﻿J. 
  W. 
  TODD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  225 
  

  

  lapilli 
  and 
  ashes, 
  or 
  the 
  vestiges 
  of 
  ancient 
  soils 
  and 
  vegetation, 
  and 
  

   the 
  indications 
  exhibited 
  by 
  the 
  surfaces 
  on 
  which 
  these 
  rock 
  masses 
  

   lie 
  of 
  having 
  been 
  subjected 
  to 
  the 
  action 
  of 
  heat. 
  

  

  One 
  of 
  the 
  most 
  striking 
  points 
  of 
  similarity 
  between 
  these 
  old 
  

   lavas 
  and 
  those 
  seen 
  to 
  be 
  actually 
  connected 
  with 
  existing 
  vol- 
  

   canoes 
  has, 
  through 
  a 
  very 
  prevalent 
  misinterpretation 
  of 
  the 
  ap- 
  

   pearances 
  presented, 
  been 
  generally 
  overlooked. 
  I 
  refer 
  to 
  that 
  re- 
  

   markable 
  peculiarity, 
  connected 
  with 
  the 
  columnar 
  structure, 
  which 
  

   is 
  nowhere 
  better 
  exhibited 
  than 
  in 
  the 
  beautiful 
  caves 
  of 
  Staffa, 
  

   and 
  which 
  has 
  been 
  so 
  clearly 
  described 
  by 
  Mr. 
  Scrops 
  as 
  giving 
  

   rise 
  to 
  such 
  conspicuous 
  features 
  at 
  Pont 
  Gribaud, 
  the 
  Coiron, 
  La 
  

   Gravenne 
  de 
  Souillols, 
  Jaujac 
  and 
  other 
  points 
  in 
  Central 
  France*. 
  

   In 
  all 
  these 
  cases 
  the 
  same 
  lava 
  stream 
  is 
  found 
  to 
  be 
  composed 
  of 
  

   two 
  portions, 
  which 
  at 
  a 
  short 
  distance 
  appear 
  to 
  be 
  very 
  distinctly 
  

   separated 
  from 
  one 
  another. 
  The 
  lower 
  of 
  these 
  divisions, 
  which 
  

   usually 
  occupies 
  about 
  one 
  third 
  of 
  the 
  thickness 
  of 
  the 
  lava 
  stream, 
  

   is 
  composed 
  of 
  very 
  regular, 
  upright, 
  and 
  generally 
  jointed 
  columns, 
  

   the 
  articulations 
  of 
  which 
  often 
  exhibit 
  remarkable 
  curved 
  surfaces 
  

   and 
  angular 
  processes. 
  The 
  upper 
  part 
  of 
  the 
  stream, 
  however, 
  pre- 
  

   sents 
  strikingly 
  different 
  characters, 
  being 
  made 
  up 
  either 
  of 
  nearly 
  

   amorphous 
  basalt 
  or 
  of 
  thickly 
  clustered 
  columns 
  of 
  small 
  diameter, 
  

   these 
  being 
  usually 
  curved 
  and 
  twisted 
  in 
  the 
  most 
  remarkable 
  

   manner. 
  All 
  who 
  have 
  visited 
  Staffa 
  will 
  at 
  once 
  call 
  to 
  mind 
  the 
  

   contrast 
  presented 
  by 
  the 
  thick 
  upright 
  pillars 
  which 
  form 
  the 
  

   " 
  Colonnade 
  " 
  and 
  the 
  sides 
  of 
  Fingal's 
  Cave, 
  and 
  the 
  thin, 
  grace- 
  

   fully 
  curved, 
  and 
  intricately 
  interwoven 
  shafts 
  which 
  form 
  the 
  

   Buchaille, 
  the 
  Clam-shell 
  Cave, 
  and 
  the 
  roof 
  of 
  Fingal's 
  Cave. 
  A 
  

   careful 
  examination 
  will 
  convince 
  the 
  geologist 
  that 
  the 
  two 
  varie- 
  

   ties 
  of 
  columnar 
  basalt 
  form 
  parts 
  of 
  the 
  same 
  lava 
  stream, 
  and 
  are 
  

   not, 
  as 
  is 
  usually 
  stated 
  in 
  guide-books 
  and 
  geological 
  manuals, 
  

   the 
  product 
  of 
  two 
  distinct 
  and 
  superposed 
  flows. 
  In 
  the 
  beautiful 
  

   basaltic 
  columns 
  of 
  the 
  Giant's 
  Causeway, 
  in 
  those 
  of 
  Carsaig, 
  

   Ulva, 
  and 
  many 
  less 
  known, 
  and 
  more 
  inaccessible 
  localities, 
  I 
  have 
  

   been 
  able 
  to 
  verify 
  the 
  correctness 
  of 
  this 
  observation. 
  

  

  Mr. 
  Scrope, 
  in 
  an 
  incidental 
  allusion 
  to 
  the 
  phenomena 
  presented 
  

   by 
  Staffa 
  f, 
  shows 
  that 
  the 
  very 
  striking 
  identity 
  of 
  its 
  features 
  with 
  

   those 
  so 
  clearly 
  described 
  by 
  him 
  as 
  occurriDg 
  in 
  Auvergne, 
  had 
  not 
  

   escaped 
  his 
  observation. 
  He 
  has 
  suggested 
  that 
  the 
  sharp 
  distinc- 
  

   tion 
  in 
  characters 
  between 
  the 
  two 
  portions 
  of 
  the 
  same 
  lava 
  

   stream, 
  is 
  due 
  to 
  the 
  different 
  conditions 
  under 
  which 
  they 
  have 
  

   parted 
  with 
  their 
  heat 
  — 
  that 
  of 
  the 
  upper 
  portion 
  having 
  ©scaped 
  

   by 
  radiation, 
  and 
  that 
  of 
  the 
  lower 
  portion 
  by 
  conduction 
  through 
  

   the 
  subjacent 
  rocks. 
  

  

  3. 
  Subaerial 
  Origin 
  of 
  these 
  old 
  Volcanic 
  rocks. 
  — 
  Accepting, 
  then, 
  as 
  

   amply 
  demonstrated, 
  the 
  conclusion 
  that 
  in 
  these 
  rocks 
  of 
  the 
  North 
  

   of 
  Ireland 
  and 
  the 
  Hebrides 
  we 
  see 
  the 
  vestiges 
  of 
  extensive 
  lava 
  

   streams, 
  the 
  next 
  problem 
  which 
  presents 
  itself 
  to 
  the 
  geologist 
  is 
  

  

  * 
  Vide 
  ' 
  The 
  Geology 
  and 
  Extinct 
  Volcanos 
  of 
  Central 
  France,' 
  2nd 
  edit. 
  

   (1858), 
  pp. 
  57, 
  163, 
  191, 
  &c. 
  

  

  f 
  ' 
  Volcanos,' 
  2nd 
  ed. 
  (1872) 
  p. 
  99. 
  

  

  