﻿NODULAR 
  EELSTONES 
  OF 
  THE 
  LLEYN. 
  255 
  

  

  evidence 
  of 
  the 
  process 
  of 
  silicification, 
  which 
  is 
  illustrated 
  in 
  all 
  the 
  

   quartz-filled 
  vesicles, 
  and 
  in 
  changes 
  superinduced 
  in 
  the 
  matrix 
  of 
  

   the 
  rocks. 
  

  

  III. 
  Nodular 
  Structures. 
  

  

  In 
  the 
  Survey 
  Memoir 
  the 
  occurrence 
  of 
  these 
  structures 
  is 
  

   noted*, 
  but 
  no 
  details 
  are 
  given 
  of 
  their 
  various 
  forms. 
  

  

  1. 
  Perlitic 
  /Spheroids 
  and 
  Concentric 
  Structures 
  in 
  Nodules. 
  — 
  

   Certain 
  of 
  the 
  nodules 
  appear 
  to 
  be 
  the 
  result 
  of 
  simple 
  contraction 
  f 
  , 
  

   since 
  they 
  are 
  lithologically 
  similar 
  to 
  the 
  rock 
  which 
  includes 
  them 
  ; 
  

   in 
  fact, 
  the 
  boundary 
  may 
  be 
  developed 
  only 
  on 
  a 
  weathered 
  surface. 
  

   In 
  a 
  slide 
  from 
  one 
  example 
  there 
  is 
  no 
  difference 
  in 
  the 
  closely 
  sphe- 
  

   rulitic 
  ground-mass 
  of 
  nodule 
  and 
  of 
  rock, 
  except 
  a 
  slightly 
  darker 
  

   staining 
  in 
  the 
  neighbourhood 
  of 
  the 
  boundary-crack. 
  

  

  Some 
  of 
  the 
  masses 
  found 
  towards 
  the 
  east 
  of 
  Careg-y-defaid 
  seem 
  

   to 
  be 
  due 
  to 
  flow-brecciation 
  ; 
  they 
  are 
  subangular, 
  varying 
  from 
  

   about 
  one 
  inch 
  to 
  three 
  feet 
  across, 
  and 
  appear 
  firmer 
  and 
  more 
  

   silicified 
  than 
  the 
  somewhat 
  schistose 
  matrix. 
  Many 
  of 
  them 
  are 
  

   slightly 
  elongated 
  in 
  the 
  direction 
  of 
  the 
  lamination, 
  and 
  are 
  marked 
  

   by 
  a 
  parallel 
  fissuring, 
  along 
  which 
  the 
  ground-mass 
  often 
  penetrates 
  

   into 
  the 
  nodules. 
  

  

  One 
  of 
  these 
  brecciated 
  masses, 
  roughly 
  rhomboidal 
  in 
  shape 
  and 
  

   rather 
  larger 
  (about 
  six 
  feet 
  by 
  three 
  and 
  a 
  half 
  feet), 
  is 
  full 
  of 
  

   half-inch 
  spheroids, 
  which 
  exhibit 
  sintery-looking 
  concentric 
  shells, 
  

   recalling 
  the 
  description 
  given 
  by 
  Zirkel 
  of 
  spherulites 
  thus 
  weather- 
  

   ing 
  %. 
  Microscope 
  sections 
  show 
  a 
  kind 
  of 
  granular 
  micrographic 
  

   growth, 
  with 
  a 
  partial 
  attempt 
  at 
  a 
  radial 
  arrangement, 
  the 
  concentric 
  

   hollows 
  apparently 
  developing 
  within 
  the 
  less 
  fibrous 
  zones. 
  If 
  an 
  

   external 
  part 
  of 
  the 
  lava 
  had 
  been 
  broken 
  up 
  in 
  the 
  flow 
  it 
  would 
  

   probably 
  be 
  vesicular 
  ; 
  and 
  it 
  is 
  conceivable 
  that, 
  under 
  the 
  altered 
  

   conditions, 
  vapours 
  which 
  were 
  contained 
  might 
  expand 
  to 
  form 
  

   the 
  irregular 
  concentric 
  hollows, 
  somewhat 
  in 
  the 
  way 
  described 
  by 
  

   von 
  Eichthofen§. 
  We 
  might 
  suppose, 
  however, 
  from 
  the 
  partial 
  

   and 
  discontinuous 
  spherulitic 
  structure 
  which 
  borders 
  the 
  internal 
  

   cavity 
  and 
  the 
  exterior 
  boundary, 
  that 
  the 
  radial 
  growth 
  was 
  pos- 
  

   sibly 
  connected 
  in 
  its 
  origin 
  with 
  contraction 
  towards 
  a 
  centre, 
  and 
  

   that 
  planes 
  of 
  weak 
  cohesion 
  might 
  have 
  thus 
  arisen, 
  as 
  Professor 
  

   Bonney 
  suggested 
  in 
  explanation 
  of 
  the 
  spherulitic 
  felsite 
  of 
  Arran 
  ||. 
  

   The 
  quickly-cooled 
  and 
  probably 
  partially-cooled 
  mass, 
  broken 
  

   up 
  and 
  carried 
  along 
  by 
  the 
  renewed 
  flow 
  of 
  the 
  lava, 
  would 
  be 
  

   doubtless 
  subject 
  to 
  more 
  than 
  one 
  series 
  of 
  contractions, 
  and 
  the 
  

  

  * 
  Geology 
  of 
  North 
  Wales, 
  p. 
  220. 
  

  

  t 
  As 
  in 
  examples 
  described 
  by 
  Professor 
  Bonney, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  

   1882, 
  vol. 
  xxxviii. 
  p. 
  295 
  ; 
  cf. 
  M. 
  A. 
  de 
  Lapparent, 
  Bull, 
  de 
  Soc. 
  Geol. 
  de 
  Fr. 
  

   1884, 
  3 
  e 
  ser. 
  t. 
  xii. 
  p. 
  287, 
  on 
  Jersey 
  nodules. 
  

  

  X 
  U. 
  S. 
  Explor. 
  of 
  the 
  40th 
  Parallel, 
  Micr. 
  Petr. 
  p. 
  212. 
  

  

  § 
  Jahrb. 
  der. 
  k.-k. 
  geol. 
  Eeichs. 
  1860, 
  p. 
  181. 
  

  

  || 
  " 
  Pitchstones 
  and 
  Felsites 
  of 
  Arran," 
  Geol. 
  Mag. 
  1877, 
  dec. 
  2, 
  vol. 
  iv. 
  p. 
  510. 
  

   In 
  specimens 
  of 
  the 
  spherulitic 
  felsite 
  which 
  Professor 
  Bonney 
  kindly 
  showed 
  

   to 
  me 
  the 
  exfoliating 
  layers 
  with 
  their 
  "white 
  dust" 
  had 
  an 
  appearance 
  not 
  

   unlike 
  in 
  miniature 
  that 
  of 
  the 
  concentric 
  shells 
  described 
  above. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  178. 
  u 
  

  

  