﻿AND 
  BANDED 
  CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  LIZARD. 
  535 
  

  

  Much 
  of 
  the 
  felspar 
  is 
  visibly 
  tri 
  clinic, 
  and 
  the 
  rest, 
  in 
  which 
  

   polysynthetic 
  twinning 
  is 
  not 
  apparent, 
  presumably 
  belongs 
  to 
  this 
  

   system. 
  

  

  As 
  compared 
  with 
  the 
  specimens 
  entered 
  in 
  the 
  table 
  of 
  " 
  granu- 
  

   litic" 
  rocks 
  (ante, 
  p. 
  532) 
  the 
  chief 
  difference 
  to 
  be 
  observed 
  is 
  the 
  

   absence 
  of 
  quartz 
  (the 
  Polbarrow 
  trap 
  is 
  the 
  only 
  exception 
  to 
  this 
  

   rule) 
  and 
  the 
  comparative 
  infrequency 
  with 
  which 
  mica 
  is 
  met 
  with 
  

   in 
  the 
  true 
  eruptive 
  diorites. 
  

  

  Saving 
  in 
  the 
  absence 
  of 
  augite 
  in 
  the 
  above 
  diorites, 
  and 
  the 
  almost 
  

   total 
  absence 
  of 
  mica 
  in 
  the 
  hornblende- 
  schists, 
  there 
  is 
  little 
  or 
  no 
  

   difference 
  in 
  mineralogical 
  composition 
  between 
  the 
  hornblende- 
  

   schists 
  and 
  the 
  true 
  intrusive 
  diorites 
  described 
  in 
  this 
  paper. 
  As 
  

   before 
  stated, 
  there 
  is 
  absolutely 
  no 
  distinction 
  to 
  be 
  made 
  oat 
  under 
  

   the 
  microscope 
  in 
  the 
  structure 
  of 
  the 
  ground-mass 
  of 
  some 
  of 
  the 
  

   porphyritic 
  diorites 
  and 
  some 
  of 
  the 
  hornblende-schists 
  — 
  a 
  result 
  

   which 
  would 
  be 
  startling 
  were 
  we 
  not 
  to 
  hold 
  that 
  the 
  latter 
  consist 
  

   of 
  the 
  recrystallized 
  products 
  of 
  volcanic 
  action. 
  

  

  The 
  difficulty 
  in 
  distinguishing 
  between 
  converted 
  ash-beds 
  and 
  

   intrusive 
  diorite, 
  when 
  the 
  latter 
  inserts 
  itself 
  between 
  the 
  planes 
  of 
  

   lamination, 
  is 
  increased 
  by 
  the 
  fact 
  that 
  granite 
  has 
  been 
  injected 
  

   into 
  the 
  intrusive 
  diorites 
  as 
  well 
  as 
  into 
  the 
  sedimentary 
  beds 
  

   of 
  the 
  " 
  gran 
  ulitic" 
  group. 
  This 
  is 
  well 
  seen 
  at 
  Kennack 
  Cove, 
  

   where 
  bands 
  of 
  porphyritic 
  diorite 
  occur 
  abundantly 
  in 
  what 
  is, 
  

   petrologically 
  considered, 
  an 
  ordinary 
  diorite. 
  Both 
  have 
  been 
  

   injected 
  with 
  granite 
  in 
  the 
  way 
  characteristic 
  of 
  the 
  " 
  granulitic 
  ? 
  

   group. 
  

  

  The 
  rocks 
  of 
  this 
  group 
  are 
  usually 
  spoken 
  of 
  as 
  a 
  banded 
  group, 
  

   and 
  appearances 
  often 
  support 
  the 
  idea 
  that 
  the 
  quasi- 
  banding 
  is 
  a 
  

   regular 
  banding 
  parallel 
  to 
  the 
  bedding. 
  When 
  the 
  eye 
  of 
  the 
  

   observer 
  becomes 
  accustomed 
  to 
  these 
  rocks, 
  however, 
  the 
  banding 
  

   is 
  seen 
  to 
  be 
  extremely 
  irregular 
  iu 
  character. 
  Not 
  only 
  do 
  some 
  

   of 
  the 
  bands 
  dwindle 
  into 
  strings 
  and 
  die 
  out 
  in 
  the 
  direction 
  of 
  

   their 
  length, 
  but 
  they 
  inosculate, 
  bifurcate, 
  and 
  entangle 
  themselves 
  

   into 
  a 
  complicated 
  network 
  of 
  meshes, 
  and 
  they 
  swell 
  in 
  places 
  into 
  

   broad 
  lacunas, 
  which, 
  in 
  their 
  turn, 
  throw 
  off 
  fine 
  veins 
  in 
  all 
  

   directions. 
  Even 
  where 
  these 
  bands 
  are 
  apparently 
  most 
  regular 
  

   they 
  will 
  often 
  be 
  found, 
  when 
  followed 
  up, 
  to 
  behave 
  like 
  injections. 
  

   An 
  example 
  is 
  given 
  in 
  fig. 
  7 
  below. 
  

  

  Eig-. 
  7. 
  — 
  Forked 
  Band 
  in 
  the 
  Granulitic 
  Group. 
  

  

  The 
  above 
  description 
  shows 
  the 
  unstable 
  character 
  of 
  the 
  

   quasi-bands 
  when 
  examined 
  in 
  the 
  direction 
  of 
  their 
  length. 
  In 
  

   the 
  direction 
  of 
  their 
  breadth 
  their 
  extension 
  is 
  equally 
  unlike 
  that 
  

  

  