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  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  tion 
  of 
  several 
  ridges 
  which 
  have 
  no 
  approximation 
  to 
  a 
  circular 
  arrangement 
  ; 
  

   and 
  they 
  do 
  not 
  form 
  the 
  entire 
  circumference 
  of 
  the 
  crater,* 
  but 
  only 
  about 
  one- 
  

   half 
  of 
  it 
  at 
  its 
  internal 
  base, 
  without 
  allowing 
  for 
  the 
  inequalities 
  which 
  even 
  

   in 
  that 
  part 
  are 
  often 
  filled 
  up 
  with 
  volcanic 
  slag, 
  and 
  the 
  enormously 
  thick 
  

   coating 
  of 
  conglomerate 
  above 
  the 
  granite 
  on 
  the 
  eastern 
  slope 
  of 
  the 
  crater. 
  

   Were 
  we 
  simply 
  to 
  project 
  the 
  points 
  where 
  the 
  granite 
  appears, 
  it 
  would 
  make 
  

   but 
  a 
  sorry 
  circus. 
  In 
  fact, 
  it 
  may 
  be 
  regarded 
  as 
  a 
  granitic 
  valley 
  choked 
  with 
  

   scoriae, 
  and 
  plastered 
  with 
  Roman 
  cement 
  till 
  it 
  forms 
  a 
  nearly 
  circular 
  cavity. 
  

   The 
  occurrence 
  of 
  a 
  crater 
  in 
  such 
  a 
  position 
  only 
  affords, 
  in 
  my 
  opinion, 
  a 
  fresh 
  

   proof 
  of 
  the 
  singular 
  perforation 
  of 
  non-volcanic 
  strata 
  (including 
  granite 
  or 
  

   gneiss 
  in 
  that 
  term), 
  by 
  subterranean 
  explosions 
  which 
  have 
  been 
  so 
  incredibly 
  

   sudden 
  and 
  violent 
  as 
  to 
  have 
  occurred 
  without 
  any 
  visible 
  general 
  disturbance 
  

   (like 
  a 
  pistol-shot 
  penetrating 
  a 
  board, 
  to 
  use 
  a 
  comparison 
  which 
  I 
  have 
  some- 
  

   where 
  read), 
  which 
  recall 
  the 
  extraordinary 
  crater 
  lakes 
  of 
  the 
  Eyffel, 
  pene- 
  

   trating 
  slaty 
  rocks 
  without 
  deranging 
  their 
  strata, 
  and 
  seldom 
  giving 
  birth 
  to 
  any 
  

   considerable 
  volcanic 
  stream. 
  

  

  Fig. 
  4 
  of 
  Plate 
  IV. 
  is 
  taken 
  from 
  a 
  rude 
  eye-sketch 
  of 
  the 
  crater 
  of 
  Pal. 
  The 
  

   chasm 
  at 
  the 
  western 
  entrance 
  gives 
  exit 
  to 
  the 
  river 
  Fontaulier, 
  which 
  takes 
  its 
  

   rise 
  here. 
  The 
  temperature 
  of 
  the 
  copious 
  spring 
  is 
  42-2, 
  the 
  height 
  above 
  the 
  sea 
  

   being 
  3900 
  feet.f 
  The 
  existence 
  of 
  a 
  spring 
  of 
  the 
  natural 
  temperature 
  (for 
  such 
  

   it 
  appears 
  to 
  be) 
  J 
  in 
  such 
  a 
  position 
  leads 
  to 
  an 
  interesting 
  reflection. 
  Its 
  great 
  

   bulk, 
  and 
  probably 
  uniform 
  temperature, 
  shew 
  that 
  it 
  rises 
  from 
  a 
  considerable 
  

   depth 
  ; 
  and 
  it 
  cannot 
  be 
  doubted 
  that 
  it 
  must 
  follow 
  the 
  course, 
  or 
  nearly 
  so, 
  by 
  

   which 
  the 
  lavas 
  amidst 
  which 
  it 
  rises 
  have 
  made 
  their 
  way 
  from 
  the 
  interior 
  of 
  

