﻿38 
  MALLET 
  : 
  VOLCANOES 
  OF 
  BARREN 
  ISLAND 
  AND 
  NARCONDAM. 
  

  

  lucent 
  felspar, 
  and 
  black 
  or 
  dark-brown 
  hornblende 
  are 
  disseminated 
  

   through 
  a 
  ground-mass, 
  which 
  is 
  (generally 
  light) 
  grey 
  in 
  unaltered 
  

   specimens, 
  but 
  pale 
  red 
  in 
  those 
  which 
  have 
  undergone 
  weathering, 
  and 
  

   in 
  which 
  the 
  iron 
  has 
  been 
  peroxidised. 
  The 
  felspar 
  crystals 
  vary 
  from 
  a 
  

   quarter 
  of 
  an 
  inch 
  in 
  length 
  downwards, 
  averaging 
  perhaps 
  one-eighth 
  or 
  

   so, 
  and 
  are 
  shown 
  by 
  the 
  polariscope 
  to 
  be 
  triclinic, 
  as 
  a 
  general 
  

   rule, 
  although 
  orthoclinic 
  felspar 
  is 
  sometimes 
  present 
  in 
  a 
  subordi* 
  

   nate 
  degree. 
  Numberless 
  inclosures 
  are 
  contained 
  in 
  the 
  felspar, 
  

   glass 
  inclosures, 
  often 
  containing 
  bubbles, 
  being 
  amongst 
  the 
  most 
  

   abundant. 
  The 
  crystals 
  of 
  hornblende 
  are 
  about 
  as 
  large 
  as 
  those 
  of 
  

   felspar, 
  but 
  are 
  far 
  less 
  numerous. 
  They 
  sometimes 
  stand 
  out 
  prominently 
  

   on 
  weathered 
  surfaces 
  of 
  the 
  rock, 
  owing 
  to 
  their 
  resisting 
  disintegration 
  

   better 
  than 
  the 
  ground-mass, 
  and 
  can 
  then 
  be 
  detached 
  for 
  examination. 
  

   The 
  combination 
  oo 
  P. 
  oo 
  ? 
  oo. 
  P. 
  P. 
  occurs 
  both 
  in 
  untwinned 
  crys- 
  

   tals 
  and 
  in 
  hemitropes, 
  while 
  ooP. 
  oo? 
  oc. 
  P. 
  was 
  observed 
  in 
  hemi- 
  

   tropes 
  only. 
  In 
  thin 
  sections 
  the 
  mineral 
  is 
  dark 
  brown 
  and 
  strongly 
  

   dichroic 
  ; 
  when 
  decomposed 
  it 
  is 
  nearly 
  or 
  quite 
  opaque. 
  

  

  Besides 
  the 
  above 
  prohiinently 
  developed 
  minerals, 
  augite 
  is 
  some- 
  

   times 
  present 
  in 
  a 
  subordinate 
  degree, 
  the 
  mineral 
  having 
  in 
  thin 
  sections 
  

   a 
  pale 
  green, 
  or, 
  when 
  decomposed, 
  reddish 
  or 
  brown 
  colour. 
  An 
  occa- 
  

   sional 
  hexagonal 
  scale 
  of 
  rubellan, 
  or 
  crystal 
  of 
  magnetite, 
  1 
  may 
  sometimes 
  

   be 
  seen 
  with 
  the 
  lens, 
  and 
  minute 
  crystals 
  of 
  apatite 
  are 
  included 
  in 
  some 
  

   instances. 
  The 
  ground-mass 
  of 
  the 
  rock 
  is 
  micro-crystalline, 
  and 
  mainly 
  

   felspathic 
  (the 
  felspar 
  being 
  similar 
  to 
  that 
  of 
  the 
  larger 
  crystals), 
  with 
  

   specks 
  of 
  hornblende, 
  magnetite, 
  opacite, 
  &c. 
  An 
  average 
  sample 
  of 
  the 
  

   lavas 
  yielded 
  58*55 
  per 
  cent, 
  of 
  silica. 
  Three 
  specimens 
  (which, 
  however, 
  

   were 
  not 
  perfectly 
  free 
  from 
  minute 
  cavities) 
  were 
  found 
  to 
  Lave 
  specific 
  

   gravities 
  of 
  2*53, 
  fc-53, 
  and 
  2-58. 
  

  

  The 
  lavas 
  of 
  Narcondam 
  are 
  essentially 
  hornblende 
  andesites, 
  and 
  are 
  

   of 
  a 
  decidedly 
  more 
  acid 
  character 
  than 
  those 
  of 
  Barren 
  Island, 
  which 
  

   is 
  shown 
  by 
  the 
  greater 
  predominance 
  of 
  felspar, 
  and 
  the 
  paler 
  color 
  of 
  

  

  1 
  Thin 
  layers 
  of 
  magnetic 
  irori-sand 
  occur, 
  here 
  and 
  there, 
  on 
  the 
  beach, 
  together 
  with 
  

   sand 
  composed 
  of 
  comminuted 
  lava, 
  coral, 
  &c. 
  Many 
  of 
  the 
  grains 
  are 
  octahedral 
  and 
  

   dodecahedral 
  crystals. 
  

  

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