﻿524 
  MAJOR-GENERAL 
  C. 
  A. 
  M'MAHON 
  ON 
  THE 
  HORNBLENDE-SCHISTS 
  

  

  of 
  hornblende-schists 
  from 
  Polurrian, 
  Cadgwith, 
  and 
  Hot 
  Point, 
  

   situated 
  on 
  the 
  east 
  and 
  west 
  coasts 
  of 
  the 
  Lizard, 
  and 
  found 
  that 
  

   they 
  also 
  contained 
  good 
  specimens 
  of 
  malacolite. 
  It 
  is 
  clear, 
  then, 
  

   that 
  the 
  presence 
  of 
  pyroxene 
  in 
  the 
  hornblende-schists 
  of 
  the 
  Lizard 
  

   is 
  not 
  a 
  mere 
  local 
  peculiarity 
  limited 
  to 
  one 
  place. 
  

  

  The 
  extracts 
  from 
  the 
  writings 
  of 
  previous 
  observers 
  giren 
  

   in 
  the 
  preceding 
  pages 
  show 
  that 
  the 
  idea 
  of 
  attributing 
  an 
  

   original 
  volcanic 
  origin 
  to 
  the 
  hornblende-schists 
  of 
  this 
  district 
  

   naturally 
  suggested 
  itself 
  to 
  those 
  first 
  in 
  the 
  field 
  ; 
  and 
  I 
  think 
  

   the 
  study 
  of 
  these 
  rocks 
  under 
  the 
  microscope 
  goes 
  a 
  long 
  way 
  

   to 
  confirm 
  this 
  diagnosis. 
  Not 
  only 
  are 
  all 
  the 
  minerals 
  found 
  

   in 
  them, 
  minerals 
  commonly 
  found 
  in 
  rocks 
  of 
  volcanic 
  origin, 
  but 
  

   the 
  microscope 
  strongly 
  supports 
  the 
  inference 
  that 
  the 
  hornblende, 
  

   which 
  now 
  enters 
  so 
  largely 
  into 
  their 
  composition, 
  is 
  a 
  secondary 
  

   product 
  after 
  augite, 
  and 
  that 
  the 
  beds, 
  when 
  originally 
  deposited, 
  

   were 
  composed 
  principally 
  of 
  felspar 
  and 
  pyroxene. 
  Malacolite 
  

   may 
  be 
  seen 
  in 
  the 
  thin 
  slices 
  described 
  above 
  in 
  every 
  stage 
  of 
  

   conversion 
  into 
  hornblende. 
  Not 
  only 
  are 
  malacolite-crystals 
  sur- 
  

   rounded 
  by 
  hornblende, 
  but 
  in 
  many 
  cases 
  a 
  single 
  crystal 
  may 
  be 
  

   seen 
  to 
  consist 
  in 
  part 
  of 
  augite 
  and 
  in 
  part 
  of 
  hornblende. 
  

  

  De 
  la 
  Beche, 
  we 
  have 
  seen, 
  was 
  disposed 
  to 
  regard 
  the 
  hornblende- 
  

   schists 
  as 
  " 
  a 
  mass 
  " 
  of 
  altered 
  " 
  ancient 
  volcanic 
  ash 
  " 
  which 
  is 
  

   "to 
  be 
  detected 
  so 
  abundantly 
  amid 
  the 
  grauwacke 
  of 
  Devon 
  and 
  

   Cornwall 
  : 
  " 
  and 
  I 
  think 
  it 
  highly 
  probable 
  that 
  not 
  only 
  the 
  

   hornblende-schists 
  but 
  also 
  a 
  portion 
  of 
  the 
  " 
  granulitic 
  group 
  " 
  

   were 
  originally 
  made 
  up 
  of 
  volcanic 
  ash 
  intermingled 
  with 
  

   beds 
  of 
  lava. 
  The 
  origin 
  of 
  ancient 
  rocks 
  so 
  highly 
  altered 
  as 
  the 
  

   Lizard 
  schists 
  are, 
  must 
  be 
  more 
  or 
  less 
  a 
  matter 
  of 
  inference. 
  

   Direct 
  proof 
  is 
  out 
  of 
  the 
  question 
  ; 
  but 
  the 
  available 
  evidence 
  

   points, 
  I 
  think, 
  in 
  one 
  direction 
  and 
  in 
  one 
  direction 
  only. 
  The 
  

   absence 
  of 
  free 
  quartz 
  is 
  very 
  much 
  against 
  the 
  supposition 
  that 
  

   they 
  were 
  originally 
  sedimentary 
  rocks 
  of 
  an 
  ordinary 
  character 
  ; 
  their 
  

   mineralogical 
  contents 
  strongly 
  suggest 
  an 
  igneous 
  origin 
  of 
  some 
  

   sort 
  ; 
  whilst 
  the 
  fact 
  that 
  they 
  are 
  bedded 
  indicates 
  that 
  they 
  belong 
  

   to 
  the 
  volcanic, 
  and 
  not 
  to 
  the 
  plutonic 
  class. 
  

  

  But, 
  admitting 
  their 
  volcanic 
  origin, 
  to 
  what 
  agency 
  are 
  we 
  to 
  

   attribute 
  the 
  present 
  foliated 
  and 
  banded 
  appearance 
  of 
  the 
  horn- 
  

   blende-schists 
  ? 
  I 
  have 
  searched 
  in 
  vain 
  for 
  evidence 
  to 
  show 
  that 
  

   the 
  banding 
  is 
  due 
  to 
  dynamic 
  deformation 
  after 
  the 
  consolidation 
  

   of 
  the 
  rocks. 
  The 
  hornblende-schists 
  and 
  the 
  " 
  granulitic 
  " 
  group, 
  

   it 
  is 
  true, 
  are 
  cut 
  up 
  by 
  numerous 
  small 
  faults 
  ; 
  they 
  are 
  cracked, 
  

   and 
  portions 
  of 
  them 
  have 
  been 
  forced 
  to 
  slide 
  over 
  other 
  portions 
  ; 
  

   but 
  these 
  slidings 
  have 
  been 
  for 
  short 
  distances 
  and 
  the 
  throw 
  of 
  

   the 
  faults 
  seems, 
  as 
  a 
  rule, 
  to 
  have 
  been 
  small*. 
  I 
  have 
  examined 
  

   under 
  the 
  microscope 
  the 
  junction 
  of 
  sliding 
  planes 
  in 
  two 
  cases 
  ; 
  

   but 
  no 
  mica 
  was 
  found 
  at 
  the 
  line 
  of 
  contact, 
  and 
  the 
  latter 
  gives 
  no 
  

   evidence 
  of 
  any 
  chemical 
  or 
  mineralogical 
  action 
  having 
  been 
  set 
  

   up 
  by 
  friction. 
  I 
  have 
  not 
  come 
  across 
  a 
  single 
  faulted 
  felspar 
  in 
  

   the 
  hornblende-schists, 
  and 
  I 
  have 
  not 
  found 
  evidence 
  to 
  prove 
  that 
  

   * 
  Bonney, 
  Quart. 
  Joum. 
  Geol. 
  Soc. 
  xxxix. 
  p. 
  5, 
  footnote. 
  

  

  