﻿466 
  ME. 
  J. 
  PAEKINSON 
  ON 
  SOME 
  [Dec. 
  1 
  89 
  7, 
  

  

  manner, 
  and 
  flies 
  under 
  the 
  hammer. 
  In 
  thin 
  section 
  it 
  is 
  micro- 
  

   crystalline, 
  with 
  some 
  quantity 
  of 
  microfelsitic 
  matter, 
  and 
  shows 
  

   an 
  approach 
  to 
  a 
  banded 
  structure. 
  Clearer 
  secondary 
  felspar 
  is 
  

   formed 
  in 
  some 
  instances, 
  filling 
  cracks 
  in 
  and 
  forming 
  a 
  zone 
  — 
  

   always 
  very 
  narrow 
  — 
  round 
  {he 
  porphyritic 
  crystals. 
  

  

  As 
  an 
  instance 
  of 
  essentially 
  similar 
  rocks 
  from 
  Cam 
  Alw 
  a 
  

   specimen 
  may 
  be 
  taken 
  of 
  a 
  medium 
  tint 
  of 
  blue-grey, 
  with 
  irregular 
  

   flecks 
  and 
  streaks 
  of 
  a 
  darker 
  colour 
  which, 
  on 
  the 
  broad 
  surface 
  of 
  

   the 
  specimen, 
  are 
  seen 
  to 
  have 
  a 
  certain 
  parallelism. 
  A 
  rock 
  of 
  this 
  

   character 
  is 
  typical 
  of 
  much 
  from 
  this 
  locality, 
  and 
  may 
  be 
  briefly 
  

   described 
  as 
  being 
  of 
  a 
  compact 
  nature, 
  rather 
  dark 
  in 
  col 
  our, 
  breaking 
  

   with 
  a 
  subconchoidal 
  fracture, 
  not 
  conspicuously 
  porphyritic, 
  and 
  

   weathering 
  white, 
  or 
  occasionally 
  to 
  a 
  pinkish 
  tinge. 
  The 
  quartz- 
  

   crystals 
  are 
  corroded, 
  and 
  contain 
  portions 
  of 
  the 
  ' 
  matrix/ 
  while 
  the 
  

   felspars 
  have 
  undergone 
  considerable 
  alteration. 
  

  

  In 
  some 
  cases 
  there 
  is 
  a 
  microcrystalline, 
  in 
  others 
  a 
  confused 
  

   crystallization, 
  which 
  shows 
  large 
  irregular, 
  though 
  often 
  fairly 
  

   definite, 
  patches 
  as 
  the 
  stage 
  is 
  rotated 
  between 
  the 
  two 
  nicols, 
  and 
  

   with 
  coarser 
  areas 
  and 
  veins, 
  having 
  a 
  tendency 
  also 
  in 
  places 
  to 
  a 
  

   radial 
  structure. 
  

  

  In 
  many 
  of 
  these 
  rocks 
  — 
  notably 
  from 
  Carn 
  Alw 
  — 
  flow-structure 
  

   is 
  beautifully 
  developed 
  (PL 
  XXXVI, 
  fig. 
  1). 
  In 
  a 
  typical 
  specimen 
  

   the 
  bands 
  are 
  very 
  regular, 
  about 
  '1 
  inch 
  across 
  on 
  an 
  average, 
  but 
  

   commonly 
  exist 
  as 
  mere 
  lines, 
  the 
  darker 
  containing 
  a 
  green 
  chloritic 
  

   mineral. 
  In 
  many 
  cases 
  this 
  may 
  be 
  seen 
  to 
  have 
  spread, 
  or 
  to 
  be 
  

   in 
  process 
  of 
  spreading, 
  over 
  the 
  major 
  part 
  of 
  the 
  band, 
  leaving 
  often 
  

   irregular 
  but 
  well-defined 
  patches, 
  having 
  a 
  roughly 
  radial 
  crystal- 
  

   lization. 
  There 
  are, 
  however, 
  bands 
  much 
  thinner 
  than 
  the 
  above, 
  

   possessing 
  a 
  crystallization 
  normal 
  to 
  their 
  length 
  ; 
  along 
  the 
  middle, 
  

   and, 
  finally, 
  all 
  over 
  which 
  the 
  green 
  mineral 
  has 
  spread. 
  The 
  

   slide 
  contains 
  porphyritic 
  felspars 
  — 
  monoclinic 
  — 
  and 
  occasionally 
  

   a 
  small 
  quartz-crystal. 
  The 
  constituent 
  particles 
  of 
  the 
  crypto- 
  

   crystalline 
  groundmass 
  are 
  generally 
  rather 
  elongated. 
  Macro- 
  

   scopically 
  there 
  is 
  a 
  considerable 
  resemblance 
  between 
  this 
  rock 
  and 
  

   those 
  from 
  Penrhyn, 
  near 
  Fishguard, 
  recently 
  described 
  by 
  Mr. 
  Cowper 
  

   Eeed. 
  1 
  The 
  latter 
  have 
  a 
  crystallization 
  rather 
  more 
  definite 
  and 
  at 
  

   the 
  same 
  time 
  finer, 
  but 
  the 
  Carn 
  Alw 
  rock 
  is 
  more 
  finely 
  fluidal. 
  

   The 
  principal 
  difference 
  exists 
  in 
  the 
  presence 
  of 
  felspar-microlites 
  

   and 
  in 
  the 
  absence 
  of 
  the 
  green 
  chloritic 
  substance 
  from 
  the 
  Penrhyn 
  

   rock, 
  both 
  facts 
  indicating, 
  of 
  course, 
  a 
  diversity 
  in 
  composition 
  ; 
  but 
  

   the 
  general 
  likeness 
  is 
  very 
  great. 
  

  

  Possessing 
  close 
  affinities 
  to 
  the 
  above 
  is 
  a 
  light 
  blue-grey 
  rock, 
  

   weathering 
  white, 
  and 
  traversed 
  with 
  some 
  regularity 
  by 
  innu- 
  

   merable 
  lighter 
  bands 
  and 
  lines, 
  which 
  vary 
  from 
  about 
  -005 
  to 
  

   nearly 
  "1 
  inch 
  in 
  breadth. 
  It 
  contains 
  porphyritic 
  crystals 
  of 
  both 
  

   quartz 
  and 
  felspar, 
  and 
  in 
  thin 
  section 
  the 
  bands 
  are 
  found 
  to 
  consist 
  

   of 
  greenish 
  microlites 
  and 
  of 
  granular 
  ill-defined 
  matter, 
  apparently 
  

   essentially 
  similar 
  to 
  the 
  microfelsitic 
  substance 
  scattered 
  more 
  

   sparsely 
  through 
  the 
  whole 
  of 
  the 
  slide. 
  Between 
  the 
  bands 
  are 
  

   1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  li. 
  (1895) 
  p 
  160. 
  

  

  