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  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  this 
  point 
  was 
  undoubtedly 
  the 
  ancient 
  issue 
  of 
  the 
  stream 
  of 
  the 
  lateral 
  valley 
  

   which 
  now 
  escapes 
  at 
  the 
  Gueule 
  d'Enfer. 
  I 
  have 
  ascertained 
  by 
  barometrical 
  

   observations 
  that 
  this 
  is 
  really 
  the 
  lowest 
  point 
  of 
  contact 
  of 
  the 
  lava 
  with 
  the 
  

   ancient 
  surface 
  of 
  the 
  granite 
  valley 
  ; 
  and 
  was 
  therefore 
  the 
  thalweg 
  or 
  water- 
  

   way 
  until 
  it 
  was 
  choked 
  by 
  the 
  lava 
  of 
  Mouleyres. 
  If, 
  then, 
  the 
  Gravenne 
  gave 
  

   birth 
  to 
  any 
  stream 
  of 
  lava, 
  however 
  small, 
  it 
  might 
  be 
  expected 
  to 
  have 
  flowed 
  in 
  

   this 
  direction. 
  If 
  this 
  change 
  of 
  the 
  water-way 
  be 
  admitted 
  (and 
  it 
  appears 
  to 
  be 
  

   unquestionable), 
  there 
  is 
  another 
  proof, 
  amounting 
  almost 
  to 
  demonstration, 
  that 
  

   the 
  Gueule 
  d'Enfer 
  has 
  been 
  excavated 
  in 
  the 
  granite 
  or 
  gneiss 
  since 
  the 
  lava 
  of 
  

   Mouleyres 
  flowed 
  ; 
  for 
  before 
  that 
  time 
  no 
  water 
  could 
  have 
  run 
  through 
  it. 
  

   The 
  following 
  are 
  the 
  results 
  of 
  my 
  barometrical 
  observations 
  : 
  — 
  

  

  Eng. 
  Ft. 
  above 
  Sea. 
  

  

  Summit 
  of 
  Lava 
  Cliff 
  at 
  Echelle 
  du 
  Roi 
  1476 
  

  

  Foot 
  of 
  ... 
  1283 
  

  

  Summit 
  of 
  Lava 
  Cliff 
  at 
  Gueule 
  d'Enfer, 
  . 
  . 
  . 
  1514 
  

  

  Foot 
  of 
  1343 
  

  

  Height 
  of 
  Cliff 
  at 
  Echelle 
  du 
  Roi 
  (A), 
  ... 
  .193 
  

  

  Gueule 
  d'Enfer 
  (B), 
  .... 
  171 
  

  

  Surface 
  of 
  Lava 
  at 
  A 
  below 
  Surface 
  at 
  B, 
  38 
  

  

  Contact 
  of 
  Lava 
  with 
  Ground 
  at 
  A 
  below 
  contact 
  at 
  B, 
  60 
  

  

  The 
  lava 
  of 
  Thuez 
  gradually 
  thins 
  out 
  as 
  we 
  ascend 
  the 
  valley 
  ; 
  continuing, 
  

   however, 
  to 
  present 
  a 
  basaltic 
  cliff 
  towards 
  the 
  stream. 
  Where 
  the 
  lava 
  ceases, 
  

   the 
  valley 
  expands 
  ; 
  and 
  the 
  river 
  has 
  a 
  wide 
  bed 
  formed 
  of 
  detritus, 
  preserving 
  a 
  

   uniform 
  level, 
  evidently 
  occasioned 
  by 
  a 
  lake 
  which 
  once 
  existed, 
  due 
  to 
  the 
  ob- 
  

   struction 
  caused 
  by 
  the 
  lava 
  to 
  the 
  waters 
  of 
  the 
  Ardeche.* 
  

  

  Montpezat. 
  

  

  A 
  short 
  walk 
  from 
  Thuez 
  leads 
  to 
  the 
  top 
  of 
  the 
  volcano 
  called 
  La 
  Gravenne 
  

   de 
  Montpezat 
  ; 
  but 
  the 
  ascent 
  is 
  rapid, 
  the 
  strata 
  of 
  pozzuolana 
  constituting 
  the 
  

   southern 
  face 
  of 
  the 
  Gravenne 
  dip 
  at 
  an 
  angle 
  of 
  about 
  30°, 
  the 
  superficial 
  debris 
  

   25°. 
  The 
  principal 
  discharge 
  of 
  lava 
  being 
  towards 
  the 
  north, 
  filling 
  the 
  valley 
  of 
  

   Montpezat, 
  we 
  shall 
  next 
  in 
  order 
  consider 
  the 
  phenomena 
  which 
  that 
  district, 
  

   watered 
  by 
  the 
  Fontaulier, 
  presents. 
  The 
  Gravenne 
  itself 
  rises 
  to 
  a 
  height 
  of 
  2727 
  

   feet. 
  The 
  view 
  from 
  it 
  is 
  extensive 
  and 
  striking. 
  It 
  has 
  an 
  exceedingly 
  well- 
  

   formed 
  crater, 
  which, 
  however, 
  is 
  seen 
  from 
  but 
  few 
  points. 
  Plate 
  III., 
  fig. 
  4, 
  

   shews 
  its 
  appearance, 
  as 
  seen 
  from 
  a 
  hill 
  to 
  the 
  north, 
  on 
  a 
  ridge 
  between 
  Mont- 
  

  

  * 
  About 
  5 
  hours' 
  walk 
  across 
  the 
  hills 
  to 
  the 
  south-west 
  of 
  Mayres, 
  or 
  7 
  hours' 
  from 
  Thuez 
  

   (passing 
  le 
  Chamhon 
  and 
  Bornes), 
  are 
  the 
  remote 
  mineral-springs 
  of 
  St 
  Laurent 
  les 
  Bains, 
  having 
  

   a 
  temperature 
  of 
  125° 
  Fahr. 
  They 
  rise 
  from 
  mica 
  slate, 
  in 
  the 
  neighbourhood 
  of 
  granite, 
  and 
  

   traversed 
  by 
  granite 
  veins. 
  They 
  contain 
  salts 
  of 
  soda 
  almost 
  exclusively, 
  and 
  particularly 
  the 
  

   carbonate. 
  

  

  