﻿NODULAR 
  FELSTONES 
  OF 
  THE 
  LLEYN. 
  257 
  

  

  traversed 
  in 
  various 
  directions, 
  often 
  radially, 
  by 
  other 
  siliceous 
  

   veins. 
  

  

  2. 
  Origin 
  of 
  the 
  Agate-nodules. 
  — 
  The 
  crust 
  of 
  the 
  true 
  agate- 
  

   nodules 
  is 
  spherulitic, 
  sometimes 
  with 
  concentric 
  zones, 
  which 
  may 
  

   be 
  marked 
  by 
  an 
  infilling 
  of 
  secondary 
  chalcedony. 
  Occasionally 
  

   traces 
  of 
  the 
  rock-lamination 
  and 
  porphyritic 
  crystals 
  are 
  contained 
  

   in 
  it, 
  proving 
  here, 
  as 
  by 
  similar 
  evidence 
  and 
  by 
  chemical 
  analysis 
  has 
  

   been 
  shown 
  elsewhere, 
  that 
  the 
  crust 
  is 
  an 
  altered 
  portion 
  of 
  the 
  

   rock-mass. 
  This 
  outer 
  part 
  is 
  generally 
  lined 
  with 
  small 
  granular 
  

   quartz, 
  or 
  with 
  mamillated 
  fibrous 
  chalcedony, 
  which 
  sometimes 
  

   forms 
  beautiful 
  spherulites, 
  and 
  may 
  fill 
  up 
  the 
  interior 
  of 
  the 
  

   nodule 
  ; 
  in 
  other 
  examples 
  the 
  heart 
  of 
  the 
  cavity 
  is 
  occupied 
  by 
  

   an 
  aggregation 
  of 
  large 
  quartz 
  grains, 
  or 
  it 
  may 
  be 
  hollow, 
  with, 
  

   crystals 
  projecting 
  inwards. 
  Cavities, 
  empty 
  or 
  filled 
  in, 
  which, 
  I 
  

   presume, 
  might 
  be 
  similar 
  to 
  the 
  interior 
  of 
  these 
  nodules, 
  have 
  

   been 
  described, 
  as 
  resulting 
  from 
  decomposition 
  of 
  the 
  central 
  part 
  

   of 
  a 
  spherulite, 
  or 
  as 
  originating 
  from 
  vesicles 
  caused 
  by 
  disengage- 
  

   ment 
  of 
  gases 
  *. 
  

  

  Among 
  the 
  difficulties 
  of 
  deriving 
  the 
  nodules 
  from 
  original 
  

   spherulites 
  by 
  a 
  process 
  of 
  hollowing-out 
  are 
  the 
  occasional 
  appear- 
  

   ance 
  of 
  neighbouring 
  felspars 
  not 
  much 
  silicified, 
  and 
  the 
  occurrence, 
  

   in 
  connexion 
  with 
  the 
  central 
  cavity, 
  of 
  sharply 
  defined 
  chalcedonic 
  

   veins, 
  which 
  traverse 
  the 
  spherulitic 
  crust. 
  These 
  veins 
  sometimes 
  

   appear 
  to 
  be 
  subsequent 
  formations, 
  but 
  the 
  history 
  in 
  certain 
  of 
  the 
  

   examples 
  is 
  clearly 
  shown. 
  Chalcedonic 
  deposit 
  lined 
  the 
  central 
  

   cavity 
  and 
  filled 
  up 
  concentric 
  rings 
  in 
  the 
  crust. 
  After 
  a 
  pause, 
  

   deposition 
  recommenced, 
  and 
  the 
  heart 
  of 
  the 
  cavity, 
  which 
  had 
  

   remained 
  vacant, 
  became 
  completely 
  filled 
  up. 
  The 
  veins 
  therefore, 
  

   in 
  this 
  case 
  at 
  least, 
  cannot 
  be 
  connected 
  with 
  any 
  action 
  of 
  decom- 
  

   position, 
  since 
  they 
  can 
  be 
  traced, 
  through 
  a 
  thick 
  envelope 
  of 
  chal- 
  

   cedonic 
  deposit, 
  to 
  the 
  small 
  cavity 
  which 
  they 
  have 
  evidently 
  sup- 
  

   plied. 
  By 
  inference, 
  we 
  should 
  therefore 
  incline 
  to 
  consider 
  similar 
  

   veins 
  elsewhere 
  as 
  feeders 
  to 
  other 
  cavities 
  within 
  nodules. 
  The 
  

   fissures 
  are 
  usually 
  part 
  of 
  a 
  general 
  system 
  of 
  brecciation, 
  which 
  

   has 
  affected 
  the 
  spherulite, 
  in 
  many 
  examples, 
  either 
  radially 
  or 
  along 
  

   concentric 
  surfaces. 
  Owing 
  to 
  the 
  existence 
  of 
  radial 
  cracks, 
  a 
  

   sector 
  may 
  be 
  displaced, 
  so 
  as 
  to 
  project 
  beyond 
  the 
  surface 
  of 
  the 
  rest 
  

   of 
  the 
  sphere 
  ; 
  and 
  such 
  sectors 
  may 
  appear 
  as 
  if 
  they 
  had 
  contracted 
  

   away 
  from 
  one 
  another 
  at 
  the 
  heart 
  of 
  the 
  spherulite. 
  The 
  stellate 
  

   outline 
  of 
  the 
  central 
  cavity 
  within 
  some 
  agate-nodules 
  may 
  very 
  

   probably 
  be 
  correlated 
  (as 
  Professor 
  Bonney 
  pointed 
  out 
  to 
  me) 
  

   with 
  this 
  tendency 
  to 
  radial 
  cracking 
  and 
  tearing 
  at 
  the 
  centre. 
  

  

  Further, 
  while, 
  as 
  Mr. 
  Cole 
  suggests, 
  the 
  centre 
  of 
  the 
  spherulite 
  

   might 
  be 
  a 
  less 
  resistant 
  part 
  f, 
  yet 
  the 
  exterior 
  would 
  seem 
  to 
  be 
  

   as 
  much 
  — 
  or 
  far 
  more 
  — 
  exposed. 
  The 
  ends 
  of 
  the 
  fibres, 
  in 
  some 
  

   instances, 
  like 
  the 
  terminations 
  of 
  cleavage-planes 
  in 
  some 
  crystals, 
  

  

  * 
  See 
  " 
  Nature 
  and 
  Origin 
  of 
  Lithophysas," 
  J. 
  P. 
  Iddings, 
  Amer. 
  Journ 
  of 
  

   Sci. 
  vol. 
  xxxiii. 
  Jan. 
  1887 
  ; 
  and 
  Mr. 
  Cole, 
  Quart. 
  Journ. 
  Geol. 
  Soc 
  1 
  885, 
  vol. 
  xli. 
  

   p. 
  162 
  ; 
  and 
  references 
  given 
  in 
  these 
  papers. 
  

  

  t 
  " 
  On 
  Hollow 
  Spherulites," 
  G-. 
  A. 
  J. 
  Cole, 
  Quart. 
  Journ. 
  Ge. 
  /Soc. 
  1885, 
  

   vol. 
  xli. 
  p. 
  166. 
  

  

  V2 
  

  

  