﻿^°1- 
  53'] 
  IGNEOUS 
  ROCKS 
  IN 
  NORTH 
  PEMBROKESHIRE. 
  469 
  

  

  In 
  some 
  cases 
  the 
  spherulites 
  are 
  not 
  so 
  clearly 
  defined 
  as 
  those 
  

   just 
  mentioned 
  ; 
  they 
  are 
  finely 
  cryptocrystalline, 
  the 
  periphery 
  is 
  

   less 
  regular, 
  and 
  the 
  radial 
  growth 
  not 
  so 
  marked. 
  

  

  The 
  separation 
  from 
  the 
  magma, 
  or 
  from 
  the 
  original 
  glass, 
  now 
  

   represented 
  by 
  the 
  inert 
  green 
  substance 
  previously 
  mentioned, 
  has 
  

   been 
  accompanied 
  by 
  the 
  formation 
  of 
  lines 
  and 
  bands, 
  white 
  in 
  the 
  

   hand-specimen, 
  in 
  contrast 
  to 
  the 
  blue-grey 
  colour 
  of 
  the 
  rest 
  of 
  the 
  

   rock, 
  and 
  weathering 
  out 
  as 
  white 
  ridges. 
  There 
  is 
  no 
  doubt 
  

   that 
  these 
  are 
  due 
  to 
  fluxional 
  action, 
  as 
  is 
  shown 
  by 
  their 
  parallel 
  

   habit, 
  and 
  the 
  manner 
  in 
  which 
  they 
  bend 
  round 
  porphyritic 
  crystals 
  ; 
  

   also 
  in 
  the 
  field 
  the 
  rock 
  passes 
  in 
  a 
  short 
  distance 
  into 
  the 
  ordinary 
  

   banded 
  type. 
  The 
  bands 
  are 
  now 
  cryptocrystalline, 
  but 
  an 
  attempt 
  

   at 
  a 
  radial 
  growth 
  is 
  shown 
  by 
  the 
  elongation 
  of 
  the 
  particles 
  

   normally 
  to 
  the 
  boundary 
  of 
  the 
  band, 
  meeting 
  in 
  a 
  median 
  line 
  of 
  

   discontinuity 
  which 
  is 
  often 
  rather 
  marked 
  between 
  crossed 
  nicols. 
  

   Accordingly 
  these 
  may 
  be 
  regarded 
  as 
  an 
  elongated 
  variety 
  of 
  axio- 
  

   lite. 
  They 
  are 
  closely 
  related 
  to 
  other 
  bands 
  — 
  in 
  fact 
  no 
  hard-and- 
  

   fast 
  line 
  can 
  be 
  drawn 
  between 
  them 
  — 
  which 
  are 
  outlined 
  by 
  

   markedly 
  scalloped 
  boundaries, 
  and 
  apparently 
  formed 
  by 
  the 
  linear 
  

   arrangement 
  and 
  mutual 
  interference 
  of 
  individual 
  spherulites. 
  

  

  It 
  follows 
  then 
  that 
  stages 
  can 
  be 
  traced 
  from 
  the 
  isolated 
  

   spherulite 
  — 
  outlined 
  by 
  the 
  green 
  inert 
  substance 
  representing 
  the 
  

   residual 
  hydrated 
  glass 
  — 
  through 
  stages 
  where 
  two 
  or 
  three 
  of 
  

   these 
  are 
  arranged 
  linearly, 
  then 
  losing 
  the 
  boundaries 
  of 
  the 
  two 
  

   sides, 
  passing 
  thus 
  into 
  an 
  axiolite, 
  or 
  into 
  a 
  flow-band 
  in 
  which 
  

   the 
  crystallization 
  is 
  normal 
  to 
  the 
  sides 
  defining 
  it 
  in 
  greater 
  or 
  

   lesser 
  degree. 
  These 
  structures 
  recall 
  the 
  ' 
  granular 
  axiolites 
  ' 
  

   mentioned 
  by 
  Zirkel, 
  1 
  which 
  in 
  section 
  show 
  two 
  series 
  of 
  roughly 
  

   wedge-shaped 
  grains 
  with 
  a 
  distinct 
  suture 
  running 
  between 
  them. 
  

   He 
  remarks 
  that 
  these 
  are 
  closely 
  related 
  to 
  the 
  fibrous 
  axiolites. 
  

   The 
  suture 
  or 
  median 
  line 
  of 
  discontinuity 
  referred 
  to 
  is 
  often 
  due 
  

   to 
  the 
  dovetailing 
  of 
  opposed 
  particles, 
  but 
  in 
  some 
  cases 
  its 
  

   straightness 
  and 
  definiteness 
  suggest 
  a 
  crack. 
  

  

  Passing 
  from 
  those 
  rocks 
  which 
  show 
  normal 
  and 
  complete, 
  to 
  

   those 
  which 
  show 
  incomplete 
  spherulites, 
  one 
  finds, 
  of 
  course, 
  the 
  

   common 
  type 
  in 
  which 
  growth 
  has 
  taken 
  place 
  from 
  several 
  centres 
  

   close 
  together, 
  so 
  that 
  mutual 
  interference 
  has 
  prevented 
  the 
  perfect 
  

   form 
  from 
  being 
  produced. 
  There 
  is, 
  however, 
  another 
  and 
  less 
  

   common 
  variety 
  which 
  is 
  characterized 
  by 
  the 
  simultaneous 
  action 
  

   of 
  fluxional 
  and 
  spherulitic 
  forces, 
  the 
  former 
  having 
  modified 
  the 
  

   complete 
  form 
  which 
  should 
  have 
  been 
  produced 
  by 
  the 
  latter. 
  

  

  As 
  an 
  example, 
  a 
  slide 
  is 
  selected 
  from 
  the 
  same 
  locality. 
  The 
  

   spherulites 
  are 
  arranged 
  in 
  bands, 
  interrupted 
  and 
  of 
  irregular 
  

   thickness, 
  parallel 
  one 
  to 
  another, 
  and 
  much 
  more 
  opaque 
  than 
  the 
  

   rock 
  in 
  which 
  they 
  lie 
  ; 
  but 
  the 
  edges 
  of 
  the 
  band 
  on 
  either 
  side 
  

   have 
  been, 
  as 
  it 
  were, 
  abruptly 
  truncated, 
  leaving, 
  however, 
  a 
  

   boundary 
  which 
  is 
  by 
  no 
  means 
  a 
  straight 
  line, 
  owing 
  to 
  the 
  

  

  14 
  Micro. 
  Petr. 
  ' 
  p. 
  174, 
  pi. 
  vii. 
  fig. 
  1. 
  Compare 
  also 
  Eutley, 
  'Felsitic 
  

   Lavas 
  of 
  England 
  & 
  Wales,' 
  Mem. 
  Geol. 
  Surv. 
  1885, 
  p. 
  5, 
  pi. 
  i. 
  figs. 
  11 
  & 
  12. 
  

  

  