﻿10 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  the 
  lava 
  wave, 
  and 
  thus, 
  as 
  it 
  were, 
  unrolls 
  a 
  continuous 
  carpet, 
  over 
  which 
  the 
  

   more 
  liquid 
  stream 
  flows 
  ; 
  here 
  and 
  there 
  breaking 
  through 
  the 
  tenacious 
  but 
  

   partially 
  yielding 
  and 
  crackling 
  crust 
  by 
  which 
  it 
  is 
  imperfectly 
  confined.* 
  This 
  

   perfectly 
  explains 
  why 
  we 
  should 
  have 
  such 
  a 
  substratum 
  of 
  scoriae 
  in 
  this 
  place, 
  

   which 
  is 
  often 
  wanting 
  below 
  the 
  basalt 
  in 
  the 
  higher 
  and 
  steeper 
  valleys 
  where 
  

   the 
  lava 
  flowed 
  more 
  rapidly, 
  and 
  it 
  also 
  accounts 
  for 
  the 
  arched 
  forms 
  of 
  the 
  

   contact-surface, 
  which 
  fig. 
  3 
  remarkably 
  displays 
  ; 
  these 
  vaults 
  being 
  due 
  to 
  the 
  

   actual 
  rolling 
  of 
  the 
  lava 
  over 
  the 
  more 
  or 
  less 
  ponderous 
  masses 
  of 
  scoriae 
  de- 
  

   posited 
  from 
  its 
  own 
  surface. 
  The 
  brick-red 
  colour 
  common 
  in 
  scoriae 
  beneath 
  

   lava 
  is 
  to 
  be 
  attributed 
  to 
  the 
  intense 
  heat 
  communicated 
  to 
  them 
  by 
  the 
  lava, 
  

   after 
  they 
  have 
  been 
  covered 
  by 
  it. 
  This 
  heat 
  must 
  have 
  been 
  retained 
  for 
  a 
  

   prodigious 
  space 
  of 
  time. 
  Brick-red 
  tints 
  are 
  usually 
  produced 
  upon 
  minerals 
  

   subjected 
  to 
  close 
  or 
  confined 
  heat. 
  The 
  casual 
  removal 
  of 
  the 
  scorise 
  has, 
  in 
  some 
  

   places, 
  left 
  grottoes 
  beneath 
  the 
  vaults. 
  Secondly, 
  These 
  vaults 
  also 
  present 
  this 
  

   remarkable 
  peculiarity, 
  that 
  the 
  columnar 
  structure 
  of 
  the 
  lava 
  (which 
  here, 
  

   as 
  elsewhere, 
  is 
  best 
  developed 
  near 
  its 
  lower 
  surface) 
  conforms 
  so 
  nicely 
  to 
  the 
  

   contact-surface 
  with 
  the 
  scoriae, 
  to 
  which 
  it 
  is 
  always 
  nearly 
  perpendicular, 
  as 
  to 
  

   give 
  quite 
  the 
  appearance 
  of 
  vaulting 
  stones 
  to 
  the 
  covering 
  of 
  the 
  grottoes. 
  This 
  

   is 
  a 
  fact 
  so 
  general 
  amongst 
  the 
  lavas 
  of 
  the 
  Vivarais, 
  as 
  to 
  deserve 
  almost 
  to 
  be 
  

   called 
  universal. 
  It 
  is 
  interesting, 
  as 
  illustrating 
  the 
  development 
  of 
  the 
  pris- 
  

   matic 
  structure 
  which 
  was 
  so 
  long 
  supposed 
  to 
  distinguish 
  ancient 
  basalts 
  from 
  

   true 
  lavas, 
  but 
  which 
  is 
  now 
  universally 
  admitted 
  to 
  characterize 
  both, 
  when 
  the 
  

   circumstances 
  of 
  cooling 
  are 
  favourable 
  to 
  their 
  production. 
  The 
  fact 
  of 
  the 
  per- 
  

   pendicularity 
  of 
  the 
  columns 
  to 
  the 
  surface 
  of 
  cooling 
  admits 
  of 
  this 
  general 
  

   illustration, 
  — 
  that 
  if 
  A 
  (Plate 
  II., 
  fig. 
  4) 
  represents 
  a 
  cold 
  mass 
  of 
  rock 
  over- 
  

   flowed 
  by 
  lava, 
  which 
  gradually 
  loses 
  its 
  heat 
  by 
  contact 
  and 
  conduction, 
  all 
  the 
  

   points 
  equidistant 
  from 
  the 
  rock, 
  as 
  a, 
  a, 
  a, 
  or 
  b, 
  b, 
  b, 
  or 
  c, 
  c, 
  c, 
  may 
  be 
  conceived 
  

   to 
  be, 
  at 
  the 
  same 
  moment, 
  in 
  the 
  same 
  condition 
  as 
  regards 
  a 
  tendency 
  to 
  con- 
  

   solidate 
  or 
  to 
  crystallize. 
  Any 
  peculiar 
  action, 
  which 
  depends 
  upon 
  a 
  particular 
  

   stage 
  of 
  cooling, 
  will 
  therefore 
  affect 
  similarly 
  all 
  the 
  points 
  a, 
  a, 
  a, 
  and 
  so 
  of 
  the 
  

   rest 
  ; 
  that 
  peculiar 
  state 
  of 
  tension 
  which 
  produces 
  the 
  columnar 
  division, 
  will 
  

   therefore 
  prevail 
  uniformly 
  over 
  any 
  one 
  of 
  these 
  isothermal 
  surfaces 
  (or 
  surfaces 
  

   equally 
  cooled) 
  at 
  a 
  given 
  time, 
  and 
  will 
  tend 
  to 
  produce 
  its 
  effect 
  everywhere 
  on 
  

   that 
  surface, 
  and 
  the 
  lines 
  or 
  planes 
  of 
  separation 
  will 
  therefore 
  seem 
  to 
  proceed 
  

   uniformly 
  from 
  the 
  surface 
  of 
  cooling 
  in 
  a 
  direction 
  perpendicular 
  to 
  it. 
  Or 
  on 
  

   the 
  less 
  probable 
  hypothesis 
  of 
  the 
  columns 
  being 
  due 
  to 
  real 
  crystallization, 
  the 
  

   crystals 
  will 
  naturally 
  begin 
  to 
  form 
  at 
  the 
  surface 
  of 
  earliest 
  consolidation, 
  

  

  * 
  That 
  such 
  is 
  the 
  mode 
  of 
  progression 
  of 
  lava 
  streams 
  at 
  a 
  great 
  distance 
  from 
  their 
  origin, 
  

   or 
  after 
  they 
  have 
  been 
  running 
  for 
  a 
  long 
  time, 
  appears 
  from 
  the 
  descriptions 
  of 
  the 
  best 
  writers 
  on 
  

   volcanoes. 
  Compare 
  Scrope's 
  description 
  of 
  the 
  lava 
  of 
  Etna 
  of 
  1819, 
  in 
  his 
  work 
  on 
  Volcanoes, 
  

   p. 
  102; 
  and 
  Auldjo's 
  figure 
  of 
  the 
  descending 
  lava 
  waves 
  of 
  1831, 
  in 
  his 
  Description 
  of 
  Vesuvius, 
  

   p. 
  92. 
  

  

  