﻿18 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  the 
  greater 
  part 
  both 
  of 
  the 
  lateral 
  and 
  principal 
  valley, 
  damming 
  back 
  the 
  river 
  

   Ardeche 
  so 
  as 
  to 
  form 
  a 
  lake, 
  which 
  was 
  only 
  drained 
  when 
  the 
  unceasing 
  and 
  all- 
  

   powerful 
  action 
  of 
  water 
  had 
  excavated 
  a 
  channel 
  in 
  the 
  hard 
  basalts, 
  and 
  even 
  

   in 
  the 
  granites 
  beneath 
  them, 
  leaving 
  a 
  cliff, 
  which 
  has 
  been 
  already 
  described, 
  

   towering 
  above 
  the 
  walls 
  of 
  primitive 
  rock, 
  between 
  which 
  the 
  stream 
  struggles, 
  

   forming 
  a 
  scene 
  far 
  more 
  varied 
  and 
  picturesque, 
  though 
  less 
  regular, 
  than 
  the 
  

   columnade 
  of 
  Jaujac 
  which 
  we 
  have 
  lately 
  quitted. 
  

  

  Dire 
  must 
  have 
  been 
  the 
  confusion 
  which 
  the 
  element 
  of 
  fire 
  wrought 
  in 
  

   this 
  quiet 
  valley. 
  Scarcely 
  less 
  appalling 
  the 
  confusion 
  and 
  organic 
  change 
  pro- 
  

   duced 
  by 
  the 
  subsequent 
  action 
  of 
  the 
  water 
  upon 
  the 
  intruded 
  masses 
  which 
  had 
  

   for 
  a 
  while 
  staid 
  its 
  course. 
  But 
  now 
  all 
  is 
  again 
  tranquil, 
  and 
  the 
  progress 
  of 
  

   events 
  is 
  marked 
  by 
  almost 
  imperceptible 
  gradation 
  ; 
  for 
  now, 
  the 
  lava 
  being 
  no- 
  

   where 
  in 
  contact 
  with 
  the 
  stream, 
  the 
  action 
  of 
  the 
  river 
  on 
  the 
  granite 
  may 
  be 
  

   considered 
  as 
  inappreciable. 
  The 
  platform 
  raised 
  by 
  the 
  lava, 
  and 
  terminated 
  by 
  a 
  

   tremendous 
  chasm 
  on 
  the 
  side 
  next 
  the 
  valley, 
  is 
  covered 
  by 
  a 
  most 
  luxuriant 
  

   vegetation. 
  The 
  village, 
  surrounded 
  by 
  its 
  vines 
  and 
  maize, 
  is 
  exactly 
  opposite 
  

   to 
  the 
  spot 
  whence 
  the 
  volcano 
  of 
  Mouleyres 
  or 
  Thuez 
  must 
  have 
  evacuated 
  the 
  

   most 
  abundant 
  part 
  of 
  the 
  fiery 
  flood 
  which 
  charged 
  it 
  ; 
  the 
  crater 
  itself, 
  which 
  

   rises 
  to 
  2026 
  feet 
  above 
  the 
  sea, 
  is 
  filled 
  (as 
  usual) 
  with 
  splendid 
  chestnut 
  trees, 
  

   and 
  its 
  porous 
  cone 
  is 
  planted, 
  like 
  the 
  sides 
  of 
  Vesuvius, 
  with 
  productive 
  vine- 
  

   yards 
  ; 
  the 
  site 
  of 
  the 
  lake 
  once 
  formed 
  by 
  the 
  waters 
  of 
  the 
  Ardeche 
  is 
  now 
  a 
  

   fertile 
  meadow 
  ; 
  and 
  the 
  chasm 
  once 
  so 
  ruined 
  and 
  bare, 
  in 
  which 
  the 
  river 
  flows 
  

   past 
  Thuez, 
  is 
  now 
  ornamented 
  with 
  luxuriant 
  wood 
  of 
  walnut, 
  chestnut, 
  and 
  beech, 
  

   which 
  give 
  a 
  great 
  charm 
  to 
  the 
  contrasted 
  outlines 
  of 
  the 
  jagged 
  granites, 
  and 
  

   the 
  alternately 
  level 
  and 
  perpendicular 
  basalts. 
  In 
  many 
  places 
  the 
  primitive 
  

   soil 
  of 
  the 
  valley, 
  the 
  granite 
  surface, 
  cleared 
  of 
  the 
  prodigious 
  load 
  of 
  black 
  rock 
  

   which 
  for 
  ages 
  covered 
  it, 
  is 
  again 
  brought 
  into 
  cultivation 
  and 
  yields 
  abundant 
  

   crops. 
  

  

  I 
  have 
  attempted 
  to 
  give 
  (in 
  Plate 
  VI., 
  fig. 
  5) 
  a 
  panoramic 
  sketch 
  of 
  the 
  

   position 
  of 
  Thuez, 
  which 
  may 
  serve 
  to 
  illustrate 
  this 
  description. 
  It 
  is 
  taken 
  from 
  

   the 
  granitic 
  heights 
  on 
  the 
  opposite 
  side 
  of 
  the 
  Ardeche. 
  In 
  the 
  centre 
  of 
  the 
  

   view, 
  in 
  the 
  middle 
  distance, 
  is 
  the 
  village 
  of 
  Thuez, 
  resting, 
  as 
  has 
  been 
  explained, 
  

   on 
  the 
  almost 
  horizontal 
  plateau 
  of 
  basalt, 
  whose 
  front 
  or 
  section 
  is 
  presented 
  to 
  

   the 
  spectator, 
  and 
  which 
  stretches 
  for 
  a 
  great 
  way 
  to 
  the 
  left, 
  or 
  up 
  the 
  valley 
  of 
  

   the 
  Ardeche, 
  with 
  gradually 
  diminishing 
  elevation. 
  In 
  the 
  nearer 
  part 
  of 
  the 
  draw- 
  

   ing, 
  on 
  the 
  left, 
  is 
  seen 
  the 
  deep 
  chasm 
  in 
  the 
  granite 
  rocks 
  through 
  which 
  the 
  

   river 
  passes. 
  Immediately 
  behind 
  Thuez, 
  at 
  the 
  closed 
  end 
  of 
  the 
  little 
  lateral 
  

   valley 
  which 
  we 
  have 
  mentioned, 
  rises 
  the 
  Gravenne 
  of 
  Montpezat, 
  a 
  volcano 
  

   which 
  has 
  thrown 
  by 
  far 
  the 
  greater 
  part, 
  if 
  not 
  all, 
  of 
  its 
  lava 
  in 
  a 
  contrary 
  direc- 
  

   tion, 
  and 
  which, 
  therefore, 
  has 
  not 
  produced 
  at 
  least 
  any 
  sensible 
  portion 
  of 
  the 
  

   basalts 
  of 
  Thuez. 
  It 
  is 
  to 
  the 
  open 
  and 
  degraded 
  crater 
  on 
  the 
  right, 
  whose 
  vivid 
  

   red 
  colour 
  contrasts 
  splendidly 
  in 
  nature 
  with 
  the 
  bright 
  green 
  of 
  the 
  trees 
  around 
  

  

  