﻿NODULAR 
  FELSTONES 
  OE 
  THE 
  LLEYN. 
  

  

  263 
  

  

  Near 
  the 
  south-east 
  corner 
  of 
  the 
  Pen-y-chain 
  promontory 
  agate- 
  

   nodules 
  occur 
  in 
  beds 
  which 
  are 
  much 
  contorted, 
  and 
  are 
  appa- 
  

   rently 
  slaggy 
  lava-flows. 
  Some 
  of 
  the 
  nodules 
  are 
  large, 
  while 
  

   others, 
  more 
  crowded 
  together, 
  vary 
  only 
  from 
  about 
  | 
  inch 
  to 
  \ 
  inch 
  

   across. 
  Close 
  by, 
  a 
  rock-mass 
  occurs, 
  thickly 
  set 
  with 
  small, 
  almond- 
  

   shaped, 
  white 
  quartz-nodules, 
  usually 
  \ 
  inch 
  or 
  smaller, 
  sometimes 
  

   a 
  hollow 
  shell 
  lined 
  with 
  pyramidal 
  quartz 
  — 
  white 
  or 
  clear 
  golden. 
  

   These 
  nodules 
  appear, 
  under 
  the 
  microscope, 
  sharply 
  limited 
  (fig. 
  8), 
  

  

  Pig. 
  8. 
  — 
  Diagram 
  of 
  Amygdaloids 
  from 
  old 
  lava-flow 
  at 
  south-east 
  

   of 
  Pen-y-Chain. 
  (Enlarged 
  16 
  diameters.) 
  

  

  

  ■if 
  V-^'fe? 
  1 
  :, 
  

  

  ^T"i 
  , 
  '?\t'>i' 
  , 
  ."-i'-.i''-,y 
  

  

  V 
  

  

  a. 
  

  

  W 
  

  

  V 
  

  

  The 
  amygdaloids 
  are 
  filled 
  mainly 
  with 
  crystalline 
  quartz. 
  The 
  outer 
  grains 
  

   are 
  small 
  ; 
  those 
  of 
  the 
  interior 
  are 
  larger 
  and 
  clearer. 
  In 
  one 
  example 
  

   the 
  cavity 
  is 
  partially 
  empty 
  (a). 
  The 
  devitrified 
  ground-mass 
  contains 
  

   remains 
  of 
  porphyritic 
  felspars 
  (b). 
  

  

  although 
  some 
  of 
  the 
  quartz-grains 
  transgress 
  the 
  boundary. 
  

   Radial 
  strise 
  of 
  small 
  enclosures 
  may 
  occur 
  towards 
  the 
  exterior, 
  and 
  

   one 
  example 
  has 
  minute 
  microliths 
  ranged 
  approximately 
  along 
  

   straight 
  lines 
  in 
  two 
  directions, 
  which 
  cross 
  at 
  a 
  high 
  angle. 
  The 
  

   opinion 
  might 
  perhaps 
  be 
  advanced 
  that 
  the 
  rounded 
  nodules 
  are 
  a 
  

   variety 
  of 
  pseud-amygdals, 
  since 
  adjacent 
  structures 
  have 
  a 
  radialized 
  

  

  