﻿468 
  he. 
  j. 
  pakkinson 
  on 
  some 
  [Dec. 
  1 
  89 
  7, 
  

  

  The 
  spherulites, 
  or 
  rather 
  axiolites, 
  for 
  they 
  are 
  often 
  twice 
  as 
  long 
  

   as 
  broad, 
  show 
  marked 
  stages 
  of 
  growth 
  even 
  macroscopically 
  ; 
  indeed, 
  

   differences 
  of 
  texture, 
  accentuated 
  by 
  an 
  appearance 
  of 
  discontinuity 
  

   between 
  the 
  inner 
  and 
  more 
  peripheral 
  parts, 
  suggest 
  an 
  interval 
  

   during 
  their 
  development 
  when 
  growth 
  ceased 
  (PI. 
  XXX 
  VT, 
  fig. 
  2). 
  

   The 
  outer 
  border 
  is 
  frequently 
  clear, 
  in 
  contrast 
  to 
  the 
  darker, 
  

   almost 
  flocculent 
  appearance 
  of 
  the 
  centre 
  ; 
  and 
  the 
  exterior 
  is 
  a 
  

   surface 
  of 
  easy 
  parting, 
  since 
  fracture 
  of 
  the 
  specimen 
  often 
  leaves 
  

   the 
  knob-like 
  surfaces 
  of 
  the 
  axiolites 
  standing 
  out. 
  

  

  The 
  matrix 
  contains 
  a 
  few 
  porphyritic 
  crystals 
  of 
  both 
  quartz 
  and 
  

   monoclinie 
  felspar, 
  and 
  a 
  second, 
  less 
  numerous 
  generation 
  of 
  smaller 
  

   spherulites. 
  In 
  thin 
  section 
  it 
  is 
  pale 
  green 
  in 
  colour, 
  slightly 
  

   dichroic, 
  very 
  feebly 
  anisotropic, 
  and 
  easily 
  marked 
  with 
  a 
  knife. 
  

   The 
  smaller 
  spherulites 
  to 
  which 
  I 
  have 
  just 
  referred 
  contain 
  centrally 
  

   a 
  greenish 
  substance, 
  no 
  doubt 
  an 
  inclusion 
  of 
  the 
  matrix 
  in 
  which 
  

   they 
  lie. 
  In 
  places 
  there 
  is 
  a 
  slightly 
  radial 
  or 
  tufted 
  habit 
  which 
  

   recalls 
  chlorite, 
  but 
  the 
  chemical 
  difficulties 
  * 
  in 
  supposing 
  the 
  whole 
  

   of 
  the 
  matrix 
  to 
  be 
  converted 
  into 
  such 
  a 
  mineral 
  are 
  considerable, 
  

   and 
  I 
  am 
  indebted 
  to 
  Prof. 
  Bonney 
  for 
  the 
  suggestion 
  that 
  this 
  

   somewhat 
  structureless 
  substance 
  is 
  probably 
  of 
  the 
  nature 
  of 
  a 
  glassy 
  

   residuum, 
  in 
  which 
  the 
  iron 
  silicates 
  have 
  been 
  hydrated, 
  and 
  which 
  

   would 
  answer 
  to 
  the 
  palagonite 
  of 
  the 
  more 
  basic 
  rocks. 
  

  

  In 
  some 
  instances, 
  this 
  hydrated 
  glass 
  is 
  reduced 
  almost 
  to 
  a 
  

   minimum 
  (PL 
  XXXVI, 
  fig. 
  3), 
  and 
  the 
  grouping 
  of 
  some 
  of 
  the 
  

   axiolites, 
  which 
  are 
  similar 
  to 
  those 
  just 
  described, 
  recalls 
  a 
  figure 
  of 
  

   Zirkel's 
  of 
  a 
  specimen 
  from 
  Nevada. 
  2 
  In 
  the 
  present 
  as 
  in 
  the 
  other 
  

   case, 
  the 
  elongated, 
  often 
  sinuous, 
  appearance 
  of 
  the 
  axiolites 
  suggests 
  

   that 
  flow-movement 
  was 
  indirectly 
  concerned 
  in 
  their 
  development, 
  

   but 
  the 
  dark, 
  granular, 
  fluidal 
  lines 
  of 
  the 
  American 
  rock, 
  forming 
  the 
  

   meshes 
  in 
  which 
  the 
  axiolitic 
  growths 
  took 
  place, 
  are 
  not 
  present 
  here. 
  

   It 
  seems, 
  therefore, 
  that 
  the 
  curving 
  and 
  at 
  the 
  same 
  time 
  complete 
  

   structure 
  is 
  best 
  explained 
  by 
  concluding 
  that 
  after 
  their 
  formation, 
  

   but 
  while 
  they 
  were 
  still 
  in 
  a 
  plastic 
  state, 
  a 
  slight 
  movement 
  in 
  

   the 
  unsolidified 
  magma 
  occurred, 
  forcing 
  together 
  and 
  bending 
  the 
  

   axiolites, 
  producing 
  effects 
  analogous 
  to 
  the 
  contortion 
  so 
  often 
  seen 
  

   in 
  a 
  flow-band, 
  and 
  allowing 
  little 
  tongues 
  of 
  glass 
  to 
  penetrate 
  the 
  

   axiolites 
  where 
  such 
  an 
  entrance 
  could 
  be 
  effected. 
  

  

  It 
  is 
  interesting 
  to 
  notice 
  that 
  in 
  one 
  or 
  two 
  cases 
  a 
  fluxional 
  

   movement, 
  which 
  affected 
  the 
  particles 
  prior 
  to 
  the 
  beginning 
  of 
  

   spherulitic 
  growth, 
  has 
  left 
  a 
  permanent 
  impress. 
  This 
  is 
  seen 
  in 
  

   the 
  attempted 
  formation 
  of 
  a 
  spherulite 
  round 
  a 
  porphyritic 
  felspar 
  ; 
  

   the 
  bending 
  of 
  the 
  flow-lines 
  round 
  the 
  crystal 
  is 
  still 
  distinctly 
  

   seen, 
  while 
  a 
  little 
  distance 
  away 
  the 
  radial 
  growth 
  and 
  zoned 
  

   structure 
  of 
  the 
  spherulite 
  are 
  more 
  conspicuous. 
  

  

  * 
  ' 
  Chemical 
  difficulties 
  ' 
  : 
  these 
  consist 
  in 
  the 
  large 
  percentage 
  of 
  silica 
  and 
  

   the 
  small 
  percentage 
  of 
  magnesia 
  which 
  rocks 
  of 
  this 
  kind 
  contain. 
  Assuming 
  

   such 
  a 
  matrix 
  to 
  be 
  changed 
  in 
  any 
  great 
  degree 
  into 
  chlorite, 
  it 
  is 
  not 
  easy 
  to 
  see 
  

   what 
  is 
  to 
  become 
  of 
  the 
  excess 
  of 
  the 
  former, 
  or 
  how 
  the 
  deficiency 
  in 
  the 
  

   latter 
  constituent 
  is 
  to 
  be 
  made 
  up. 
  

  

  2 
  ' 
  Micro. 
  Petr.' 
  p. 
  171, 
  pi. 
  vi. 
  fig. 
  2. 
  

  

  