﻿EXCAVATION 
  OF 
  THE 
  LAVA 
  OF 
  THUEZ. 
  

  

  21 
  

  

  and 
  Sir 
  R. 
  I. 
  Murchison, 
  in 
  a 
  paper 
  published 
  many 
  years 
  ago, 
  on 
  this 
  very 
  subject,* 
  

   in 
  which 
  the 
  excavation 
  of 
  the 
  granite, 
  as 
  well 
  as 
  of 
  the 
  lava 
  of 
  Thuez, 
  is 
  cited 
  in 
  

   additional 
  confirmation. 
  But 
  none 
  of 
  these 
  authors, 
  so 
  far 
  as 
  I 
  recollect, 
  have 
  

   referred 
  to 
  the 
  singular 
  manner 
  in 
  which 
  this 
  lake 
  of 
  lava 
  has 
  been, 
  as 
  it 
  were, 
  

   suspended 
  in 
  the 
  middle 
  of 
  a 
  valley 
  which 
  presents 
  so 
  great 
  a 
  declivity. 
  The 
  

   section 
  just 
  given 
  in 
  the 
  Gueule 
  d'Enfer 
  seems 
  to 
  shew 
  that 
  that 
  ravine 
  must 
  

   have 
  been 
  entirely 
  excavated 
  since 
  the 
  lava 
  was 
  consolidated! 
  There 
  are 
  few 
  

   phenomena, 
  geologically 
  so 
  recent, 
  which 
  appear 
  more 
  unaccountable, 
  more 
  dis- 
  

   proportioned 
  to 
  the 
  means 
  by 
  which 
  apparently 
  they 
  must 
  have 
  been 
  produced. 
  

   The 
  facts 
  before 
  us 
  recal, 
  in 
  a 
  striking 
  manner, 
  the 
  parellel 
  roads 
  of 
  Glen 
  Roy 
  in 
  

   Scotland 
  ; 
  lake 
  terraces 
  apparently 
  of 
  a 
  similar 
  age 
  to 
  the 
  basalts 
  of 
  the 
  Vivarais 
  

   (that 
  is, 
  posterior 
  to 
  many 
  of 
  the 
  ordinary 
  river 
  alluvia), 
  and 
  which 
  must 
  have 
  

   required 
  barriers 
  far 
  exceeding 
  in 
  dimension 
  those 
  which 
  dammed 
  up 
  the 
  lava 
  of 
  

   Thuez. 
  But 
  in 
  Glen 
  Roy, 
  whatever 
  were 
  the 
  barriers, 
  they 
  were 
  certainly 
  not 
  

   composed 
  of 
  solid 
  rock. 
  Here, 
  on 
  the 
  contrary, 
  they 
  would 
  appear 
  at 
  least 
  in 
  a 
  

   great 
  measure 
  to 
  have 
  been 
  so. 
  

  

  As 
  we 
  issue 
  from 
  the 
  Gueule 
  d'Enfer, 
  we 
  find 
  a 
  tolerably 
  wide 
  and 
  cultivated 
  

   ravine, 
  entirely 
  based 
  on 
  the 
  primitive 
  rock 
  ; 
  whilst 
  the 
  mural 
  precipice 
  of 
  lava, 
  

   in 
  some 
  places 
  200 
  feet 
  high, 
  extends 
  for 
  about 
  a 
  mile 
  parallel 
  to 
  the 
  river 
  on 
  

   the 
  right. 
  The 
  contact 
  of 
  the 
  lava 
  with 
  the 
  ancient 
  soil 
  may 
  almost 
  everywhere 
  

   be 
  traced. 
  The 
  lower 
  part 
  is 
  usually 
  composed 
  of 
  vertical 
  columns 
  ; 
  the 
  upper 
  

   part 
  is 
  (as 
  at 
  Jaujac) 
  only 
  very 
  imperfectly 
  prismatic 
  ; 
  but 
  the 
  whole 
  is 
  evidently 
  

   the 
  result 
  of 
  one 
  eruption. 
  The 
  lower 
  part 
  of 
  the 
  stream, 
  where 
  it 
  touches 
  the 
  

   soil, 
  has 
  in 
  some 
  places 
  a 
  very 
  singular 
  appearance, 
  glistening 
  and 
  coaly 
  ; 
  pro- 
  

   bably 
  composed 
  of 
  nearly 
  pure 
  augite 
  mixed 
  with 
  carbonaceous 
  matter, 
  of 
  which 
  

   I 
  found 
  a 
  singular 
  proof 
  in 
  a 
  portion 
  of 
  a 
  vegetable 
  stem, 
  of 
  which 
  a 
  cast 
  has 
  been 
  

   made 
  by 
  the 
  lava.f 
  

  

  The 
  general 
  phenomenon 
  of 
  the 
  perpendicularity 
  of 
  the 
  lava 
  prisms 
  to 
  the 
  

   surface 
  of 
  cooling 
  is 
  everywhere 
  exemplified; 
  but 
  nowhere 
  so 
  beautifully 
  as 
  near 
  

   the 
  part 
  of 
  the 
  cliff 
  called 
  the 
  echelle 
  du 
  Roi 
  (from 
  a 
  narrow 
  steep 
  passage 
  formed 
  

   by 
  a 
  dyke 
  or 
  vein 
  in 
  the 
  lava, 
  and 
  by 
  which 
  the 
  cliff 
  may 
  be 
  ascended). 
  Here 
  there 
  

   occurs 
  beneath 
  the 
  prismatic 
  lava 
  a 
  shapeless 
  mass, 
  apparently 
  of 
  old 
  lava, 
  slaggy, 
  

   and 
  not 
  at 
  all 
  columnar 
  ; 
  of 
  which 
  the 
  new 
  lava 
  has 
  formed 
  an 
  exact 
  cast, 
  and 
  

   fringed 
  it 
  all 
  round 
  with 
  columns 
  perpendicular 
  to 
  its 
  own 
  most 
  irregular 
  surface. 
  

   This 
  is 
  represented 
  in 
  Plate 
  III. 
  fig. 
  3, 
  from 
  a 
  rather 
  careful 
  drawing 
  made 
  on 
  

   the 
  spot. 
  I 
  have 
  often 
  been 
  inclined 
  to 
  think 
  that 
  the 
  old 
  lava, 
  which 
  must 
  evi- 
  

   dently 
  have 
  been 
  cold 
  when 
  it 
  was 
  overflowed 
  by 
  the 
  other, 
  may 
  be 
  derived 
  from 
  

   the 
  Gravenne 
  of 
  Montpezat 
  ; 
  and 
  this 
  idea 
  is 
  confirmed 
  by 
  the 
  consideration, 
  that 
  

  

  * 
  Jameson's 
  Edinburgh 
  Philosophical 
  Journal, 
  1829. 
  

  

  \ 
  The 
  specimen 
  illustrating 
  this 
  curious 
  fact, 
  and 
  others 
  referred 
  to 
  in 
  this 
  paper, 
  are 
  now 
  placed 
  

   in 
  the 
  Museum 
  of 
  the 
  Royal 
  Society 
  of 
  Edinburgh. 
  

  

  VOL. 
  XX. 
  PART 
  I. 
  p 
  

  

  