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  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  in 
  the 
  washing 
  of 
  the 
  sides 
  of 
  the 
  valley 
  by 
  the 
  scorise 
  to 
  a 
  considerable 
  height 
  

   on 
  either 
  hand 
  ; 
  (2.) 
  from 
  the 
  nicety 
  with 
  which 
  it 
  has 
  filled 
  every 
  angle 
  of 
  this 
  

   irregular 
  channel 
  ; 
  (3.) 
  from 
  its 
  almost 
  total 
  disappearance 
  for 
  several 
  miles 
  from 
  

   its 
  course, 
  shewing 
  (I 
  apprehend) 
  that 
  it 
  ran 
  over 
  the 
  ground 
  almost 
  without 
  con- 
  

   solidating. 
  

  

  Feet. 
  

  

  The 
  Height 
  of 
  the 
  Pont 
  de 
  la 
  Beaume 
  above 
  the 
  Sea 
  is 
  . 
  1073 
  

   Village 
  of 
  Burzet* 
  ... 
  ... 
  . 
  . 
  1815 
  

  

  Rise 
  of 
  the 
  Valley 
  in 
  8 
  miles, 
  ...... 
  742 
  

  

  Mean 
  Slope 
  (1 
  in 
  57, 
  or) 
  1° 
  0' 
  

  

  At 
  the 
  crater 
  of 
  Fiollonge 
  the 
  barometer 
  stood, 
  on 
  the 
  26th 
  June 
  1841, 
  at 
  

   6707 
  millimetres. 
  Its 
  height 
  is 
  3678 
  English 
  feet. 
  If 
  we 
  deduct 
  from 
  this 
  

   300 
  feet, 
  as 
  a 
  rough 
  estimate 
  of 
  the 
  height 
  of 
  the 
  crater 
  above 
  the 
  bed 
  of 
  the 
  

   river 
  of 
  Burzet, 
  at 
  the 
  foot 
  of 
  the 
  cascade 
  of 
  Raipis, 
  we 
  have 
  for 
  the 
  rise 
  in 
  

   the 
  upper 
  8 
  miles 
  of 
  the 
  valley 
  from 
  the 
  village 
  of 
  Burzet, 
  1563 
  feet. 
  

  

  Mean 
  Slope 
  (1 
  in 
  27, 
  or) 
  2° 
  7' 
  

  

  Even 
  on 
  my 
  first 
  visit 
  in 
  1839, 
  I 
  was 
  so 
  much 
  struck 
  by 
  the 
  fluidity 
  which 
  

   this 
  lava 
  stream 
  must 
  have 
  possessed, 
  that 
  I 
  made 
  some 
  experiments 
  during 
  the 
  

   following 
  winter, 
  with 
  the 
  kind 
  permission 
  and 
  aid 
  of 
  the 
  late 
  Mr 
  Edington 
  of 
  

   Glasgow, 
  upon 
  the 
  flow 
  of 
  cast-iron 
  in 
  very 
  small 
  and 
  tortuous 
  channels 
  at 
  

   different 
  slopes. 
  The 
  results 
  are 
  scarcely 
  worthy 
  of 
  minute 
  detail 
  ; 
  but 
  they 
  left 
  

   the 
  impression 
  on 
  my 
  mind 
  that 
  the 
  temperature 
  and 
  the 
  liquidity 
  of 
  the 
  lava 
  of 
  

   Burzet 
  must 
  have 
  at 
  least 
  equalled 
  that 
  of 
  pure 
  cast-iron, 
  a 
  result 
  which 
  strik- 
  

   ingly 
  contrasts 
  with 
  the 
  extreme 
  viscidity 
  of 
  most 
  modern 
  lavas, 
  which 
  only 
  attain 
  

   a 
  long 
  course, 
  aided 
  by 
  very 
  steep 
  inclinations, 
  or 
  by 
  their 
  great 
  volume, 
  which 
  

   generates 
  a 
  vast 
  moving 
  power 
  as 
  well 
  as 
  retains 
  the 
  high 
  temperature. 
  I 
  found 
  

   that 
  in 
  tortuous 
  channels 
  of 
  half-an-inch 
  broad, 
  and 
  under 
  slopes 
  of 
  about 
  1 
  in 
  

   120, 
  the 
  hottest 
  iron 
  used 
  in 
  ordinary 
  casting 
  solidified 
  before 
  it 
  had 
  run 
  a 
  course 
  

   of 
  many 
  feet. 
  The 
  form 
  of 
  the 
  current, 
  too, 
  was 
  the 
  same 
  as 
  in 
  the 
  lava 
  ; 
  the 
  sur- 
  

   face 
  was 
  concave, 
  except 
  near 
  the 
  lower 
  termination 
  of 
  the 
  stream, 
  where 
  it 
  be- 
  

   came 
  convex, 
  owing 
  to 
  the 
  pressure 
  from 
  behind, 
  and 
  the 
  accumulation 
  due 
  to 
  the 
  

   increasing 
  friction 
  occasioned 
  by 
  the 
  crystallization 
  of 
  the 
  iron 
  which 
  appeared 
  to 
  

   be 
  rather 
  suddenly 
  effected. 
  Near 
  the 
  source 
  a 
  mere 
  trail 
  of 
  slag 
  was 
  left 
  (often 
  

   hollow), 
  and 
  the 
  slag 
  bathed 
  the 
  sides 
  of 
  the 
  channel 
  to 
  some 
  height, 
  as 
  described 
  

   in 
  the 
  case 
  of 
  the 
  lava. 
  

  

  In 
  making 
  these 
  experiments, 
  I 
  little 
  thought 
  that 
  in 
  a 
  few 
  years 
  I 
  should 
  

   have 
  occasion 
  to 
  return 
  to 
  the 
  subject 
  of 
  the 
  motion 
  of 
  viscous 
  fluids 
  in 
  narrow 
  

   channels, 
  in 
  connection 
  with 
  the 
  seemingly 
  opposite 
  subject 
  of 
  glaciers 
  ; 
  but, 
  in 
  

   fact, 
  the 
  forms 
  of 
  my 
  cast-iron 
  models 
  (which 
  I 
  still 
  possess), 
  recall 
  strikingly 
  

  

  * 
  Barom. 
  7156 
  mm. 
  26th 
  June 
  1841. 
  

  

  