﻿NODTJLAK 
  FELSTONES 
  OF 
  THE 
  LLETN". 
  259 
  

  

  Some 
  intermingling 
  in 
  structure 
  of 
  the 
  outer 
  crust 
  and 
  of 
  the 
  

   quartz-deposit 
  within 
  seems 
  traceable 
  in 
  a 
  few 
  nodules 
  ; 
  but 
  if 
  the 
  

   present 
  condition 
  of 
  the 
  spherulites 
  had 
  resulted 
  from 
  gradual 
  change 
  

   and 
  concentration 
  of 
  silica 
  *, 
  or 
  other 
  slow 
  alteration, 
  we 
  should 
  

   expect 
  such 
  gradations 
  to 
  be 
  much 
  more 
  marked 
  and 
  general. 
  In 
  

   the 
  large 
  number 
  of 
  examples 
  the 
  agate 
  contained 
  would 
  usually 
  be 
  

   considered 
  to 
  have 
  been 
  formed 
  by 
  the 
  filling 
  of 
  a 
  cavity 
  already 
  exist- 
  

   ing 
  as 
  a 
  cavity, 
  and 
  the 
  transitional 
  zone, 
  traceable 
  only 
  when 
  highly 
  

   magnified, 
  does 
  not 
  seem 
  more 
  than 
  might 
  arise 
  in 
  the 
  process 
  of 
  

   infilling 
  and 
  siliceous 
  alteration. 
  Such 
  modification 
  would 
  be 
  facili- 
  

   tated 
  if 
  there 
  was 
  originally 
  a 
  vesicular 
  nucleus, 
  to 
  act, 
  as 
  Prof. 
  

   Judd 
  has 
  said, 
  as 
  a 
  " 
  laboratory 
  of 
  synthetic 
  mineralogy 
  " 
  f 
  . 
  In 
  

   some 
  cases 
  the 
  cavity 
  may 
  have 
  had 
  a 
  frothy 
  pumiceous 
  substance 
  

   around 
  or 
  within 
  it, 
  the 
  disorganization 
  of 
  which 
  may 
  have 
  caused 
  

   the 
  chalcedonic 
  deposit 
  to 
  be, 
  at 
  places, 
  less 
  clean 
  and 
  clear. 
  

  

  3. 
  Examples 
  of 
  Agate-nodules. 
  — 
  Some 
  masses 
  of 
  flow-brecciation 
  

   at 
  Careg-y-defaid, 
  near 
  those 
  already 
  mentioned, 
  contain 
  small 
  

   rounded 
  nodules, 
  filled 
  with 
  chalcedony 
  and 
  bounded 
  by 
  a 
  thin 
  

   crust. 
  Near 
  by 
  are 
  larger 
  nodules 
  in 
  a 
  somewhat 
  trapezoidal 
  space 
  

   on 
  the 
  beach, 
  about 
  ten 
  feet 
  by 
  fifteen 
  feet, 
  which 
  has 
  the 
  appearance 
  

   of 
  a 
  conglomerate, 
  or 
  of 
  a 
  surface 
  paved 
  with 
  close-set 
  whitish-looking 
  

   pebbles. 
  The 
  ground-mass, 
  as 
  in 
  the 
  surrounding 
  rock, 
  is 
  schistose, 
  

   with 
  a 
  " 
  glistening 
  pale 
  green 
  mineral," 
  like 
  the 
  structure 
  described 
  

   by 
  Professor 
  Bonney 
  from 
  near 
  Conway 
  Palls 
  J, 
  and 
  a 
  firm 
  band 
  

   crossing 
  the 
  tract 
  has 
  slickensided 
  surfaces 
  ; 
  so 
  that 
  the 
  area 
  may 
  

   have 
  been 
  displaced 
  and 
  modified 
  by 
  crushing. 
  The 
  nodules, 
  too 
  

   (the 
  " 
  agate-like 
  nodules," 
  I 
  suppose, 
  of 
  the 
  Survey 
  memoir 
  §), 
  may 
  

   owe 
  their 
  generally 
  ellipsoidal 
  shape 
  and 
  parallel 
  arrangement 
  to 
  

   the 
  action 
  of 
  pressure. 
  A 
  longitudinal 
  section 
  shows, 
  in 
  one 
  case, 
  

   a 
  squeezed-looking 
  X-shaped 
  interior, 
  and 
  in 
  others, 
  as 
  in 
  the 
  am- 
  

   monite-like 
  lithophysse 
  of 
  von 
  Richthofen 
  [|, 
  three 
  or 
  more 
  irregular 
  

   chambers 
  superimposed 
  (fig. 
  5), 
  as 
  if 
  vesicles 
  of 
  adjacent 
  layers 
  had 
  

   been 
  included 
  in 
  one 
  nodule. 
  The 
  cavities 
  are 
  filled 
  with 
  chalcedony 
  ; 
  

   the 
  crust 
  contains 
  porphyritic 
  crystals, 
  and 
  its 
  spherulitic 
  fibres 
  

   traverse, 
  in 
  the 
  microscope 
  slide, 
  curious 
  quartzose 
  spheres, 
  which 
  

   are 
  probably 
  incipient 
  segregatory 
  structures. 
  

  

  Turning 
  to 
  Pen-y-chain, 
  we 
  find 
  agate-nodules 
  at 
  four 
  or 
  five 
  

   localities 
  %. 
  They 
  are 
  thickly 
  developed 
  on 
  the 
  east 
  coast, 
  along 
  

   both 
  sides 
  of 
  a 
  small 
  cleft, 
  which 
  seems 
  to 
  mark 
  a 
  fault. 
  The 
  

   overlying 
  beds 
  are 
  finely 
  laminated 
  : 
  the 
  ground-mass 
  has 
  a 
  perlitic 
  

   structure 
  beautifully 
  chalcedonized, 
  and 
  is 
  crowded 
  with 
  clear 
  sphe- 
  

   rulites 
  exhibiting 
  the 
  black 
  cross. 
  Large 
  nodules, 
  sometimes 
  

  

  * 
  J. 
  Szabo, 
  Jahrb. 
  k.-k. 
  geol. 
  Eeichs. 
  1866, 
  p. 
  89. 
  

  

  f 
  " 
  Tertiary 
  Gabbros," 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  1886, 
  vol. 
  xlii. 
  p. 
  83. 
  

  

  I 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  1882, 
  vol. 
  xxxviii. 
  p. 
  290. 
  

  

  § 
  Geology 
  of 
  North 
  Wales, 
  p. 
  220. 
  

  

  || 
  Jahrb. 
  k.-k. 
  geol. 
  Eeicbs. 
  1860, 
  p. 
  181. 
  

  

  ■Jf 
  Among 
  the 
  specimens 
  which 
  Mr. 
  T. 
  Davies 
  kindly 
  allowed 
  me 
  to 
  examine 
  

   was 
  a 
  slide 
  much 
  resembling 
  the 
  Pen-y-chain 
  nodules. 
  This 
  specimen 
  was. 
  

   from 
  the 
  Cleeve- 
  Church 
  boulder, 
  which 
  had 
  been 
  noted 
  by 
  Mr 
  Davies, 
  and 
  

   has 
  been 
  suggested 
  as 
  probably 
  Welsh 
  in 
  its 
  origin. 
  Min. 
  Mag. 
  1879, 
  p. 
  119. 
  

  

  