﻿8 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  many 
  miles 
  to 
  the 
  south 
  of 
  Montpezat 
  to 
  form 
  one 
  considerable 
  river, 
  — 
  the 
  Ar- 
  

   deche, 
  which 
  gives 
  its 
  name 
  to 
  the 
  department, 
  — 
  it 
  will 
  be 
  most 
  convenient 
  to 
  

   suppose 
  the 
  traveller 
  ascending 
  the 
  Ardeche 
  from 
  the 
  town 
  of 
  Aubenas, 
  one 
  of 
  

   the 
  most 
  considerable 
  in 
  the 
  province, 
  and 
  arriving 
  at 
  a 
  point 
  called 
  Font 
  de 
  la 
  

   Beaume 
  (see 
  the 
  map), 
  where 
  the 
  river 
  Fontaulier, 
  whose 
  rise 
  we 
  have 
  taken 
  

   notice 
  of, 
  joins 
  the 
  Ardeche, 
  having 
  previously 
  passed 
  close 
  to 
  the 
  village 
  of 
  

   Montpezat, 
  and 
  immediately 
  under 
  the 
  volcano 
  of 
  the 
  same 
  name, 
  subsequently 
  

   receiving 
  two 
  minor 
  tributaries, 
  which 
  we 
  shall 
  presently 
  have 
  to 
  describe. 
  

  

  From 
  the 
  neighbourhood 
  of 
  the 
  Pont 
  de 
  la 
  Beaume, 
  the 
  greater 
  part 
  of 
  the 
  

   valleys 
  of 
  which 
  we 
  are 
  to 
  speak 
  diverge 
  almost 
  like 
  the 
  rays 
  of 
  a 
  fan. 
  As 
  we 
  

   look 
  up 
  the 
  course 
  of 
  the 
  Ardeche, 
  Montpezat 
  occupies 
  nearly 
  the 
  centre 
  of 
  the 
  

   fan. 
  Now, 
  in 
  all 
  these 
  valleys 
  there 
  is 
  a 
  remarkable 
  uniformity 
  of 
  constitution, 
  

   and, 
  in 
  some 
  respects, 
  of 
  general 
  appearance. 
  The 
  substratum 
  of 
  the 
  whole 
  is 
  a 
  

   primitive 
  rock, 
  granite, 
  or 
  in 
  some 
  places 
  gneiss. 
  The 
  distinction 
  is 
  not 
  very 
  im- 
  

   portant 
  in 
  connection 
  with 
  the 
  phenomena 
  which 
  we 
  are 
  to 
  describe 
  ; 
  and 
  I 
  have 
  

   not 
  attempted 
  to 
  determine 
  the 
  limits 
  of 
  the 
  more 
  crystalline 
  granites, 
  as 
  dis- 
  

   tinguished 
  from 
  those 
  whose 
  slaty 
  structure 
  may 
  allow 
  them 
  to 
  be 
  considered 
  as 
  

   having 
  a 
  regular 
  cleavage 
  and 
  direction 
  of 
  beds, 
  subordinate 
  to 
  which 
  hornblende 
  

   slate 
  also 
  occurs. 
  When 
  I 
  speak, 
  therefore, 
  of 
  granite 
  forming 
  the 
  predominant 
  

   rock 
  (uncoloured 
  on 
  the 
  map), 
  I 
  would 
  not 
  be 
  understood 
  to 
  do 
  so 
  always 
  with 
  

   precise 
  mineralogical 
  accuracy. 
  So 
  far 
  as 
  I 
  know, 
  there 
  is 
  no 
  peculiarity 
  in 
  the 
  

   volcanic 
  action 
  in 
  the 
  granitic 
  districts, 
  compared 
  with 
  that 
  in 
  gneiss. 
  A 
  small 
  

   patch 
  of 
  the 
  coal-formation 
  appears 
  near 
  Jaujac 
  on 
  the 
  Alignon, 
  but 
  it 
  is 
  sur- 
  

   rounded 
  by 
  granite 
  or 
  gneiss, 
  which 
  again 
  is 
  succeeded 
  by 
  the 
  lias 
  or 
  oolite 
  for- 
  

   mations 
  near 
  Aubenas. 
  The 
  coal-formation 
  occurs 
  in 
  patches 
  nearly 
  all 
  round 
  

   the 
  great 
  primitive 
  plateau 
  of 
  central 
  France. 
  It 
  is 
  extensively 
  worked 
  on 
  the 
  

   east 
  side 
  at 
  St 
  Ettienne 
  ; 
  and, 
  in 
  some 
  places 
  at 
  least, 
  its 
  strata 
  lie 
  horizontally 
  

   against 
  the 
  granite, 
  shewing 
  the 
  anterior 
  date 
  of 
  the 
  elevation 
  of 
  the 
  latter* 
  

   The 
  valleys 
  we 
  have 
  to 
  describe 
  farther 
  agree 
  in 
  this 
  extraordinary 
  particular, 
  — 
  

   that, 
  as 
  surely 
  as 
  they 
  contain 
  water 
  they 
  contain 
  a 
  stream 
  of 
  lava 
  or 
  basalt, 
  or 
  

   the 
  remains 
  of 
  one, 
  which 
  stream 
  has 
  accommodated 
  itself 
  perfectly 
  to 
  the 
  

   sinuosities 
  of 
  the 
  channel 
  of 
  primitive 
  rock 
  in 
  which 
  it 
  has 
  run, 
  the 
  possession 
  

   of 
  which 
  it 
  contests 
  yard 
  by 
  yard 
  with 
  the 
  water 
  ; 
  these 
  lava 
  streams 
  are 
  

   sometimes 
  attenuated 
  to 
  a 
  surprising 
  degree, 
  leaving 
  but 
  small 
  relics 
  for 
  the 
  

   space 
  of 
  miles 
  ; 
  in 
  other 
  places 
  they 
  accumulate 
  to 
  an 
  astonishing 
  thickness 
  and 
  

   breadth, 
  altering 
  the 
  configuration 
  of 
  the 
  valley, 
  the 
  stagnant 
  pool 
  of 
  lava 
  hav- 
  

   ing, 
  in 
  the 
  first 
  instance, 
  created 
  a 
  lake 
  of 
  water, 
  and 
  compelling 
  the 
  river 
  to 
  

   alter 
  its 
  course 
  and 
  to 
  excavate 
  a 
  new 
  channel. 
  The 
  tributary 
  of 
  each 
  valley 
  

   commonly 
  unites 
  with 
  others, 
  accumulating 
  near 
  the 
  points 
  of 
  junction; 
  but 
  the 
  

  

  * 
  BuRAT, 
  p. 
  4. 
  

  

  