﻿296 
  LT.-GEN. 
  C. 
  A. 
  MCHAUON 
  & 
  CAPT. 
  A. 
  H. 
  MCMAHON 
  [Allg. 
  1 
  897, 
  

  

  water, 
  but 
  the 
  specific 
  gravity 
  even 
  after 
  these 
  precautions 
  had 
  been 
  

   taken 
  was 
  in 
  almost 
  every 
  case 
  too 
  low. 
  The 
  specimens 
  doubtless 
  

   contained 
  hidden 
  vesicles 
  filled 
  with 
  air, 
  or 
  gas, 
  which 
  the 
  water 
  

   failed 
  to 
  reach. 
  The 
  specific 
  gravities 
  given 
  above 
  were 
  obtained 
  

   by 
  the 
  use 
  of 
  a 
  specific-gravity 
  bottle. 
  The 
  rock 
  was 
  reduced 
  to 
  

   powder 
  and 
  boiled, 
  and 
  the 
  weighing 
  was 
  done 
  with 
  the 
  aid 
  of 
  a 
  

   chemical 
  balance. 
  

  

  The 
  low 
  specific 
  gravities 
  obtained 
  are, 
  I 
  think, 
  innate 
  and 
  

   characteristic 
  of 
  these 
  rocks. 
  In 
  one 
  case 
  the 
  low 
  density 
  is 
  due 
  

   to 
  the 
  alteration 
  of 
  the 
  felspars, 
  which 
  in 
  this 
  sample 
  were 
  mere 
  

   pseudomorphs. 
  Fragments 
  of 
  them 
  were 
  isolated, 
  and 
  their 
  specific 
  

   gravity 
  determined 
  with 
  the 
  aid 
  of 
  a 
  heavy 
  liquid 
  and 
  a 
  Westphal's 
  

   balance. 
  It 
  was 
  too 
  low 
  for 
  any 
  felspar. 
  I 
  do 
  not 
  think, 
  however, 
  

   that 
  this 
  explanation 
  applies 
  to 
  the 
  rocks 
  generally, 
  for 
  the 
  felspars 
  

   in 
  them 
  appear 
  fairly 
  fresh. 
  Oligoclase 
  appears 
  to 
  predominate, 
  

   and 
  the 
  density 
  of 
  this 
  species 
  ranges 
  from 
  2*65 
  to 
  2-67. 
  The 
  low 
  

   specific 
  gravity 
  of 
  these 
  rocks 
  is 
  due, 
  I 
  think, 
  to 
  the 
  acid 
  character 
  

   of 
  the 
  felspars 
  ; 
  to 
  the 
  fact 
  that 
  they 
  contain 
  inclusions 
  of 
  glass 
  ; 
  

   and 
  to 
  the 
  presence 
  of 
  a 
  glassy 
  base. 
  The 
  mean 
  density 
  of 
  pitch- 
  

   stone 
  is 
  2-34 
  and 
  of 
  obsidian 
  2-40. 
  Glass 
  of 
  low 
  specific 
  gravity 
  

   seems 
  to 
  be 
  present 
  in 
  sufficient 
  quantity 
  to 
  balance 
  the 
  basic 
  ferro- 
  

   magnesian 
  silicates, 
  and 
  leave 
  the 
  rock, 
  as 
  a 
  whole, 
  at 
  the 
  density 
  

   of 
  the 
  dominant 
  felspar 
  contained 
  in 
  them, 
  which 
  in 
  these 
  rocks 
  I 
  

   would 
  not 
  put 
  higher 
  than 
  2-65. 
  

  

  The 
  andesites 
  grouped 
  together 
  in 
  the 
  above 
  list 
  are 
  mainly 
  

   composed 
  (the 
  accessory 
  minerals 
  will 
  be 
  alluded 
  to 
  later) 
  of 
  

   idiomorphic 
  crystals 
  and 
  microliths 
  of 
  felspar 
  embedded 
  in 
  a 
  base 
  

   which 
  appears 
  amorphous 
  in 
  ordinary 
  light, 
  but 
  which 
  when 
  revolved 
  

   between 
  crossed 
  nicols 
  remains 
  dark 
  in 
  some 
  cases, 
  and 
  in 
  others 
  

   breaks 
  up 
  into 
  cryptocrystallinc 
  or 
  into 
  microgranular 
  felsitic 
  

   material. 
  In 
  one 
  of 
  the 
  first-named 
  cases 
  the 
  glassy 
  base 
  has 
  been 
  

   converted 
  into 
  yellow 
  palagonite, 
  but 
  in 
  places 
  the 
  original 
  purple- 
  

   brown 
  coloured 
  glass 
  remains. 
  The 
  glassy 
  base 
  during 
  the 
  later 
  

   stages 
  of 
  the 
  lava's 
  history 
  exercised 
  a 
  distinctly 
  solvent 
  action 
  on 
  the 
  

   comparatively 
  basic 
  minerals 
  that 
  had 
  crystallized 
  out 
  from 
  the 
  

   still 
  fluid 
  magma, 
  for 
  all 
  the 
  original 
  minerals 
  have 
  been 
  more 
  

   or 
  less 
  corroded 
  by 
  it. 
  

  

  The 
  felspar-microliths 
  vary 
  in 
  numbers 
  very 
  much 
  in 
  different 
  

   slices. 
  In 
  some 
  the 
  base 
  is 
  crowded 
  with 
  them, 
  but 
  in 
  one 
  or 
  two 
  

   they 
  are 
  sparse. 
  They 
  do 
  not 
  exhibit 
  in 
  their 
  orientation 
  indications 
  

   of 
  fluxion 
  except 
  partially, 
  and 
  locally, 
  in 
  their 
  relation 
  to 
  the 
  

   larger 
  felspars. 
  In 
  all 
  cases 
  they 
  belong 
  to 
  the 
  oligoclase-felspar 
  

   species. 
  Binary 
  twinning, 
  combined 
  with 
  simultaneous 
  extinction, 
  

   is 
  common, 
  though, 
  in 
  some 
  cases, 
  more 
  than 
  two 
  macles 
  are 
  to 
  be 
  

   seen. 
  The 
  high 
  refraction 
  of 
  those 
  with 
  binary 
  twins, 
  as 
  compared 
  

   with 
  the 
  refraction 
  of 
  the 
  Canada 
  balsam 
  of 
  the 
  slide, 
  shows 
  that 
  

   they 
  are 
  not 
  orthoclase. 
  

  

  The 
  felspars 
  larger 
  than 
  microliths 
  cannot 
  be 
  classed 
  as 
  felspars 
  

   of 
  first 
  and 
  second 
  generation, 
  for, 
  as 
  a 
  rule, 
  they 
  dwindle 
  gradually 
  

   in 
  size 
  from 
  large 
  tabular 
  crystals, 
  or 
  aggregates 
  of 
  crystals, 
  down 
  

  

  