﻿14 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  to 
  the 
  plane 
  of 
  the 
  stratum 
  which 
  they 
  occupy, 
  which 
  is 
  inclined 
  about 
  9 
  to 
  the 
  

   horizon.* 
  Some 
  of 
  the 
  pillars 
  are 
  a 
  little 
  twisted 
  in 
  their 
  upper 
  parts, 
  although 
  

   quite 
  straight 
  below, 
  a 
  phenomenon 
  which 
  gives 
  them 
  an 
  appearance 
  of 
  having 
  

   yielded 
  under 
  the 
  pressure 
  of 
  the 
  load 
  above 
  them 
  ; 
  this 
  occurs 
  a 
  little 
  to 
  the 
  east 
  

   of 
  Fingal's 
  Cave, 
  and 
  is 
  correctly 
  represented 
  in 
  one 
  of 
  Dr 
  MacCulloch's 
  views 
  of 
  

   the 
  island. 
  If 
  we 
  compare 
  the 
  lava-cliff 
  of 
  Jaujac 
  with 
  Staffa 
  as 
  a 
  mere 
  columnar 
  

   display, 
  we 
  must 
  give 
  the 
  former 
  a 
  decided 
  preference. 
  The 
  columns 
  are 
  more 
  

   numerous, 
  more 
  extended, 
  higher, 
  slenderer, 
  better 
  pointed, 
  and, 
  in 
  every 
  respect, 
  

   more 
  perfect. 
  It 
  is 
  probable 
  that 
  this, 
  as 
  well 
  as 
  their 
  incomparable 
  superiority 
  

   to 
  any 
  known 
  product 
  of 
  altogether 
  recent 
  volcanoes, 
  is 
  due 
  to 
  the 
  more 
  perfect 
  

   composition 
  and 
  fusion 
  of 
  the 
  material 
  of 
  which 
  they 
  are 
  formed. 
  

  

  But 
  this 
  grand 
  phenomenon 
  might 
  have 
  been 
  lost 
  for 
  ever 
  to 
  human 
  sight 
  

   had 
  not 
  the 
  excavating 
  action 
  of 
  the 
  stream 
  made 
  the 
  section 
  which 
  we 
  have 
  been 
  

   contemplating 
  ; 
  and, 
  is 
  it 
  not 
  interesting 
  to 
  inquire 
  in 
  what 
  manner 
  the 
  degrada- 
  

   tion 
  or 
  destruction 
  of 
  so 
  large 
  a 
  mass 
  of 
  intensely 
  hard 
  rock 
  was 
  effected, 
  and 
  

   whether 
  it 
  is 
  going 
  on 
  still 
  ? 
  Now, 
  as 
  to 
  the 
  present 
  action 
  of 
  the 
  elements, 
  there 
  

   are 
  abundant 
  proofs 
  that 
  it 
  is 
  going 
  on 
  with 
  great 
  energy. 
  Just 
  below 
  Jaujac, 
  a 
  

   small 
  stream, 
  named 
  Rioclat, 
  joins 
  the 
  Alignon, 
  and, 
  opposite 
  to 
  their 
  junction, 
  a 
  

   great 
  mass 
  of 
  the 
  principal 
  lava-cliff 
  fell 
  about 
  three 
  weeks 
  before 
  my 
  visit 
  in 
  

   1841. 
  The 
  scale 
  of 
  this 
  operation, 
  which 
  is 
  the 
  removal 
  of 
  about 
  2000 
  cubic 
  

   yards 
  (40 
  yards 
  in 
  front, 
  3 
  from 
  back 
  to 
  front, 
  and 
  16 
  high), 
  makes 
  it 
  very 
  in- 
  

   teresting, 
  and 
  its 
  recent 
  occurrence, 
  as 
  well 
  as 
  that 
  of 
  another 
  great 
  eboidement 
  

   opposite 
  Souillols, 
  throws 
  light 
  upon 
  the 
  natural 
  mode 
  of 
  proceeding. 
  In 
  both 
  these 
  

   cases 
  the 
  atmosphere 
  seems 
  to 
  have 
  acted 
  alone. 
  At 
  Jaujac 
  (the 
  case 
  represented 
  

   in 
  Plate 
  VI., 
  fig. 
  3), 
  the 
  river 
  did 
  not 
  touch 
  the 
  base 
  of 
  the 
  lava 
  at 
  all 
  ; 
  Ave 
  must, 
  

   therefore, 
  distinguish 
  two 
  methods 
  of 
  disintegration. 
  1st, 
  The 
  atmospheric 
  water 
  

   penetrating 
  freely 
  amongst 
  the 
  countless 
  fissures 
  which 
  the 
  imperfect 
  columnar 
  

   structure 
  of 
  the 
  upper 
  bed 
  presents, 
  detaches 
  it 
  gradually 
  by 
  the 
  usual 
  effect 
  of 
  

   liquid 
  pressure, 
  more 
  rarely 
  perhaps, 
  by 
  congelation. 
  The 
  principal 
  tendency 
  to 
  

   fracture 
  being 
  exactly 
  in 
  a 
  vertical 
  direction, 
  the 
  cliff 
  has 
  a 
  continual 
  tendency 
  to 
  

   instability. 
  A 
  few 
  hundred 
  yards 
  from 
  the 
  spot 
  last 
  described, 
  there 
  was, 
  at 
  the 
  

   time 
  of 
  my 
  visit, 
  another 
  huge 
  mass 
  in 
  the 
  process 
  of 
  separation 
  from 
  the 
  cliff 
  by 
  

   a 
  gap 
  already 
  two 
  feet 
  wide 
  at 
  the 
  top. 
  I 
  passed 
  below 
  it 
  during 
  a 
  tremendous 
  

   thunder-storm, 
  when 
  the 
  rain-water 
  was 
  gushing 
  in 
  torrents 
  from 
  the 
  joints 
  of 
  

   the 
  pillars, 
  threatening 
  instant 
  precipitation. 
  2d, 
  The 
  lower 
  part 
  of 
  the 
  lava 
  

   being 
  always 
  regularly 
  columnar, 
  and 
  having, 
  therefore, 
  very 
  little 
  lateral 
  cohe- 
  

   sion, 
  and 
  probably, 
  like 
  a 
  table 
  with 
  many 
  legs, 
  all 
  the 
  pillars 
  not 
  bearing 
  equal 
  

   shares 
  of 
  the 
  load, 
  the 
  erosive 
  action 
  of 
  the 
  water 
  must 
  necessarily 
  detach 
  them 
  ; 
  

   and, 
  we 
  almost 
  always, 
  if 
  not 
  invariably, 
  find 
  that 
  the 
  upper 
  part 
  of 
  the 
  cliff 
  pro- 
  

  

  * 
  A 
  description 
  of 
  the 
  Western 
  Islands 
  of 
  Scotland, 
  vol. 
  ii. 
  

  

  