﻿34 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  Aysac 
  is 
  very 
  perfect, 
  with 
  a 
  nearly 
  circular 
  crater, 
  which 
  rises 
  to 
  a 
  height 
  of 
  2040 
  

   feet 
  above 
  the 
  sea, 
  and 
  1257 
  above 
  the 
  bridge 
  across 
  the 
  Volane 
  at 
  Antraigues. 
  The 
  

   crater 
  is 
  extremely 
  well 
  formed, 
  and 
  I 
  estimated 
  its 
  circumference 
  at 
  about 
  a 
  

   mile. 
  It 
  is 
  broken 
  down 
  at 
  one 
  point 
  only, 
  on 
  the 
  NW. 
  side, 
  by 
  which 
  the 
  lava 
  

   stream 
  has 
  run 
  down 
  the 
  steep 
  face 
  of 
  the 
  hill 
  opposite 
  to 
  the 
  hamlet 
  of 
  Aysac, 
  

   where 
  it 
  enters 
  the 
  bed 
  of 
  the 
  torrent, 
  forming 
  a 
  prismatic 
  mass 
  exactly 
  at 
  the 
  foot 
  of 
  

   the 
  cone. 
  It 
  is 
  from 
  this 
  point 
  that 
  the 
  view 
  of 
  La 
  Coupe, 
  in 
  Faujas's 
  work, 
  is 
  

   taken, 
  and 
  the 
  general 
  features 
  are 
  exact 
  enough. 
  It 
  is 
  therefore 
  without 
  reason 
  

   that 
  Mr 
  Scrope 
  has 
  charged 
  him 
  with 
  a 
  gross 
  and 
  absurd 
  blunder 
  in 
  representing 
  

   the 
  crater 
  in 
  a 
  direction 
  whence 
  it 
  could 
  not 
  possibly 
  be 
  seen. 
  Mr 
  Scrope 
  supposes 
  

   the 
  basalts 
  in 
  the 
  foreground 
  to 
  be 
  those 
  which 
  he 
  has 
  himself 
  figured 
  in 
  the 
  

   ravine 
  opposite 
  Antraigues 
  ; 
  whereas 
  the 
  view 
  of 
  Faujas 
  is 
  entirely 
  confined 
  to 
  the 
  

   little 
  lateral 
  valley 
  leading 
  to 
  the 
  Collet 
  d' 
  Aysac, 
  as 
  clearly 
  appears 
  from 
  his 
  

   ample, 
  and 
  generally 
  accurate 
  description.* 
  

  

  The 
  lava 
  of 
  Aysac, 
  after 
  filling 
  up 
  in 
  great 
  part 
  the 
  basin 
  of 
  Antraigues, 
  has 
  

   followed 
  the 
  course 
  of 
  the 
  Volane 
  for 
  several 
  miles, 
  as 
  the 
  numerous 
  basaltic 
  

   patches 
  between 
  that 
  village 
  and 
  Vals 
  sufficiently 
  attest. 
  These 
  basalts 
  are 
  per- 
  

   haps 
  equalled 
  by 
  none 
  in 
  the 
  Vivarais, 
  or 
  in 
  any 
  other 
  part 
  of 
  Europe, 
  as 
  regards 
  

   the 
  exquisite 
  perfection 
  of 
  the 
  pillars 
  of 
  which 
  they 
  are 
  composed 
  ; 
  which, 
  though 
  

   they 
  attain 
  no 
  great 
  height, 
  as 
  at 
  Jaujac 
  and 
  Thuez, 
  are 
  sj'mmetrically 
  polygonal, 
  

   small 
  in 
  diameter, 
  and 
  beautifully 
  jointed, 
  affording 
  beautiful 
  cabinet 
  specimens, 
  

   particularly 
  at 
  the 
  bridge 
  across 
  the 
  Volane, 
  two 
  miles 
  above 
  Vals. 
  This 
  lava, 
  

   like 
  that 
  of 
  Burzet, 
  appears 
  to 
  have 
  been 
  very 
  fluid, 
  and 
  to 
  have 
  accumulated 
  in 
  the 
  

   gorges 
  to 
  a 
  greater 
  height 
  than 
  it 
  could 
  afterwards 
  retain. 
  We 
  find 
  it, 
  accordingly, 
  

   applied 
  in 
  thin 
  sheets 
  upon 
  the 
  steep 
  granite 
  walls 
  of 
  the 
  valley, 
  forming, 
  as 
  usual, 
  

   pillars 
  perpendicular 
  to 
  the 
  cooling 
  surface, 
  as 
  in 
  fig. 
  2 
  of 
  Plate 
  V. 
  

  

  Before 
  finally 
  quitting 
  Antraigues, 
  I 
  must 
  mention 
  that 
  the 
  summit 
  of 
  the 
  

   hill, 
  a 
  little 
  to 
  the 
  ESE. 
  of 
  the 
  village, 
  is 
  occupied 
  by 
  a 
  formation 
  of 
  basalt 
  (which 
  

   is 
  indicated 
  in 
  the 
  map 
  as 
  a 
  lava), 
  but 
  which 
  appears 
  rather 
  to 
  belong 
  to 
  the 
  an- 
  

   cient 
  basalts 
  of 
  the 
  Coyrans 
  and 
  Mesilhac. 
  It 
  is 
  composed 
  chiefly 
  of 
  loose 
  masses, 
  

   excessively 
  hard 
  and 
  heavy, 
  containing 
  olivine, 
  chalcedony, 
  and 
  other 
  substances, 
  

   and 
  decomposing 
  into 
  an 
  earthy 
  matter. 
  It 
  gives 
  no 
  indication 
  of 
  having 
  flowed 
  

   into 
  the 
  valley, 
  and 
  was 
  probably 
  formed 
  before 
  the 
  valleys 
  existed. 
  

  

  The 
  village 
  of 
  Vals 
  is 
  pleasantly 
  situated 
  on 
  the 
  river 
  Volane, 
  below 
  the 
  last 
  

   traces 
  of 
  the 
  lava 
  of 
  Aysac, 
  and 
  not 
  far 
  from 
  the 
  junction 
  of 
  the 
  gneiss 
  with 
  the 
  

   secondary 
  rocks. 
  Two 
  mineral 
  springs 
  issue 
  from 
  the 
  gneiss 
  at 
  the 
  rivers 
  side, 
  

   having 
  temperatures 
  of 
  59 
  0, 
  5 
  and 
  61°. 
  The}' 
  are 
  charged 
  with 
  carbonic 
  acid, 
  and 
  

   contain 
  a 
  little 
  iron 
  and 
  soda. 
  They 
  are 
  frequented 
  in 
  summer 
  by 
  visitors 
  even 
  

   from 
  a 
  considerable 
  distance, 
  and, 
  in 
  consequence, 
  the 
  inn 
  or 
  hotel 
  at 
  Vals 
  is 
  the 
  

   only 
  tolerable 
  one 
  in 
  the 
  whole 
  district, 
  and, 
  as 
  such, 
  will 
  be 
  duly 
  estimated 
  by 
  

  

  * 
  Recherclies, 
  p. 
  296-298. 
  

  

  