﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  279 
  

  

  less 
  regular 
  form, 
  while 
  they 
  often 
  extend 
  to 
  much 
  greater 
  length 
  

   than 
  those 
  of 
  the 
  latter 
  class, 
  unlike 
  which 
  again 
  they 
  are 
  never 
  di- 
  

   vided 
  into 
  regular 
  blocks 
  by 
  equidistant, 
  curved, 
  joint-planes. 
  One 
  

   of 
  the 
  best 
  examples 
  of 
  the 
  columnar 
  structure 
  among 
  the 
  old 
  lavas 
  

  

  Fig. 
  8.— 
  Cliff-section, 
  8. 
  W. 
  of 
  Oban, 
  exhibiting 
  very 
  fine 
  Columnar 
  

   Structure 
  in 
  the 
  Porphyrite 
  Lavas 
  of 
  Lorn. 
  

  

  of 
  Lorn 
  is 
  that 
  exhibited 
  in 
  a 
  cliff 
  about 
  two 
  miles 
  south-west 
  of 
  

   Oban, 
  where 
  the 
  columns 
  are 
  of 
  great 
  length 
  and 
  beautifully 
  curved 
  

   (see 
  woodcut, 
  fig. 
  8) 
  ; 
  but 
  more 
  or 
  less 
  perfect 
  examples 
  of 
  the 
  same 
  

   structure 
  are 
  exposed 
  on 
  the 
  face 
  of 
  Beinn 
  Lora 
  and 
  at 
  other 
  points. 
  

  

  Some 
  of 
  the 
  volcanic 
  rocks 
  of 
  Lorn 
  present 
  characters 
  which, 
  at 
  

   first 
  sight, 
  appear 
  not 
  a 
  little 
  anomalous 
  and 
  puzzling 
  ; 
  but 
  this 
  is 
  a 
  

   circumstance 
  which, 
  when 
  we 
  remember 
  the 
  metamorphic 
  processes 
  

   to 
  which 
  lavas, 
  in 
  common 
  with 
  all 
  other 
  rocks, 
  are 
  subject, 
  need 
  

   scarcely 
  occasion 
  surprise. 
  Certain 
  of 
  the 
  lavas 
  appear 
  after 
  their 
  

   emission 
  to 
  have 
  weathered 
  into 
  the 
  characteristic 
  spheroidal 
  and 
  

   concentric 
  forms, 
  while 
  others 
  have 
  evidently 
  decomposed 
  into 
  a 
  

   " 
  wackose 
  " 
  condition 
  ; 
  and 
  in 
  both 
  cases 
  the 
  rocks 
  which 
  result 
  from 
  

   the 
  re-induration 
  of 
  such 
  masses, 
  present 
  very 
  peculiar 
  features, 
  and 
  

   may, 
  in 
  some 
  instances, 
  be 
  mistaken 
  for 
  consolidated 
  "trap-tuffs." 
  

  

  The 
  separate 
  lava-flows 
  were 
  often 
  of 
  enormous 
  thickness. 
  As 
  a 
  

   general 
  rule 
  the 
  great 
  body 
  of 
  each 
  of 
  these 
  streams 
  is 
  made 
  up 
  of 
  a' 
  

   compact, 
  often 
  highly 
  porphyritic 
  rock 
  ; 
  but 
  towards 
  the 
  upper 
  and 
  

   under 
  surfaces 
  of 
  the 
  mass, 
  it 
  usually 
  assumes 
  the 
  amygdaloidal 
  

   structure. 
  The 
  amygdaloidal 
  cavities, 
  which 
  have 
  evidently 
  served 
  

   as 
  chemical 
  laboratories 
  in 
  which 
  very 
  complex 
  operations 
  have 
  been 
  

   carried 
  on, 
  are 
  frequently 
  deprived 
  of 
  their 
  contents 
  by 
  recent 
  wea- 
  

   thering 
  operations 
  ; 
  and 
  the 
  original 
  structure 
  of 
  the 
  rocks 
  is 
  thus 
  to 
  a 
  

   great 
  extent 
  restored. 
  Then 
  is 
  made 
  clearly 
  apparent 
  the 
  originally 
  

   highly 
  vesicular 
  character 
  of 
  the 
  upper 
  and 
  under 
  portions 
  of 
  these 
  an- 
  

   cient 
  lava 
  streams, 
  the 
  vesicles 
  being 
  often 
  seen 
  to 
  be 
  drawn 
  out 
  in 
  the 
  

  

  x2 
  

  

  