﻿12 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  down 
  (which 
  is 
  commonly 
  due 
  to 
  the 
  pressure 
  of 
  lava) 
  on 
  the 
  south-west 
  side. 
  

   A 
  very 
  careful 
  examination 
  in 
  1841 
  changed 
  my 
  opinion 
  ; 
  and, 
  notwithstanding 
  

   the 
  seeming 
  improbability 
  of 
  the 
  conclusion, 
  we 
  must, 
  I 
  believe, 
  admit 
  that 
  the 
  

   lava 
  of 
  Jaujac 
  terminates 
  almost 
  exactly 
  where 
  the 
  lava 
  ofNeyrac 
  commences, 
  

   filling 
  the 
  valley 
  to 
  almost 
  the 
  same 
  depth, 
  and 
  with 
  similar 
  matter. 
  And 
  this 
  

   must 
  be 
  conceded 
  for 
  the 
  following 
  amongst 
  other 
  reasons 
  -.—First, 
  There 
  is 
  a 
  

   break 
  in 
  the 
  continuity 
  of 
  the 
  lava 
  stream 
  a 
  little 
  above, 
  and 
  opposite 
  to 
  the 
  

   village 
  of 
  Souillols, 
  and 
  in 
  the 
  cliff 
  formed 
  by 
  the 
  river. 
  The 
  granite 
  of 
  the 
  

   country 
  may 
  be 
  traced 
  in 
  the 
  interval. 
  Secondly, 
  Up 
  to 
  this 
  point 
  the 
  character 
  

   of 
  the 
  lava, 
  as 
  displayed 
  continually 
  in 
  the 
  cliff 
  on 
  the 
  right 
  bank 
  of 
  the 
  stream, 
  

   is 
  remarkably 
  uniform 
  upwards 
  from 
  the 
  point 
  of 
  section 
  (fig. 
  2) 
  opposite 
  the 
  

   Castle 
  of 
  Mayras. 
  Only 
  a 
  small 
  portion 
  of 
  the 
  lower 
  part 
  is 
  columnar, 
  sur- 
  

   rounded 
  in 
  the 
  greater 
  part 
  of 
  its 
  thickness 
  by 
  basalt 
  nearly 
  amorphous 
  or 
  

   slightly 
  columnar. 
  The 
  columnar 
  part 
  diminishes 
  in 
  thickness 
  as 
  we 
  ascend 
  

   the 
  course 
  of 
  the 
  river, 
  and 
  opposite 
  Souillols 
  it 
  is 
  only 
  three 
  or 
  four 
  feet 
  in 
  

   height. 
  But 
  when 
  the 
  lava 
  cliff 
  reappears 
  after 
  the 
  break 
  alluded 
  to 
  (a 
  break, 
  

   however, 
  so 
  slight, 
  that 
  it 
  might 
  easily 
  escape 
  notice), 
  it 
  presents 
  a 
  very 
  different 
  

   front 
  to 
  the 
  river. 
  The 
  cliff 
  is 
  now 
  130 
  feet 
  in 
  height,* 
  of 
  which 
  not 
  less 
  than 
  

   two-thirds, 
  and 
  in 
  some 
  places 
  nearly 
  the 
  whole 
  is 
  composed 
  of 
  a 
  single 
  range 
  of 
  

   perfectly 
  continuous 
  basaltic 
  pillars 
  ; 
  and 
  the 
  level 
  of 
  the 
  prismatic 
  boundary 
  is 
  

   again 
  gradually 
  depressed, 
  as 
  we 
  approach 
  the 
  undoubted 
  origin 
  of 
  this 
  part 
  of 
  

   the 
  stream, 
  namely, 
  the 
  crater 
  of 
  Jaujac. 
  Thirdly, 
  The 
  volcano 
  of 
  Neyrac 
  does 
  

   exhibit 
  a 
  streak 
  of 
  ashes 
  and 
  slag 
  down 
  its 
  southern 
  face. 
  Now 
  the 
  ashes 
  may 
  

   be 
  seen 
  to 
  pass 
  into 
  the 
  slag, 
  and 
  the 
  slag 
  into 
  the 
  lava 
  of 
  the 
  Alignon 
  near 
  

   Souillols, 
  — 
  a 
  convincing 
  argument. 
  

  

  We 
  now 
  come 
  to 
  describe 
  more 
  particularly 
  the 
  lava 
  of 
  Jaujac, 
  which 
  ex- 
  

   tends 
  along 
  the 
  bottom 
  of 
  the 
  valley, 
  from 
  a 
  short 
  distance 
  above 
  the 
  village 
  of 
  

   that 
  name, 
  to 
  nearly 
  opposite 
  Souillols, 
  at 
  least 
  two 
  miles 
  farther 
  down. 
  All 
  this 
  

   space 
  has 
  been 
  raised 
  from 
  the 
  natural 
  bed 
  of 
  the 
  stream 
  to 
  a 
  vertical 
  height 
  of 
  

   perhaps 
  100 
  feet 
  on 
  an 
  average, 
  throughout 
  the 
  entire 
  breadth 
  of 
  the 
  valley, 
  and 
  

   now 
  presents 
  a 
  cultivated 
  and 
  wooded 
  plateau, 
  whose 
  extraneous 
  origin 
  would 
  

   hardly 
  be 
  suspected 
  but 
  for 
  the 
  deep 
  incision 
  made 
  by 
  the 
  river 
  near 
  the 
  foot 
  of 
  

   the 
  hills 
  which 
  bound 
  it 
  to 
  the 
  north. 
  This 
  section 
  displays 
  the 
  wonderful 
  colon- 
  

   nade 
  already 
  referred 
  to, 
  of 
  which 
  so 
  correct 
  a 
  representation 
  has 
  been 
  given 
  in 
  

   Mr 
  Scrope's 
  beautiful 
  work 
  on 
  Central 
  France. 
  

  

  That 
  representation 
  exhibits 
  well 
  the 
  remarkable 
  fact 
  of 
  the 
  gradual 
  rise 
  of 
  

   the 
  lower 
  perfectly-columnar 
  stratum 
  into 
  the 
  higher 
  or 
  imperfect 
  one, 
  in 
  pro- 
  

   portion 
  as 
  we 
  follow 
  the 
  section 
  down 
  the 
  valley, 
  or 
  farther 
  from 
  the 
  point 
  of 
  

  

  * 
  Measured 
  by 
  the 
  fall 
  of 
  a 
  stone, 
  and 
  confirmed 
  by 
  the 
  authority 
  of 
  a 
  person 
  who 
  told 
  me 
  that 
  

   he 
  had 
  measured 
  it 
  with 
  a 
  string. 
  

  

  