   the 
  earth. 
  The 
  source 
  of 
  the 
  spring 
  is 
  therefore 
  in 
  the 
  direction 
  of 
  the 
  volcanic 
  

   focus. 
  It 
  is 
  evident, 
  then, 
  that 
  the 
  mass 
  of 
  rock 
  at 
  that 
  depth, 
  which 
  was 
  

   once 
  incandescent, 
  has 
  had 
  time 
  to 
  cool 
  to 
  (sensibly) 
  the 
  temperature 
  of 
  the 
  

   air. 
  How 
  old, 
  then, 
  must 
  be 
  the 
  date 
  of 
  this 
  geological 
  recent 
  eruption 
  ? 
  It 
  

   would 
  not 
  be 
  difficult 
  to 
  submit 
  it 
  to 
  calculation 
  ; 
  but 
  the 
  uncertainty 
  of 
  the 
  date 
  

   would 
  render 
  numerical 
  results 
  of 
  little 
  value. 
  Every 
  one 
  comprehends 
  that 
  a 
  

   mountain 
  of 
  granite, 
  with 
  a 
  nucleus 
  of 
  incandescent 
  lava, 
  could 
  not 
  have 
  been 
  

   completely 
  cooled 
  unless 
  in 
  a 
  period 
  of 
  time 
  of 
  Yery 
  great 
  length. 
  

  

  * 
  I 
  am 
  sorry 
  to 
  say, 
  that 
  the 
  view 
  in 
  M. 
  Burat's 
  interesting 
  work 
  on 
  Central 
  France 
  (Plate 
  

   VII.), 
  is 
  altogether 
  exaggerated 
  and 
  inexact. 
  

  

  t 
  1839, 
  June 
  3. 
  Spring 
  42°-0 
  (Therm, 
  marked 
  A. 
  1.). 
  1841, 
  June 
  25. 
  Spring 
  42°-2 
  

   (Therm, 
  marked 
  A. 
  3.). 
  Now 
  the 
  correction 
  of 
  A. 
  3 
  is 
  + 
  0°-2. 
  That 
  of 
  A. 
  1 
  was 
  0°-0 
  in 
  1838. 
  

   By 
  a 
  singular 
  coincidence 
  the 
  barometer 
  on 
  these 
  two 
  occasions 
  marked 
  the 
  same 
  tenth 
  of 
  a 
  millimetre. 
  

   The 
  temperature 
  of 
  the 
  air 
  was 
  also 
  within 
  1° 
  Fahr. 
  of 
  being 
  the 
  same 
  both 
  days. 
  

  

  % 
  The 
  mean 
  temperature 
  of 
  Viviers 
  on 
  the 
  Rhone, 
  which 
  is 
  in 
  the 
  same 
  Department, 
  and 
  only 
  57 
  

   metres 
  above 
  the 
  sea, 
  is 
  55°-25 
  according 
  to 
  Cotte, 
  as 
  given 
  in 
  Dove's 
  Tables. 
  The 
  crater 
  of 
  Pal 
  is 
  

   at 
  1186 
  metres 
  above 
  the 
  sea, 
  or 
  1129 
  above 
  Viviers. 
  Now, 
  in 
  France 
  generally 
  (according 
  to 
  

   Martins), 
  the 
  decrease 
  of 
  temperature 
  with 
  height 
  is 
  1° 
  cent, 
  for 
  180 
  metres, 
  or 
  1° 
  Fahr. 
  for 
  

   100 
  metres 
  exactly; 
  or 
  ll°-29 
  for 
  1129 
  metres, 
  whence 
  the 
  mean 
  temperature 
  of 
  Pal 
  should 
  be 
  

   55°-25 
  — 
  ll°-29 
  = 
  43°-96. 
  The 
  temperature 
  of 
  the 
  spring 
  was 
  somewhat 
  lower 
  in 
  the 
  month 
  

   of 
  June, 
  but 
  the 
  approximation 
  is 
  a 
  fair 
  one. 
  

  

  