﻿6 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVAKAIS. 
  

  

  lets 
  of 
  melted 
  matter 
  were 
  far 
  more 
  widely 
  spread 
  than 
  this 
  idea 
  of 
  a 
  central 
  vol- 
  

   cano 
  would 
  permit. 
  Perhaps 
  it 
  may 
  added 
  that 
  the 
  close 
  proximity 
  of 
  .the 
  

   granite 
  to 
  the 
  surface, 
  wherever 
  the 
  volcanic 
  materials 
  have 
  been 
  wasted 
  suffi- 
  

   ciently 
  to 
  expose 
  it, 
  and 
  that 
  particularly 
  in 
  the 
  valley 
  of 
  the 
  Salliouse, 
  as 
  shewn 
  

   in 
  M. 
  Bertrand's 
  Geological 
  Map, 
  almost 
  close 
  to 
  the 
  Cirque 
  de 
  Clusels, 
  seems 
  at 
  

   variance 
  with 
  the 
  supposition 
  that 
  Mont 
  Mezenc 
  is 
  to 
  be 
  regarded 
  as 
  the 
  sole 
  vol- 
  

   canic 
  centre 
  which 
  gave 
  rise 
  to 
  such 
  widely-spread 
  phenomena. 
  

  

  I 
  must 
  still 
  more 
  emphatically 
  dissent 
  from 
  the 
  theory 
  that 
  the 
  hills 
  of 
  

   phonolite, 
  forming 
  the 
  north-east 
  barrier 
  of 
  the 
  basin 
  of 
  Le 
  Puy, 
  are 
  the 
  relics 
  of 
  

   a 
  felspathic 
  eruption 
  proceeding 
  from 
  the 
  Mezenc* 
  Not 
  to 
  multiply 
  arguments 
  

   against 
  so 
  very 
  bold, 
  and, 
  in 
  itself, 
  I 
  must 
  think, 
  so 
  improbable 
  a 
  supposition, 
  I 
  

   will 
  only 
  observe, 
  flrst, 
  the 
  rarity 
  of 
  any 
  appearances, 
  in 
  trachytic 
  mountains, 
  in- 
  

   dicating 
  that 
  the 
  matter 
  of 
  which 
  they 
  are 
  composed 
  has 
  been 
  sufficiently 
  liquid 
  

   to 
  flow, 
  after 
  the 
  manner 
  of 
  a 
  current, 
  over 
  any 
  extent 
  of 
  country. 
  Secondly, 
  the 
  

   absence 
  of 
  any 
  appearance 
  of 
  a 
  current 
  in 
  the 
  chain 
  of 
  detached 
  sugar-loaf 
  

   shaped 
  eminences 
  here 
  referred 
  to. 
  Thirdly, 
  that 
  if 
  it 
  were 
  a 
  current, 
  it 
  would 
  

   not 
  have 
  occupied 
  the 
  axis 
  of 
  a 
  granitic 
  elevation, 
  constituting 
  one 
  of 
  the 
  oro- 
  

   graphic 
  features 
  of 
  the 
  country 
  ; 
  and, 
  lastly, 
  that 
  the 
  direction 
  of 
  the 
  chain, 
  coin- 
  

   cident 
  with 
  that 
  of 
  other 
  important 
  chains, 
  and 
  especially 
  of 
  the 
  chain 
  separating 
  

   the 
  Allier 
  and 
  the 
  Loire, 
  and 
  the 
  chain 
  of 
  La 
  Margeride, 
  beyond 
  the 
  former, 
  evi- 
  

   dently 
  points 
  out 
  an 
  axis 
  of 
  elevation 
  which, 
  in 
  other 
  instances, 
  in 
  this 
  singular 
  

   country, 
  is 
  the 
  fertile 
  source 
  of 
  local 
  explosions 
  and 
  eruptions, 
  to 
  which 
  these 
  in- 
  

   sulated 
  phonolitic 
  peaks 
  may, 
  in 
  my 
  opinion, 
  be, 
  with 
  far 
  more 
  probability, 
  

   ascribed. 
  

  

  The 
  route, 
  southwards 
  from 
  the 
  Mezenc, 
  presents 
  some 
  singular 
  features. 
  

   We 
  follow 
  a 
  ridge, 
  sometimes 
  composed 
  of 
  trachyte 
  or 
  phonolite, 
  sometimes 
  of 
  

   basalt, 
  which 
  separates 
  the 
  gentle 
  slopes 
  towards 
  the 
  basin 
  of 
  the 
  Loire 
  and 
  the 
  

   more 
  precipitous 
  ones 
  towards 
  that 
  of 
  the 
  Rhone. 
  The 
  views, 
  in 
  the 
  latter 
  direc- 
  

   tion, 
  are 
  eminently 
  singular, 
  and 
  even 
  romantic 
  ; 
  a 
  country 
  intersected 
  with 
  deep 
  

   ravines, 
  and 
  divided 
  by 
  deep 
  hilly 
  ranges, 
  often 
  capped 
  by 
  fantastic 
  summits, 
  

   stretches 
  away 
  for 
  many 
  miles 
  in 
  the 
  direction 
  of 
  Chalangon 
  and 
  La 
  Voulte. 
  

   Some 
  idea 
  of 
  the 
  scenery 
  may 
  be 
  formed 
  from 
  the 
  sketch 
  in 
  Plate 
  V., 
  fig. 
  5, 
  

   taken 
  from 
  a 
  spot 
  commanding 
  a 
  view 
  towards 
  the 
  Mezenc, 
  across 
  the 
  country 
  

   now 
  referred 
  to. 
  The 
  form 
  of 
  the 
  phonolitic 
  summits 
  (those 
  whose 
  names 
  

   are 
  given 
  on 
  the 
  drawing) 
  marks 
  their 
  composition 
  in 
  a 
  manner 
  scarcely 
  to 
  

   be 
  mistaken 
  ; 
  but 
  the 
  basis 
  of 
  the 
  whole 
  is 
  granite 
  or 
  gneiss, 
  as 
  pointed 
  out 
  

   in 
  M. 
  Bertrand's 
  map, 
  and 
  in 
  the 
  new 
  Geological 
  Map 
  of 
  France. 
  It 
  is 
  pro- 
  

  

  * 
  " 
  The 
  uniformly 
  progressive 
  declination 
  of 
  this 
  series 
  of 
  phonolitic 
  summits 
  from 
  the 
  Mezenc 
  

   to 
  the 
  hed 
  of 
  the 
  river 
  where 
  they 
  terminate, 
  proves 
  them, 
  in 
  my 
  opinion, 
  to 
  he 
  the 
  remains 
  of 
  a 
  

   single 
  enormous 
  lava 
  current 
  prior 
  in 
  date 
  to 
  the 
  excavation 
  of 
  the 
  actual 
  channel 
  of 
  the 
  Loire, 
  

   and 
  far 
  the 
  most 
  considerable 
  in 
  hulk 
  and 
  extent 
  of 
  any 
  which 
  I 
  have 
  had 
  occasion 
  to 
  observe 
  in 
  the 
  

   phlegrsean 
  fields 
  of 
  France." 
  — 
  Scrope's 
  Geology 
  of 
  Central 
  France, 
  p. 
  129. 
  

  

  