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  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  way 
  to 
  the 
  Pont 
  de 
  la 
  Beaume, 
  and 
  is 
  no 
  doubt 
  the 
  same 
  which 
  we 
  have 
  already 
  

   described 
  as 
  underlying 
  the 
  great 
  coulee 
  of 
  the 
  valley 
  of 
  the 
  Ardeche. 
  

  

  Having 
  now 
  traced 
  the 
  course 
  of 
  the 
  lava 
  of 
  the 
  Gravenne, 
  we 
  shall 
  point 
  out 
  

   some 
  other 
  volcanic 
  appearances 
  in 
  the 
  valley 
  of 
  Montpezat. 
  And, 
  first, 
  we 
  have 
  

   a 
  dyke 
  of 
  basalt, 
  in 
  the 
  granite 
  of 
  the 
  left 
  bank 
  of 
  the 
  Fontaulier, 
  near 
  the 
  hamlet 
  

   of 
  Les 
  Plantas, 
  about 
  half-way 
  between 
  the 
  Castle 
  of 
  Pourchirol 
  and 
  the 
  entrance 
  

   of 
  the 
  valley 
  of 
  Burzet. 
  This 
  is 
  a 
  phenomenon 
  which 
  we 
  should 
  naturally 
  ex- 
  

   pect 
  to 
  meet 
  with 
  very 
  frequently 
  in 
  a 
  country 
  which, 
  with 
  a 
  soil 
  entirely 
  primi- 
  

   tive, 
  has 
  been 
  pierced 
  at 
  so 
  many 
  points 
  within 
  a 
  short 
  space 
  by 
  volcanic 
  ducts, 
  

   which 
  can 
  hardly 
  have 
  been 
  formed 
  without 
  an 
  intense 
  pressure 
  from 
  below, 
  which 
  

   might 
  have 
  been 
  expected 
  to 
  rend 
  the 
  strata 
  of 
  gneiss 
  in 
  all 
  directions, 
  and 
  then 
  

   to 
  have 
  filled 
  the 
  rents 
  with 
  melted 
  matter, 
  constituting 
  true 
  dykes, 
  such 
  as 
  those 
  

   which 
  occur 
  in 
  Monte 
  Somma, 
  near 
  Naples. 
  The 
  reverse 
  is, 
  however, 
  the 
  case 
  : 
  

   and 
  the 
  dyke 
  of 
  Les 
  Plantas 
  is 
  the 
  only 
  one 
  which 
  I 
  have 
  met 
  with 
  in 
  this 
  part 
  of 
  the 
  

   Vivarais 
  (amongst 
  the 
  older 
  volcanic 
  formations 
  of 
  Le 
  Puy 
  and 
  the 
  Coyrons 
  they 
  

   are 
  numerous). 
  No 
  doubt, 
  many 
  may 
  have 
  escaped 
  me 
  ; 
  and 
  I 
  should 
  never 
  have 
  

   known 
  of 
  this 
  one, 
  owing 
  to 
  its 
  remote 
  and 
  concealed 
  position, 
  had 
  I 
  not 
  been 
  fortu- 
  

   nately 
  directed 
  to 
  it 
  by 
  a 
  resident 
  at 
  Montpezat. 
  It 
  occurs 
  in 
  a 
  small 
  ravine 
  (see 
  

   the 
  map), 
  running 
  in 
  a 
  direction 
  nearty 
  north 
  and 
  south, 
  and 
  is 
  said 
  to 
  have 
  been 
  

   traced 
  for 
  a 
  mile. 
  Its 
  plane 
  is 
  vertical. 
  Its 
  breadth 
  varies 
  from 
  1 
  to 
  4 
  feet 
  ; 
  and 
  

   it 
  sends 
  forth 
  small 
  veins 
  into 
  the 
  rock, 
  and 
  includes 
  portions 
  of 
  granite 
  in 
  its 
  

   substance. 
  In 
  composition, 
  it 
  resembles 
  the 
  lava 
  of 
  the 
  district 
  ; 
  black 
  in 
  colour, 
  

   with 
  concretions 
  of 
  olivine. 
  

  

  The 
  village 
  of 
  Montpezat, 
  charmingly 
  situated 
  on 
  the 
  rivulet 
  Pourseuille, 
  

   at 
  the 
  height 
  of 
  1857 
  feet 
  above 
  the 
  sea, 
  does 
  not 
  stand 
  upon 
  the 
  lava 
  of 
  

   the 
  Gravenne, 
  but 
  about 
  half-a-mile 
  beyond 
  where 
  it 
  terminates 
  at 
  the 
  

   parish 
  church 
  of 
  St 
  Pierre. 
  The 
  valley 
  is 
  filled 
  by 
  a 
  prodigious 
  multitude 
  

   of 
  rolled 
  blocks 
  of 
  granite; 
  which 
  here, 
  as 
  in 
  the 
  lateral 
  valley 
  of 
  Thuez, 
  

   appear 
  to 
  be 
  out 
  of 
  all 
  proportion 
  to 
  the 
  extent 
  of 
  the 
  ravine 
  from 
  which 
  they 
  

   must 
  have 
  been 
  derived, 
  and 
  of 
  which 
  the 
  countless 
  multitude 
  contrasts 
  with 
  

   the 
  comparative 
  rarity 
  of 
  basaltic 
  blocks. 
  At 
  the 
  head 
  of 
  the 
  valley, 
  near 
  the 
  

   source 
  of 
  the 
  Pourseuille, 
  rises 
  a 
  mass 
  of 
  volcanic 
  cinders, 
  well 
  distinguished 
  at 
  a 
  

   distance 
  by 
  its 
  colour 
  ; 
  and 
  which 
  is 
  figured 
  by 
  Mr 
  Scrope, 
  in 
  his 
  view 
  of 
  the 
  valley 
  

   of 
  Montpezat 
  (Plate 
  XV. 
  of 
  his 
  work), 
  who 
  describes 
  it 
  as 
  an 
  anonymous 
  volcanic- 
  

   cone 
  which 
  has 
  not 
  produced 
  any 
  lava 
  stream. 
  He 
  had 
  evidently 
  not 
  visited 
  the 
  

   place 
  ; 
  for 
  the 
  true 
  description 
  is 
  very 
  different. 
  I 
  shall 
  now 
  detail 
  the 
  chief 
  re- 
  

   sults 
  of 
  a 
  patient 
  examination 
  of 
  this 
  locality, 
  both 
  in 
  1839 
  and 
  ] 
  841 
  ; 
  there 
  is 
  no 
  

   point 
  in 
  the 
  Vivarais 
  which 
  I 
  have 
  more 
  narrowly 
  investigated. 
  

  

  Montpezat, 
  we 
  have 
  said, 
  is 
  placed 
  upon 
  an 
  immense 
  bed 
  of 
  granite 
  boulders, 
  

   which 
  extend 
  upwards, 
  occupying 
  the 
  whole 
  bed 
  of 
  the 
  valley, 
  to 
  a 
  great 
  thick- 
  

   ness 
  ; 
  but 
  near 
  the 
  village 
  of 
  Le 
  Fau 
  we 
  find 
  a 
  mass 
  of 
  lava, 
  which, 
  with 
  other 
  

  

  