﻿10 
  MALLET 
  : 
  VOLCANOES 
  OF 
  BARREN 
  ISLAND 
  AND 
  NARCONDAM. 
  

  

  Specimens 
  of 
  the 
  older 
  lavas 
  (those 
  of 
  the 
  outer 
  cone) 
  were 
  collected 
  

   from 
  various 
  localities, 
  both 
  in 
  the 
  interior 
  of 
  the 
  crater, 
  where 
  the 
  most 
  

   ancient 
  flows 
  visible 
  are 
  exposed, 
  and 
  from 
  several 
  points 
  around 
  the 
  coast. 
  

   Generally 
  speaking, 
  they 
  are 
  more 
  or 
  less 
  vesicular, 
  with 
  small 
  crystals 
  

   of 
  white 
  translucent 
  felspar, 
  and 
  small 
  granules 
  and 
  crystals 
  of 
  bottle-green 
  

   translucent 
  augite, 
  disseminated 
  through 
  a 
  dark 
  grey 
  ground-mass. 
  

   Brownish-yellow 
  olivine 
  is 
  frequently 
  present 
  also. 
  Magnetite 
  is 
  rarely 
  

   visible 
  to 
  the 
  naked 
  eye, 
  or 
  with 
  a 
  lens, 
  but 
  the 
  specimens, 
  almost 
  without 
  

   exception, 
  act 
  on 
  the 
  needle, 
  showing 
  that 
  the 
  mineral 
  is 
  present 
  in 
  "a 
  fine- 
  

   ly 
  divided 
  state. 
  Sections 
  of 
  the 
  lava, 
  viewed 
  by 
  polarised 
  light, 
  show 
  the 
  

   felspar 
  to 
  be 
  triclinic 
  ; 
  indeed 
  striae 
  can 
  sometimes 
  be 
  seen 
  with 
  the 
  help 
  

   of 
  a 
  lens 
  only. 
  The 
  crystals 
  often 
  contain 
  glass 
  inclosures, 
  and 
  numerous 
  

   black 
  opaque 
  particles 
  many 
  of 
  which 
  have 
  the 
  section 
  of 
  magnetite 
  

   crystals. 
  Similar 
  inclosures 
  also 
  occur, 
  although 
  less 
  plentifully, 
  in 
  the 
  

   augite. 
  Under 
  the 
  microscope 
  the 
  ground-mass 
  is 
  seen 
  to 
  have 
  a 
  micro- 
  

   crystalline 
  structure, 
  and 
  to 
  be 
  made 
  up 
  of 
  minute 
  crystals 
  of 
  triclinic 
  

   felspar, 
  with 
  augite, 
  and 
  nearly 
  or 
  quite 
  opaque 
  specks, 
  which 
  appear 
  to 
  

   consist, 
  at 
  least 
  in 
  part, 
  of 
  more 
  or 
  less 
  decomposed 
  augite, 
  with 
  some 
  

   magnetite. 
  Molybdate 
  of 
  ammonium 
  indicates 
  the 
  presence 
  of 
  apatite, 
  in 
  

   some 
  specimens 
  at 
  least. 
  Whilst 
  the 
  lavas 
  are 
  commonly 
  rather 
  finely 
  

   vesicular, 
  the 
  vesicles 
  at 
  times 
  are 
  more 
  largely 
  developed. 
  Sometimes 
  

   ao»ain 
  the 
  rock 
  is 
  quite 
  compact. 
  1 
  In 
  some 
  specimens 
  the 
  porphyritic 
  

   felspar 
  crystals 
  are 
  thickly 
  strewn 
  ; 
  in 
  others 
  they 
  are 
  comparatively 
  

   few, 
  and 
  the 
  same 
  is 
  true 
  of 
  the 
  augite. 
  The 
  latter 
  mineral 
  is 
  at 
  times 
  

   partially 
  converted 
  into 
  a 
  dull 
  red 
  opaque 
  substance, 
  through 
  the 
  peroxi- 
  

   dation 
  of 
  the 
  iron. 
  Frequently 
  olivine 
  is 
  present 
  also, 
  and 
  in 
  rare 
  cases 
  it 
  

   replaces 
  the 
  augite 
  almost 
  completely. 
  The 
  ground- 
  mass 
  also 
  varies 
  in 
  

   the 
  ratio 
  it 
  bears 
  towards 
  the 
  porphyritic 
  portion 
  of 
  the 
  rock, 
  and 
  in 
  

   the 
  relative 
  proportions 
  of 
  the 
  minerals 
  of 
  which 
  it 
  is 
  composed. 
  When 
  

   these 
  have 
  been 
  altered, 
  and 
  the 
  iron 
  peroxidised, 
  the 
  ground-mass 
  

   acquires 
  a 
  dull 
  red 
  tint. 
  An 
  average 
  sample 
  of 
  the 
  lavas 
  yielded 
  49*55 
  per 
  

  

  * 
  On 
  some 
  compact 
  lava 
  near 
  the 
  landing-place 
  a 
  few 
  green 
  cupreous 
  stains 
  were 
  ob- 
  

   served, 
  the 
  copper 
  being, 
  apparently, 
  in 
  the 
  state 
  of 
  silicate. 
  

  

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