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  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  shews 
  the 
  extreme 
  liquidity 
  of 
  the 
  stream, 
  which 
  must 
  have 
  shot 
  through 
  this 
  

   narrow 
  and 
  tortuous 
  valley 
  with 
  a 
  rapidity 
  truly 
  astonishing, 
  leaving 
  its 
  scum 
  

   and 
  slag 
  to 
  mark 
  the 
  height 
  which 
  it 
  attained 
  ; 
  but 
  the 
  main 
  flood 
  of 
  lava 
  not 
  

   having 
  time 
  to 
  solidify, 
  was 
  propelled 
  ever 
  onwards, 
  leaving 
  often 
  but 
  a 
  narrow 
  

   thread 
  in 
  the 
  channel 
  of 
  the 
  stream, 
  by 
  which 
  to 
  trace 
  its 
  passage. 
  

  

  One 
  circumstance 
  struck 
  me 
  very 
  forcibly 
  as 
  regards 
  the 
  mineral 
  character 
  

   of 
  this 
  lava. 
  It 
  contains 
  most 
  abundant 
  nodules 
  of 
  olivine, 
  which 
  have 
  not 
  in 
  the 
  

   least 
  the 
  appearance 
  of 
  being 
  formed 
  in 
  the 
  lava 
  by 
  a 
  slow 
  crystallizing 
  process 
  ; 
  

   but, 
  on 
  the 
  contrary, 
  the 
  irregularity 
  of 
  then- 
  forms 
  is 
  decidedly 
  fragmentary, 
  often 
  

   as 
  if 
  rolled 
  ; 
  and 
  from 
  the 
  manifest 
  impurities 
  which 
  they 
  present, 
  I 
  am 
  of 
  opinion 
  

   that 
  they 
  are 
  nothing 
  but 
  fragments 
  of 
  granite 
  in 
  a 
  peculiar 
  metamorphic 
  condi- 
  

   tion. 
  This 
  idea 
  first 
  struck 
  me 
  several 
  years 
  before 
  I 
  visited 
  the 
  Vivarais, 
  in 
  an 
  

   examination 
  of 
  the 
  lavas 
  of 
  Clermont 
  ; 
  where 
  I 
  found 
  olivine 
  masses 
  near 
  La 
  

   Barraque, 
  having 
  at 
  first 
  sight 
  the 
  aspect 
  of 
  granite 
  fragments, 
  but 
  when 
  examined, 
  

   they 
  appear 
  to 
  be 
  only 
  olivine, 
  with 
  some 
  adhering 
  reddish 
  micaceous 
  and 
  clayey 
  

   matter.* 
  The 
  olivine 
  of 
  Burzet 
  presents 
  two 
  varieties 
  of 
  a 
  pale 
  yellow 
  green, 
  and 
  

   of 
  a 
  peculiar 
  orange 
  colour. 
  They 
  have 
  no 
  concretionary 
  structure 
  depending 
  

   on 
  their 
  form. 
  Similar 
  phenomena 
  are 
  common 
  in 
  different 
  parts 
  of 
  the 
  Vivarais, 
  

   but 
  less, 
  I 
  think, 
  in 
  lavas 
  which 
  are 
  decidedly 
  columnar. 
  The 
  frequency 
  of 
  ap- 
  

   parently 
  ejected 
  masses 
  of 
  almost 
  pure 
  olivine 
  in 
  many 
  volcanic 
  countries, 
  and 
  

   most 
  especially 
  in 
  Upper 
  Eyffel 
  (as 
  at 
  the 
  crater 
  of 
  Dreiser- 
  Weyer) 
  gives 
  an 
  ad- 
  

   ditional 
  probability 
  to 
  the 
  hypothesis 
  that 
  they 
  are 
  foreign 
  substances 
  expelled 
  in 
  

   an 
  altered 
  condition. 
  Long 
  after 
  making 
  these 
  remarks, 
  I 
  noticed 
  a 
  passage 
  in 
  Mr 
  

   Scrope's 
  work 
  on 
  Central 
  France, 
  tending 
  to 
  the 
  same 
  conclusion 
  ; 
  namely, 
  that 
  

   these 
  olivine 
  nodules 
  are 
  altered 
  masses 
  of 
  pinite 
  from 
  the 
  granite, 
  f 
  

  

  In 
  1839 
  I 
  pursued 
  the 
  traces 
  of 
  the 
  lava 
  of 
  Burzet 
  about 
  4 
  miles 
  beyond 
  

   the 
  village 
  of 
  that 
  name, 
  or 
  at 
  least 
  8 
  from 
  the 
  opening 
  of 
  the 
  valley, 
  without 
  

   seeing 
  a 
  trace 
  of 
  a 
  crater 
  ; 
  and 
  my 
  time 
  did 
  not 
  then 
  allow 
  me 
  to 
  prosecute 
  the 
  

   search. 
  But 
  it 
  was 
  one 
  of 
  the 
  objects 
  of 
  my 
  second 
  visit 
  in 
  1841 
  to 
  resume 
  it 
  ; 
  

   and, 
  accordingly, 
  accompanied 
  by 
  my 
  friend 
  Mr 
  John 
  Mackintosh, 
  I 
  slept 
  

   at 
  Burzet 
  in 
  very 
  uncomfortable 
  quarters, 
  in 
  order 
  to 
  have 
  the 
  whole 
  day 
  for 
  

   our 
  excursion, 
  resolving 
  to 
  ascend 
  as 
  high 
  as 
  the 
  great 
  ridge 
  which 
  separates 
  the 
  

   waters 
  of 
  the 
  Ardeche 
  and 
  of 
  the 
  Erieux. 
  We 
  then 
  readily 
  found 
  the 
  point 
  

   which 
  I 
  had 
  before 
  reached, 
  where 
  the 
  lava 
  temporarily 
  disappears 
  from 
  the 
  

   valley, 
  though 
  there 
  is 
  no 
  appearance 
  of 
  a 
  crater 
  ; 
  and 
  this 
  disappearance 
  con- 
  

  

  * 
  M. 
  de 
  Buch 
  in 
  his 
  early 
  writings 
  (Journal 
  des 
  Mines, 
  XIII. 
  , 
  p. 
  251) 
  notices 
  the 
  olivine 
  of 
  La 
  

   Barraque, 
  without 
  saying 
  anything 
  as 
  to 
  its 
  origin, 
  except 
  that 
  he 
  considers 
  it 
  an 
  exclusive 
  character 
  

   of 
  old 
  basalt, 
  as 
  contrasted 
  with 
  lavas, 
  at 
  least 
  in 
  that 
  country. 
  

  

  t 
  Scrope, 
  p. 
  150. 
  Faujas 
  St 
  Fond 
  speaks 
  of 
  the 
  lava 
  at 
  Beaume 
  containing 
  "petits 
  eclats 
  de 
  

   granit 
  en 
  chrysolite,' 
  1 
  '' 
  which 
  seems 
  to 
  indicate 
  a 
  similar 
  opinion. 
  (Recherches, 
  p. 
  300.) 
  He 
  also 
  

   describes 
  masses 
  of 
  olivine 
  existing 
  in 
  the 
  basalt 
  of 
  the 
  valley 
  of 
  Burzet, 
  in 
  the 
  meadows 
  below 
  the 
  

   village 
  of 
  St 
  Pierre 
  Colombier, 
  weighing 
  as 
  much 
  as 
  30 
  pounds. 
  This 
  remarkable 
  statement 
  deserves 
  

   verification. 
  See 
  Recherches 
  sur 
  les 
  Volcans 
  eteints, 
  pp. 
  249 
  and 
  312. 
  

  

  