﻿26 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  culties. 
  It 
  must 
  be 
  a 
  stream 
  from 
  the 
  Crater 
  of 
  Pal, 
  already 
  adverted 
  to 
  in 
  de- 
  

   scribing 
  the 
  route 
  by 
  the 
  Bauzon 
  from 
  Le 
  Puy 
  to 
  Montpezat. 
  This 
  well-formed, 
  

   nearly 
  circular 
  crater 
  occurs 
  a 
  little 
  behind 
  the 
  granite 
  rocks 
  and 
  cliffs 
  which 
  com- 
  

   pose 
  the 
  back-ground 
  of 
  the 
  ravine 
  of 
  Chambon 
  ; 
  and 
  so 
  far 
  it 
  would 
  appear 
  to 
  

   be 
  a 
  very 
  natural 
  origin 
  of 
  the 
  lava. 
  But 
  the 
  singular 
  circumstance 
  is, 
  that 
  the 
  

   crater 
  in 
  question 
  belongs 
  to 
  a 
  different 
  system 
  of 
  valleys 
  ; 
  inasmuch 
  as 
  the 
  water 
  

   issuing 
  from 
  it, 
  forms 
  the 
  source 
  of 
  the 
  Fontaulier, 
  and 
  reaches 
  the 
  vicinity 
  of 
  

   Montpezat 
  after 
  a 
  long 
  circuit 
  (see 
  the 
  General 
  Map, 
  Plate 
  I.). 
  It 
  is 
  also 
  pretty 
  

   open 
  to 
  the 
  north 
  ; 
  but 
  it 
  seems 
  completely 
  cut 
  off 
  from 
  the 
  ravine 
  of 
  Chambon 
  by 
  

   a 
  considerable 
  granite 
  ridge 
  running 
  NW. 
  and 
  SE., 
  forming 
  the 
  Col 
  or 
  passage 
  o~ 
  

   Le 
  Pal, 
  and 
  rising 
  to 
  a 
  height 
  of 
  4537 
  feet, 
  exactly 
  between 
  the 
  crater 
  of 
  Pal 
  and 
  

   lava 
  of 
  Chambon, 
  being 
  G44 
  feet 
  higher 
  than 
  the 
  former, 
  and 
  403 
  feet 
  above 
  the 
  

   extreme 
  point 
  of 
  the 
  latter. 
  There 
  are 
  therefore 
  two 
  difficulties 
  to 
  be 
  accounted 
  

   for 
  ; 
  first, 
  the 
  passage 
  of 
  a 
  great 
  mass 
  of 
  lava 
  and 
  other 
  volcanic 
  matters 
  over 
  

   this 
  barrier 
  ; 
  secondly, 
  the 
  excavation 
  of 
  the 
  ravine 
  of 
  Chambon, 
  and 
  the 
  removal 
  

   of 
  all 
  the 
  upper 
  part 
  of 
  the 
  lava 
  stream. 
  

  

  With 
  regard 
  to 
  tike 
  first 
  difficulty, 
  it 
  is 
  important 
  to 
  remark 
  that 
  most 
  of 
  the 
  

   granite 
  heights 
  between 
  Le 
  Pal 
  and 
  Chambon 
  are 
  covered 
  more 
  or 
  less 
  thickly 
  

   with 
  a 
  volcanic 
  conglomerate 
  forming 
  horizontal 
  beds 
  (one 
  granite 
  top 
  which 
  has 
  

   escaped, 
  is 
  uncoloured 
  on 
  the 
  map). 
  This 
  conglomerate 
  descends 
  to 
  the 
  crater 
  of 
  

   Pal, 
  and 
  stops 
  short 
  abruptly 
  on 
  the 
  face 
  nearest 
  Le 
  Chambon 
  (see 
  section, 
  

   Plate 
  IV., 
  fig. 
  2). 
  Consequently, 
  the 
  hills 
  about 
  C 
  (fig. 
  2, 
  Plan) 
  did 
  not 
  probably 
  

   always 
  form 
  the 
  highest 
  part 
  of 
  the 
  edge 
  of 
  the 
  crater 
  ; 
  for 
  the 
  conglomerate 
  is 
  of 
  

   too 
  compact 
  a 
  character 
  to 
  have 
  been 
  formed 
  by 
  a 
  dejection 
  of 
  volcanic 
  materials 
  

   merely 
  ; 
  and 
  the 
  same 
  cause 
  which 
  permitted 
  the 
  deposition 
  of 
  these 
  conglo- 
  

   merates 
  (such 
  as 
  the 
  heightening 
  of 
  the 
  crater 
  by 
  the 
  formation 
  of 
  a 
  cone 
  which 
  

   has 
  since 
  disappeared), 
  would 
  permit 
  equally 
  the 
  passage 
  of 
  a 
  part 
  of 
  the 
  fused 
  

   materials 
  across 
  the 
  ridge 
  into 
  the 
  neighbouring 
  valley. 
  And 
  the 
  fact 
  of 
  the 
  oc- 
  

   currence 
  of 
  abundant 
  conglomerates 
  in 
  both 
  positions, 
  confirms, 
  if 
  it 
  does 
  not 
  

   render 
  absolutely 
  necessary, 
  this 
  supposition 
  ; 
  although 
  I 
  am 
  aware 
  that 
  there 
  is 
  

   some 
  mineralogical 
  difference 
  in 
  their 
  composition,— 
  that 
  of 
  Chambon 
  being 
  more 
  

   friable, 
  and 
  more 
  generally 
  composed 
  of 
  volcanic 
  ingredients, 
  containing, 
  however, 
  

   granitic 
  masses, 
  — 
  that 
  of 
  Pal 
  being 
  chiefly 
  of 
  granite 
  boulders, 
  cemented 
  by 
  a 
  very 
  

   hard 
  volcanic 
  basis. 
  The 
  posteriority 
  of 
  the 
  conglomerate 
  eruption 
  may 
  account 
  

   (together 
  with 
  the 
  great 
  declivity) 
  for 
  the 
  absence 
  of 
  any 
  trace 
  of 
  the 
  lava 
  flow 
  

   on 
  the 
  granite 
  heights 
  of 
  Le 
  Pal. 
  

  

  The 
  second 
  difficulty, 
  that 
  of 
  the 
  excavation 
  of 
  the 
  ravine 
  of 
  Chambon 
  since 
  

   the 
  lava 
  flowed, 
  must 
  remain, 
  I 
  fear, 
  unanswered, 
  upon 
  any 
  theory. 
  Of 
  the 
  fact 
  

   there 
  appears 
  to 
  be 
  no 
  reasonable 
  doubt. 
  This 
  (geologically) 
  recent 
  excavation 
  

   of 
  a 
  perfect 
  mountain 
  of 
  hard 
  granite, 
  at 
  the 
  head 
  of 
  a 
  ravine 
  which 
  possesses 
  no 
  

   drainage 
  sufficient 
  to 
  procure 
  a 
  powerful 
  current 
  of 
  water, 
  and 
  which 
  is 
  near 
  the 
  

   culminating 
  point 
  of 
  three 
  pretty 
  extensive 
  mountain-ridges, 
  is 
  merely 
  one 
  of 
  a 
  

  

  