﻿16 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  the 
  whole 
  exterior 
  of 
  the 
  volcanic 
  cone, 
  and 
  found 
  the 
  sandstone 
  everywhere, 
  ex- 
  

   cept 
  in 
  the 
  narrow 
  space 
  occupied 
  by 
  the 
  issue 
  of 
  the 
  lava-stream 
  from 
  the 
  crater 
  

   (see 
  Plate 
  II., 
  fig. 
  5), 
  which 
  abuts 
  against 
  a 
  hillock 
  of 
  sandstone, 
  marked 
  C 
  on 
  the 
  

   plan. 
  It 
  does 
  not, 
  however, 
  appear 
  to 
  reach 
  the 
  banks 
  of 
  the 
  Alignon, 
  and 
  ter- 
  

   minates 
  abruptly 
  against 
  the 
  two 
  hills 
  of 
  gneiss, 
  A 
  and 
  B, 
  which 
  form 
  part 
  of 
  

   the 
  boundary 
  of 
  the 
  valley, 
  which 
  is 
  everywhere 
  of 
  primitive 
  rock. 
  Near 
  A, 
  the 
  

   junction 
  of 
  the 
  coal-formation 
  and 
  the 
  gneiss 
  is 
  well 
  seen. 
  The 
  strata 
  of 
  sand- 
  

   stone 
  and 
  gneiss 
  resemble 
  each 
  other 
  so 
  much, 
  that, 
  being 
  nearly 
  conformable, 
  it 
  

   was 
  some 
  time 
  before 
  I 
  could 
  assure 
  myself 
  that 
  the 
  fine 
  granite 
  veins 
  which 
  occur 
  

   there 
  were 
  not 
  in 
  the 
  coal-formation. 
  

  

  The 
  crater 
  is 
  probably 
  the 
  largest 
  in 
  the 
  Vivarais. 
  It 
  is 
  low, 
  strong, 
  and 
  of 
  

   an 
  elliptical 
  form, 
  and 
  has 
  burst 
  at 
  one 
  end 
  of 
  the 
  longer 
  axis, 
  being 
  that 
  to- 
  

   wards 
  the 
  village 
  of 
  Jaujac. 
  From 
  the 
  firmness 
  and 
  dimension 
  of 
  the 
  lava 
  walls, 
  

   I 
  presume 
  that 
  it 
  must 
  have 
  been 
  lofty, 
  as 
  is 
  indeed 
  probable, 
  if 
  it 
  contained 
  any 
  

   large 
  part 
  of 
  the 
  prodigious 
  flow 
  of 
  lava 
  which 
  proceeds 
  from 
  it, 
  and 
  which 
  is 
  

   evidently 
  the 
  result 
  of 
  one 
  eruption, 
  and 
  probably 
  the 
  only 
  one 
  which 
  this 
  vol- 
  

   cano 
  has 
  experienced. 
  The 
  interior 
  of 
  the 
  crater 
  is 
  filled 
  with 
  clay 
  and 
  ashes, 
  

   which 
  sustain, 
  however, 
  a 
  magnificent 
  growth 
  of 
  chestnut 
  trees. 
  The 
  open 
  lip 
  is 
  

   narrow, 
  and 
  just 
  gives 
  vent 
  to 
  a 
  stream 
  of 
  fine 
  compact 
  lava 
  with 
  little 
  slag, 
  

   which 
  then 
  fills 
  the 
  valley 
  ; 
  the 
  town 
  of 
  Jaujac 
  stands 
  on 
  the 
  extreme 
  upper 
  

   part 
  of 
  it. 
  Near 
  the 
  mouth 
  of 
  the 
  crater, 
  and 
  1531 
  feet 
  above 
  the 
  sea, 
  is 
  a 
  spring 
  

   plentifully 
  charged 
  with 
  carbonic 
  acid, 
  whose 
  temperature 
  was 
  53 
  -2. 
  A 
  spring, 
  

   issuing 
  from 
  below 
  the 
  basalt, 
  at 
  the 
  junction 
  of 
  the 
  Alignon 
  and 
  Rioclat, 
  had 
  a 
  

   temperature 
  of 
  54-7 
  (23d 
  June 
  1841). 
  These 
  temperatures 
  are 
  both 
  above 
  the 
  

   mean 
  of 
  the 
  place. 
  The 
  extreme 
  height 
  of 
  the 
  Coupe 
  (by 
  the 
  mean 
  of 
  barome- 
  

   trical 
  observations 
  in 
  1839 
  and 
  1841) 
  is 
  1923 
  English 
  feet 
  above 
  the 
  sea. 
  The 
  

   level 
  of 
  the 
  surface 
  of 
  the 
  basaltic 
  formation 
  at 
  Jaujac 
  is 
  about 
  1400 
  feet 
  ; 
  and 
  the 
  

   top 
  of 
  the 
  coulee 
  of 
  Neyrac, 
  at 
  the 
  junction 
  of 
  the 
  Ardeche 
  and 
  Alignon, 
  is 
  1117 
  feet. 
  

  

  Valley 
  of 
  the 
  Ardeche 
  — 
  Thuez. 
  

  

  If, 
  instead 
  of 
  pursuing 
  the 
  course 
  of 
  the 
  Alignon 
  to 
  Jaujac, 
  we 
  follow 
  the 
  

   principal 
  stream 
  of 
  the 
  Ardeche 
  to 
  Thuez, 
  we 
  first 
  of 
  all 
  seem 
  to 
  leave 
  all 
  traces 
  

   of 
  basalts 
  in 
  the 
  valley. 
  Very 
  soon, 
  however, 
  patches 
  of 
  volcanic 
  formation 
  

   appear 
  upon 
  the 
  right 
  of 
  the 
  road 
  ; 
  whilst, 
  on 
  our 
  left 
  beyond 
  the 
  river, 
  we 
  have 
  

   the 
  volcano 
  of 
  Neyrac 
  or 
  Souillols, 
  formerly 
  mentioned 
  as 
  occupying 
  the 
  ridge 
  

   between 
  the 
  two 
  valleys, 
  and 
  as 
  having 
  unquestionably 
  thrown 
  out 
  its 
  principal 
  

   stream 
  into 
  that 
  of 
  the 
  Alignon. 
  The 
  summit 
  of 
  the 
  volcano 
  (which 
  is 
  at 
  a 
  

   height 
  of 
  2178 
  feet 
  above 
  the 
  sea, 
  barometrically 
  determined 
  in 
  1839) 
  commands 
  

   a 
  very 
  fine 
  view 
  of 
  the 
  upper 
  district 
  of 
  the 
  Vivarais, 
  and 
  of 
  the 
  volcanoes 
  which 
  

  

  