﻿266 
  

  

  MISS 
  C. 
  A. 
  RAISIN 
  ON 
  SOME 
  

  

  broken 
  and 
  moved 
  along 
  in 
  a 
  viscous 
  state 
  by 
  a 
  renewed 
  now 
  of 
  the 
  

   rock, 
  and 
  thus 
  caused 
  to 
  assume 
  an 
  oval 
  form, 
  with 
  more 
  or 
  less 
  

   lamination 
  of 
  the 
  crust. 
  

  

  At 
  Pen-y-chain 
  I 
  found 
  in 
  situ 
  one 
  illustration 
  of 
  spheroidal 
  forma- 
  

   tion 
  induced 
  around 
  a 
  foreign 
  centre,where 
  the 
  nucleus 
  was 
  a 
  rounded 
  

   agate-nodule, 
  three 
  inches 
  across, 
  with 
  a 
  thick 
  crust 
  and 
  an 
  interior 
  

   filled 
  with 
  white 
  quartz. 
  Around 
  this, 
  cutting 
  across 
  the 
  lamination 
  

   of 
  the 
  perlitic 
  and 
  spherulitic 
  rock, 
  was 
  a 
  crack 
  defining 
  a 
  sphere 
  some 
  

   13 
  inches 
  in 
  diameter. 
  The 
  part 
  within 
  the 
  crack 
  was 
  slightly 
  

   darkened, 
  but 
  otherwise 
  similar 
  to 
  the 
  mass 
  of 
  the 
  rock. 
  Prof. 
  

   Bonney 
  describes, 
  in 
  a 
  microscopic 
  slide, 
  a 
  cracking 
  similarly 
  

   caused 
  by 
  a 
  strain 
  around 
  spherulites 
  in 
  a 
  devitrified 
  glass 
  *. 
  

  

  I 
  might 
  mention 
  also, 
  in 
  connexion 
  with 
  these 
  specimens, 
  an 
  

   example 
  which 
  I 
  found 
  some 
  years 
  ago 
  in 
  a 
  road-heap 
  near 
  Dolgelly. 
  

   The 
  rock 
  is 
  a 
  kind 
  of 
  diabase, 
  and 
  has 
  a 
  dark 
  ground-mass, 
  in 
  which 
  

   are 
  imbedded 
  grey, 
  flinty-looking, 
  oval- 
  shaped 
  nodules 
  without 
  

   radial 
  structure. 
  The 
  matrix 
  is 
  a 
  slaggy-looking 
  mass 
  with 
  deposit 
  

   of 
  viridite, 
  and 
  contains 
  small 
  felspars 
  of 
  plagioclase 
  form. 
  The 
  

   nodules 
  are 
  similar, 
  except 
  that 
  viridite 
  is 
  absent 
  from 
  their 
  devit- 
  

   rified 
  ground- 
  mass, 
  although 
  it 
  occasionally 
  spreads 
  along 
  cracks, 
  or 
  

   aggregates 
  around 
  what 
  are 
  apparently 
  central 
  cavities 
  filled 
  with 
  

   quartz. 
  Porphyritic 
  felspars, 
  both 
  orthoclase 
  and 
  plagioclase, 
  occur, 
  

   generally 
  normal 
  in 
  appearance 
  ; 
  but 
  one 
  crystal 
  seems 
  to 
  be 
  replaced 
  

   by 
  quartz-grains. 
  There 
  is 
  small 
  epidote, 
  occurring 
  in 
  connexion 
  with 
  

   dark 
  opaque 
  crystals. 
  

  

  Summary. 
  

  

  I. 
  At 
  both 
  the 
  headlands 
  of 
  Pen-y-chain 
  and 
  Careg-y-defaid 
  the 
  

   character 
  of 
  the 
  rocks 
  clearly 
  negatives 
  the 
  theory 
  of 
  intrusion. 
  

   They 
  are 
  old 
  lava-flows, 
  once 
  glassy, 
  now 
  devitrified, 
  and, 
  at 
  Pen-y- 
  

   chain, 
  with 
  interbedded 
  agglomeratic 
  and 
  ashy 
  strata. 
  The 
  pro- 
  

   portion 
  of 
  silica 
  has 
  probably 
  undergone 
  subsequent 
  alteration, 
  but 
  the 
  

   frequency 
  of 
  porphyritic 
  plagioclase 
  suggests 
  that 
  the 
  rocks 
  approach 
  

   nearer 
  to 
  dacites 
  than 
  to 
  rhyolites 
  f 
  . 
  As 
  to 
  the 
  slratigraphical 
  

   position 
  of 
  the 
  Pen-y-chain 
  rocks, 
  apart 
  from 
  any 
  vague 
  suggestion 
  

   of 
  lithological 
  resemblance, 
  the 
  general 
  dip 
  and 
  the 
  indications 
  of 
  a 
  

   succession 
  near 
  Llym 
  gwyn 
  seem 
  to 
  afford 
  a 
  tolerable 
  certainty 
  of 
  

   the 
  Bala 
  age 
  of 
  these 
  volcanic 
  accumulations. 
  The 
  dip 
  is 
  towards 
  

   the 
  north 
  or 
  east 
  of 
  north, 
  and 
  the 
  rocks 
  therefore 
  belong 
  to 
  the 
  

   southern 
  part 
  of 
  the 
  synclinal 
  in 
  which 
  the 
  Bala 
  beds 
  of 
  the 
  Lleyn 
  

   are 
  arranged. 
  As 
  suggested 
  in 
  the 
  Survey 
  memoir 
  J, 
  this 
  synclinal 
  

   may 
  be 
  traced 
  eastwards, 
  and 
  the 
  ashy 
  beds 
  near 
  Pwllheli 
  may 
  be 
  

   taken 
  to 
  represent 
  similar 
  strata 
  of 
  Snowdon 
  and 
  Moel 
  Hebog. 
  In 
  

   like 
  manner 
  we 
  may 
  probably 
  correlate 
  the 
  Pen-y-chain 
  rocks 
  with 
  

   felsites 
  exposed 
  in 
  those 
  mountains. 
  The 
  mass 
  of 
  felstone 
  also 
  near 
  

   Pwllheli, 
  examined 
  either 
  in 
  the 
  field 
  or 
  by 
  the 
  microscope, 
  shows 
  

  

  * 
  Pres. 
  Adtfr. 
  Geol. 
  Soc. 
  1885, 
  p. 
  64. 
  

  

  t 
  Cf. 
  Kosenbusch, 
  ' 
  Mikr. 
  Pkys. 
  der 
  rnassigen 
  Gesteine,' 
  ii. 
  Abth. 
  p. 
  418. 
  

  

  + 
  Geol. 
  of 
  North 
  Wales, 
  p. 
  218. 
  

  

  