﻿20 
  PKOFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  by 
  a 
  rough 
  footway 
  beneath 
  the 
  bridge 
  across 
  the 
  Gueule 
  d'Enfer. 
  Here 
  we 
  are 
  

   on 
  the 
  exact 
  boundary 
  of 
  the 
  immense 
  lava 
  plateau 
  and 
  the 
  granite 
  to 
  the 
  east, 
  — 
  

   that 
  is, 
  the 
  granite 
  is 
  under 
  our 
  feet 
  and 
  to 
  our 
  left 
  in 
  descending, 
  the 
  lava 
  is 
  

   above 
  us 
  and 
  to 
  the 
  right. 
  The 
  remarkable 
  section 
  (Plate 
  III., 
  fig. 
  2) 
  of 
  the 
  

   Gueule 
  d'Enfer 
  shews 
  plainly 
  that 
  the 
  lava 
  must 
  have 
  had 
  a 
  support 
  which 
  

   piled 
  it 
  up, 
  when 
  fluid, 
  to 
  the 
  level 
  which 
  it 
  still 
  retains, 
  and 
  the 
  position 
  of 
  

   the 
  barrier 
  is 
  conclusively 
  shewn 
  by 
  the 
  direction 
  of 
  the 
  stratum 
  of 
  basaltic 
  

   columns, 
  whose' 
  axes 
  are 
  as 
  usual 
  perpendicular 
  to 
  its 
  surface, 
  and 
  which 
  point 
  

   out 
  with 
  mathematical 
  accuracy 
  the 
  figure 
  of 
  the 
  retaining 
  wall 
  now 
  removed, 
  

   and 
  replaced 
  by 
  the 
  deep 
  ravine 
  on 
  the 
  right 
  of 
  the 
  spectator, 
  who 
  looks 
  

   up 
  the 
  defile 
  as 
  in 
  fig. 
  2. 
  We 
  are 
  therefore 
  compelled 
  not 
  only 
  to 
  admit 
  

   the 
  excavation 
  of 
  the 
  Gueule 
  d'Enfer 
  since 
  the 
  lava 
  was 
  consolidated, 
  but 
  we 
  

   must 
  suppose 
  that 
  a 
  barrier 
  of 
  some 
  kind 
  stretched 
  across 
  the 
  valley 
  of 
  the 
  

   Ardeche 
  itself, 
  in 
  order 
  to 
  retain 
  the 
  prodigious 
  lava 
  flow 
  at 
  the 
  great 
  elevation 
  

   which 
  it 
  has 
  attained, 
  and 
  which 
  causes 
  its 
  bared 
  cliffs 
  now 
  to 
  overhang 
  the 
  

   valley 
  to 
  a 
  height 
  of 
  250 
  feet, 
  reckoning 
  from 
  the 
  bed 
  of 
  the 
  stream. 
  A 
  careful 
  

   examination 
  of 
  the 
  panoramic 
  view 
  will 
  clearly 
  prove 
  the 
  surprising 
  dilemma 
  

   in 
  which 
  we 
  are 
  placed. 
  The 
  almost 
  perfect 
  horizontality 
  of 
  the 
  whole 
  remain- 
  

   ing 
  surface 
  of 
  the 
  lava 
  proves 
  that 
  it 
  consolidated 
  tranquilly 
  at 
  that 
  level 
  ; 
  and 
  

   yet 
  we 
  find 
  to 
  the 
  right 
  nothing 
  but 
  a 
  wide 
  open 
  valley, 
  which 
  presents 
  no 
  trace 
  

   of 
  a 
  support, 
  and 
  from 
  which 
  the 
  lava 
  itself 
  has 
  totally 
  vanished 
  ; 
  for 
  the 
  most 
  

   scrupulous 
  examination 
  of 
  the 
  bed 
  of 
  the 
  Ardeche 
  has 
  shewn 
  me 
  that 
  there 
  is 
  

   not 
  a 
  volcanic 
  vestige 
  in 
  its 
  neighbourhood 
  so 
  far 
  down 
  as 
  the 
  environs 
  of 
  Neyrac 
  ; 
  

   and, 
  though 
  it 
  is 
  as 
  plain 
  as 
  the 
  truth 
  of 
  hydrostatics 
  that 
  the 
  basalt 
  must 
  have 
  

   once 
  filled 
  up 
  the 
  whole 
  bed 
  of 
  the 
  Ardeche 
  at 
  this 
  place, 
  and 
  abutted 
  against 
  the 
  

   granite-hill 
  opposite 
  (from 
  whence 
  the 
  panoramic 
  view 
  in 
  Plate 
  VI. 
  is 
  taken), 
  there 
  

   is 
  now 
  not 
  a 
  trace 
  of 
  it 
  on 
  the 
  southern 
  side 
  of 
  the 
  river. 
  How 
  astonishing, 
  then, 
  

   must 
  have 
  been 
  the 
  excavating 
  power 
  which 
  has 
  not 
  merely 
  disintegrated 
  the 
  mass 
  

   of 
  lava 
  which 
  has 
  disappeared, 
  but 
  has 
  destroyed 
  the 
  barrier, 
  by 
  means 
  of 
  which 
  it 
  

   was 
  accumulated 
  to 
  the 
  level 
  which 
  it 
  retains 
  ! 
  It 
  is 
  certainly 
  conceivable 
  that 
  this 
  

   barrier 
  might 
  have 
  been 
  partly 
  composed 
  of 
  the 
  dejections 
  of 
  the 
  volcano 
  which, 
  

   when 
  much 
  higher 
  than 
  at 
  present, 
  may 
  possibly 
  have 
  extended 
  its 
  cone 
  so 
  as 
  

   partly 
  to 
  close 
  the 
  valley 
  ; 
  yet 
  the 
  whole 
  circumstances 
  appear 
  to 
  shew 
  that 
  the 
  

   forms 
  of 
  these 
  volcanoes 
  have 
  not 
  materially 
  changed 
  since 
  the 
  completion 
  of 
  their 
  

   eruptions, 
  and 
  that 
  certainly 
  no 
  vast 
  or 
  powerful 
  streams 
  of 
  water, 
  sweeping 
  over 
  

   the 
  whole 
  country 
  after 
  the 
  manner 
  of 
  a 
  debacle, 
  can 
  be 
  invoked 
  in 
  explanation 
  of 
  the 
  

   last 
  excavation 
  of 
  the 
  valleys 
  ; 
  for 
  the 
  loose 
  texture 
  of 
  the 
  ashes, 
  which 
  repose 
  upon 
  

   every 
  volcanic 
  cone, 
  would 
  have 
  given 
  way 
  at 
  once 
  before 
  the 
  action 
  of 
  a 
  flood, 
  

   however 
  gentle. 
  This 
  argument 
  has 
  been 
  effectively 
  used 
  by 
  Mr 
  Scrope, 
  to 
  prove 
  

   that 
  the 
  removal 
  of 
  the 
  lava 
  beds 
  can 
  be 
  ascribed 
  only 
  to 
  the 
  action 
  of 
  water 
  

   following 
  the 
  channels 
  of 
  the 
  present 
  rivers 
  ; 
  and 
  has 
  been 
  enforced 
  by 
  Sir 
  C. 
  Lyell 
  

  

  