﻿ORIGIN 
  OF 
  COLUMNAR 
  STRUCTURE 
  — 
  JAUJAC 
  — 
  STAFFA. 
  13 
  

  

  emission 
  (the 
  Coupe 
  de 
  Jaujac, 
  near 
  the 
  village 
  of 
  that 
  name). 
  Near 
  Jaujac, 
  

   where 
  the 
  cliff 
  may 
  be, 
  perhaps, 
  from 
  60 
  to 
  70 
  feet 
  high, 
  only 
  one-fourth 
  of 
  the 
  

   height 
  is 
  occupied 
  by 
  the 
  columnar 
  range 
  ; 
  down 
  the 
  course 
  of 
  the 
  valley 
  it 
  rises 
  

   to 
  one-third 
  and 
  one-half 
  of 
  the 
  height 
  of 
  the 
  cliff 
  (which 
  is 
  also 
  increasing), 
  it 
  

   then 
  divides 
  into 
  two 
  columnar 
  beds, 
  with 
  an 
  amorphous 
  portion 
  between, 
  and 
  

   finally 
  presents 
  the 
  magnificent 
  display 
  of 
  unbroken 
  columns 
  in 
  almost 
  its 
  entire 
  

   height, 
  already 
  referred 
  to. 
  

  

  We 
  have 
  seen, 
  that, 
  sharp 
  as 
  the 
  separation 
  appears 
  of 
  the 
  columns 
  from 
  the 
  

   amorphous 
  or 
  slightly-columnar 
  part, 
  it 
  is 
  impossible 
  to 
  deny 
  that 
  both 
  are 
  the 
  

   production 
  of 
  a 
  single 
  eruption. 
  The 
  cause 
  of 
  the 
  abruptness 
  of 
  the 
  separation 
  

   appears, 
  in 
  the 
  present 
  state 
  of 
  our 
  knowledge, 
  altogether 
  inexplicable. 
  Mr 
  

   Scrope, 
  indeed, 
  gives 
  the 
  following 
  elucidation, 
  but 
  I 
  doubt 
  whether 
  it 
  can 
  be 
  

   considered 
  as, 
  in 
  any 
  degree, 
  satisfactory. 
  " 
  [This] 
  singular 
  difference 
  of 
  struc- 
  

   ture 
  may 
  be 
  accounted 
  for, 
  I 
  conceive, 
  by 
  supposing 
  the 
  elastic 
  vapour, 
  contained 
  

   in 
  the 
  lava, 
  to 
  have 
  escaped 
  from 
  the 
  upper 
  part, 
  through 
  fissures 
  of 
  retreat, 
  

   formed 
  in 
  irregular 
  directions 
  as 
  the 
  mass 
  cooled 
  ; 
  while 
  the 
  immense 
  pressure, 
  

   acting 
  on 
  the 
  lower 
  part, 
  would, 
  at 
  the 
  same 
  time, 
  prevent 
  the 
  escape 
  of 
  the 
  con- 
  

   tained 
  vapour, 
  and 
  effect 
  its 
  condensation, 
  thereby 
  allowing 
  the 
  mass 
  to 
  consoli- 
  

   date 
  in 
  a 
  regular 
  and 
  tranquil 
  manner, 
  such 
  as 
  would 
  facilitate 
  the 
  establishment 
  

   of 
  straight 
  and 
  vertical 
  axes 
  of 
  contraction, 
  and 
  the 
  formation 
  of 
  very 
  regular 
  

   hexahedral 
  columnar 
  concretions."* 
  Whatever 
  be 
  the 
  cause, 
  it 
  would 
  appear 
  to 
  

   become 
  more 
  energetic 
  after 
  the 
  lava 
  has 
  flowed 
  to 
  a 
  considerable 
  distance 
  ; 
  and 
  

   from 
  the 
  regularity 
  with 
  which 
  the 
  limit 
  of 
  the 
  columns 
  follows 
  the 
  sinuosities 
  of 
  

   the 
  base 
  (see 
  Plate 
  VI., 
  fig. 
  3), 
  it 
  would 
  seem 
  to 
  depend 
  more 
  upon 
  the 
  rapidity 
  

   of 
  the 
  cooling 
  action 
  below 
  than 
  upon 
  the 
  superincumbent 
  load 
  of 
  lava. 
  

  

  It 
  is 
  a 
  very 
  curious 
  circumstance 
  that 
  this 
  sharp 
  limit 
  of 
  columnar 
  forma- 
  

   tions 
  occurs 
  also 
  in 
  circumstances 
  which 
  would 
  appear 
  very 
  different 
  from 
  the 
  

   volcanic 
  flood 
  of 
  Jaujac, 
  which 
  undoubtedly 
  occurred 
  in 
  the 
  open 
  air 
  exactly 
  like 
  

   any 
  modern 
  eruption. 
  A 
  recent 
  visit 
  to 
  the 
  Island 
  of 
  Staffa 
  has 
  satisfied 
  me 
  that 
  

   the 
  basaltic 
  bed 
  in 
  which 
  Fingal's 
  Cave 
  occurs, 
  is 
  constituted 
  precisely 
  like 
  the 
  

   lavas 
  of 
  the 
  Vivarais, 
  and 
  that 
  the 
  sharp 
  cessation 
  of 
  the 
  columns, 
  and 
  the 
  abrupt 
  

   transition 
  to 
  a 
  nearly 
  shapeless 
  bed 
  of 
  basalt, 
  is 
  not 
  due 
  to 
  a 
  superposition 
  of 
  

   other 
  matter, 
  but 
  simply 
  to 
  the 
  limited 
  sphere 
  of 
  action 
  of 
  the 
  crystallizing 
  or 
  

   concretionary 
  forces 
  ; 
  we 
  have 
  there, 
  as 
  at 
  Jaujac, 
  perfect 
  evidence 
  of 
  the 
  con- 
  

   temporaneous 
  formation 
  of 
  both 
  beds; 
  and, 
  indeed, 
  the 
  apparent 
  section 
  of 
  the 
  two 
  

   would 
  be 
  perfectly 
  identical, 
  the 
  compact 
  part 
  commonly 
  projecting 
  over 
  the 
  colum- 
  

   nar 
  part, 
  as 
  in 
  Plate 
  II., 
  fig. 
  6. 
  Dr 
  MacCulloch, 
  who 
  makes 
  no 
  allusion 
  to 
  the 
  con- 
  

   temporaneity 
  of 
  the 
  beds, 
  or 
  to 
  the 
  cause 
  of 
  their 
  distinction 
  of 
  character, 
  minutely 
  

   specifies 
  the 
  exact 
  perpendicularity 
  of 
  the 
  principal 
  range 
  of 
  the 
  columns 
  of 
  Staffa 
  

  

  * 
  Geology 
  of 
  Central 
  France, 
  p. 
  152. 
  

   VOL. 
  XX. 
  PART 
  I. 
  D 
  

  

  