
  in 
  a 
  rock 
  permeated 
  by 
  siliceous 
  infiltrations 
  would 
  

   be 
  likely 
  to 
  give 
  rise 
  to 
  some 
  modification 
  of 
  the 
  surrounding 
  crust 
  ; 
  

   and 
  where 
  this 
  was 
  already 
  spherulitic, 
  the 
  next 
  stage 
  of 
  alteration 
  

  

  * 
  Am. 
  Jonrn. 
  of 
  Bra. 
  vol 
  wviii. 
  p. 
  36. 
  It 
  is 
  possible 
  that 
  some 
  earlier 
  stage, 
  

   now 
  masked, 
  might 
  bear 
  out, 
  for 
  some 
  of 
  the 
  example?, 
  the 
  dehydration 
  theory 
  

   proposed 
  by 
  Mr. 
  ladings, 
  as 
  in 
  the 
  isolated 
  ridged 
  nodules 
  which 
  I 
  hare 
  

   described 
  in 
  an 
  old 
  obsidian 
  cliff 
  at 
  Pen-y-chain. 
  

  

  t 
  GeaL 
  Mag. 
  1877j 
  fee. 
  2. 
  vol 
  iv. 
  p. 
  510. 
  It 
  seemed 
  worth 
  considering 
  whether 
  

   in 
  the 
  formation 
  of 
  concentric 
  surfaces 
  there 
  might 
  hare 
  been 
  a 
  shrinking 
  

   due 
  to 
  the 
  passage 
  from 
  hydrated 
  to 
  anhydrous 
  glass, 
  in 
  the 
  course 
  of 
  crystal- 
  

   lization, 
  as 
  indicated 
  by 
  Mr. 
  Iddings 
  : 
  but 
  in 
  that 
  case 
  the 
  concentric 
  rings 
  

   should 
  accompany 
  the 
  more 
  marked 
  spherulitic 
  or 
  crystalline 
  structure, 
  which 
  

   does 
  not 
  seem 
  always 
  in 
  accordance 
  with 
  the 
  facts. 
  TTnless 
  we 
  could 
  suppose 
  

   that 
  the 
  primary 
  contraction 
  was 
  centripetal, 
  causing 
  incipient 
  crystallization 
  

   along 
  an 
  outer 
  limit, 
  and 
  that 
  the 
  subsequent 
  growth 
  resulted 
  in 
  a 
  drawing 
  

   together 
  of 
  the 
  crystalline 
  elements 
  towards 
  the 
  outer 
  fibrous 
  rim 
  which 
  had 
  

   already 
  "set." 
  

  

  | 
  Pres. 
  Addr. 
  Geol. 
  Boa, 
  Prof. 
  Bonney. 
  1885, 
  pp. 
  65, 
  67. 
  

  

  