﻿GEOLOGY 
  OF 
  MONT 
  MEZENC. 
  5 
  

  

  the 
  fragments 
  of 
  trachyte 
  found, 
  as 
  already 
  mentioned, 
  completely 
  imbedded 
  in 
  

   the 
  basalt, 
  near 
  the 
  church 
  at 
  Fay-le-froid. 
  

  

  That 
  the 
  Mountain 
  of 
  Mezenc 
  is 
  situated 
  near 
  what 
  was 
  once 
  a 
  centre 
  of 
  vol- 
  

   canic 
  action 
  is 
  highly 
  probable, 
  not 
  only 
  from 
  being 
  the 
  most 
  elevated 
  summit 
  of 
  

   the 
  whole 
  country, 
  and 
  itself 
  composed 
  of 
  volcanic 
  rocks, 
  but 
  from 
  the 
  man- 
  

   ner 
  in 
  which 
  the 
  basaltic 
  plateaux 
  which 
  have 
  since 
  been 
  broken 
  up, 
  and 
  in 
  

   great 
  part 
  removed 
  by 
  denuding 
  causes, 
  may 
  be 
  traced 
  from 
  the 
  very 
  environs 
  

   of 
  Le 
  Puy 
  up 
  to 
  this 
  chain, 
  and 
  also 
  from 
  the 
  peculiar 
  evidences 
  of 
  local 
  fire 
  

   which 
  the 
  torrefied 
  materials 
  exhibit 
  in 
  the 
  section 
  just 
  presented. 
  I 
  am, 
  how- 
  

   ever, 
  disposed 
  to 
  agree 
  with 
  those 
  geologists 
  who 
  consider 
  that 
  since 
  the 
  produc- 
  

   tion 
  (perhaps 
  in 
  a 
  good 
  measure 
  by 
  elevation) 
  of 
  the 
  Mont 
  Mezenc, 
  the 
  contour 
  of 
  

   the 
  ground 
  has 
  been 
  so 
  completely 
  altered 
  and 
  disfigured 
  as 
  to 
  leave 
  no 
  ground 
  

   for 
  inferring 
  that 
  we 
  are 
  to 
  trace 
  in 
  its 
  lineaments 
  the 
  actual 
  point 
  of 
  ejection, 
  

   still 
  less 
  that 
  the 
  Cirque 
  de 
  Chisels 
  or 
  Boutieres 
  is 
  really 
  a 
  " 
  crater 
  of 
  elevation," 
  

   for 
  it 
  appears 
  to 
  want 
  the 
  essential 
  characters 
  of 
  a 
  crater 
  at 
  all. 
  It 
  would 
  be 
  dif- 
  

   ficult 
  to 
  prove 
  that 
  the 
  strata, 
  (ill-defined 
  even 
  where 
  they 
  exist) 
  dip 
  away 
  uni- 
  

   versally 
  from 
  a 
  common 
  centre 
  ; 
  for 
  only 
  the 
  northern 
  and 
  western 
  portions 
  of 
  the 
  

   supposed 
  circus 
  can 
  be 
  found 
  ; 
  and 
  the 
  argument 
  which 
  has 
  been 
  drawn* 
  from 
  the 
  

   precipitousness 
  of 
  the 
  rocks 
  in 
  this 
  place 
  compared 
  to 
  their 
  gentle 
  slope 
  in 
  other 
  

   directions 
  is 
  worth 
  little, 
  since 
  the 
  same 
  reasoning 
  would 
  apply 
  to 
  the 
  whole 
  eastern 
  

   flank 
  of 
  the 
  range, 
  between 
  Mont 
  Mezenc 
  and 
  the 
  Gerbier 
  des 
  Jones, 
  compared 
  to 
  

   the 
  western, 
  which 
  depends 
  upon 
  some 
  general, 
  but 
  probably 
  posterior 
  cause 
  of 
  

   denudation 
  of 
  which 
  I 
  can 
  give 
  no 
  account. 
  That 
  the 
  expansive 
  action 
  of 
  the 
  

   basalt, 
  whilst 
  fluid, 
  elevated 
  the 
  previously 
  existing 
  beds 
  of 
  trachyte, 
  and 
  thus 
  

   contributed 
  to 
  give 
  to 
  the 
  Mezenc 
  its 
  present 
  height, 
  I 
  think 
  there 
  can 
  be 
  no 
  

   reasonable 
  doubt 
  ; 
  and 
  the 
  dikes 
  of 
  basalt 
  occurring 
  here 
  and 
  elsewhere 
  appear 
  to 
  

   confirm 
  this 
  opinion. 
  It 
  is, 
  however, 
  worthy 
  of 
  note, 
  that 
  if 
  this 
  be 
  cited 
  as 
  an 
  

   argument 
  in 
  favour 
  of 
  " 
  elevation 
  craters," 
  and 
  as 
  confirming 
  the 
  usual 
  chronology 
  

   of 
  volcanic 
  rocks 
  of 
  different 
  mineral 
  characters, 
  in 
  the 
  Cantal 
  at 
  least, 
  we 
  find 
  a 
  

   mountain 
  of 
  phonolite, 
  the 
  Puy 
  Griou, 
  subjacent 
  to 
  the 
  basalts 
  which 
  it 
  is 
  sup- 
  

   posed 
  to 
  have 
  elevated. 
  

  

  Whilst 
  I 
  admit, 
  with 
  M. 
  Bertrand, 
  that 
  immense 
  lava 
  streams 
  proceeded 
  

   from 
  some 
  point 
  or 
  fissure 
  near 
  the 
  Mont 
  Mezenc, 
  and 
  flooded 
  the 
  fresh-water 
  for- 
  

   mations 
  of 
  a 
  great 
  part 
  of 
  the 
  basin 
  of 
  Le 
  Puy, 
  I 
  should 
  hesitate 
  before 
  ascribing, 
  

   with 
  Mr 
  Scrope, 
  the 
  immense 
  basaltic 
  plateaux 
  of 
  the 
  Coyrons 
  in 
  the 
  lower 
  Vi- 
  

   varais, 
  with 
  the 
  least 
  degree 
  of 
  certainty, 
  or 
  even 
  of 
  probability, 
  to 
  an 
  eruption 
  

   of 
  the 
  Mezenc.f 
  Amongst 
  other 
  arguments, 
  the 
  numerous 
  dikes 
  of 
  basalt, 
  tra- 
  

   versing 
  granite, 
  and 
  other 
  formations 
  in 
  this 
  country, 
  seem 
  to 
  shew 
  that 
  the 
  out- 
  

  

  * 
  Burat 
  Terrains 
  Volcaniques 
  de 
  la 
  France 
  Centrale, 
  p. 
  230. 
  

  

  •j" 
  Mr 
  Scrope's 
  General 
  Map 
  gives 
  an 
  altogether 
  erroneous 
  idea 
  of 
  the 
  proximity 
  and 
  mutual 
  

   dependence 
  of 
  the 
  basaltic 
  formations 
  of 
  the 
  Mezenc 
  and 
  those 
  of 
  the 
  Coyrons. 
  

  

  VOL. 
  XX. 
  PART 
  I. 
  B 
  

  

  